Molecular Biology Of The Cell Flashcards

Question

Name the 2 enzymes involved in fatty acid biosynthesis

Answer 1

1) Acetyl CoA Carboxylase | 2) Fatty acid synthase

Answer 2

In adults, de novo biosynthesis is restricted mainly to the liver, adipose tissue and lactating breast. Evidence suggests that reactivation of fatty acid synthesis also occurs in certain cancer cells. The inhibition of FASN by cerulenin (an antifungal antibiotic) has shown to reduce Timor growth of obstinate cancer cells.

Answer 3

A family of different acyl-CoA-dehydrogenase catalyse the initial step in each cycle of beta-oxidation within the mitochondrial matrix. Each acyl-CoA-dehydrogenase can bind a fatty acid chain of varying lengths: >Short-chain acyl-CoA-dehydrogenase (<6C) >Medium-chain acyl-CoA-dehydrogenase (6C-12C) >Long-chain acyl-CoA-dehydrogenase (13C-21C) >Very long-chain acyl-CoA-dehydrogenase (>22C)

Answer 4

This is an autosomal recessive disorder mainly occurring in Caucasians. It occurs in 1 in 10,000 births in the UK per year. If undiagnosed, it can be fatal. It is thought to account for 1% of deaths from Sudden Infant Death Syndrome (SIDS). If diagnosed, using a MCADD screening (heel prick test on newborns), patients should never go without food for longer than 10-12 hours and adhere to a high carbohydrate diet. Patients with an illness resulting in appetite loss or severe vomiting may need i.v. glucose to make sure that the body is not dependent on fatty acids for energy.

Answer 5

The brain and nervous system accounts 2% of the body’s total mass, but uses 20% of resting metabolic rate. It requires a continuous supply of glucose, as it cannot metabolise fatty acids. Ketone bodies (e.g. beta-hydroxybutyrate) can partially substitute for glucose. Too little glucose (hypoglycaemia) causes faintness and coma. Too much glucose (hyperglycaemia) can cause irreversible damage. Even in the fasting state, glucose remains the main metabolic fuel of the brain.

Answer 6

Skeletal muscle account for about 40% of the body’s total mass. It’s ATP requirements vary depending on exercise. The requirements of light contraction of skeletal muscle, are met by oxidative phosphorylation, oxygen, together with glucose and fatty acids in the blood being used as fuel for the muscle. During vigorous contraction, oxygen becomes a limiting factor as ATP consumption is faster than the ATP supply from oxidative phosphorylation. This leads to the muscle stores of glycogen being broken down to produce ATP. Under anaerobic conditions, pyruvate is converted to lactate, which leaves the muscle and reaches the liver via the blood.

Answer 7

The heart account for 1% of total body mass but uses 10% of resting metabolic rate. As it must beat constantly, it is therefore designed for completely aerobic respiration, making it rich in mitochondria, utilising the TCA cycle substrates (e.g, free fatty acids, ketone bodies). Loss of oxygen supply to the heart can therefore be devastating, as it leads to cell death and myocardial infarction when the energy demand becomes much greater than the energy supply. The main metabolic fuel of the heart in the fasting state is fatty acids.

Answer 8

The liver undertake a wide repertoire of metabolic processes: 1) It is highly metabolically active 2) It can convert nutrient types. 3) It plays a central role in maintains blood glucose at 4.0-5.5 mM. 4) It is a storage organ, storing glucose as glycogen. 5) It plays a key role in lipoprotein metabolism (the transport of triglycerides and cholesterols).

Answer 9

1) Carbohydrates are broken down into simple sugars and enter the glycolytic pathway leading to the production of pyruvate. 2) Decarboxylation and reduction of pyruvate produces acetyl CoA which can enter the TCA cycle. This cycle produces reduced co-factors which are reoxidised by the electron transport chain which in turn is coupled to ATP production (Oxidative phosphorylation). 3) During extreme exercise, the ATP demands of the muscle outstrip the oxygen supply needed for aerobic respiration and lactate is produced. 4) During fasting, rather than enter the TCA, much of the acetyl CoA produced results in ketone body production.

Answer 10

Excess glucose-6-phosphate can be used to generate glycogen in liver and muscle. Similarly, excess Acetyl CoA can be used to generate fatty acids, which are stored as triglycerides in adipose tissue.

Answer 11

1) Pyruvate and other TCA cycle intermediates can also be a source of some amino acids. The backbone of these molecules can be used to used to make nucleotides. 2) Glucose-6-phosphate via the pentose phosphate pathway can also be used as a source for nucleotide production in a pathway that generates the bulk of the NADPH needed for anabolic pathways e.g. cholesterol synthesis.

Answer 12

During fasting, if plasma glucose concentrations fall below 3mM then the body will enter a hypoglycaemic coma. In the short term, to avoid hypoglycaemia the body can: 1) Breakdown of liver glycogen stores occurs to maintain plasma glucose levels. 2) Releases free fatty acids from adipose tissue. 3) Convert Acetyl CoA into ketone bodies via the liver.

Answer 13

Both fatty acids and ketone bodies can be used by muscle, making more of the plasma glucose available for the brain. However, within 12-18 hr all glycogen stores are typically exhausted, hence the need for another pathway to generate glucose – gluconeogenesis.

Answer 14

1) Lactate is generated by skeletal muscle during strenuous exercise, when the rate of glycolysis exceeds the rate of the TCA cycle and the electron transport chain. Lactate can be taken up by the liver and utilised to regenerate pyruvate by lactate dehydrogenase (LDH), also known as the Cori cycle. 2) Amino acids can be derived from the diet or during times of starvation (e.g. from the breakdown of skeletal muscle). 3) Triglyceride hydrolysis yields fatty acids and glycerol, the glycerol backbone being used to generate dihydroxyyacetone phosphate (DHAP) a molecule you may recall from step 5 of glycolysis.

Answer 15

As the three essentially irreversible reactions, catalysed by the kinases hexokinase, phosphofructokinase and pyruvate kinase in glycolysis, have to be bypassed with four additional enzymes in gluconeogenesis. ΔG for the straight reversal of glycolysis would be +90 kJ/mol which is energetically unfavourable. ΔG for gluconeogenesis is -38 kJ/mol.

Answer 16

1) Pyruvate 2) Acetyl CoA, 3) Acetoacetyl CoA, 4) α-Ketoglutarate, 5) Succinyl CoA, 6) Fumerate and 7) Oxaloacetate. Urea is lost as a waste product.

Answer 17

Their skeletons can give rise to glucose via gluconeogenesis.

Answer 18

Give rise to skeletons which cannot enter gluconeogenesis but can be used to synthesise fatty acids and ketone bodies. Fatty acids can also be converted into ketone bodies and used by tissues such as muscle and brain.

Answer 19

Triglycerides are broken down into fatty acids and glycerol. Glycerol can be converted to DHAP and enter the gluconeogenic pathway upstream.

Answer 20

Fatty acids cannot be converted into glucose by gluconeogenesis. 2C atoms enter the TCA cycle as acetyl CoA by combining with oxaloacetate to form citrate. As the cycle progresses, two carbon atoms are sequentially lost as CO2 before oxaloacetate is eventually regenerated. Hence, no net synthesis of oxaloacetate or pyruvate is possible from acetyl CoA .

Answer 21

During moderate levels of exercise, where oxygen supply is adequate, the ATP demands of muscle can be met by oxidative phosphorylation using glucose and other substrates as fuels. Glucose is transported from the blood into muscle cells where it can undergo metabolism by glycolysis and the TCA cycle to ultimately generate ATP by the re-oxidation of cofactors.

Answer 22

As muscle contracts, the demand for ATP increases (e.g. requirements of muscle actomyosin ATPase and cation balance). Increased demand for glucose is met by an increase in the number of glucose transporters on the membranes of muscle cells.

Answer 23

Adrenalin plays a key role in meeting the demand for ATP by increasing the rate of glycolysis in muscle, increasing the rate of gluconeogensis by the liver and increasing the release of fatty acids from adipocytes.

Answer 24

Under anaerobic conditions, the demands of the contracting muscle for ATP cannot be met by oxidative phosphorylation and similarly, the transport of glucose from the blood cannot keep up with the demands of glycolysis. Glycogen within the muscle is therefore broken down to meet these demands. To replenish NAD+ levels and maintain glycolysis, pyruvate is taken up by the liver and converted into lactate by lactate dehydrogenase. Lactate can then be used by the liver to generate glucose by gluconeogenesis.

Answer 25

Blood glucose concentrations are typically maintained at around 4mM. Control of metabolic pathways is typically centred around reactions that are irreversible steps. At these points, increases in the rate of enzyme activity greatly increases the rate of the downstream steps. For the greatest levels of control it is desirable that these control steps are reasonably early in the pathway. Control can be at several levels including: 1) product inhibition 2) under the influence of signalling molecules such as hormones

Answer 26

Hexokinase catalyses the first irreversible step in the glycolysis pathway. Muscles and the liver contain suitably different forms (isoforms) of this enzyme. Both isoforms catalyse the same reaction. However they are maximally active at different concentrations of glucose.

Answer 27

Parameters such as the Michaelis constant (KM) allows comparison of relative activities of enzymes, by indicating the concentration of substrate at which an enzyme functions at a half-maximal rate (Vmax).

Answer 28

The KM of Hexokinase I found in muscle is 0.1mM, which means it is active at low concentrations of glucose and is essentially operating at maximal velocity at all times.

Answer 29

Hexokinase I is also highly sensitive to inhibition by the product glucose-6-phosphate. This means that under anaerobic conditions when the rate of the TCA cycle drops, and glycolysis therefore slows, Hexokinase I is inhibited by accumulating levels of glucose-6-phosphate.

Answer 30

Hexokinase IV found in liver, has a high KM of around 4mM and is much less sensitive to blood glucose concentrations than Hexokinase I. It is also less sensitive to the inhibitory effects of glucose-6-phosphate.

Answer 31

Glucose 6-phosphatase, found in the liver but not in muscle, can catalyse the reverse reaction to hexokinase, generating glucose from glucose-6-phosphate.

