Cellular Physiology

Question 1

Hyperaldosteronism

Answer 1

Hyperaldosteronism will increase sodium reabsorption and potassium secretion. This will increase the concentration of extracellular sodium. Fluids will flow from intracellular fluid to extracellular fluid due to the concentration gradient differences. Furthermore, sodium retention by the kidneys will increase bicarbonate reabsorption and acid excretion. Oedema is not found in hyperaldosteronism as may be expected.

Question 2

Equilibrium potential

Answer 2

Equilibrium potential for a single ion system is synonymous with reversal potential. It is defined as the membrane potential at which there is no net inward or outward movement of an ion across the cell membrane. It depends on the ratio of the concentrations of that ion inside and outside the cell. It is the potential that the membrane potential would approach if the membrane became freely permeable to that ion. It would be zero if the concentrations of that ion on each side of the membrane were equal.

Sodium’s equilibrium potential is positive, whereas potassium and chloride have negative equilibrium potentials. In normal circumstances, the resting membrane potential is close to the equilibrium potential of potassium. Increasing the membrane permeability to sodium and potassium will depolarize the membrane. As mentioned above, sodium has a positive equilibrium potential, so increasing its permeability would increase the membrane depolarization to the maximum. If increased sodium permeability is coupled with potassium, it will decrease the effect of sodium on membrane depolarization.

Bicarbonate is mainly an extracellular anion where its concentration ranges from 22-28 mmol/L. The intracellular HCO3- level is 10 mmol/L. It is a major part of the intracellular and extracellular buffer apparatus to maintain the pH. Intracellular bicarbonate ions play an important role in maintaining membrane potential as well.

The Na+ gradient across a nerve cell membrane is used as an energy source for the transport of other ions. It is not the result of the Donnan equilibrium, which is defined as an equilibrium between two solutions separated by a semi-permeable membrane. The sodium gradient is not significantly changed during an action potential. Potassium gradient is the primary determinant of the resting membrane potential. The Na+ gradient is maintained by Na+/K+-ATPase pump.

The bicarbonate ion is mainly an extracellular anion. Its intracellular concentration is less than the extracellular concentration. Sodium, chloride, and calcium ions are also mainly extracellular ions. The intracellular concentration of potassium ions, magnesium ions, cholesterol, and amino acids is higher than the extracellular concentration. The intracellular concentration of proteins is four times higher than the extracellular concentration.

Only small, uncharged molecules can diffuse freely across the cell membrane, for example O2, CO2, and H2O. Larger uncharged polar molecules like glucose, amino acids, and chylomicrons cannot pass freely. Inorganic ions, such as Na+, K+, and Ca++, pass across the cell membrane through ion channels which form open pores to allow their free passage. Small molecules such as glucose and amino acids are transported by carrier proteins. Urea can pass passively across cell membranes. However, due to its low solubility in the lipid bilayer, its transport is accelerated by urea transporters, which transport the urea by facilitated diffusion. Chylomicrons are released from the intestinal cells through a process of exocytosis.

Question 3

Skeletal muscle

Answer 3

Skeletal muscle is composed of a collection of muscle fibers. Skeletal muscle cells have a peripheral nucleus, use glycogen as their main energy store, and have multiple mitochondria to meet energy needs. Muscle fibers are classified based on their color, metabolism, and fatigability in I, IIa, IIb, or IIx. A skeletal muscle fiber is innervated by only one neuron that causes neuromuscular transmission, resulting in the release of intracellular Ca2+ from the sarcoplasmic reticulum and recruitment and cycling of cross-bridges.

Skeletal muscles originate embryologically through the fusion of individual myoblasts. Hence, these cells are fused and multinucleated forming a syncytium. Myofibril is the basic rod-like unit of a microfiber. Myofibrils contain myofilaments. Under the electron microscope, a repeating sequence of striations is seen. Each sequence constitutes a sarcomere. Each sarcomere is limited by a Z-band. Each sarcomere is supported by two transverse tubules (T tubules).

Question 4

Calcium storage cell

Answer 4

Extracellular calcium has an inward electrical and concentration gradient because it’s more abundant in the extracellular fluid than the intracellular fluid, and is a strong, positively-charged ion compared to the negatively-charged cells.

Intracellular calcium is stored in the organelles, especially mitochondria and the endoplasmic reticulum. Calcineurin may limit Ca2+ influx through dihydropyridine-sensitive Ca2+ channels in the plasma membrane by dephosphorylating the channel or a closely-associated protein and inactivating it.

