All Free Biology MCQs with Answers

331. The flagellum of a eukaryotic cell differs from that of a bacterium in that it

• A. is made of the protein flagellin
• B. rotates like a propeller at its base
• C. is shorter and never used for movement
• D. has a 9 plus 2 arrangement of microtubules enclosed by the plasma membrane

Explanation: A eukaryotic flagellum, as in a sperm, is an extension of the cell containing nine outer pairs of microtubules around a central pair, and it beats by the sliding of these microtubules using ATP. A bacterial flagellum is a much simpler solid filament of flagellin outside the cell that rotates like a screw. The two structures are not homologous despite sharing a name.

332. Active transport differs from facilitated diffusion in that active transport

• A. does not require a carrier protein
• B. requires ATP and can move a substance against its concentration gradient
• C. occurs only in plant cells
• D. moves only water molecules

Explanation: Both use carrier proteins, but facilitated diffusion is passive and can only run down a gradient, whereas active transport spends ATP and can therefore concentrate a substance on one side of the membrane. This is how glucose is fully reabsorbed in the kidney tubule and how root hair cells take in mineral ions from very dilute soil water. Cells performing much active transport are rich in mitochondria.

333. Osmosis is best defined as the movement of water molecules

• A. from a region of low water potential to one of high water potential
• B. against a concentration gradient using ATP
• C. from a region of higher water potential to one of lower water potential through a partially permeable membrane
• D. along with dissolved solutes through a partially permeable membrane

Explanation: Water always moves down its own water potential gradient, that is from the more dilute solution to the more concentrated one, and the membrane is what stops the solute simply moving the other way. Osmosis is a special case of diffusion, so it is passive and needs no ATP. Defining it in terms of water potential rather than concentration avoids the confusion of talking about the concentration of water itself.

334. A plant cell placed in pure water does not burst because

• A. the rigid cellulose wall resists further expansion once the cell is turgid
• B. water cannot enter the cell at all
• C. the vacuole absorbs the excess water permanently
• D. the plasma membrane is impermeable to water

Explanation: Water enters by osmosis until the wall pushes back with a pressure equal to the inward osmotic pull, and the cell stops taking in more water and is described as turgid. Turgor is what supports herbaceous plants, so wilting follows when it is lost. An animal cell has no wall and therefore does burst in the same conditions.

335. A red blood cell placed in distilled water will

• A. shrink and become crenated
• B. remain unchanged
• C. become turgid but survive
• D. swell and burst

Explanation: Water enters the cell by osmosis because the cytoplasm has the lower water potential, and with no cell wall to resist the pressure the membrane stretches until it ruptures, a process called haemolysis. Placed in a concentrated salt solution the same cell loses water and becomes crenated instead. This is why fluids given intravenously must be isotonic with blood plasma.

336. Plasmolysis is said to occur when

• A. a plant cell takes in water and becomes turgid
• B. the protoplast of a plant cell shrinks away from the cell wall in a concentrated solution
• C. the cell wall dissolves completely
• D. the nucleus divides into two

Explanation: In a solution of lower water potential than the cell sap, water leaves the vacuole, the protoplast contracts and pulls away from the wall, and the cell becomes flaccid and then plasmolysed. Because the wall is fully permeable, the external solution simply fills the space between it and the shrunken protoplast. The process can be reversed if the cell is returned to water soon enough.

337. A karyotype is

• A. the complete set of genes in a gamete
• B. the sequence of bases in one chromosome
• C. a display of all the chromosomes of a cell arranged in pairs by size and shape
• D. the number of chromatids in a chromosome

Explanation: Chromosomes are photographed at metaphase, when they are most condensed, then cut out and arranged in numbered pairs, which makes an abnormal number or a structural change immediately visible. This is how Down syndrome is confirmed as an extra copy of chromosome 21. It shows chromosome number and shape only, not the base sequence or the individual genes.

338. Of the 23 pairs of chromosomes in a human somatic cell, the number of pairs of autosomes is

• A. 22
• B. 23
• C. 21
• D. 44

Explanation: Twenty two pairs are autosomes, which carry the genes for body characters and are identical in appearance in both sexes, and the remaining pair are the sex chromosomes, XX in a female and XY in a male. The figure 44 is the total number of individual autosomes rather than the number of pairs, which is the usual careless error. A gamete carries 22 autosomes plus one sex chromosome.

339. Peroxisomes protect the cell because they contain the enzyme catalase, which

• A. digests worn out organelles
• B. synthesises hydrogen peroxide for defence
• C. adds sugar groups to newly made proteins
• D. breaks down toxic hydrogen peroxide into water and oxygen

Explanation: Hydrogen peroxide is a damaging by product of several oxidation reactions, and catalase destroys it almost as fast as it forms, making it one of the fastest enzymes known. Peroxisomes are also active in the oxidation of fatty acids and, in liver cells, in detoxifying alcohol. Digestion of worn out organelles is the work of lysosomes, which is the usual confusion.

340. Chemically, almost all enzymes are

• A. globular proteins
• B. lipids
• C. polysaccharides
• D. nucleotides

Explanation: An enzyme is a globular protein whose precisely folded tertiary structure creates the active site, which is why anything that unfolds the protein destroys the catalytic power. The globular rather than fibrous form matters because it makes the molecule soluble and compact. The one exception is the small group of catalytic RNA molecules called ribozymes.