Quiz 3 13/11/18

Question 1

Concerning the neuromuscular junction: The duration of the depolarising phase of the action potential in skeletal muscle is approximately 250 msec.

Answer 1

FALSE
-The action potential in skeletal muscle is much shorter, lasting only 1-2 msec. The action potential in cardiac muscle may however last 250 msec, due to prolonged influx of calcium ions.

Question 2

Concerning the neuromuscular junction: The endplate potential is curtailed by an inhibitory postsynaptic potential.

Answer 2

FALSE
-Unlike most central nervous system synapses, no IPSP is generated at the neuromuscular junction. In many ways it can be considered to be a very simple synapse.

Question 3

Concerning smooth muscle: Smooth muscle is innervated by the somatic nervous system.

Answer 3

FALSE
-The somatic nervous system innervates only skeletal (voluntary) muscle. Smooth muscle is often spontaneously active and its activity may be modulated by the autonomic nervous system, hormones, and other local factors.

Question 4

T lymphocytes (T cells): Have class I MHC molecules on their cell surface.

Answer 4

TRUE

-Just like all nucleated cells.

Question 5

Natural killer (NK) cells: Functionally, can kill tumour cells or cells infected with viruses.

Answer 5

TRUE
-This is their main function. They can recognise viral or tumour antigens on the surface of cells, and subsequently kill the infected or tumour transformed cells.

Question 6

Concerning blood: A haematocrit of 58% would be normal in a man living at high altitude.

Answer 6

TRUE
- The normal haematocrit range for a male is 40-52%. However the normal acclimatisation process to living at altitude (where oxygen is less abundant) involves an increase in the number of circulating red blood cells which increases the oxygen carrying capacity of the blood. If the red blood cell number increases then haematocrit, which defines the percentage of red blood cells in whole blood, will increase.

Question 7

The three ‘gluconeogenic sources’ are: certain fatty acids, glycerol, and lactate.

Answer 7

FALSE
-The three ‘non-carbohydrate sources’ from which glucose can be generated are certain amino-acids (not fatty acids), glycerol and lactate.

Question 8

Glucokinase has a high Km for glucose, and hexokinase a low Km for glucose.

Answer 8

FALSE
-The liver enzyme glucokinase has a lower affinity for glucose than does its isozyme (isoenzyme) hexokinase present in other tissue. This means that the removal of blood glucose by the liver is restricted largely to circumstances in which the plasma [glucose] is high, as occurs following a carbohydrate-rich meal.

Question 9

One function of the pentose phosphate pathway is the production of NADP+.

Answer 9

FALSE

-One of the functions of the pentose phosphate pathway is the production of NADPH, not NADP+.

Question 10

Concerning reflexes: The primary sensory ending of a muscle spindle in a voluntary muscle sends its information via the dorsal columns to the postcentral gyrus.

Answer 10

TRUE
-This pathway is in addition to the local spinal circuit that mediates the monosynaptic stretch reflex. This ascending pathway provides information that contributes to the conscious perception of the position of the limb.

Question 11

Concerning drugs acting at the skeletal neuromuscular junction: Hemicholinium inhibits activity at the NMJ.

Answer 11

TRUE

-By blocking choline transport into ACh synaptic vesicles.

Question 12

The range of movement at a synovial joint is limited by the synovial capsule.

Answer 12

FALSE
-The capsule is protective but limitation of movement is not affected by the capsule itself. Limitation is mostly due to the structure of the articulating bones, ligaments and sometimes muscle bulk getting in the way.

Question 13

Concerning skeletal muscle: Actin and myosin filaments shorten during contraction.

Answer 13

FALSE
-This is the crux of the sliding filament theory – the actin and filaments do not shorten, but slide past each other, hence shortening the muscle as a whole.

Question 14

Concerning the major histocompatibility complex (MHC): Class II MHC molecules are present on all classical antigen presenting cells.

Answer 14

TRUE
-Although, in certain circumstances, many cells can be induced to express class II MHC molecules, only classical APCs such as monocytes/macrophages, B cells, Langerhans cells in the skin epidermis, and interdigitating cells (but not follicular dendritic cells) in lymph nodes express these glycoproteins constitutively.

Question 15

Natural killer cells (NK) cells: Are of lymphoid origin and bone marrow divided.

Answer 15

TRUE

-They are derived from bone marrow lymphoid cells.