Calcium has many effects, such as bone remodeling and neural activity. It has an opposing effect based on its concentration. For example, in a normal concentration, calcium ions play a major part in muscle contraction. However, when the concentration of calcium ions rises above normal (hypercalcemia), muscle contraction can’t occur, causing a person to become hypotonic.

The increase in intracellular calcium ions is the initiating stimulus for smooth muscle contraction. However, its role differs in smooth muscles as compared to skeletal muscles. In skeletal muscles, contraction is initiated when calcium binds to troponin. In smooth muscles, calcium binds to calmodulin, and calcium–calmodulin complex initiates the contraction. The source of activation of calcium ions, mechanism of the force of contraction, source of energy, and nature of contractile protein are the same.

Question 5

Cell ATP consumption

Answer 5

Protein synthesis 75% of ATP use

Question 6

Paracrine, autocrine, endocrine, exocrine

Answer 6

Paracrine signalling is a form of cell signalling or cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behavior of those cells.

Autocrine signalling is a form of cell signalling in which a cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell.

Endocrine signalling occurs when endocrine cells release hormones that act on distant target cells in the body.

Exocrine signalling occurs when cells secrete signalling molecules into the circulation.

Question 7

Things that are never incorporated into proteins

Answer 7

Taurine, l dopa, thyroxine, serotonin - never incorporated into proteins

Question 8

IDL’s

Answer 8

ntermediate-density lipoproteins (IDLs) are VLDLs from which a share of the triglycerides has been removed, so the concentrations of cholesterol and phospholipids are increased. LDLs are especially important in the different stages of phospholipid and cholesterol transport from the liver to the peripheral tissues. The heart has virtually no glycogen reserves. Fatty acids are the heart’s main source of fuel. Adipose tissue lipoprotein lipase activity is high after feeding and low during fasting and stress.

Question 9

Protein diet and calcium absorption

Answer 9

Researchers have found that a high-protein diet increases intestinal calcium absorption. Alkalosis promotes tetany by causing hypocalcemia. Parathormone indirectly exerts its effect on intestinal calcium absorption via its effects on vitamin D metabolism. 45% of calcium circulates as free calcium, whereas 40% is bound to albumin, and the remaining 15% is bound to organic and inorganic anions. Most of the renal calcium reabsorption takes place in proximal convoluted tubules.

Question 10

Brown fat

Answer 10

Brown fat is also present and metabolically active in adult humans. It is located between the shoulder blades, surrounding the kidneys, the neck, and supraclavicular area, and along the spinal cord. Upon leaving fat cells, fatty acids ionize strongly in the plasma and the ionic portion combines immediately with albumin molecules of the plasma proteins. While passing through the intestinal epithelial cells, the monoglycerides and fatty acids are resynthesized into new molecules of triglycerides that enter the lymph as minute, dispersed droplets called “chylomicrons”. The VLDLs transport triglycerides synthesized in the liver mainly to the adipose tissue.

Question 11

Thermal body receptors

Answer 11

Deep body thermal receptors are found predominantly in the spinal cord, in the great veins in the upper abdomen and thoracic cavity and the abdominal viscera. They are sensitive to the body’s core temperature. They detect mainly cold rather than warmth and are concerned with preventing low body temperature.

Question 12

Cell membrane permeability factors

Answer 12

Cell membrane permeability is directly proportional to the concentration gradient across the cell membrane as shown by the formula: Net diffusion is proportional to (Co−Ci) where Co is the concentration outside the cell membrane and Ci is the concentration inside the cell membrane. Decreased thickness of the membrane provides a smaller distance for the molecules to travel and hence increases cell membrane permeability. As the cell membrane is a lipid bilayer, it is more permeable to lipophilic molecules. Large membrane surface area and increased temperature also increase cell membrane permeability.

Question 13

Fat metabolism

Answer 13

Fats cannot be converted to glucose because the pyruvate dehydrogenase reaction is irreversible and there are no alternative pathways that will generate pyruvate from acetyl-CoA. Acetyl-CoA is already an activated molecule and it cannot be activated further to overcome the free energy difference involved in pyruvate formation.

Question 14

RNA polymerase

Answer 14

RNA polymerase is an enzyme involved in the assembly of the RNA molecule. It attaches to the promoter sequence of the DNA strand and initiates RNA formation. Then, it causes unwinding of two turns of DNA helix and separates the two strands. It forms a hydrogen bond between the end base of the DNA strand and the RNA nucleotide base. It recognizes the chain-terminating sequence in DNA and breaks away from the DNA strand. Once finished on a particular strand of DNA, it can be used on further DNA molecules.

