Biochemistry and Molecular Biology solutions

Question 1

Which of the following statements about triacylglycerols is correct?

Your answer:

1. a) Triacylglycerols are carried in the blood bound to albumin.

Correct answer:

1. d) Triacylglycerols are hydrolysed to glycerol and fatty acids.

Feedback:

Fat in the form of triacylglycerol is stored in the adipose cells. This fat storage is apparently unlimited. Triacylglycerols (TAGs) are not hydrated, are more highly reduced than is glycogen and therefore has a higher energy/mass ratio. Under catabolic conditions (such as occurs in fasting) glucagon levels are high causing increased hydrolysis of TAGs in the adipose cells since the hormone sensitive lipase is activated. Three fatty acid molecules and one glycerol molecule are released from each triacylglycerol molecule. The free fatty acids are carried in the blood reversibly adsorbed to serum albumin. Fatty acids are oxidized to acetyl-CoA in cells with mitochondria (with the exception of brain cells) providing energy via the citric acid cycle and the electron transport chain. The glycerol is converted to glucose in the liver via the gluconeogenesis pathway.
Page reference: Page 216

Question 2

Which of the following statements about free fatty acids is correct?

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Correct answer:

1. b) Fatty acids are oxidised to acetyl-CoA.

Feedback:

Under catabolic conditions, when glucagon levels are high, three fatty acid molecules and one glycerol molecule are released from each triacylglycerol molecule in adipose tissue by the action of the hormone sensitive lipase. The free fatty acids are carried in the blood reversibly bound to serum albumin. Fatty acids can be oxidized to acetyl-CoA in cells with mitochondria (with the exception of brain cells) to provide energy via the citric acid cycle and the electron transport chain. The glycerol released from the hydrolysis of triacylglycerol is converted to glucose in the liver via gluconeogenesis.
Page reference: Page 216

Question 3

Which of the following statements about the activation of fatty acids is correct?

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Correct answer:

1. d) Fatty acid activation requires energy in the form of ATP.

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Fatty acids released from triacylglycerols stores under catabolic conditions are converted to fatty acyl-CoAs by a process known as fatty acid activation before they are oxidised. The activations occur on the outer mitochondrial membrane catalyzed by fatty acyl-CoA synthetases. ATP is hydrolysed to AMP and pyrophosphate, the hydrolysis of the latter making the reaction strongly exergonic and irreversible. These acyl derivatives are then transported into the mitochondria by carnitine (without the CoA), handed to mitochondrial coenzyme A, and become fatty acyl-CoAs again with the high energy nature of the acyl bond preserved. Fatty acyl-CoA derivatives are broken down two carbon atoms at a time forming acetyl-CoA, by a process known as β-oxidation. FADH2 and NADH + H+ are generated in the process. The latter feed into the electron transport chain while the acetyl-CoA feeds into the citric acid cycle.
Page reference: Page 217

Question 4

Which of the following statements about β-oxidation of fatty acids is correct?

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Correct answer:

1. d) ?-oxidation of fatty acyl-CoAs occurs in the mitochondrial matrix.

Feedback:

Fatty acyl-CoA derivatives from fatty acid activation are broken down two carbon atoms at a time to form acetyl-CoA. This process, β-oxidation, occurs in the mitochondrial matrix. The reactions are an FAD-dependent dehydrogenation producing one FADH2, a hydration, an-NAD+-dependent dehydrogenation producing one NADH + H+ and a split catalysed by a thiolase to produce one fatty acyl-C0A two carbons shorter than the original, and an acetyl-CoA. The latter feeds into the citric acid cycle. The FADH2 and NADH + H+ feed into the electron transport chain and drive ATP synthesis. Oxidation of odd-numbered carbon-chain fatty acids produces the three-carbon molecule propionyl-CoA as the penultimate product. This is converted to succinyl-CoA and then fed into the citric acid cycle.
Page reference: Page 218

Question 5

Which of the following statements about the ketone body acetoacetate is not correct?

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Correct answer:

1. a) Acetoacetate is oxidised to ?-hydroxybutyrate.

