All Free Biology MCQs with Answers

581. In the small intestine, maltose is converted into glucose by the enzyme

• A. lipase
• B. trypsin
• C. maltase
• D. pepsin

Explanation: Maltase is one of the intestinal disaccharidases that finish carbohydrate digestion, alongside sucrase and lactase. Trypsin and pepsin act on protein, and lipase on fat.

582. Certain vitamins, including vitamin K, are synthesised for our benefit by

• A. the villi of the small intestine
• B. bacteria living in the large intestine
• C. the pancreas
• D. the gall bladder

Explanation: The resident flora of the colon manufacture vitamin K and some B vitamins, which are absorbed there, one reason the gut microbiome matters. Human tissues cannot make vitamin K themselves.

583. The elimination of undigested solid waste from the body is called

• A. defecation
• B. excretion
• C. secretion
• D. absorption

Explanation: Defecation, or egestion, removes material that never entered the body's tissues. Excretion is different: it removes metabolic wastes such as urea that came from inside cells.

584. Humans cannot digest cellulose because they

• A. find it toxic
• B. digest it before it reaches the stomach
• C. absorb it without digestion
• D. lack the enzyme cellulase

Explanation: The beta glycosidic bonds of cellulose need cellulase, which human cells do not produce, so it passes through as fibre. Ruminants rely on symbiotic microbes that supply the enzyme for them.

585. Inflammation of the blind ending tube attached to the caecum causes the condition known as

• A. ulcer
• B. hernia
• C. appendicitis
• D. cholera

Explanation: The vermiform appendix can become infected and swollen, producing pain in the lower right abdomen, and may need surgical removal. Cholera is an infection of the small intestine, and ulcers and hernias are unrelated structures.

586. An enzyme that is secreted and works outside the cell that made it is called an extracellular enzyme. An example is

• A. trypsin
• B. DNA polymerase
• C. catalase
• D. RNA polymerase

Explanation: The pancreas exports trypsin into the duodenum, where it digests food outside any cell. DNA and RNA polymerases and catalase all work inside the cells that make them, which is what makes them intracellular enzymes.

587. The protein part of a conjugated enzyme, which on its own is catalytically inactive, is called the

• A. prosthetic group
• B. apoenzyme
• C. holoenzyme
• D. co-substrate

Explanation: The apoenzyme needs its non-protein partner, the cofactor, to become the active holoenzyme. Calling the whole active complex the apoenzyme is a common slip, so remember that holoenzyme equals apoenzyme plus cofactor.

588. Ribozymes are unusual biological catalysts because they are

• A. lipid molecules with enzyme activity
• B. synthetic polymers made in the laboratory
• C. RNA molecules that catalyse reactions
• D. proteins permanently fixed to ribosomes

Explanation: The discovery that certain RNA molecules can catalyse reactions, such as self splicing, showed that not all enzymes are proteins. This evidence supported the idea of an early RNA world in which RNA stored information and catalysed reactions.

589. The ending most commonly used when naming an enzyme is

• A. -ose
• B. -in
• C. -ol
• D. -ase

Explanation: Enzyme names end in -ase added to the substrate or reaction type, giving urease, lipase and dehydrogenase. The ending -ose belongs to sugars such as glucose and maltose, which is the mix-up the examiner is testing.

590. One molecule of carbonic anhydrase can hydrate up to a million molecules of carbon dioxide every second. This demonstrates the enzyme's

• A. instability at body temperature
• B. gradual consumption during the reaction
• C. very high turnover number
• D. resistance to all inhibitors

Explanation: The turnover number is how many substrate molecules one enzyme molecule converts per second, and carbonic anhydrase has one of the highest known. The enzyme itself is unchanged and reused, so the huge rate reflects catalytic power, not consumption.