Question 15

Resting membrane potential

Answer 15

Resting membrane potential (RMP) is the voltage (charge) difference across the cell membrane when the cell is at rest. RMP occurs when extracellular fluid (ECF) is positively charged and intracellular fluid (ICF) is negatively charged, with a difference of around 90 mV. The ion flow through the cell membrane is essential to maintain the RMP. Na+ and Cl- will flow through the membrane from the ECF to ICF. In contrast, K+ will flow from the ICF to the ECF. In the resting state, the cell membrane is more permeable to K+ and Cl- than to Na+. Because of the high outflow of K+ and inflow of Cl-, the transmembrane potential will remain negative.

Question 16

ATP

Answer 16

Adenine triphosphate (ATP) is the most important energy source for cells; it releases energy by breaking its high-energy phosphate bonds, producing adenine diphosphate and then monophosphate. Phosphocreatine can donate its phosphate to form new ATP. Acetyl-CoA is another high-energy compound, important in the citric acid cycle. Anaerobic metabolism of glycogen has a net production of 3 mol of ATP, whereas the net gain from metabolising glucose is only 2 mol of ATP. The reason for this difference is that 1 mol of ATP is used to phosphorylate glucose when it enters the cell from the blood. Glycogen is already present in the cell.

ATP synthase, also known as “mitochondrial complex V”, is located in the inner surface of the mitochondrial inner membrane (F0 domain) and the mitochondrial matrix (F1 domain). ATP synthase is responsible for the synthesis of ATP from ADP using the energy provided by the electron transport chain, and its activity increases in cells when the ATP/ADP ratio is low. Cardiac and skeletal contractility are strongly dependent on the mitochondria.

Question 17

Golgi apparatus

Answer 17

The Golgi apparatus is an organelle found in almost all eukaryotic cells. Conspicuously, erythrocytes do not have a golgi apparatus. It has two sides (cis and trans) and the membranes at one end of the stack differ in both composition and in thickness from those at the opposite end. The Golgi apparatus receives protein synthesized by the RER (rough endoplasmic reticulum), processes it in many post-translational modifications (i.e. glycosylation and phosphorylation), and packages it in secretory vesicles.

Question 18

Citric acid cycle

Answer 18

The citric acid cycle takes place inside the mitochondria. The cycle uses 1 molecule of acetyl-CoA as a significant carbon input in each cycle. Per 1 molecule of glucose, 2 molecules of acetyl-CoA are produced in glycolysis, generating 2 ATP, 6 NADH, 2 FADH2, 4 CO2, and 6 H+ in the citric acid cycle. NADH and FADH2 molecules are used in the electron transport chain, resulting in the net synthesis of 12 ATP per acetyl-CoA. The cycle can be entered through intermediates other than acetyl-CoA, for example, citrate and succinate.

The tricarboxylic acid (TCA) cycle is an aerobic metabolic pathway that takes place in the mitochondria. It oxidizes acetyl-CoA to CO2 and generates high amounts of redox cofactors (NADH/FADH2) that are utilized for ATP synthesis in the electron transport chain. Intermediates in the TCA cycle include citrate, alpha ketoglutarate, succinyl-CoA, malate, and oxaloacetate. Oxaloacetate is the primary precursor for gluconeogenesis in the fasting state. Glucose is converted into pyruvate through glycolysis.

During aerobic respiration, glucose is catabolized in 3 metabolic pathways: glycolysis, the tricarboxylic acid cycle (TCA or Krebs cycle), and oxidative phosphorylation. Most ATP is formed in the inner mitochondrial membrane by oxidative phosphorylation (34 ATP). Glycolysis occurs in the cytosol of cells and results in a net gain of 2 ATP per 1 molecule of glucose. The Krebs cycle occurs in the mitochondrial matrix and produces 3 NADH, 1 FADH2, 1 GTP, and 2 ATP.

The citric acid cycle is the major metabolic hub of the cell. Its intermediates are used in the biosynthesis of various compounds. Oxaloacetate is used for the synthesis of amino acids. These non-carbohydrate substrates can be further used in gluconeogenesis. Citrate is utilized in the synthesis of fatty acids and sterols. Succinyl-CoA provides carbon atoms for porphyrins.

Question 19

Cyclic AMP

Answer 19

Cyclic AMP is a second messenger molecule formed from ATP by the enzyme adenylyl cyclase. It is inactivated by another enzyme, phosphodiesterase. Phosphodiesterase may be inhibited by caffeine and theophylline. Cyclic AMP works by activating protein kinase A.