Question 16

Dietary disaccharides are digested in the stomach.

Answer 16

FALSE
-The two main disaccharidases are lactase and sucrase, and they are found on the surface of epithelial cells in the small intestine.

Question 17

Concerning the autonomic nervous system: Activation of the sympathetic nervous system releases epinephrine into the blood system.

Answer 17

TRUE

-Epinephrine release from the adrenal glands is part of the fight-or-flight response.

Question 18

Concerning the neuromuscular junction: Acetylcholine is packaged into vesicles in the presynaptic terminal.

Answer 18

TRUE
-Ca2+ entry into the presynaptic terminal of the motor neurone triggers vesicles to fuse with the presynaptic membrane and hence release acetylcholine into the synaptic cleft by exocytosis.

Question 19

Concerning skeletal muscle: The removal of Ca2+ into the sarcoplasmic reticulum requires ATP.

Answer 19

TRUE
-Ca2+ is actively sequestered into the sarcoplasmic reticulum by a calcium pump. This moves calcium ions up their concentration gradient and therefore requires energy which is released from ATP.

Question 20

As electrons pass along the terminal respiratory system, they are pumped across the inner membrane of the mitochondrion.

Answer 20

FALSE
-The passage of electrons along the terminal respiratory system results in protons being ‘pumped’ across the inner membrane to generate a gradient in which the [H+] outside the membrane is greater than the [H+] inside.

Question 21

Haemoglobin is one of the redox components of the terminal respiratory system.

Answer 21

FALSE
-Haemoglobin, of course, is an oxygen-carrying protein of blood, not part of the terminal respiratory system. However, the terminal respiratory system does include redox components which, like haemoglobin, contain haem groups: these are the cytochromes.

Question 22

McArdle’s disease is a genetic deficiency in skeletal muscle glucose 6-phosphatase.

Answer 22

FALSE
-Skeletal muscle quite normally lacks glucose 6-phosphatase. McArdle’s disease is a genetic lack of skeletal muscle glycogen phosphorylase.

Question 23

The three kinase-catalysed reactions of glycolysis all generate ATP from ADP during glycolytic flow.

Answer 23

The first two, catalysed by hexokinase (or glucokinase in liver) and phosphofructokinase accomplish phosphorylation of glucose and fructose 6-phosphate respectively, and use ATP. They can be regarded as investing energy in the process of glycolysis, by preparing the sugar molecule for its later, ATP-providing splitting. The third kinase-catalysed reaction occurs at the end of the process. Here, pyruvate kinase catalyses the dephosphorylation of phosphoenolpyruvate (which essentially is just pyruvate phosphate) into pyruvate. This is one of the ATP-yielding steps of glycolysis. Notice that the enzyme is named for the reaction as it occurs in the opposite direction to that actually occurring during glycolysis. Kinases catalyse phosphorylation of something, with the phosphate coming from ATP. In theory, like all enzymes, pyruvate kinase can catalyse its reaction in either direction, but in actuality, in cells, net flow always occurs in the direction of phosphoenolpyruvate to pyruvate (and of ADP to ATP). Nevertheless, the enzyme is named as though it were catalysing the reaction in the opposite direction.

Question 24

Concerning skeletal muscle: A decrease in cytosolic Ca2+ concentration will initiate muscle contraction.

Answer 24

FALSE
-Ca2+ released from the sarcoplasmic reticulum binds to troponin, which shifts tropomyosin out of the way and allows cross bridges between actin and myosin to be formed. A fall in cytosolic calcium ion concentration will stop the process of cross bridge formation.

Question 25

In eukaryotic cells, parts of the citric acid cycle operate in the cytosol, and parts in the mitochondrion.

Answer 25

FALSE

-In eukaryotic cells, the citric acid cycle occurs exclusively in mitochondria.

Question 26

Someone without access to water but with access to food could delay the symptoms of dehydration by eating fats.

Answer 26

TRUE
- Fats are highly reduced molecules (i.e. they contain many H atoms). Catabolism of fats therefore generates, through β-oxidation and the citric and cycle, much NADH and FADH2.When these pass their electrons along the terminal respiratory system, much water is produced as the electrons finally reach oxygen. So, the person in this slightly surreal situation uses H atoms from the fatty food and oxygen breathed in to generate body water, and hence prevent dehydration.

