All Free Biology MCQs with Answers

71. The theoretical total yield of ATP from the complete aerobic oxidation of one molecule of glucose is about

• A. 2
• B. 4
• C. 12
• D. 38

Explanation: Two ATP come from glycolysis, two from the two turns of the Krebs cycle and the remainder from oxidative phosphorylation of the ten NADH and two FADH2, giving the classical figure of 38, or 36 if the cost of transporting cytoplasmic NADH into the mitochondrion is counted. The contrast with the two ATP of anaerobic respiration is the point: aerobic respiration is roughly nineteen times more efficient.

72. Carbon dioxide is NOT released during

• A. the link reaction
• B. the Krebs cycle
• C. glycolysis
• D. alcoholic fermentation

Explanation: Glycolysis only rearranges and splits the six carbons of glucose into two three carbon pyruvate molecules, so no carbon is lost as gas. Decarboxylation begins with the link reaction, which removes one carbon per pyruvate, and continues twice more in each turn of the Krebs cycle. Alcoholic fermentation also releases carbon dioxide when pyruvate is converted to ethanal.

73. The electron transport chain of a eukaryotic cell is located in the

• A. inner mitochondrial membrane
• B. outer mitochondrial membrane
• C. mitochondrial matrix
• D. cytosol

Explanation: The cytochromes and other carriers are embedded in the inner membrane, whose folding into cristae provides the surface area, and the same membrane is impermeable enough to hold the proton gradient that chemiosmosis depends on. The outer membrane is freely permeable to small molecules and so could never maintain a gradient. In prokaryotes the equivalent chain sits on the plasma membrane.

74. FADH2 yields fewer ATP than NADH in oxidative phosphorylation because

• A. FADH2 carries fewer electrons than NADH
• B. FADH2 is produced only in the cytoplasm
• C. FADH2 is never reoxidised
• D. FADH2 delivers its electrons at a later point in the chain, so fewer protons are pumped

Explanation: Both carriers deliver a pair of electrons, but FADH2 feeds them in past the first proton pumping complex, so one fewer pump operates and roughly two ATP rather than three are produced. The number of protons moved across the membrane, not the number of electrons carried, is what sets the yield. FADH2 is made inside the mitochondrion in the Krebs cycle and is certainly reoxidised.

75. Cyanide is rapidly lethal because it blocks

• A. glycolysis at the phosphofructokinase step
• B. the ATP synthase channel
• C. cytochrome oxidase, the last carrier of the electron transport chain
• D. the Krebs cycle at citrate synthase

Explanation: Cyanide binds the iron of cytochrome oxidase, so electrons can no longer be passed to oxygen, the whole chain backs up and oxidative phosphorylation stops even though the blood is fully oxygenated. Cells are left with only the two ATP of glycolysis, which the brain and heart cannot survive on. Blocking glycolysis alone would be far less immediately fatal, since aerobic tissues could still use other substrates.

76. The formation of ATP by direct transfer of a phosphate group from a substrate molecule to ADP is called

• A. oxidative phosphorylation
• B. substrate level phosphorylation
• C. photophosphorylation
• D. chemiosmosis

Explanation: This is how ATP is made twice in glycolysis and once per turn of the Krebs cycle, and it needs no membrane, no oxygen and no proton gradient. Oxidative phosphorylation is the indirect route through the electron transport chain and ATP synthase, and chemiosmosis is the mechanism behind it. Photophosphorylation is the equivalent light driven process in chloroplasts.

77. Lactic acid that accumulates in a vigorously exercising muscle is mainly

• A. excreted unchanged in the urine
• B. converted back to glucose within the muscle fibre itself
• C. completely oxidised in the muscle within seconds
• D. carried in the blood to the liver and reconverted to glucose

Explanation: Muscle lacks the enzymes to run gluconeogenesis, so lactate diffuses into the blood and the liver rebuilds it into glucose in the Cori cycle, using ATP from oxygen taken in after exercise. That extra oxygen consumption is the oxygen debt, which is why heavy breathing continues after a sprint has finished. Only a very small fraction of lactate is lost in urine or sweat.

78. Which respiratory substrate releases the greatest amount of energy per gram when oxidised?

• A. Fat
• B. Carbohydrate
• C. Protein
• D. Nucleic acid

Explanation: Fat yields roughly 9 kcal per gram against about 4 for carbohydrate and protein, because its carbon atoms are far more reduced and so give up more electrons to the transport chain. This energy density is why fat rather than glycogen is the body's long term store and why migrating birds and hibernating mammals rely on it. Nucleic acids are informational molecules and are not used as fuel.

79. Which feature of mitochondria supports the view that they arose from free living prokaryotes?

• A. They are bounded by a single membrane
• B. They contain digestive enzymes like a lysosome
• C. They carry 80S ribosomes identical to those of the cytoplasm
• D. They contain circular DNA and 70S ribosomes and divide by binary fission

Explanation: Circular DNA, 70S ribosomes and division by fission are all bacterial characteristics, and they are exactly what the endosymbiotic theory predicts for an organelle descended from an engulfed prokaryote. A mitochondrion has two membranes, not one, the outer one being the vesicle the host cell wrapped it in. Its ribosomes are 70S, so the presence of 80S ribosomes would argue against the theory rather than for it.

80. Recombinant DNA is DNA that

• A. has been copied many times in a test tube
• B. has been joined together from two different sources
• C. has lost one of its two strands
• D. has been broken down into nucleotides

Explanation: A gene cut from one organism is inserted into the DNA of another, usually a bacterial plasmid, so the molecule carries sequences that never occurred together in nature. Because the genetic code is universal, the host reads the foreign gene and makes the protein it codes for. Copying DNA repeatedly in a tube is the polymerase chain reaction, a different technique altogether.