Vasopressin stimulates adenylate cyclase via the activation of the vasopressin 2 receptor (V2R), more officially called “arginine vasopressin receptor 2” (“AVPR2”). AVPR2 belongs to the subfamily of G-protein–coupled receptors. Its activity is mediated by the Gs type of G-proteins, which stimulate adenylate cyclase. AVPR2 is expressed in the kidney tubule, predominantly in the membrane of cells of the distal convoluted tubule and collecting ducts, in fetal lung tissue and lung cancer, the last two being associated with alternative splicing. AVPR2 is also expressed outside the kidney in vascular endothelium.

Glucagon stimulates adenylate cyclase via the activation of the glucagon receptor. The glucagon receptor is a 62 kDa protein that is activated by glucagon and is a member of the class B G-protein–coupled family of receptors, coupled to G alpha i, Gs, and (to a lesser extent) G alpha q. Glucagon receptors are mainly expressed in the liver and kidneys, with lesser amounts found in the heart, adipose tissue, spleen, thymus, adrenal glands, pancreas, cerebral cortex, and gastrointestinal tract.

Calcitonin stimulates adenylate cyclase. The calcitonin receptor, localized to osteoclasts, the kidney, and regions of the brain, is a G-protein–coupled receptor. It is coupled by Gs to adenylate cyclase, and thereby to the generation of cAMP in target cells.

Estrogen does not stimulate adenylate cyclase.

Question 20

Fast twitch and slow twitch muscle fibres

Answer 20

Fast-twitch muscle fibers have a lower number of mitochondria as compared to slow muscle fibers. Therefore, their oxidative capacity is low. They also differ from slow fibers in that they have more extensive sarcoplasmic reticulum, larger fibers, larger amounts of glycolytic enzymes, and a less extensive blood supply in the form of low capillary density.

Question 21

Biological oxidation

Answer 21

Biological oxidation is a process in which cells use an electron transport chain to transfer electrons from substrates to oxygen. Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for energy production. The end products of the flavoprotein–cytochrome system are carbon dioxide and water, which are exhaled in each breath. In oxidative phosphorylation, NADH is oxidized to NAD+.

Question 22

Potassium

Answer 22

Potassium is mainly an intracellular cation. Increasing the extracellular potassium concentration would result in depolarization of the membrane potentials. This is the result of an increase in the equilibrium potential of potassium. The depolarization activates voltage-gated sodium channels but also decreases sodium influx. This depolarization does not generate an action potential; rather, it leads to a phenomenon called “accommodation”. Due to the decrease in a gradient across the cell membrane, the hyperpolarizing outward permeability of potassium ions will decrease, and inward potassium current will increase. The activity of the sodium-potassium ATPase pump is also increased.

Question 23

DNA bases

Answer 23

Adenine and guanine are purines. Cytosine, thymine and uracil are pyrimidines. In DNA, adenine binds to thymine, and guanine to cytosine. In RNA, adenine binds to uracil.

Each DNA strand is composed of simple monomeric units called “nucleotides”. Each nucleotide consists of the sugar deoxyribose, a phosphate group, and a nitrogen-containing nucleobase. Each phosphate group is attached to the sugar of the next nucleotide by covalent bonds. The base of one DNA strand pairs with its opposite by weak cross-linkages. Adenine binds to thymine, and cytosine binds to guanine. It takes three bases to code for a single amino acid.

Question 24

Sweating volume changes

Answer 24

When a person is sweating excessively due to exercise, the body excretes more water than sodium. It causes extracellular fluid (ECF) to become more hypertonic. Due to the concentration gradient differences, fluid from the intracellular fluid (ICF) will flow to the ECF by osmosis. This will decrease the ICF volume. When that person drinks 2 liters of water, the absorbed water will cause the ECF to become more hypotonic. This will allow water flow from ECF to ICF, thus the ICF volume increases.

Question 25

Deamination

Answer 25

Deamination is defined as the process of removing the amino group from amino acids via a group of enzymes called “deaminases”. Deamination occurs in situation where amino acids are utilized for energy production. The vast majority of deaminations of amino acids take place in the liver. The products of deamination are carbon, hydrogen and ammonia. Carbon and hydrogen are recycled and utilized for energy production, whilst ammonia is detoxified via several chemical processes and excreted in urine.

Question 26

Beta 1 receptors

Answer 26

Actions of beta-1 receptors vary depending on the tissue and organs. In the heart, they increase cardiac output, heart rate, contractility, conduction, and automaticity. In adipose tissue, they promote lipolysis. In juxtaglomerular cells, they increase renin secretion.