Question 27

The first thing that happens to glucose when it enters a cell is that it is phosphorylated.

Answer 27

TRUE
-Phosphorylation traps the glucose inside the cell, so, whatever its subsequent metabolic fate, this is the first process it undergoes. The product is glucose in which the -OH group attached to carbon 6 of the hexose is converted into a phosphate group. The phosphorylation is a group transfer reaction, the phosphate coming from ATP, which is therefore converted to ADP. Notice that trapping the glucose in the cell requires energy. The reaction is catalysed by hexokinase in most tissues, and by an isozyme (isoenzyme) of hexokinase, glucokinase, in liver.

Question 28

The main enzyme of glycogen catabolism is glycogen phosphatase.

Answer 28

FALSE
-Phosphatases are enzymes that catalyse the removal of phosphate groups. Glucose 6-phosphatase, which catalyses conversion of glucose 6-phosphate to glucose and inorganic phosphate, is an example. The main enzyme of glycogen catabolism is glycogen phosphorylase. This adds inorganic phosphate onto glucose monomers as they are removed from a glycogen chain.

Question 29

McArdle’s disease is a genetic deficiency in skeletal muscle glucose 6-phosphatase.

Answer 29

FALSE
-Skeletal muscle quite normally lacks glucose 6-phosphatase. McArdle’s disease is a genetic lack of skeletal muscle glycogen phosphorylase.

Question 30

Concerning contrast media: 1 in 100,000 patients die from an allergic reaction to contrast media.

Answer 30

TRUE
- Iodine is safe and is easily attached to molecules of varying sizes. However, a very small number of patients can have an allergic reaction to the iodine. These reactions range from mild (nausea, feeling of heat, urticaria), moderate (wheeze, hypotension, bradycardia), to severe (angiooedema, bronchospasm, cardiovascular collapse).

Question 31

Concerning smooth muscle: The contractile filaments in smooth muscle are not arranged into sarcomeres.

Answer 31

TRUE
-The filaments in skeletal and cardiac muscle are arranged into sarcomeres, whereas the arrangement of filaments in smooth muscle is more random.

Question 32

Immunoglobulin M (IgM): Is the key antibody produced in a primary immune response.

Answer 32

TRUE
-In a primary antibody response IgM is the major immunoglobulin type produced. In a secondary or subsequent response, a switch to production of other types generally occurs. This is usually to IgG but the exact type produced is a result of a complex interplay of factors involving the nature of the antigen, the helper T cells, the cytokines, and the route of entry of antigen into the body.

Question 33

The citric acid cycle is a process concerned with catabolism.

Answer 33

TRUE
-As a cycle, the cycle is concerned solely with the catabolism of acetyl CoA (derived from fuel molecules) to carbon dioxide. During flow through the cycle, there is no net gain (or loss) of any of the eight intermediates, so none can be ‘tapped off’ for anabolic purposes.

Question 34

Aldolase B catalyses the cleavage of fructose 1-phosphate.

Answer 34

TRUE
-Fructose may be phosphorylated to fructose 1-phosphate in the fructokinase-catalysed reaction. This is then cleaved to dihydroxyacetone phosphate and glyceraldehyde in the aldolase B-catalysed reaction. Both products may then enter glycolysis (the glyceradehyde having first been phosphorylated). The aldolase B-catalysed reaction is similar to the familiar, sugar-splitting, aldolase-catalysed reaction of glycolysis, except that, in the latter, it is fructose 1, 6-bisphosphate that is cleaved, and the two products are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

Question 35

Digestion of dietary polysaccharides and disaccharides involves hydrolysis reactions.

Answer 35

TRUE
-The glycosidic bond found in polysaccharides and disaccharides is broken when the ‘elements of water’ are added across the bond in a hydrolysis reaction. Conversely, the formation of the bond is, chemically, a condensation reaction, in which water is released. Many types of biopolymers are formed by condensation reactions and degraded by hydrolysis reactions.

Question 36

Concerning reflexes: The primary sensory ending of a muscle spindle in a voluntary muscle is stimulated when gamma efferent fibres to the spindle are activated.

Answer 36

TRUE
-gamma efferent fibres innervate the intrafusal fibres, which lie in series with the muscle spindles. Contraction of just the intrafusal fibres will therefore stretch, and stimulate, the sensory ending of the muscle spindle. This will in turn trigger reflex contraction of extrafusal muscle fibres.