Answer 32

1) Insulin: secreted when glucose levels rise: it stimulates uptake and use of glucose and storage as glycogen and fat. It also suppresses the production of glucagon. 2) Glucagon: secreted when glucose levels fall: it stimulates production of glucose by gluconeogenesis and breakdown of glycogen and fat. (both are secreted by islets of the pancreas). 3) Adrenalin (or epinephrine): has strong and fast metabolic effects to mobilise glucose for “flight or fight”. 4) Glucocorticoids: steroid hormones which increase synthesis of metabolic enzymes concerned with glucose availability.

Answer 33

On having a meal, blood glucose levels initially rise which is controlled by increased secretion of insulin (and reduced glucagon) from islets. This has several effects including: 1) Increased glucose uptake by liver – used for glycogen synthesis and 2) Glycolysis (acetyl-CoA produced is used for fatty acid synthesis). 3) Increased glucose uptake and glycogen synthesis in muscle. 4) Increased triglyceride synthesis in adipose tissue. 5) Increased usage of metabolic intermediates due to a general stimulatory effect on the body's synthesis and growth.

Answer 34

After a meal blood glucose levels start to fall and are controlled by: 1) Increased glucagon secretion (and reduced insulin) from islets. 2) Glucose production in liver resulting from glycogen breakdown and gluconeogenesis. 3) Utilisation of fatty acid breakdown as alternative substrate for ATP production (important for preserving glucose for brain).

Answer 35

Adrenalin also stimulates glucose production in the liver, but also stimulates skeletal muscle towards glycogen breakdown and glycolysis, and adipose tissue towards fat lipolysis to provide other tissues with alternative substrate to glucose.

Answer 36

After prolonged fasting (i.e. longer than can be covered by glycogen reserves): 1) The glucagon/insulin ratio increases further 2) Adipose tissue begins to hydrolyse triglyceride to provide fatty acids for metabolism 3) TCA cycle intermediates are reduced in amount to provide substrate for gluconeogenesis 4) Protein breakdown provides amino acid substrates for gluconeogenesis 5) Ketone bodies are produced from fatty acids and amino acids in liver to substitute partially the brain’s requirement for glucose

Answer 37

Diabetes mellitus is a disorder of insulin release and signalling, resulting in an impaired ability to regulate blood glucose concentrations. The overall effect is that metabolism is controlled as if the person is undergoing starvation, regardless of dietary glucose uptake.

Answer 38

1) Type I diabetes in which individuals fail to secrete enough insulin (β-cell dysfunction). 2) Type II diabetes in which individuals fail to respond appropriately to insulin levels (insulin resistance).

Answer 39

1) Hyperglycaemia with progressive tissue damage (e.g. retina, kidney, peripheral nerves) 2) Increase in plasma fatty acids and lipoprotein levels with possible cardiovascular complications 3) Increase in ketone bodies with the risk of acidosis 4) Hypoglycaemia with consequent coma if insulin dosage is imperfectly controlled

Answer 40

1) Glucagon is important in protection against hypoglycaemia. 2) A major site of action is the liver where glucagon stimulates gluconeogenesis and glycogenolysis. 3) Insulin deficiency and relative excess of glucagon leads to increased hepatic output of glucose and, thus, hyperglycaemia.

Answer 41

It is a complex network of macromolecules (proteins and carbohydrates) filling spaces between cells and compromises of both fibrillar and non-fibrillar components. It is formed from the material deposited by cells which forms the “insoluble” part of the extracellular environment. It is generally composed of fibrillar (or reticular) proteins (e.g. collagens, elastin) embedded in a hydrated gel (proteoglycans or “ground substance”). It may be poorly organised (e.g. loose connective tissue) or highly organised (e.g. tendon, bone, basal lamina)

Answer 42

1) Provides physical support 2) Determines the mechanical and physicochemical properties of the tissue 3) Influences the growth, adhesion and differentiation status of the cells and tissues with which it interacts 4) Essential for development, tissue function and organogenesis

Answer 43

These are particularly rich in Extracellular matrix and component cells. All connective tissues have a distinct spectrum of collagens (types I, II, III and IV - basement membrane), multi-adhesive glycoproteins (fribronectin, fibrinogen and laminin - basement membrane) and proteoglycans (aggrecan, versivan, decorin and perlecan - basement membrane), that make them unique. Matrix components interact with specific cell surface receptors.

Answer 44

The different types and arrangement of collagen coupled with the presence or absence of different components of the Extracellular matrix, gives a wide variety of connective tissues: 1) Vitreous humour (jelly that fills the interior of the eye): relatively soft and transparent. 2) Tendon and skin: tough and flexible 3) Bone: hard and dense 4) Cartilage: resilient and shock absorbing

Answer 45

1) Gene mutations affecting matrix proteins 2) Gene mutations affecting ECM catabolism 3) Fibrotic disorders due to excessive ECM deposition 4) Excessive loss of ECM

Answer 46

They are a family of fibrous proteins found in all multicellular organisms. They are major proteins in bone, tendon and skin, and are the most abundant proteins in mammals (25% of total protein mass).

Answer 47

1) Skin: successive layers nearly at right angles to each other 2) Mature bone and cornea: same arrangement These tissues resist tensile force in all directions

Answer 48

28 collagen types exist in humans, designated by roman numerals. There are 42 genes encoding collagens in humans. Each collagen molecule comprises three alpha chains, forming a triple helix. Collagen molecules can be composed of one or more different alpha chains.

Answer 49

Type I collagen is a heterotrimer, has chains from two different genes - its composition is [alpha1(I)]2 [alpha2(I)].

Answer 50

Types II and III collagen are homotrimers, having only one chain type each – their compositions are, therefore, [alpha1(II)]3 and [alpha1(III)]3, respectively.

Answer 51

It has a characteristic gly-x-y repeat, where x is often proline and y is often hydroxyproline. In fibrillar collagens, each alpha chain is approximately 1000 amino acids, forming a left-handed helix. Three alpha chains form a stiff triple helical structure – every third position must be occupied by glycine, which is small enough to occupy the interior (H side chain).

Answer 52

Three alpha chains form a triple-stranded collagen molecule, which can associate to form fibrils. Fibrils can then come together to form collagen fibre.

Answer 53

All newly synthesised collagen chains have non-collagenous domains at N- and C-termini. These domains are removed after secretion in the case of fibrillar collagens but remain part of the collagen in most other types.

Answer 54

Crosslinking provides tensile strength and stability. Both lysine and hydroxy-lysine residues are involved. The type and extent of cross-links is tissue specific and changes with age.

Answer 55

Prolyl and lysyl hydroxylation, in protein produces hydrolysine and hydroxyproline. Their hydroxylases require Fe2+ and vitamin C and contributes to interchain hydrogen bond formation. Lysine and hydroxylysine are also modified in the formation of covalent crosslinkages. This takes place only after the collagen has been secreted. Vitamin C-deficiency results in underhydroxylated collagens, with dramatic consequences for tissue stability (scurvy).

Answer 56

Ehlers–Danlos syndromes (EDS) are a group of inherited connective tissue disorders whose symptoms include stretchy skin and loose joints.Several disoders arise due to mutations in collagen, which negatively affect: •collagen production •collagen structure •collagen processing

Answer 57

No, some collagens do not form fibrils. These include: 1) Fibril-associated collagens (e.g types IX and XII): associate with fibrillar collagens and regulate the organisation of collagen fibrils. 2) Type IV collagen: a network-forming collagen and is present in all basement membranes, though its molecular constitution varies from tissue to tissue.

Answer 58

Basement membranes (BMs, also called basal laminae) are flexible, thin mats of extracellular matrix underlying epithelial sheets and tubes. They surround muscle, peripheral nerve and fat cells and underlie most epithelia. They are highly specialised extracellular matrices containing a distinct repertoire of collagens, glycoproteins and proteoglycans. In the kidney, they form a key part of the filtration unit as the Glomerular basement membrane (GBM).

Answer 59

Whereas collagens are important for the tensile strength of tissues, elastic fibres are important for the elasticity of tissues, such as skin, blood vessels and lungs. Often, collagen and elastic fibres are interwoven to limit the extent of stretching. Elastic fibres consist of a core made up of the protein elastin, and microfibrils, which are rich in the protein fibrillin.

Answer 60

The integrity of elastic fibers depends upon microfibrils, containing the protein fibrillin. Mutations in the protein fibrillin-1 are associated with Marfan’s syndrome which has some diverse manifestations, involving primarily the skeletal, ocular, and cardiovascular systems.

Answer 61

Elastin is an unusual protein consisting of two types of segments that alternate along the polypeptide chain: hydrophobic regions, and α-helical regions rich in alanine and lysine. Many lysine side chains are covalently cross-linked.

Answer 62

Laminins are heterortrimeric proteins made up of an α chain, a β chain and a γ (gamma) chain, which form a cross shaped molecule. Laminins are very large proteins with each chain having a molecular weight of between 160 and 400 kDa. Laminins are multi-adhesive proteins which can interact with a variety of cell surface receptors including integrins and dystroglycan. They can self-associate as part of the basement membrane matrix, but can also interact with other matrix components such as type IV collagen, nidogen and proteoglycans.

Answer 63

Fibronectins are a family of closely related glycoproteins of the extracellular matrix which are also found in body fluids. They can exist either as an insoluble fibrillar matrix or as a soluble plasma protein. They are derived from a single gene, with alternate splicing of mRNAs giving rise to the different types. Like laminin, fibronectins are multi-adhesive proteins, made up of a large multidomain molecule linked together by disulphide bonds.

Answer 64

Again, like laminin, fibronectins are able to interact with cell surface receptors and other matrix molecules. They play important roles in regulating cell adhesion and migration in a variety of processes, notably embryogenesis and tissue repair. They are also important for wound healing, helping to promote blood clotting. Fibronectins can also bind multiple ligands and cell receptors.

Answer 65

Proteoglycans are core proteins to which are covalently attached to one or more glycosaminoglycan (GAG) chains. Small proteoglycans can have a single GAG chain attached, whereas some large proteoglycans carry up to 100 GAG chains.

Answer 66

GAG chains are made up of repeating disaccharide units with one of the two sugars being an amino sugar (a sugar in which a hydroxyl group is replaced with an amine group). Many GAGs are sulfated or carboxylated, and as a result carry a high negative charge. This charge attracts a cloud of cations including Na+, resulting in large amounts of water being sucked into the extracellular matrix.