Question 27

Carotid baroreceptors and aortic bodies

Answer 27

Common carotid arteries have carotid baroreceptors which sense blood pressure changes. Occlusion of both common carotid arteries will cause reflex changes in blood pressure. There is a decrease in the carotid sinus pressure which leads to less firing by the carotid baroreceptors. The vasomotor centers in the brain become disinhibited. This results in increased aortic arterial pressure and increased heart rate.

Carotid and aortic bodies are peripheral chemoreceptors. They send signals mostly in response to hypoxia, as opposed to changes in pH or carbon dioxide concentration. They are composed of type 1 glomus cells and glia-like type II cells. They send output to the dorsal respiratory area of the medulla via the carotid sinus nerve and IX and X nerves. They vigorously respond to hypoxia driven by exercise and high altitude as compared to anemic hypoxia. Hypercapnia has direct stimulatory effects on the central respiratory centers, and less than 20% of ventilatory drive due to hypercapnia is mediated by these peripheral chemoreceptors.

Question 28

Adipose cells

Answer 28

Adipose cells are also called “adipocytes” or “fat cells”. They consist of water, triglycerides, cholesterol esters, and proteins. After water, the next most abundant substance in adipocytes is proteins, which are secreted by the fat cells as adipokines. They include leptin, apelin, adiponectin, and resistin. Protein enzymes present in the adipocytes are involved in the hydrolysis of triglycerides.

Question 29

Prostaglandins

Answer 29

Arachidonic-acid–derived mediators, also called “eicosanoids”, are synthesized by two major classes of enzymes: cyclooxygenases (which generate prostaglandins) and lipoxygenases (which produce leukotrienes and lipoxins). Eicosanoids bind to G-protein–coupled receptors on many cell types and can mediate virtually every step of inflammation.

The most important ones in inflammation are PGE2, PGD2, PGF2α, PGI2 (prostacyclin), and TxA2 (thromboxane).

Serotonin (5-hydroxytryptamine) is a preformed vasoactive mediator with actions similar to those of histamine.

Question 30

Endoplasmic reticulum

Answer 30

Rough endoplasmic reticulum (RER) is one of two types of endoplasmic reticulum. It is an organelle that functions as a manufacturing and packaging system in a cell. It takes part in polypeptide-chain production, especially on its surface which is studded with ribosomes. Smooth endoplasmic reticulum (SER) functions mainly to produce steroid hormones; they are synthesized from cholesterol in the gonads or the adrenal glands. RER does not have any polarity.

Smooth (or “agranular”) endoplasmic reticulum functions to synthesize lipids and steroid molecules. The nucleus is the control center of the cell and contains large quantities of DNA. Ribosomes are attached to the rough endoplasmic reticulum and they are involved in the synthesis of proteins. Golgi apparatus functions to synthesize lysosomes, secretory vesicles, and other cytoplasmic constituents. Mitochondria are called the “powerhouse of the cell”, as they extract all energy from the nutrients.

Question 31

Fatty acids

Answer 31

Essential fatty acids are fatty acids that humans and other animals require for maintaining health, but are unable to synthesize. Humans and other animals must gain EFAs from their diet. There are only two fatty acids that are considered as EFAs in humans. These are alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).

Arachidonic acid (AA, sometimes ARA) is a polyunsaturated omega-6 fatty acid. It is not considered an essential fatty acid because it can be formed in the body from linoleic acid. However, it does become essential if a deficiency in linoleic acid exists or if an inability to convert linoleic acid to arachidonic acid occurs.

Oleic acid is the most common fatty acid in nature, mainly found in various animal and vegetable fats and oils. It is an odorless, colorless oil, although commercial samples may be yellowish. In chemical terms, oleic acid is classified as a monounsaturated omega-9 fatty acid. Oleic acid can be biosynthesized by the help of enzyme stearoyl-CoA 9-desaturase acting on stearoyl-CoA.

Question 32

Mitochondria

Answer 32

Most DNA is located in the cell nucleus, making it known as “nuclear DNA”. A small amount of DNA can also be found in the mitochondria, where it is called “mitochondrial DNA” or “mtDNA”.

The mitochondrion is a unique cytoplasmic organelle that contains circular DNA, which encodes tRNAs, rRNAs, and some proteins. The mitochondrial genetic system uses a slightly different genetic code, which is distinct from the universal genetic code. The mitochondrion resembles bacteria and contains an outer membrane and an inner membrane but does not contain microsomes and ribosomes. The mitochondrion is involved in ATP synthesis through the tricarboxylic acid cycle and oxidative phosphorylation. The initial phases of glucose metabolism (glycolysis) occur in the cytosol.