Feedback:

Fatty acids are oxidised to acetyl-CoA in most cells. This provides energy via the citric acid cycle and the electron transport chain. However under starvation conditions when glycogen stores have been exhausted, or in uncontrolled diabetes mellitus type 1 when fat breakdown predominates (high glucagon/insulin ratios) due to lack of insulin production, the excessive levels of acetyl-CoA formed from β-oxidation of fatty acids in the liver cannot be fed into the citric acid cycle. Acetoacetate is produced along with its reduced product β-hydroxybutyrate. These molecules derived from fatty acid oxidation are known as ketone bodies. Acetoacetate spontaneously decarboxylates to acetone which is exhaled and not metabolised. Acetoacetate is converted back to acetyl-CoA and can be used by peripheral tissues with mitochondria to generate energy.
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Question 1

Which of the following statements about glycogen breakdown is correct?

Your answer:

1. a) Glycogen phosphorylase splits off glucose from the reducing end of a glycogen chain.

Correct answer:

1. b) Glucose-6-phosphatase is present in liver but not in muscle.

Feedback:

The enzyme glycogen phosphorylase splits off glucose units from the non-reducing end of glycogen chains using inorganic phosphate to release glucose-1-phosphate. This is converted to glucose-6-phosphate by phosphoglucomutase. The enzyme glucose-6-phosphatase, located in the membrane of the endoplasmic reticulum of the liver, hydrolyses glucose-6-phosphate to give free glucose which is released into the blood and inorganic phosphate that stays in the liver cytoplasm.
The enzyme glucose-6-phosphatase is present in liver (and kidneys although their small size makes them insignificant in this respect), but not in muscle. Consequently muscle does not release glucose into the blood. Fatty acids are the preferred fuel for the liver so glucose resulting from glycogen breakdown is released rather than being metabolised.
Page reference: Page 165

Question 2

Which of the following statements about glycogen synthesis is correct?

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Correct answer:

1. c) Glycogen synthesis occurs when there is a high insulin to glucagon ratio

Feedback:

Glycogen is synthesized from glucose which is first phosphorylated to glucose-6-phosphate (using ATP) then converted to glucose-1-phosphate by the enzyme phosphoglucomutase. This enzyme catalyses a reversible reaction that is used in both glycogen synthesis and breakdown. The glucose-1-phosphate reacts with UTP to form UDP-glucose which donates glucose units to the nonreducing ends of glycogen chains. The released UDP is recycled back to UTP using ATP as the phosphoryl donor. Glycogen synthase is active when insulin levels are high since insulin is an anabolic hormone that promotes storage.
Page reference: Page 162

Question 3

Which of the following statements about galactose is correct?

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Correct answer:

1. d) Galactosaemia is a serious genetic disease that requires the removal of galactose from the diet of newborn infants.

Feedback:

Lactose, milk sugar, is a disaccharide of galactose and glucose. Galactose is a glucose epimer (and an isomer of fructose). Specific enzymes use each of these sugars as separate substrates. After absorption, the galactose released in digestion is phosphorylated to galactose-1-phosphate by the liver enzyme galactokinase and reacts with UDP-glucose to form UDP-galactose and glucose-1-phosphate. The reaction is catalysed by the enzyme galactose-1-phosphate uridyltransferase. Glucose-1-phosphate is converted to glucose-6-phosphate and enters the mainstream of glucose metabolism while UDP-galactose is epimerised to UDP-glucose in the liver by UDP-galactose epimerase. The most common galactosaemia is due to a deficiency of the enzyme galactose-1-phosphate uridyltransferase causing the accumulation of toxic derivatives of galactose-1-phosphate that cause irreparable brain damage. This can be avoided in infants by restricting galactose from the diet.
Page reference: Page 168

Question 4

Which of the following statements about the enzymes, glucokinase and/or hexokinase is correct?

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Correct answer:

1. b) Hexokinase has a high affinity for glucose.

Feedback:

Glucokinase is found in the liver (and pancreas), and hexokinase is found in most other tissues. They are isoenzymes. Both enzymes form glucose-6-phosphate from glucose; glucokinase has a low affinity for glucose and functions when the intracellular concentration of glucose in the liver is high (such as after a meal). The liver is not dependent on glucose for energy as it can use fatty acids. It stores glucose as glycogen, and when these stores are filled, converts it to fat.
Hexokinase has a high affinity for glucose so can function even when glucose levels are low. It is present in brain and other tissues. Depending on the tissue, glucose-6-phosphate can either provide energy via glycolysis or it can be converted to glucose-1-phosphate for the synthesis of glycogen.
Page reference: Page 167

Question 5

Which of the following statements about circulating lipoproteins is correct?