Answer 67

1) Basement membrane proteoglycans : e.g. perlecan 2) Aggregating proteoglycans (interact with hyaluronan): e.g. aggrecan 3) Small leucine-rich proteoglycans: e.g.decorin 4) Cell surface proteoglycans: e.g. syndecans 1-4

Answer 68

Cartilage has a matrix rich in collagen with large quantities of GAGs trapped within the meshwork. The balance of swelling pressure is negated by the tension in the collagen fibres, generating great tensile strength. For example the cartilage lining the knee joint (synovial cartilage) can support pressures in excess of hundreds of Kg/cm2.

Answer 69

1) Hyaluronan 2) Chondroitin sulfate and dermatan sulfate 3) Heparan sulfate 4) Keratan sulfate

Answer 70

Hyaluronan (also called hyaluronic acid) is found in the extracellular matrix of soft connective tissues. It is distinct form the other GAGs as it is simply a carbohydrate chain without a core protein. It is unsulfated and made up of repeating disaccharides which can number up to 25,000 sugars. It can undergo a very high degree of polymerization, typically in the range of 10,000 disaccharides creating molecules of enormous sizes. This means that hyularonan chains can occupy a relatively large volume. It is typically of high viscosity e.g. in the vitreous humour of the eye and in synovial fluid of joints. In the latter location, hyaluronan plays a key role in protecting the cartilaginous surface from damage.

Answer 71

Aggrecan is a major constituent of the cartilage extracellular matrix. In aggrecan, the GAGs are highly sulfated, increasing their negative charge. Also present are large numbers of negatively carboxyl groups. These multiple negative charges attract cations such as Na+ that are osmotically active. This in turn leads to large quantities of water being retained by the highly negatively charged environment. Under compressive load, water is given up, but regained once the load is reduced. Therefore, aggrecan in the cartilage matrix is perfectly suited to resist compressive forces.

Answer 72

These are a group or groups of cells whose type, organisation and architecture are integral to its function. Tissues are made up of cells, extracellular matrix and fluid.

Answer 73

1) Connective tissue cells: fibroblasts (many tissues), chondrocytes (cartilage), osteocytes (bone). Mesenchymal (connective tissue and muscle) cancers are sarcomas. 2) Contractile tissues: skeletal muscle, cardiac muscle, smooth muscle. 3) Haematopoietic cells: blood cells, tissue-resident immune cells, and the cells of the bone marrow from which they are derived. Haemopoietic cancers are leukaemias (from bone marrow cells) or lymphomas (from lymphocytes). 4) Neural cells: cells of the nervous system having two main types; neurones (carry electrical signals) and glial cells (support cells). Neural cell cancers are neuroblastomas (from neurones) or gliomas (from glial cells). 5) Epithelial cells: cells forming continuous layers, these layers line surfaces and separate tissue compartments and have a variety of other functions. Epithelial cancers are carcinomas.

Answer 74

Epithelial cells make organised, stable cell-cell junctions to form continuous, cohesive layers. Epithelial layers line internal and external body surfaces and have a variety of functions (e.g. transport, absorption, secretion, protection). Cell-cell junctions are key to the formation and maintenance of epithelial layers.