Question 33

Osmosis

Answer 33

Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. In biological systems, the solvent is typically water, but osmosis can occur in other liquids, supercritical liquids, and even gases.

Question 34

Equations

Answer 34

The equilibrium potential is the difference in electrical potential across a membrane, at which the electrical and concentration gradients acting on an ion are balanced, and there is no net movement of an ion out of or into the cell. The Nernst equation can be used to calculate this for a given ion.

The Gibbs–Donnan equation demonstrates how, in the presence of a non-diffusible ion (e.g. proteins), diffusible ions will distribute themselves across the membrane such that their ratios of concentration on either side of the membrane are equal.

The Henderson–Hasselbach equation is used to estimate the pH of a buffer solution.

The Dirac equation is an equation in particle physics and has no application in human physiology.

The Euler–Lagrange equation is a mathematical equation and has no application in human physiology.

Question 35

Peptide hormone bonding

Answer 35

Binding of a peptide hormone to its receptor leads to a series of events that generate second messengers. These include reversible hydrophilic interactions, allosteric regulation, hydrogen bonding, and coupling with an active site. Allosteric modulators bind to the allosteric binding site of the receptor and modify the effects of agonists. Covalent linkages result in irreversible antagonism at the receptor site which is not seen with peptide hormones.

Question 36

CAMs

Answer 36

Cells are attached to the basal lamina and to each other by cell adhesion molecules (CAMs) that are prominent parts of the intercellular connections.
The CAMs can be divided into four broad families: integrins, adhesion molecules of the IgG superfamily of immunoglobulins, cadherins, and P-selectins. Proteoglycans are also considered to be a class of CAMs. Cadherins and integrins link the cell surface with the cytoskeleton through binding to actin and intermediate filaments. The cadherin adhesion complex bridges neighboring cells and the actin-myosin cytoskeleton, and thereby contributes to mechanical coupling between cells, which drives many morphogenetic events and tissue repair. Myosins, kinesins, and dyneins are included in the family of cytoskeletal motor proteins. Dyneins use energy from ATP hydrolysis to power a wide variety of cellular functions.

Question 37

Innervation of blood vessels

Answer 37

Almost all vessels except the capillaries are innervated by the sympathetic system. Precapillary sphincters and metarterioles receive minimal sympathetic innervation in mesenteric circulation. Sympathetic innervation of small arteries and arterioles increases their resistance. Sympathetic innervation of veins decreases the pooling of blood in the venous system and helps with blood flow to the heart, thus maintaining circulation.

Question 38

Urea cycle

Answer 38

The urea cycle occurs in the liver, primarily in the cytoplasm of hepatocytes, though part of it also occurs in mitochondria. Principally, NH4+ forms carbamoyl phosphate, which is combined with ornithine in the mitochondria to form citrulline. Citrulline is converted to arginine; urea is then released and ornithine regenerated. The whole process requires 3 ATP per cycle.

Question 39

Muscle contraction

Answer 39

Muscle fiber contains myofibrils. Myofibrils contain myofilaments which are of two types. Thin filaments consist of actin, a globular protein, arranged in a double helix. Along the length of actin are troponin and tropomyosin molecules. Thick filaments consist of myosin. Actin and myosin filaments are arranged parallel to each other, producing an overlapping repeating pattern. Each repeating unit is called a sarcomere, which is bound on two sides by the Z-line. The H-band or H-zone is a zone of thick filaments that has no actin, and has a lighter appearance. It shortens during contraction. The M-line is the center of thick filament where reversal of polarity occurs, so that cross bridges extending left and right draw the thin filaments toward the M-line during the contraction phase.

Question 40

Connexon

Answer 40

A connexon is an assembly of six proteins called “connexins” that form the pore for a gap junction between the cytoplasm of two adjacent cells. Connexons permit bidirectional substance and ion flow without entering the extracellular fluid. Connexons do not maintain a cell’s polarity. In some cells, the hemichannel itself is active as a conduit between the cytoplasm and the extracellular space, allowing the transference of ions and small molecules lower than 1–2 kDa.