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Correct answer:

1. c) Chylomicrons contain mainly dietary triacylglycerols in their core.

Feedback:

Chylomicrons are synthesised in intestinal cells from triacylglycerols and cholesterol esters during absorption of food digestion products. VLDL are synthesised in the liver from triacylglycerols produced from excess glucose. When these lipoproteins are released into the blood, lipoprotein lipase located on the surface of endothelial cells of capillaries, hydrolyses the triacylglycerols in their core to fatty acids and glycerol. The fatty acids diffuse into the nearby adipose and other tissues to be stored as triacylglycerols, or to be used as fuel, and the glycerol travels to the liver. VLDL released from the liver are modified to IDL then to LDL by lipoprotein lipase. They acquire cholesterol ester from HDL. =Cholesterol is then acquired by most cells from the endocytosis of LDL which contain high levels of cholesterol ester.
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Question 1

Which of the following statements about the digestion of proteins is correct?

Your answer:

1. a) Protein digestion begins in the small intestine.

Correct answer:

1. b) Protein digestion begins in the stomach.

Feedback:

Dietary proteins are first denatured by the hydrochloric acid secreted by the parietal cells of gastric glands (the weak non-covalent interactions that hold proteins in their specific 3-D configuration are disrupted exposing many peptide bonds that were initially shielded inside the protein). The cells of the stomach epithelium secrete pepsinogen (stimulated by the hormone gastrin) which is activated to pepsin initially by the hydrochloric acid, then by autocatalysis by pepsin itself. Pepsin then begins the specific partial hydrolysis of the denatured proteins in the stomach where the pH of the environment is 1-2 (optimum pH of pepsin). Pepsin is an endopeptidase that only hydrolyses those peptide bonds in which an aromatic amino acid supplies the NH of the peptide bond.
Trypsin is synthesised in the pancreas as trypsinogen (together with other proenzymes) and released into the small intestine. It is not present in the stomach.
Page reference: Page 147

Question 2

Which of the following statements about the digestion of starch is correct?

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Correct answer:

1. b) The digestion of starch by ?-amylase is an exergonic process.

Feedback:

All digestive reactions are hydrolytic reactions. They are exergonic processes that do not require energy, but release it. α-amylase hydrolyses α-1, 4-glycosidic bonds that make up the long chains of the starch molecules. Other enzymes complete the digestion. The Gibbs free energy change for the reaction is sufficiently negative to push the equilibrium to the side of hydrolysis. The glucose released from the complete digestion of starch is absorbed into epithelial cells of the intestine by an energy dependent process and is transported into the blood by facilitated diffusion.
Page reference: Page 149

Question 3

Which of the following statements about the digestion of dietary triacylglycerols is correct?

You did not answer the question.

Correct answer:

1. c) Dietary triacylglycerols are digested by pancreatic lipase.

Feedback:

Dietary fats or triacylglycerols are hydrolysed to two fatty acid molecules and one monoacylglycerol molecule by pancreatic lipase. These hydrolytic products are absorbed into the brush border cells of the small intestine and re-esterified to triacylglycerols that become part of the hydrophobic core of chylomicrons, which are one class of lipoproteins. Free fatty acids are not directly released into the blood.
Lipoprotein lipase is an extra cellular enzyme on endothelial cells of blood capillaries. It hydrolyses the triacylglycerols in circulating plasma lipoproteins.
Page reference: Page 151

Question 4

Which of the following statements about hormonal situations in the body is correct?

You did not answer the question.

Correct answer:

1. b) During the post-absorptive phase, high insulin levels mediate glucose uptake in muscles.

Feedback:

During the post-absorptive phase, insulin is released from the pancreas and glucagon secretion is inhibited. High insulin levels mediate glucose uptake in muscle and adipose tissues. There is uptake of all foods from the blood when insulin is secreted and those tissues that can store foods will do so. However insulin is not required for glucose uptake in the liver since glucose enters it in an insulin independent manner. During emergency situations when adrenalin is released, glycogen breakdown occurs in both liver and muscle, but although the liver releases glucose into the blood, the muscle cannot do so. The glucose-6-phosphate formed from glycogen breakdown via glucose-1-phosphate is hydrolysed in the liver (by glucose-6-phosphatase) to release glucose into the blood, but in muscle, it provides ATP (via glycolysis) for muscle processes such as contraction as will be seen in later chapters. Glucagon affects the liver but not muscle.
Page reference: Page 155

Question 5

Which of the following statements about food storage in the body is correct?