Answer 75

``` The two main criteria of epithelial classification are: >their shape: •squamous - flattened plate-shape •cuboidal - cuboid •columnar - arranged in columns > Their layering: •single layer - simple epithelium •multi-layered - stratified epithelium This classification is related to types of epithelial function. ```

Answer 76

These arrangements are found in the lung alveolar (air sac) epithelium, mesothelium (lining major body cavities), endothelium lining blood vessels and other blood spaces). They form a thin epithelium that allows exchange to occur (e.g. gas exchange in the alveoli).

Answer 77

These epithelial cell arrangements are typical of the linings found in ducts (e.g. those lining the kidney collecting ducts).

Answer 78

These epithelial cell arrangements are typical of surfaces involved in absorption and secretion of molecules (e.g. enterocytes lining the gut, involved in the take up of the breakdown products of digestion).

Answer 79

1) Keratinizing: Epithelial cells which produce keratin and in doing so die becoming thicker, stronger, protective structures (e.g. epidermis - skin epithelium). Such cells lose their cellular organelles and nuclei, which are not visible under light microscopy. Keratinizing epithelium can form thick layers that protect underlying tissues for various physical and chemical insults (e.g. heat, cold, solvents (alcohol), abrasion, etc). 2) Non-keratinizing: Epithelial cells which do not undergo keratinisation. They retain their nuclei and organelles (e.g. epithelium lining the mouth, oesophagus, anus, cervix and vagina). In the various layers, the cell shapes vary. The squamous classification relates to the surface cells.

Answer 80

This epithelium appears to be multi-layered and are found in: the airway (trachea and bronchi) epithelium, various ducts in the urinary and reproductive tracts. On close examination, the surface cells have contact with the basal lamina.

Answer 81

In a typical epithelial cell, the membrane can be seen to be organised into discrete domains by the formation of junctions. This membrane polarity is the key to generating a distinct polarity, with an apical domain at the lumenal (open) surface and a basolateral domain. The basal surface in contact with the extracellular matrix. The membrane between these two surfaces, where membranes of adjacent cells appose each other, is the lateral membrane. Most epithelial functions are directional, e.g. secretion, fluid and solute transport and absorption. These processes are not random but are highly organised. Epithelial polarity is required to give the directionality needed for epithelial function. Polarity in epithelial cells is is seen as different regions of the cell surface being different from one another, with discretely organised cellular contents.

Answer 82

The pumps and channels involved in transporting ions and fluids across epithelial layers, need to be polarised. When they’re not polarised, that means that they’re present in all parts of the plasma membrane, so they’re pumping apically and basal laterally. Consequently, the direction flow is in all directions meaning that there is no desired net directional flow. If the transporters and channels are polarised, then directionality can’t be achieved because only one aspect of the plasma membrane is being pumped on, allowing the flow to be in the desired direction.

Answer 83

Most epithelial secrete in one direction only, either to the apical aspect into a lumen, or to a basal aspect into the interstitial space. In order to do that, the secretory machinery has to be polarised. If it were unpolarised, epithelial cells would secrete into both the apical and basal compartments, which could be catastrophic if secreting digestive enzymes into the basal aspect, as the body would digest its own tissues.

Answer 84

1) Tight junction: form a belt, usually around the apical lateral membrane. They seal the gaps between the cells. 2) Adherens junction: essentially the master junction which controls the formation of all of the other junctions. 3) Desmosomes: spot junctions, scattered throughout the lateral membrane, that form mechanically tough junctions between cells. They are important in tissues that require to resist mechanical stresses. 4) Gap junction: a channel forming junction, that form pores between cells and allow cells to exchange and share materials. These act as communicating junctions to allow cells to form communities and synchronise a number of activities.

Answer 85

In these epithelia, the plasma membranes contain high concentrations of ion transporters. Typically, mitochondria are closely associated with extensive basal membrane infoldings, providing energy for active transport across the abundant membranes. The infoldings increase the amount of basal membrane that can pump ions and water. Mitochondria are concentrated in the basal aspect of the cell, close to the basal infoldings which contain the active transporters. Because active transport is mainly confined to the basal membranes, ion and water transport will move directionality.

Answer 86

The interior surface of the wall of the small intestine is folded into numerous finger-like processes that point into the interior, called villi. The villi are covered with intestinal epithelial cells. The dense-microvillus brush-border, contains large amounts of active transporters and channels for the uptake of nutrients from the lumen of the gut. As the concentration of nutrients increases in the cytoplasm of the absorptive cells, it diffuses down its concentration gradient into the basal interstitial space to be collected in the capillaries and distributed in the circulation.

Answer 87

In tissues whose main purpose is secretion, the epithelium is often arranged in tubules and glands of varying complexity. However, in many epithelial tissues, individual, dispersed secretory cells can be present in the epithelium. There are two mains types of secretion: exocrine (into a duct or lumen) and endocrine (into the bloodstream). Endocrine and exocrine cells can be seen to have distinct arrangements of their organelles. In an exocrine secretory cell, the organelles are arranged for secretion from the apical plasma membrane. Endocrine cells secrete their contents to the basal aspect. The basal aspects of endocrine secretory cells surround a thin-walled capillary. The secretory vesicles are positioned so that when their contents are released, they have close access to the blood circulation.

Answer 88

1) Constitutive – secretory vesicles, as they are formed, move directly to the plasma membrane and release their contents, e.g. production of plasma proteins by hepatocytes (constitutive endocrine secretion). 2) Stimulated – secretory vesicles are stored in the cytoplasm and only fuse with the plasma membrane to release their contents, e.g. the release of adrenaline from cells of the adrenal medulla after a fight-or-flight stimulus (stimulated endocrine secretion); when stomach contents enter the duodenum, pancreatic acinar cells are stimulated to release their digestive enzymes into ducts (stimulated exocrine secretion).

Answer 89

As a cell migrates up the villus epithelium, new cells are constantly being produced by the crypt stem cells, to replace the cells constantly being lost from the villus tip.

Answer 90

This occurs in cancer chemotherapy and results in loss of the finger-like intestinal villi and flattening of the intestinal mucosa. This is responsible for many of the gastro-intestinal disturbances that are side-effects of chemotherapy. Cell loss from the villus tips continues as normal, but the failure to produce new cells to replace the lost cells results in a loss of tissue and the villi shorten.

Answer 91

The epidermis is the keratinising stratified squamous epithelium of the body’s surface. Surface cells are constantly being lost, but are replaced by new cells being formed in the basal layer which migrate up while undergoing a programme of differentiation that eventually leads to them flattening out and keratinising. Each layer replaces the one above as the layers are lost from the surface.

Answer 92

In contrast to a loss of proliferation, hyperproliferation of epithelial cells results in increased cell numbers and a thickening of cell layers. This can be in response to repeated or constant pressure. If the increase in cell production is greater than the cell loss from the surface, cells will accumulate creating an increased thick hard layer e.g. pressure and abrasion to areas of the skin results in local hyperproliferation leading to “hard skin” or “corns”. Infectious agents such as papilloma virus can also induce hyperproliferation. They do this by hijacking the cellular machinery of stratified squamous epithelia and inducing increased cell proliferation, which results in a surface growth, e.g. a wart as shown below.

Answer 93

Cholesterol is a steroid composed of 27 carbon atoms. It is composed of cyclic rings with a hydrophobic tail. The steroid ring structure is planar. Apart from the hydroxyl group at position 3, the molecule is very hydrophobic, consisting only of carbon and hydrogen atoms.

Answer 94

Cholesterol is a vital component of cell membranes, indeed, more than 90% of the cholesterol in our bodies is found in cell membranes. A key property of cholesterol is that it can increase and decrease membrane stiffness, depending on the temperature and the nature of the membrane.

Answer 95

Dietary cholesterol uptake in humans is limited to around 500mg/day. Given the great need for cholesterol as a membrane component, all physiological requirements for cholesterol are supplied by the liver through de novo synthesis of cholesterol from acetyl-CoA.

Answer 96

1) Synthesis of isopentenyl pyrophosphate, an activated isoprene unit which serves as a key building block (in the cytoplasm). 2) Condensation of six molecules of isopentenyl pyrophosphate to form squalene (in the cytoplasm). 3) Cyclisation and demethylation of squalene by monooxygenases to give cholesterol (in the ER).

Answer 97

2 molecules of Acetyl-CoA condense to form the molecule Acetoacetyl CoA under the catalysis of the enzyme beta-ketothiolase.

Answer 98

Another molecule of Acetyl CoA condenses with Acetoacetyl CoA, under the catalysis of HMG-CoA synthase, to form 3-Hydroxy-3methylglutaryl CoA (HMG-CoA).

Answer 99

HMG CoA is reduced, under the catalysis of HMG-CoA reductase, to generate Mevalonate. HMG-CoA reductase is under negative feed back control by the end product cholesterol, the intermediate mevalonate and bile salts.

Answer 100

Mevalonate undergoes sequential phosphorylation at the hydroxyl groups at position 3 and 5, followed by decarboxylation to form 3-Isopentenyl pyrophosphate. This activated isoprene unit is a useful building block for further synthesis.

Answer 101

Dolichol phosphate is a specialized lipid molecule located in the ER membrane and involved in N-linked glycosylation of proteins. Likewise, proteins can undergo lipid modifications such as prenylation (addition of farnesyl or a geranyl-geranyl moiety to C-terminal cysteine residues) which gives them affinity for lipid bilayers. The same lipophilic properties of the isoprene unit confine ubiquinone to the inner membrane of mitochondria.

Answer 102

Via an isomerization reaction, Dimethylallyl pyrophosphate can be produced from isopentyl PP. This can condense with a unit of Isopentenyl-PP to form the C10 compound Geranyl-PP. A third isopentenyl-PP molecule is added to form the C15 intermediate farnesyl-PP.

Answer 103

Two farnesyl-PP molecules condense to form C30 squalene plus 2 molecules of pyrophosphate.

Answer 104

Squalene is cyclized to cholesterol, in 3 steps (2 which are in phase 7): 1) Squalene is first reduced in the presence of oxygen and NADPH to form squalene epoxide which has a different C=C bond distribution priming the molecule for carbon ring fusion. 2) The enzyme squalene epoxide lanosterol-cyclase catalyses the formation of Lanosterol. A series of 1,2-methyl group and hydride shifts along the chain of the squalene molecule result in the formation of the four rings.

Answer 105

3) Lanosterol is subsequently reduced and three methyl units removed (demethylated) to generate cholesterol.

Answer 106

They are key in emulsifying dietary fats due to their hydrophobic and hydrophillic faces. Bile salts are the major breakdown products of cholesterol and account for about half of the 800 mg of cholesterol made each day by the liver.

Answer 107

Cholesterol can be converted by a series of reactions into the primary bile salt, glycocholate and also taurocholate. The polar groups attached to the cholesterol core unit, form the hydrophilic face of the bile salts.

Answer 108

The precursor pregnenolone is generated from cholesterol by the action of the enzyme desmolase. All five classes of steroid hormones come from pregnenolone: Progestagens, glucocorticoids, mineralocorticoids, androgens and oestrogens.

Answer 109

Vitamin D is a collective term for a group of steroids which are vital for the intestinal absorption of important ions needed for bone development, namely calcium, phosphate and magnesium. The bulk of the Western diet is low in vitamin D and so the main source is from the activity of UV light upon 7-dehydrocholesterol in the epdermis of the skin. A deficient of Vitamin D3 in childhood leads to rickets, a defect of bone development in children.

Answer 110

Calcitriol is the most active vitamin D metabolite and plays a key role in calcium metabolism. It functions as a steroid hormone, binding to vitamin D response elements (VDREs) in the promoter of target genes and inducing key genes involved in bone metabolism.

Answer 111

It is a monogenic dominant trait, in which cholesterol transportation is defective. Individuals who carry a single copy of a mutant gene (heterozygotes) have cholesterol levels approximately 2-3 times higher than in normal people and are susceptible to atherosclerosis (hardening of the arteries) in middle age. Homozygotes who carry two copies of a mutant gene are severely affected. Their serum cholesterol levels are five times higher than in healthy individuals and severe atherosclerosis and coronary infarction may be observed in adolescence. Deposition of LDL-derived cholesterol in the coronary arteries can lead to their occlusion and myocardial infarction.

Answer 112

Cholesterol - in the form of LDL - is taken up by a specific receptor molecules on the cell surface - the LDL receptor (LDLR). Fibroblasts from patients with severe FH, lacked functional LDLRs. Mutations in several domains of the LDL receptor lead to FH; indeed this seems to be a bit of a "hotpot" in the genome for mutations with over 1000 different LDLR mutations reported in the literature. This can affect either receptor expression, LDL binding or LDLR endocytosis and recycling, all of which can manifest in FH.

Answer 113

1) Inhibition of de novo cholesterol synthesis by the liver, using HMG-CoA reductase inhibitors/statins (e.g. Lipitor (Pfizer), Crestor (Astra Zeneca). These are some of the best-selling pharmaceuticals in history! Pfizer reported peak annual sales of almost $13 billion in the 2000s before they went off patent in 2011. 2) The reduction of dietary cholesterol absorption by the intestine, using resins or sequestrants (e.g. Cholestyramine - brand names: Questran, Prevalite). These bind or sequester bile acid-cholesterol complexes preventing their reabsorption by the intestine. They can lower LDL ("bad" cholesterol) by 15 -30% and raise HDL ("good" cholesterol) by 3 - 5%.

Answer 114

1) Lethal: produces cell death | 2) Sublethal: produces injury not amounting to cell death. May be reversible or progress to cell death.

Answer 115