Question 41

Endocytosis and phagocytosis

Answer 41

In endocytosis, the material to be internalized is surrounded by an area of plasma membrane, which then buds off inside the cell to form a vesicle containing the ingested material. There are various types of endocytosis:

1. Phagocytosis (“cell eating”) is the process by which bacteria, dead tissue, or other bits of microscopic material are engulfed by cells such as the polymorphonuclear leukocytes of the blood. The material makes contact with the cell membrane, which then invaginates. During phagocytosis (endocytosis), the invagination is pinched off, leaving the engulfed material in the membrane-enclosed vacuole and the cell membrane intact.
2. Pinocytosis (“cell drinking”) is a similar process where the vesicles are much smaller in size, and the substances ingested are in solution.
3. Clathrin-mediated endocytosis, also known as “receptor-mediated endocytosis”, occurs at membrane indentations where the protein clathrin accumulates. It is responsible for the internalization of many receptors and the ligands bound to them, including, for example, nerve growth factor and low-density lipoproteins. It also plays a major role in synaptic function.

One pathway of clathrin-independent endocytosis involves the uptake of molecules in small invaginations of the plasma membrane (50 to 80 nm in diameter) called “caveolae”. Caveolae are enriched in lipid rafts of cholesterol and sphingolipids, and possess a distinct coat formed by a protein called “caveolin”. They have been implicated in cell signaling and a variety of transport processes, including endocytosis.

Exocytosis is the reverse of endocytosis. Exocytosis proceeds by two main pathways: constitutive and non-constitutive.

Question 42

Beta 3 receptors

Answer 42

Beta3 receptors are mainly found in the brown adipose tissue, gallbladder, and urinary bladder. Two main actions of the beta3 receptors are enhancement of lipolysis in adipose tissue and thermogenesis in skeletal muscle.

Question 43

Stretch reflex

Answer 43

Whenever a muscle is stretched suddenly, as in muscle stretch reflex, excitation of the muscle spindles takes place. Type 1a nerve fiber originates from the muscle spindle and reaches the dorsal root of the spinal cord. It gives a branch to the anterior motor neuron in the anterior horn of gray matter which innervates the same muscle. This causes reflex contraction of the large skeletal muscle fibers of the stretched muscle. This stretch reflex has both dynamic and static components.

Question 44

UPS

Answer 44

The ubiquitin–proteasome system (UPS) is a highly regulated mechanism of intracellular protein degradation and turnover. Through the concerted actions of a series of enzymes, proteins are marked for proteasomal degradation by being linked to the polypeptide co-factor ubiquitin. The impact of the UPS in some neurological disorders may be related to deficits in the clearance of misfolded proteins leading to intracellular protein aggregation, cytotoxicity, and cell death.

Question 45

Body fluids

Answer 45

Interstitial fluid accounts for around 15% of body weight.

Overall, 60% of body weight is accounted for by water, 18% by protein, 7% by minerals and 15% by fat in the average male.

40% of body weight is intracellular water and 20% is extracellular water. A quarter of this is in the plasma and the rest is interstitial fluid—so 15% of body weight.

Question 46

Plasminogen

Answer 46

Tissue plasminogen activator antigen is released in response to endothelium-dependent vasodilation in normotensive and hypertensive patients during muscarinic receptor stimulation.
Plasminogen is the inactive precursor of plasmin, which is a potent serine protease. It helps in dissolution of fibrin clots. Plasminogen is inhibited by plasminogen activator inhibitor 1.

Question 47

Relative and absolute refractory periods

Answer 47

The neuronal action potential has two refractory periods: relative and absolute. While the absolute refractory period is a period during which a second action potential cannot be initiated, the relative refractory period is an interval during which initiation of a second action potential is inhibited, but not impossible. During the relative refractory period, the rate of depolarization decreases and takes a larger than normal depolarization to produce the second action potential.

Question 48

Cell nucleus

Answer 48

The cell nucleus is an essential cellular organelle in eukaryotic cells. The nucleus has a nuclear envelope (NE) which separates the chromatin from the cytoplasm. NE is a membrane in continuity with the endoplasmic reticulum. The nucleus contains DNA, ribosomal RNA (rRNA), small nucleolar RNA (snoRNA), and nuclear proteins. Centrioles are involved in the mitotic spindle and cytokinesis. Intranuclear cytoplasmic inclusions are rare and are frequently associated with neoplasia.

Question 49

Heat loss body

Answer 49

Heat is lost from the body to the surroundings by various mechanisms. Heat loss from the body is almost 60% by radiation in the form of infrared heat rays. 3% of heat loss occurs via conduction to solid surfaces, 15% occurs via conduction to air, and 22% via evaporation. Heat lost via conduction to air is carried away by the convection air currents.