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Correct answer:

1. c) Fat is a more efficient form of fuel storage than glycogen.

Feedback:

More glycogen is stored per unit mass of liver than per unit mass of muscle. However since humans have a greater muscle mass than liver mass, there is more glycogen stored in the muscle than in the liver. The polymerisation of glucose to form glycogen reduces its osmotic pressure but its storage in the liver is limited. In fasting the liver glycogen stores are exhausted in about 24 hours.
Fat (triacylglycerol) storage appears to be unlimited. It is not hydrated and is hydrophobic and insoluble. Fat is more reduced than glycogen. It stores more energy per unit weight than does glycogen.
There is no real protein storage. Dietary amino acids are utilised to make tissue proteins such as enzymes and neurotransmitters, but there is no form of storage as there is for glucose and fat.
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Question 1

Which of the following statements about nicotinamide adenine dinucleotide (NAD+) is correct?

Your answer:

1. a) NAD+ is the initial electron donor in many metabolic oxidation reactions.

Correct answer:

1. c) NAD+ is the initial electron acceptor in many metabolic oxidation reactions.

Feedback:

Nicotinamide adenine dinucleotide (NAD+) contains two nitrogenous bases, adenine and nicotinamide. It is a coenzyme for many dehydrogenases and oxidases. In the oxidised form (NAD+) it can accept two electrons and a proton as an electron carrier, and a second proton is liberated in solution forming (NADH + H+). This reduced form can then diffuse to a second enzyme and reduce another molecule in the process of which it is reoxidised to NAD+.
Page reference: Page 177

Question 2

Which of the following statements about flavin adenine dinucleotide (FAD) is correct?

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Correct answer:

1. b) FAD is derived from riboflavin, a B-group vitamin (vitamin B2)

Feedback:

Flavin adenine dinucleotide, FAD, is a prosthetic group permanently attached to specific apoenzymes of dehydrogenases and oxidases making them active enzymes involved in oxidation and/or reduction reactions. Riboflavin, a B-group vitamin (vitamin B2) is a precursor of flavin adenine dinucleotide. FAD accepts two hydrogen atoms when reduced to FADH2.
Page reference: Page 179

Question 3

Which of the following statements about glycolysis is correct?

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Correct answer:

1. d) Although glycolysis is a stage in the oxidation of glucose it can occur anaerobically.

Feedback:

Glycolysis is the first stage of glucose oxidation in the cell. It does not involve oxygen. The products of aerobic glycolysis are pyruvate, NADH and ATP. The pyruvate is then taken up into the mitochondrial matrix for further oxidation while the NADH is reoxidised via shuttle mechanisms. In anaerobic glycolysis the pyruvate is reduced to lactate to regenerate NAD+. This reaction is catalysed by the enzyme lactate dehydrogenase and is essential because the amount of NAD+/NADH in cells is limited and NAD+ needs to be regenerated if the cell has insufficient oxygen available.
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Question 4

Which of the following statements about the enzyme lactate dehydrogenase is correct?

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Correct answer:

1. b) Lactate dehydrogenase catalyses the reduction of pyruvate to lactate to regenerate NAD+.

Feedback:

During anaerobic glycolysis lactate dehydrogenase catalyses the reduction of pyruvate to lactate ensuring NAD+ is regenerated from the NADH formed in glycolysis. Each cell only contains limited amounts of NAD+/NADH. If NAD+ is not available glycolysis will not be possible.
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Question 5

Which of the following statements about mitochondria is correct?

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Correct answer:

1. d) The inner mitochondrial membrane is relatively impermeable to most solutes.

Feedback:

Mitochondria are organelles located in the cytoplasm. They are surrounded by two membranes. They are the main energy-generating units of the cell. The inner mitochondrial membrane is invaginated forming cristae to increase the capacity for ATP generation. The extent of invagination reflects the ATP needs of the cell. This membrane contains many proteins and enzymes that are part of the energy-generating complexes as well as transport mechanisms for metabolites. The outer mitochondrial membrane is relatively permeable but the inner one is impermeable to most solutes.
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Question 1

Which of the following statements about fatty acid synthesis is correct?