1) Oxygen deprivation 2) Chemical agents 3) Infectious agents 4) Immunological reactions 5) Genetic defects 6) Nutritional imbalances 7) Physical agents 8) Aging

Answer 116

1) The type of injury 2) The duration of the injury 3) The severity of the injury

Answer 117

1) The type of cell injured | 2) The status of the injured cell

Answer 118

1) Cell membrane integrity 2) ATP generation 3) Protein synthesis 4) The integrity of the genetic apparatus

Answer 119

The structural and biochemical components of a cell are so integrally related that multiple secondary effects rapidly occur. Cellular function is lost before cell death occurs which in turn occurs before the morphological changes are seen.

Answer 120

Shrinkage in the size of the cell (or organ) by the loss of cell substance.

Answer 121

Increase in the size of cells and consequently an increase in the size of the organ. This can be physiological or pathological. It is caused either by increased functional demand or specific hormonal stimulation.

Answer 122

It increase in the number of cells in an organ, that can be physiological or pathological. Physiological hyperplasia can be either hormonal or compensatory. Pathological hyperplasia is usually due to excessive hormonal or growth factor stimulation.

Answer 123

A reversible change in which one adult cell type is replaced by another. It may be physiological or pathological.

Answer 124

Precancerous cells which show the genetic and cytological features or malignancy but not invading the underlying tissue.

Answer 125

1) Fatty change 2) Cellular swelling These are examples of degenerative changes (i.e. changes associated with cell and tissue damage).

Answer 126

Confluent cell death associated with inflammation (accompanying infiltration of neutrophils).

Answer 127

1) Coagulative necrosis (e.g myocardial infarction) 2) Liquefactive necrosis (e.g. old cerebral infarction) 3) Caseous necrosis (e.g. pulmonary tuberculosis) 4) Fat Necrosis (e.g. acute pancreatitis)

Answer 128

It is programmed cell death

Answer 129

1) Embryogenesis 2) Deletion of auto-reactive T cells in the thymus 3) Hormone-dependent physiological involution 4) Cell deletion in proliferating populations 5) A variety of mild injurious stimuli that cause irreparable DNA damage that, in turn, triggers cell suicide pathways

Answer 130

1) Apoptosis may be physiological 2) Apoptosis is an active energy dependent process 3) Apoptosis is not associated with inflammation

Answer 131

It is programmed cell death associated with inflammation. It has many causes, including viral infections.

Answer 132

It is an orderly sequence of events in which a cell duplicates its contents and divides in two: 1) Duplication - cell growth and chromosome replication 2) Co-ordination - chrome segregation 3) Division

Answer 133

Different cells divide at different rates depending on: 1) Embryonic vs adult cells (early frog embryo cells - 30 min) 2) Complexity of system (yeast cells - 1.5 - 3 h) 3) Necessity for renewal (intestinal epithelial cells - ~20 h, hepatocytes - ~1 year) 4) State of differentiation (some cells never divide - i.e. neurons and cardiac myocytes) 5) If they’re tumour cells

Answer 134

G0 is the quiescent phase. In the absence of stimulus, cells go into G0

Answer 135

Control at multiple checkpoints guards against disastrous progression through the cycle. Checkpoints monitor the external environment for nutrients and growth factors. They can also pause for DNA repair or to undergo apoptosis. 1) The G1 checkpoint allows the cycle to enter the S phase, it checks for damaged DNA and whether the environment is favourable. 2) The S phase checkpoint allows the cycle to enter G2, and check for damaged or incompletely replicated DNA. 2) The G2 checkpoint allows the cycle to enter mitosis, and checks whether the DNA is replicated and if DNA damage has been repaired. 3) During Mitosis chromosomes are pulled apart, and it’s checkpoint checks whether all chromosomes are properly attached to the mitotic spindle.

Answer 136

Cells leave due to signalling cascades, achieved by: 1) Response to extracellular factors (e.g. growth factors stimulate entry from Go into the G1 phase). 2) Signal amplification 3) Signal integration/ modulation by other pathways 4) Ras/Raf/MEK/ERK

Answer 137

For the cell cycle to progress, the cell needs to double in size. Intracellular signaling pathways drive protein synthesis. Also protein degradation is inhibited. Net increase in protein synthesis Growth factor signaling pathways induces the expression of c-Myc. c-Myc promotes G0 to G1 transition and as it is a transcription factor, it also stimulates the expression of cell cycle genes. It is an oncogene, meaning that it is overexpressed in many tumours.

Answer 138

There are 4 types: CDK1, CDK2, CDK4, CDK6. They are present in proliferating cells throughout cell cycle. Although they allow exquisite control of events for Serine, Threonine and Tyrosine, their activity is regulated by interaction with cyclin and phosphorylation. This means that they are only active when a cyclin is bound. Sequential phosphorylation and dephosphorylation activates CDKs.

Answer 139

Cyclins (CyclinA, CyclinB, CyclinD, CyclinE ) are so named because their concentrations within the cell fluctuate as they are only transiently expressed at specific points in the cell cycle. They are regulated at level of expression and are synthesised, then degraded, meaning that only the cyclin-MDK complex is present during mitosis. Hence, cyclin levels fall during interphase.

Answer 140

A growth factor stimulates the production of C-Myc, which in turn stimulates the production of cyclin. This then binds with CDK to form an inactive complex. The activating protein phosphatase phosphorylates the complex, thereby activating it. Positive feedback then drives the cycle forward, allowing the complex to be formed. This is due to the fact that CDKz are sequentially active and stimulate synthesis of genes required for next phase (e.g. cyclin D/Cdk4/6 stimulates expression of cyclin E), giving direction and timing to cycle The ubuquitination of cyclin then leads to the inactivation and destruction of the complex.

Answer 141

Frequently the protein regulated by a kinase is another kinase, and so on. This leads to: 1) Signal amplification 2) Diversification 3) Opportunity for regulation

Answer 142

It is a protein acting as a molecular “brake”. It was first identified through studies of a childhood eye tumour, where it was found to be missing/inactive. It acts as a tumour suppressor and is abundant in all nucleated cells.

Answer 143

In a resting cell, active Rb sequesters a transcription factor (TF) in an inactive form (e.g. E2F family of TFs), which allows cell cycle progression. The TFs cannot turn on genes needed for cell cycle progression (e.g. DNA polymerase and Thymidine kinase), as E2F family members regulate the expression of these genes.

Answer 144

In a proliferating cell, the activation of intracellular signaling leads to production of G1-Cdk and G1/S–Cdk complexes. They can phosphorylate Rb inducing the inactivation of Rb and release of the TF. Target genes such as DNA polymerase and thymidine kinase can now be activated.

Answer 145

p53 arrests cells with damaged DNA (e.g. due to X Rays) in G1, as it is activated when activated protein kinase phosphorylates it. Active p53 then binds to the regulatory region of the p21 gene, thereby activating it.

Answer 146

This inhibits the function of CDK-cyclin complexes, by binding to the active complex.

Answer 147

Oncogenes: 1) EGFR/HER2: mutationally activated or over expressed in breast cancers. TheHerceptin antibody is used for the treatment of HER2+ metastatic breast cancer. 2) Ras: mutationally activated in many cancers. 3) Cyclin D1: overexpressed in 50% of breast cancers. 4) C-Myc: overexpressed in many tumours. Tumour suppressor 1) Rb: loss of function mutations in 80% of small cell lung cancers. 2) p53: loss of function mutations in over 50% of all human cancers.

Answer 148

1) Nerve fibres of the central and peripheral nervous system (a rapid almost instantaneous response). 2) The blood vessels of the cardiovascular system (a slower more versatile regulation).

Answer 149

Saline is used to treat ketoacidosis, which is the uncontrolled production of ketone bodies.

Answer 150

Inflammation is a rapid non-specific response protective biological process designed to remove damaged cells (endothelial and epithelial) and clear threats such as infections and toxins, It is a complex and tightly regulated process that can occur in any vascularised tissue. It leads to structural changes in the microvasculature and involves not only cells at the site of damage but also the recruitment immune cells, fluid and molecular components from the circulation.

Answer 151

It is caused by many things, including: pathogens, allergens, auto-antigens, physical damage and extreme temperatures. However, it is most commonly initiated when cellular damage, non-apoptotic cell death, leads to the release of damage associated molecular patterns (DAMPs) or the body detects pathogen associated molecular patterns (PAMPs).

Answer 152

The release of PAMPs and DAMPs causes the cells in the damaged tissue to secrete a range of signals designed to induce inflammation including molecules that alter the structure of nearby blood vessels and chemokines that promote the recruitment of immune cells to the site of injury.

Answer 153

The aims are to clear the source of the initial inflammatory stimuli, and the eventual resolution and repair of the inflamed tissue. This is the reason why inflammation has a characteristic pathology associated with the presence of increased fluid and leukocyte numbers (neutrophils, macrophages, lymphocytes, eosinophils and mast cells).

Answer 154

The majority of inflammation is acute, with a rapid onset and resolution, and characterised primarily by the recruitment of innate cells into the tissue, in particular neutrophils. This leads to a a decrease in airspace but also the thickening of the blood vessels.

Answer 155

If the acute inflammatory response cannot remove the inflammatory stimuli, then other immune cells, including adaptive immune cells, are recruited and a state of chronic inflammation can occur.

Answer 156

While acute inflammation often resolves without any substantive damage to the surrounding tissue, chronic inflammation can lead to repetitive rounds of inflammation, tissue damage and repair, resulting in scarring and loss of tissue function.

Answer 157

1) Infection 2) Autoimmunity 3) Hypersensitivity 4) Trauma 5) Fibrotic disease 6) Cancer

Answer 158

1) Inflammatory signals (e.g. non-apoptotic cell death) and the detection of foreign material. 2) Vasodilators, histamine and nitric oxide, are released. 3) Vascular changes occur such as: increased permeability, vasodilation, reduced flow and plasma leakage.

Answer 159

Its principal sources are mast cells, basophils and platelets. Its function is causing vasodilation, increasing vascular permeability, and endothelial activation.

Answer 160

Their principal sources are mast cells and leukocytes. Their function is causing vasodilation, pain and fever.

Answer 161

Their principal sources are macrophages, endothelial cells and mast cells. Their function is endothelial activation (adhesion molecules), fever, malaise, pain, anorexia and shock.

Answer 162

Their principal sources are leukocytes and activated macrophages. Their function is chemotaxis and leukocyte activation.

Answer 163

Their principal source is the plasma (produced in the liver). Their function is leukocyte chemotaxis and activation, vasodilation (mast cell stimulation), opsonisation.

Answer 164

It is defined as fluid, proteins and cells that have seeped out of a blood vessel.

Answer 165

1) Recruitment and inflammation signals at the site of damage (e.g. chemokines produced). 2) Chemokines diffuse out to form a gradient. 3) Leukocytes expressing complementary chemokine receptors migrate toward the chemokine source.

Answer 166

1) Chemo-attraction: Cytokines -> endothelial upregulation of adhesion molecules (e.g. selectins). 2) Rolling adhesion: Carbohydrate ligands in a low affinity state on neutrophils bind selectins (e.g. PSGL1 (selectin P ligand) binds P and E-selectins). 3) Tight adhesion: Chemokines promote low to high affinity switch in integrins LFA-1, Mac-1 – enhance binding to ligands (e.g. ICAM-1/2). 4) Transmigration: Cytoskeletal re-arrangement and extension of pseudopodia. Mediated by PECAM interactions on both cells.

Answer 167

1) Pathogen recognition: TLR4 and CD14 are used to identify lipopolysaccharides (LPS) present in gram-negative bacteria. 2) Pathogen clearance (e.g. phagocytosis and netosis). 3) Cytokine secretion: recruitment and activation of other immune cells.

Answer 168

Large particles are engulfed into membrane bound vesicles, called phagosomes. Phagosomes fuse with lysosomes, vesicles containing the enzymes elastase and lysozyme, forming phagolysosome. Also degrade pathogens by exposure to reactive oxygen species (ROS), phagocyte NADPH oxidase. Antimicrobial peptides – e.g. defensins.

Answer 169

1) Pathogen recognition: Immune cells (e.g. neutrophils) and antimicrobials (e.g. antibodies) kill infections or particulates. 2) Short half life: neutrophils (especially activated) have a rapid half-life and inflammatory mediators are turned over rapidly. 3) Macrophages: clear apoptotic cells and produce anti-inflammatory mediators. 4) Repair/wound healing

Answer 170

1) A molecule or molecular structure that can be recognised by an antibody 2) Any substance to which your immune system can mount an antibody or adaptive immune response

Answer 171

1) Foreign antigen: an antigen derived from molecules not found in the body 2) Self antigen: an antigen derived from molecules produced by our bodies 3) Immunogen: an antigen independently capable of driving an immune response in the absence of additional substances 4) Hapten: a small molecule that alone does not act as an antigen but when bound to a larger molecule can create an antigen

Answer 172

It is can be caused 4 different ways: 1) Persistent/prolonged infection (e.g. TB, hepatitis B/C) 2) Persistent toxic stimuli (e.g. allergens, pollutants) 3) Unclearable particulates (e.g. silica) 4) Autoimmunity (e.g. self antigens)

Answer 173

1) Inflammatory macrophages 2) T cells (and other lymphocytes) 3) Plasma (antibody secreting) cells

Answer 174

1) There is no clearance of the inflammatory agent. 2) Bystander tissue is destroyed. 3) There are concurrent repair processes (fibrosis and angiogenesis).

Answer 175

Macrophages can recruited as monocytes to the site of inflammation, but also tissue resident. Good macrophages include: phagocytes, cytotoxic, and anti-inflammatory (e.g. TGF-β, IL-10). Bad macrophages include: cytotoxic, inflammatory and pro-fibrotic.

Answer 176

1) Reactive oxygen and nitrogen species 2) Proteases 3) Cytokines, including chemokines 4) Coagulation factors 5) Amino acid metabolites

Answer 177

1) Growth factors (e.g. PDGF, FGF and TGF-beta) 2) Fibronogenic cytokines 3) Angiogenic cytokines 4) “Remodelling” collagenesis

Answer 178

1) T cells: pro-inflammatory (e.g. TNF, IL-17, IFN-γ), cytotoxic (e.g. granzymes, perforin), regulatory (e.g. TGF-β). 2) B cells: generate plasma cells that secrete antibody, aid in clearing infection, drive inflammatory reactions against self and can either be local to inflammatory site, or operate remotely.

Answer 179

It is chronic inflammation with distinct pattern of granuloma formation. It is triggered by strong T cell responses and resistant agents (e.g. mycobaterium, tumour). The aggregation of activated macrophages forms barrier designed for clearance.

Answer 180