Question 50

Glucokinase

Answer 50

Glucose-6-phosphate cannot be stored for long in the body; it must either be converted to glycogen for storage or metabolised for energy. Glycogenolysis is the breakdown of glycogen to glucose; glycolysis is the process by which glucose is converted to pyruvate with the release of energy.. Glucokinase is only found in the liver

Question 51

Heart energy substrates

Answer 51

Amino acids and ketones provide 3%–5% of energy.
Fatty acids are the main fuel for the heart (60%–90% of energy). 10%–40% is from pyruvate oxidation formed from glycolysis and lactate.

Question 52

Electron transport chain

Answer 52

Oxidative phosphorylation is the culmination of cellular respiration. It is made up of 2 parts: the electron transport chain (ETC) and chemiosmosis. The mitochondrial ETC uses oxidation–reduction reactions to create an electrochemical proton (H+ ions) gradient across the inner membrane that leads to the phosphorylation of ADP. ETC uses energy-rich molecules (e.g. NADH and FADH2) to reduce molecular oxygen to water and generate ATP. Iron is utilized for the synthesis of cofactors essential to the function of ETC, for example electron-transfer flavoproteins.

Question 53

Ribosomes

Answer 53

Ribosomes are attached to the rough endoplasmic reticulum, and they are involved in the synthesis of proteins. The nucleus is the control center of the cell and contains large quantities of DNA. Smooth (or “agranular”) endoplasmic reticulum functions to synthesise lipids and steroid molecules. Golgi apparatus functions to synthesise lysosomes, secretory vesicles, and other cytoplasmic constituents. Mitochondria are called “the powerhouse of the cell” as they extract all energy from the nutrients.

Question 54

Eosinophils

Answer 54

Eosinophils are visible as red granulated leucocytes when stained by an eosin dye. The small granules of eosinophils contain many chemical mediators, such as eosinophil peroxidase, ribonuclease (RNase), deoxyribonucleases (DNases), lipase, plasminogen, eicosanoid, and major basic protein. All these mediators are important to eosinophil-induced inflammation responses.

Question 55

Central nervous system cells

Answer 55

In the central nervous system, myelin is produced by oligodendrocytes. Schwann cells are the myelin-producing cells in the peripheral nervous system. A neuron is a nerve cell that is the basic unit of the nervous system. It has a cell body, dendrites, and a single axon. Each neuron may have multiple dendrites but only a single axon. Astrocytes have many functions, including the formation of the blood–brain barrier. They aid in maintaining the blood–brain barrier through their end-feet encircling endothelial cells. Microglia are a type of glial cell with an active role in immune defense in the central nervous system. They act as macrophages in the nervous system.

Question 56

Nucleolus

Answer 56

The nucleolus tends to be smaller in a cell that synthesizes little protein, and it becomes significantly enlarged when the cell is actively synthesizing proteins. The genetic code comprises successive “triplets” of bases. Three successive bases code for an amino acid. Messenger RNA is single-stranded RNA that contains codons which are complementary to the code triplets of the DNA genes. Transfer RNA transports activated amino acids to the ribosomes. Ribosomes do not show specificity for different types of mRNA.

Question 57

Iron transport

Answer 57

Iron circulates in the blood bound to a protein called “transferrin”. An average adult’s daily diet contains 15 mg of iron, of which only 1 mg of iron is absorbed per day. Iron is absorbed slowly from the proximal small intestine. Iron in the ferric form is reduced by the enzyme ferric reductase to the ferrous form for absorption. Iron absorption is an active process regulated by a protein called “divalent metal transporter 1” (“DMT 1”).

Question 58

Cerebral autoregulation

Answer 58

The process of autoregulation regulates the blood supply to the vital organs. Cerebral circulation changes the least during exercise because of cerebral autoregulation of blood flow. In cutaneous, renal, and splanchnic circulation, the blood flow is decreased by vasoconstriction and is diverted to the muscles. During exercise, vasodilation occurs in the muscles to increase blood flow. Blood flow increases to approximately 2 L/min to the active muscle groups during exercise.

Question 59

Lysozymes

Answer 59

Lysozymes are membrane-enclosed organelles that contain an array of enzymes capable of breaking down all types of biological polymers—proteins, nucleic acids, carbohydrates, and lipids.
All lysosomal enzymes are acid hydrolases, which are active at the acidic pH (approximately 5) that is maintained within lysosomes but not at the neutral pH (approximately 7.2) characteristic of the rest of the cytoplasm. The requirement of an acidic pH for these lysosomal hydrolases provides double protection against uncontrolled digestion of the contents of the cytosol; even if the lysosomal membrane were to break down, the released acid hydrolases would be inactive at the neutral pH of the cytosol.