Your answer:

1. a) Fatty acids can be used to synthesise glucose.

Correct answer:

1. b) Fatty acids can be synthesised from glucose.

Feedback:

Fatty acids are synthesized from acetyl-CoA, two carbons at a time, so most naturally occurring fatty acids contain an even number of carbon atoms. When the liver glycogen storage limit has been reached, excess glucose (from dietary carbohydrates) is converted to fat via acetyl-CoA formed from pyruvate by pyruvate dehydrogenase. Therefore glucose can be converted to fatty acids. The reverse cannot occur in animals for there is no mechanism by which fatty acids can be used to give a net pyruvate increase, needed for glucose synthesis. Fat can also be formed from alcohol and certain amino acids.
Page reference: Page 223

Question 2

Which of the following statements about the mechanism of synthesis of fatty acids is correct?

You did not answer the question.

Correct answer:

1. b) Malonyl-CoA is the active donor of two carbon atoms in fatty acid synthesis.

Feedback:

The first step in the synthesis of fatty acids is the carboxylation of acetyl-CoA to malonyl-CoA (with CO2 and ATP) by the enzyme acetyl-CoA carboxylase that uses biotin as a prosthetic group. Malonyl-CoA is the active donor of two carbon atoms in fatty acid synthesis. The acyl group carrier of the intermediates in fatty acid synthesis here is not coenzyme A, but a protein called ACP or acyl carrier protein to which the growing fatty acyl chain is attached. ACP, like coenzyme A contains the group 4-phosphopantotheine that forms a thiol ester with the acyl group. The enzyme that synthesizes fatty acids is a large multifunctional complex in mammals.
Page reference: Page 226

Question 3

Which of the following statements about the synthesis of fatty acids is correct?

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Correct answer:

1. c) NADPH is used for fatty acid synthesis.

Feedback:

The β-ketoacyl groups attached to ACP (the acyl carrier protein) are reduced by NADPH-dependent reactions. The NADH and NADPH coenzymes are a metabolic compartmentation allowing the cell to separate reductive power for energy ATP generation from reductive power needed for synthetic reactions. In 'times of plenty' citrate is transported to the cytosol where it is cleaved back to oxaloacetate and acetyl-CoA for fatty acid synthesis since there is no transport mechanism for the acetyl-CoA. The oxaloacetate is reduced to malate using NADH, and malate is decarboxylated to pyruvate and CO2 by the malic enzyme reducing NADP+ to NADPH for use in fatty acid synthesis. NADPH is also produced in the pentose phosphate pathway (see later).
Page reference: Page 225

Question 4

Which of the following statements about the use of citrate in fatty acid synthesis is correct?

You did not answer the question.

Correct answer:

1. d) Citrate transports acetyl groups for fatty acid synthesis from the mitochondrion to the cytosol.

Feedback:

Fatty acid synthesis occurs mainly in the cytosol of liver and adipose tissues, two carbons at a time, from acetyl-CoA. The major source of the acetyl-CoA is the mitochondrial pyruvate dehydrogenase reaction. Acetyl-CoA is then converted to citrate by citrate synthase. When the demand for ATP is low, the oxidation of acetyl-CoA via the citric acid cycle and oxidative phosphorylation is reduced, and citrate accumulates. It is transported to the cytosol where it is cleaved back to acetyl-CoA and oxaloacetate by the ATP-citrate lyase since there is no transport mechanism for the acetyl-CoA. The breakdown of ATP here makes the reaction irreversible. Oxaloacetate is reduced to malate using NADH, and malate is decarboxylated to pyruvate and CO2 by the malic enzyme reducing NADP+ to NADPH for use in fatty acid synthesis.
Page reference: Page 226

Question 5

Which of the following statements about prostaglandins is not correct?

You did not answer the question.

Correct answer:

1. a) Prostaglandins are eicosanoids made from saturated fatty acids.

Feedback:

Prostaglandins are a group of eicosanoids derived from arachidonic acid by cyclooxygenase enzymes. Arachidonic acid is an unsaturated fatty acid released from membrane phospholipids by a phospholipase in response to various signals. These prostaglandins are released from cells immediately after synthesis and act like local hormones on adjacent cells by combining with receptors. They have a wide range of physiological effects. Some can cause pain, inflammation, fever, smooth muscle contraction, while others have a protective effect such as suppressing acid production in the stomach or controlling blood pressure.
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