1) Clearing the inflammatory agent 2) Removing damaged cells 3) Restoring normal tissue function

Answer 181

1) Excess tissue damage 2) Scarring 3) Loss of organ function, leading to organ failure

Answer 182

It causes extracellular matrix (e.g. collagen) deposition.

Answer 183

1) Redness (Rubor) 2) Heat (Calor) 3) Swelling (Tumor) 4) Pain (Dolor) 5) Loss of function (Functio laesa)

Answer 184

Vasodilation as a result of signalling by mast cell derived histamine and nitric oxide on the vascular endothelium leading to a breakdown in tight junctions. Vascular leakage increases blood flow into the inflamed tissue, leading to swelling (fluid build-up ) and accumulation of blood contents including red blood cells causes redness.

Answer 185

Heat results from the increased presence of fluid at core body temperature at a site that would otherwise have a limited exposure to this. During inflammation infiltrating immune cells are also highly metabolically active, which may also contribute to the generation of heat as a by-product.

Answer 186

Many of the same mediators that signal to endothelial cells and other immune cells during inflammation, also signal on local nerve cells. During acute inflammation molecules such as histamine and the prostogladins (PGEs) released by mast cells and neutrophils drive pain sensitization in local nociceptor neurons. At later stages macrophages and lymphocytes can also contribute to this process releasing pro-inflammatory cytokines such as interleukin-6, tumour necrosis factor and interleukin-1beta.

Answer 187

This is not usually noticed during acute inflammation. Fluid build-up and immune cell infiltration often result in the inability of that area of tissue to carry out its primary function. For example, inflammation of the lung parenchyma during respiratory infection, if immune cells and fluid build-up in the alveoli, a barrier is created which prevents efficient gas exchange between the capillaries and the air.

Answer 188

Sodium is the most plentiful cation in the plasma whist conversely, potassium is the most plentiful cation inside cells.

Answer 189

Extracellular chloride is found at much higher concentrations in the plasma than inside cells.

Answer 190

The internally high concentration of potassium inside the cell is also neutralised by a variety of anions e.g. proteins, nucleic acids, phosphorylated proteins.

Answer 191

Organic phosphate is the main intracellular anion.

Answer 192

It is a key intracellular metabolite with roles in ATP production and also cell signalling. It also phosphorylates, proteins which is key for their activation and inactivation.

Answer 193

Proteins are anions, with many having a net negative charge. Although they are found at relatively low concentrations, they can have a highly negative charge.

Answer 194

In terms of pH (conc. of protons), the inside of the cell is slightly more acidic (0.3 pH units) than the plasma. Given that pH is a log scale, this translates into around a 2-fold difference in proton concentration between the plasma and the intracellular compartments.

Answer 195

The osmolarity between the blood and the intracellular compartment is identical, so there is not normally a significant osmotic effect. One exception to this rule is the regions of the kidney where fluids are concentrated.

Answer 196

The spontaneous movement of a solute down a concentration gradient until the solute molecules reach an equilibrium.

Answer 197

The movement of water down its own concentration gradient. Osmosis moves water toward an area of higher osmolarity. It can therefore change cell volume with consequences for cell function and survival. Osmo = outside osmolarity // Osmi = intracellular osmolarity

Answer 198

The number of moles of solute that contribute to the osmotic pressure of a solution. The osmolarity of a solution containing 125 mmol.L-1 KCl, 5 mmol.L-1 MgCl2 and 30 mmol.L-1 glucose would be 295mmol.L-1: 125 x 2 (K+ and Cl-)+ 5 x 3 (Mg2+ and 2 x CL-) +30 (glucose).

Answer 199

The “strength” of a solution as it affects the final cell volume. Tonicity depends on both cell membrane permeability and the solution composition.

Answer 200

In a hypertonic solution, the osmolarity of the impermeant solutes outside the cell are greater than those inside the cell. The cell therefore shrinks in the solution.

Answer 201

In a hypotonic solution, the osmolarity of the impermeant solutes outside the cell are less than those inside the cell. The cell therefore swells in the solution.

Answer 202

In an isotonic solution, the osmolarity of the impermeant solutes outside the cell is identical to those inside the cell. The cell volume therefore remains the same.

Answer 203

The cells don’t burst because the Na+K+-ATPase maintains the concentration of Na+ ions much lower inside the cell than outside. The ATPase makes the membrane “effectively impermeable” to Na+ because any Na+ that diffuses in down the Na+ concentration gradient is actively pumped out again. Thus there is no net movement of Na+ across the membrane. The intracellular osmolarity of impermeant solutes (mainly proteins at high concentration and low concentration Na+) balances the extracellular osmolarity of impermeant solutes (mainly high concentration Na+).

Answer 204

Gases (e.g. O2, N2, CO2) and hydrophobic molecules (e.g. steroids) can diffuse across the lipid bilayer.

Answer 205

Most molecules require particular proteins for transportation across a biological membrane. This uses ATP hydrolysis in the case of active transport against an electrochemical gradient (e.g. Na+K+-ATPase ) or is passive, facilitating the flow of molecules down an electrochemical gradient.

Answer 206

When any tissue loses its blood supply, ischaemic changes occur.

Answer 207

Ischaemic changes can be significantly slowed by rapid cooling of the tissue/organ to +4°C, by perfusing them with cold solutions via the arterial supply. Even when cooled, tissues/organs deteriorate.

Answer 208

The composition of the perfusion solution can reduce the deterioration in hypothermia, prolonging the time available to transport and keeping the organ viable.

Answer 209

Na+K+-ATPase stops functioning below 15°C which is compounded by the fact that without circulation there is little O2 and therefore little ATP to fuel the pump. Unless precautions are taken, Na+ will enter the cell (along with Cl-) and water will also enter as K+ exits. Cells are likely to swell and their membranes bleb, resulting in cell death.

Answer 210

University of Wisconsin solution (UW) is a perfusion solution, formulated to reduce hypothermic cell swelling and enhance preservation.

Answer 211

1) Lack of Na+ or Cl- (therefore no influx possible). 2) Presence of extracellular impermeant solutes (lactobionate ions, raffinose). 3) Presence of a macromolecular colloid (starch) Allopurinol and glutathione act as antioxidants, helping to protect the organs from damage from reactive oxygen species (ROS).

Answer 212

All blood vessels (e.g. arteries, capillaries, veins, lymphatics) are lined by endothelial cells which have pores.

Answer 213

Each day, 8L of plasma leaks out of blood vessels. Since the volume of blood plasma is around 3L, the entire plasma volume must pass into the interstitial space and back into the blood circulation every 9 hours.

Answer 214

1) Lipid soluble substances pass through the endothelial cells 2) Small, water soluble substances pass through the pores between cells 3) Exchangeable proteins are moved across by vesicular transport

Answer 215

Plasma proteins cannot generally cross the endothelial cell membranes and cannot get through the pores between cells.

Answer 216

It separates the circulating blood from the brain is tightly sealed.

Answer 217

In a normal capillary, higher concentrations of plasma proteins inside the capillary than outside, generates an osmotic pressure known as the colloid osmotic pressure (COP). The flow of blood through the vessel also generates a hydrostatic pressure inside the vessel which is greater than that in the tissues though which it is passing. Thus, there is a tendency to "push" molecules though the capillary pores.

Answer 218

In a normal capillary although the COP draws solute and fluid into the vessel, the slightly greater hydrostatic pressure results in net leakage from the capillary.

Answer 219

The term is used to describe the accumulation of fluids within tissues. Oedema is also one of the cardinal signs of inflammation, as infectious and inflammatory stimuli often results in oedema.

Answer 220

Oedema results due to an imbalance in the normal cycle of fluid exchange in tissues causing fluid to accumulate in the interstitial spaces: 1) A common cause of oedema is an increase in the permeability of capillary walls. In a leaky capillary, proteins are lost through an increase in pore size which reduces the COP and so fluids are more readily pushed out from the capillary. 2) When the leakage of plasma into the interstitium exceeds the capacity of the lymphatics to collect and return it to the circulation, oedema will result as fluid accumulates in the interstitial space.

Answer 221

They collect interstitial fluid that is destined for return to the blood circulation. They are able to do this, as they are blind ended and have a low internal pressure which results into the net flow of fluids from tissues into the lymphatic capillaries.

Answer 222

It returns either via the lymphatic ducts in the subclavian region or via lymph nodes.

Answer 223

Insect bites which can cause local blood vessels to become leaky. Swelling occurs because the rate of leakage from the vessels is greater than the rate at which the lymphatics can drain it.

Answer 224

High blood pressure means increased hydrostatic pressure in vessels. This pushes more fluid out of the vessels, and can lead to accumulation of interstitial fluid.

Answer 225

A breast cancer survivor is likely to have had axillary (armpit) lymph nodes removed as part of her diagnosis or treatment. This can remove the pathway of drainage from the upper limb on the affected side, resulting in the accumulation of fluid.

Answer 226

In elephantiasis, parasitic worms can block lymphatic vessels, thereby preventing drainage of the lymph.

Answer 227

1) A signal, called an action potential, is formed by the influx of Na+ (membrane depolarisation) and potassium efflux (repolarisation). The action potential moves along the cell membrane. 2) When it reaches the pre-synaptic terminal, the action potential causes voltage-gated Ca2+ channel to open. The calcium influx into the terminal, allows calcium to bind to pre-synaptic vesicles and causes the expulsion of the contents of the vesicles (exocytosis). 3) The neurotransmitter is now present within the synaptic cleft, travelling across it to bind to receptors on the post-synaptic membrane. This signal is therefore transmitted to the post synaptic cell.

Answer 228

Blood vessels provide a transportation route not only for blood cells but for numerous other chemical messengers (hormones)

Answer 229

Hormones are produced by most of the major organs of the body however there are a few organs that play a more prominent role than others, such as the hypothalamus and the pituitary gland.

Answer 230

This occurs when hormones travel within blood vessels to act on a distant target cell. An example of this is the physiological response of hypoglycaemia.

Answer 231

This occurs when hormones act on an adjacent cell. An example of this is the physiological response of hyperglycaemia.

Answer 232

This occurs when plasma membrane proteins on adjacent cells interact. An example of this is a blood borne virus (e.g. Hepatitis C), being detected and digested by an antigen presenting cell.

Answer 233

This occurs when signalling molecules act on the same cell. An example of this is activated T-Cell rectories initiating a cascade of reaction with the T-Cell itself.

Answer 234

These are chemical messages or molecules, they exert their effects through binding to receptors.

Answer 235

These are usually proteins that bind the chemical mediators known as ligands and upon activation they elicit an effect within a cell.

Answer 236

This is a separate entity from the receptor or the ligand that causes the intracellular effect evoked by an occupied receptor.

Answer 237

These transmembrane receptors (have a central pore incorporated within their quaternary structure. When the appropriate ligand attaches to the ‘ligand-binding domain’ on the external surface of the protein the pore will open.

Answer 238

1) The ligand binds to the receptor protein 2) A change in conformation of the channel protein results in the opening of a pore, which spans the cell membrane. 3) The pore allows ions to move in or out of the cell according to their respective concentration gradients. For example if there is a higher extracellular concentration of Na+ ions than there is an intracellular concentration, upon opening of Na+ channels, Na+ ions move into the cell.

Answer 239

They bear this name as the channel protein crosses the cell membrane 7 times. They are linked to an intracellular G protein complex, which consists of an alpha (α) subunit, a beta-gamma (βγ) subunit and an associated GDP molecule. There is significant variation between the Gα subunits and these can be broadly separated into three categories, which are associated with alternative signal transduction pathways. The Gβγ subunit is also physiologically active.

Answer 240

1) Ligand binding causes the G protein complex to associate with the receptor resulting in the GDP molecule being phosphorylated to a GTP molecule 2) The Gα subunit dissociates from the Gβγ subunit. 3) Both Gα and Gβγ can act as second messengers. 4) When the ligand dissociates from the receptor, internal GTPase on the Gα subunit hydrolyses GTP to GDP. 5) The Gα and Gβγ subunits re-associate and are once again available to the receptor.

Answer 241

They ordinarily only consist of one transmembrane domain, which has the ligand-binding domain on the outside and specialised enzymes (usually tyrosine kinase enzymes) on the inside. These receptors do not ordinarily work alone and require clustering of more than one receptor protein to activate the intracellular enzyme. Once activated the intracellular enzymes trigger a signalling cascade within the cell.

Answer 242

1) Ligand binding results in receptors clustering. 2) Receptor clustering activates enzyme activity within the cytoplasmic domain. 3) The enzymes phosphorylate the receptor. 4) This phosphorylation leads to the binding of signalling proteins to the cytoplasmic domain. 5) These signalling proteins recruit other signalling proteins and a signal is generated within the cell. 6) The signal is terminated when a phosphatase dephosphorylates the receptor.

Answer 243

Steroid hormones are membrane permeable (i.e. hydrophobic, lipophilic) and therefore exert their actions on intracellular receptors. Intracellular receptors are essentially transcription factors and therefore regulate mRNA and protein synthesis.

Answer 244

These receptors are located within the cytosolic compartment and are associated with chaperone molecules (normally heat shock proteins, hsp). Once the hormone binds to the receptor, the hsp molecule dissociates allowing the hormone-receptor complex to form a homodimer with another identical hormone-receptor complex. The homodimer subsequently translocates to the nucleus where it binds to DNA and acts as a transcription factor.

Answer 245

These receptors are located within the nucleus of a cell and are often already bound to DNA. Binding of the hormone ligand to the receptor usually results in direct transcriptional regulation by the activated hormone-receptor complex.

Answer 246

``` A histopathologist deals with tissues. She/he will examine sections, noting the architecture of the tissue and asking what it tells us about the condition. The information will be used in diagnosis and also to inform as to the efficacy of a particular treatment. A histopathologist works with the following tissue samples: Biopsies Resection specimens Frozen sections Post-mortems ```

Answer 247

Biopsies are small sections of tissues that are removed from the patient and typically placed in a formalin solution which preserves the tissues by cross-linking proteins. They are then embedded in paraffin wax to allow very thin sections (2-3μm thick) to be cut by an instrument known as a microtome. These are mounted on a glass microscope slide for further preparation prior to analysis. The time for a biopsy result from the histopathology lab to reach the clinician is 2-3 days.

Answer 248

Haemotoxylin and Eosin (H&E) staining can be used to identify the nuclei and cytoplasmic granules of leukocytes within tissues. Similarly, the Ziehl-Neelsen stain will stain acid-fast bacteria red, aiding in the diagnosis of tuberculosis infection.

Answer 249

Resection specimens are taken from tissue that has been removed as part of a surgical procedure and can be processed as for a biopsy. The time for a resection specimen result from the histopathology lab to reach the clinician is 5-7 days.

Answer 250

Resections are used primarily to look at the stage the disease, so the progression, malignancy and removal of the disease. Tissue from resection specimens can also be donated to biobanks and used to inform genomic studies of the disease process, with the input of other medical professionals such as immunologists and microbiologists.

Answer 251

Frozen sections are taken during surgical procedures and are are examined by pathologists in real time while the patient is being operated upon. The freshly taken tissue is frozen by a machine known as a cryostat, cut then mounted on glass slides and stained as for biopsies. The procedure can give a rapid diagnosis in minutes which can be relayed back to the surgeon to inform the surgery. The time for a frozen section result from the histopathology lab to reach the clinician is 30 minutes.

Answer 252

Cytopathologists work with cells which are collected and then smeared onto a microscope slide. The slide can then be stained and examined.

Answer 253

A fine needle used to get into a lesion and suck out (aspirate) the cells which can then be analysed as for a smear. This is a very powerful technique as the needle can penetrate relatively inaccessible tissues (e.g. a thyroid nodule and assess the suspect mass without the need for surgery). The downside of this technique is that the cytopathologist is only looking at cells and is unable to comment upon the likely architecture of the tissue.

Answer 254

Antibodies are versatile tools for diagnosis. The immunology labs offer 86 different tests of which 53 are used to detect the levels of specific antibodies circulating in patients. They can test the degree of the immune response using the level of antibodies, for with double stranded DNA generated such as: systemic lupus erythematosus (SLE), Sjögren's syndrome and rheumatoid arthritis.

Answer 255

Histochemistry uses manufactured antibodies that specifically detect molecules.

Answer 256

The Fc region on antibodies attaches to a variety of different molecules, the attachments are called conjugates.

Answer 257

Antibody conjugates can take the form of: 1) Enzymes (e.g. peroxidase, alkaline phosphatase): enzyme can turn colour substrate into a product wherever the antibodies bind. 2) Fluorescent probes: These can allow the rapid measurement of the levels of molecules within a sample. Also allows multiplexing, which is several antibodies with different fluorophores (colours) to measure several molecules in a sample. 3) Magnetic beads: used for the purification of cell types). 4) Drugs (e.g. Kadcyla, an anti-HER2 antibody linked to the cytotoxic chemical emtansine): can be delivered to tumorous cells that are over expressing.

Answer 258

Antibodies are re,actively easy to generate and can be used to detect: proteins, carbohydrates and lipids. 1) Direct detection: a conjugates binds to the (primary) antibody itself, that recognises the target antigen. 2) Indirect detection: the (primary) antibody itself that binds the antigen is unlabelled (unconjugated), the secondary antibody detects the primary antibody and has a conjugate attached.

Answer 259

1) Blood group serology (Haem -Blood transfusion). 2) Immunoassays (e.g. detection of hormones and circulating antibodies/antigens). 3) Immunodiagnosis (e.g. infectious diseases, antibody levels and IgE.

Answer 260

1) Clinical samples (typically serum samples), are allowed to adhere to a plastic plate. 2) They are then probed with the specific antibody raised against the molecule of interest. 3) The antibody is then conjugated an enzyme, which generates a coloured product. 4) With reference to standard curve, precise concentrations of the molecule in the sample can be determined.

Answer 261

The technique of flow cytometry allows the detection of specific cells, notably lymphocyte sub populations. It works by having fluorescently conjugated antibodies that are specific for leukocyte antigens but also have different colours (fluorophores). The cells run as a stream through a laser beam. The colour of light emitted and the forward or side scatter of the laser beam denotes the identity of the cell surface molecules expressed and the size and granularity of the cells.

Answer 262

Typically, molecules expressed on the surface of the cell are looked for (e.g. CD3 - the T cell receptor, CD4 - helper T cell, CD8 - cytotoxic T cell, C19 - B cell marker and CD56 - natural killer cell).

Answer 263

Any kind of mass forming lesion. It may be neoplastic (e.g. lung cancer), hamartomatous or inflammatory/non-plastic (e.g. nasal polyps). Many malignant tumours rarely cause death (especially skin cancers) and that some benign tumours do kill (usually because of their location, e.g. the brain)

Answer 264

The autonomous growth of tissue which have escaped normal constraints on cell proliferation. Neoplasms may be either benign (remain localised) or malignant (invade locally and/or spread to distant sites). Cancers are malignant neoplasms.

Answer 265

These are localised benign overgrowths of one of more mature cell types (e.g. in the lung). They represent architectural but not cytological abnormalities (e.g. lung hamartomas are composed of cartilage and bronchial tissue).

Answer 266

These are normal tissue being found in parts of the body where they are not normally present (e.g. pancreas in the wall of the large intestine).

Answer 267

The primary description of a neoplasm is based on the cell origin and the secondary description is whether it is benign or malignant. For example, tumours of cartilage are either chondromas (if benign) and chondrosarcomas (if malignant.) The “chondro” stem means derived from cartilage the suffix “oma” means a benign tumour and the suffix ”sarcoma” means a malignant (soft tissue) tumour.

Answer 268

1) Squamous epithelium (e.g. skin, oesophagus and cervix): form squamous epithelioma or papilloma, as benign tumours. Form squamous cell carcinoma as a malignant tumour. 2) Glandular epithelium (e.g. breast, pancreas, colon and thyroid): form adenoma, as a benign tumour. Form adenocarcinoma as a malignant tumour. 3) Transitional epithelium (e.g. bladder): form transitional papilloma, as a benign tumour. Form transitional cell carcinoma as a malignant tumour.

Answer 269

1) Smooth muscle (e.g. uterus and colon): form leiomyoma, as a benign tumour. Form leiomyosarcoma as a malignant tumour. 2) Bone (e.g. arm and leg): form osteoma, as a benign tumour. Form osteosarcoma (osteogenic sarcoma) as a malignant tumour.

Answer 270

1) Lymphocytes (e.g. lymphoma and stomach): the formation of a benign tumour is extremely uncommon. Form lymphoma as a malignant tumour. 2) Bone marrow (e.g. acute lymphoblastic leukaemia and chronic myeloid leukaemia): the formation of a benign tumour is extremely uncommon. Form leukaemia as a malignant tumour.

Answer 271

These are tumours derived from germ cells and can contain tissue derive from all three for 3 germ cell layers. They may contain mature and/or mature tissue and even cancers.

Answer 272

1) (Malignant) Lymphoma 2) (Malignant) Melanoma 3) Hepatoma (more commonly called liver cell cancer) 4) Teratoma (not all)

Answer 273

1) Invasion: This means direct extension into the adjacent connective tissue and /or other structures e.g. blood vessels. This is what distinguishes dysplasia/carcinoma in situ from cancer. Invasive cancer cells often secrete proteases that enable them to degrade the extracellular matrix at a tissue's boundary. Proteases also give cancer cells the ability to create new passageways in tissues. 2) Metastasis: This means spread via blood vessels or the lymphatic system, to other parts of the body. All malignant tumours have the capacity to metastasise although they may be diagnosed before they have done so. 3) Differentiation: This means how much do the cells of the tumour resemble the cells of the tissue it is derived from.Tumour cells tend to have larger nuclei (and hence a higher nuclear-cytoplasmic ratio) and more mitoses than the normal tissue they are derived from. They may have abnormal mitoses (e.g. tripolar) and marked nuclear pleomorphism (variability in nuclear size and shape). 4) Growth pattern: This means how much does the architecture of the tumour resembles the architecture of the tissue it is derived from. Tumours have less well defined architecture than the tissue they are derived from.

Answer 274

This system is used to evaluate or “grade” prostate cancer cells obtained by needle biopsy. The cells are assigned a number between 1-5: nearly normal cells are assigned Grade 1 and the most abnormal cells are Grade 5. Then, the scores of the two most common cell patters are added together, making the scores range from 2-10. The higher the grade, the more aggressive the cancer.

Answer 275

1) Direct extension: This is associated with a stromal response to the tumour. This includes fibroblastic proliferation (“ a desmoplastic response”), vascular proliferation (angiogenesis) and an immune response. 2) Haematogenous: This is via blood vessels. The blood vessels usually invaded are the venules and capillaries because they have thinner walls. Most sarcomas metastasise first via the blood vessels. 3) Lymphatic: This is via lymphatics to lymph nodes and beyond. The pattern of spread is dictated by the normal lymphatic drainage of the organ in question. Most epithelial cancers metastasise first via the lymphatics. 4) Transcoelomic: This is via seeding of body cavities. The commonest examples are the pleural cavities (for intrathoracic cancers) and the peritoneal cavities (for intra-abdominal cancers). 5) Perineural: This is via nerves, an underappreciated route of cancer spread.

Answer 276

1. Clinically 2. Radiologically 3. Pathologically

Answer 277

It is described using the “TNM" system and the details are different for each kind of cancer: > T = Tumour: the tumour size or extent of local invasion > N = Nodes: number of lymph nodes involved > M = Metastases: presence of distant metastases

Answer 278

Grade describes how differentiated the tumour is. Whereas, Stage describes how far the tumour has spread. In terms of tumour prognosis, Stage is more important than Grade.

Answer 279

Cells become cancerous after mutations accumulate in the various genes that control cell proliferation, as a result of independent events. Consequently, the path to cancer involves multiple steps and is often views as a microevolutionary process. When a mutation gives a cancer cell a growth advantage, it can make more copies of itself than a normal cell can — and its offspring can outperform their noncancerous counterparts in the competition for resources. Later, a second mutation might provide the cancer cell with yet another reproductive advantage, which in turn intensifies its competitive advantage even more. And, if key checkpoints are missed or repair genes are damaged, then the rate of damage accumulation increases still further.

Answer 280

1) Growth-promoting genes: such as the gene for the signaling protein Ras, become super-active and producing cells that are too strongly stimulated by growth receptors. 2) Tumour suppressor genes: genes that suppress cell proliferation or those that signal the need for apoptosis. These genes normally function like brakes on proliferation, and both copies within a cell must be mutated in order for uncontrolled division to occur.

Answer 281

Some chemotherapy drugs work to counteract growth-promoting gene mutations by blocking the action of growth-signaling proteins. The breast cancer drug Herceptin, for example, blocks overactive receptor tyrosine kinases (RTKs), and the drug Gleevec blocks a mutant signaling kinase associated with chronic myelogenous leukemia.

Answer 282

It is a protein (serine protease) composed of 241 amino acids arranged in three peptide chains (A, B and C), that are linked by disulphide bridges.

Answer 283

Chymotrypsin is secreted by the pancreas as the pro-enzyme chymotrypsinogen. This inactive form undergoes proteolysis in the duodenum, to form active chymotrypsin.

Answer 284

As it is found in digestive system of mammals, it can hydrolyse peptide bonds and aid protein digestion, facilitating their absorption. Protease also play a key role in the degradation of the extra cellular matrix to facilitate the migration of cells. Proteases are also key for regulation of other enzymes and aid in protein turnover (replacing old proteins with new). The requirements for recognition by chymotrypsin are an aromatic side chain such as phenylalanine, tyrosine or tryptophan with cleavage taking place on the carboxyl side of the peptide bond. Chymotrypsin also has specificity for bulky hydrophobic side chains.

Answer 285

Chymotrypsin catalyses the hydrolysis of N-Glutaryl-L-phenylalanine p-nitroanilide (GPNA), generating N-glutaryl-L-phenyl alanine and the bright yellow product, p-nitroaniline. This reaction can be followed continuously by spectrophotometry, since p-nitroaniline has significant absorbance at 410nm, unlike GPNA. The absorption of p-nitroaniline obeys the Beer-Lambert Law with a molar absorption coefficient of 8.8 μmol. ml cm-1. The production of p-nitroaniline can then be measured as a function of time using several different concentrations of GPNA.

Answer 286

In enzyme kinetics, the substrate is referred to as S, and the concentration of substrate is denoted as [S].

Answer 287

KM is known as the Michaelis Constant and is defined as the concentration of substrate at which a particular enzyme works at half its maximal velocity.

Answer 288

Biochemically, the KM value is useful as a means of comparing the strength of Enzyme-Substrate complexes.

Answer 289

1) A low KM indicates tight binding of a substrate to an enzyme. 2) A high KM is indicative of weak binding of a substrate to an enzyme.

Answer 290

The rate of reaction is denoted by velocity (V) in enzyme kinetics. As increasing substrate is added, there is a linear rate of increase (1/2Vmax), directly proportional to the substrate concentration. However, when enough substrate is added, so that the binding sites in enzymes are saturated, then the rate begins to plateau (Vmax).

Answer 291

They both rely on the initial velocity of the reaction (V0), this is because during the initial phase of the reaction, as long as the reaction velocity remains constant, the reaction is in a steady state. This means that the enzyme-substrate complex is formed and consumed at the same rate.

Answer 292

This plot can be generated by calculating V0 and knowing the substrate concentration. It is a double reciprocal plot of 1/V0 (y) against 1/[S] (x). 1) The intercept on the y axis = 1/Vmax 2) The intercept on the x axis = -1/KM 3) The slope or gradient = KM/Vmax

Answer 293

Creating kinase (CK) is the enzyme that facilitates the conversion of creatine phosphate to creatine and ATP. It is probably present in all cells, but is present in particularly high concentrations in muscle cells and brain cells. Following the damage or death of cells, CK is leaked into the circulation.

Answer 294

Two different subunits M and B provide the following isoenzymes: MM, MB and BB. The only human tissue where the MB form is found is the myocardium (heart), where MB represents about 15% of total creatine kinase. The brain only expresses the B gene, and hence only the BB form can be generated. The MM form is the only one made in skeletal muscle cells.

Answer 295

The three isoenzymes can be separated by electrophoresis on cellulose acetate strips. The M and B monomers have approximately the same molecular weight, but differ in their pH (isoelectric point), allowing them to be separated by charge. MM can be seen to move the furthest towards the negative electrode.

Answer 296

Atherosclerotic plaques reduce the blood supply to the heart, causing cell necrosis and CK to be leaked into circulation. The death of cardiac muscle fibres can be determined if the BM isoform of CK can be detected in the serum. The levels of the BM isoform are directly proportional to the level of cell death in the heart, as each myocyte is approximately of equal volume. As each cell does, it releases a “quantum” of CK into the extracellular fluid and thence into the serum.

Answer 297

CK activity in serum can be detected by a coupled assay, leading to the generation of detectable products, NADPH, which has an absorption spectrum distinct from NADP+.

Answer 298

The time course of serum CK is relatively short after myocardial infarction, peaking after only one day then falling dramatically. This means that there’s only a small window of time for the CK signal to be detected.

Answer 299

1) Serum glutamate oxaloacetate transaminase (SGOT) can also be useful for diagnosis of myocardial damage, having a smaller signal than CK but a longer window, peaking after 2 days. 2) Cardiac Tropinin is a very specific marker for myocardial infarction, appearing 48 hours after infarction, and persisting for 5 days. However, its an expensive assay to measure and is found generally in specialised hospitals.