All Free Biology MCQs with Answers

11. Compared with carbohydrates, lipids yield more energy per gram because they

• A. contain more oxygen atoms per carbon
• B. are more highly reduced, with more carbon-hydrogen bonds per gram
• C. are soluble in water
• D. are broken down without any oxygen

Explanation: Energy is released when carbon-hydrogen bonds are oxidised, and a fatty acid chain is almost entirely carbon and hydrogen with very little oxygen already attached. Carbohydrates are partly oxidised to begin with, so less energy remains to be released, which is why fat yields about 9 kcal per gram against roughly 4 for carbohydrate. Lipids are insoluble in water and their breakdown is fully aerobic.

12. A phospholipid molecule is described as amphipathic because it has

• A. two hydrophobic ends
• B. a hydrophilic head and hydrophobic tails
• C. two hydrophilic ends
• D. no charged groups at all

Explanation: The phosphate head is charged and interacts with water, while the two fatty acid tails are non-polar and are excluded from it. Placed in water, the molecules therefore arrange themselves into a bilayer with the tails facing inward, which is the basis of every cell membrane. A molecule with only one kind of end could not form that structure.

13. Which statement about RNA is correct?

• A. It contains thymine in place of uracil
• B. It contains ribose and is usually single stranded
• C. It is always double stranded like DNA
• D. It contains deoxyribose and uracil

Explanation: RNA carries ribose, which has a hydroxyl group at carbon 2, and it is normally a single strand that can fold back on itself, as tRNA does into its clover leaf shape. Uracil replaces thymine in RNA, not the other way round, and deoxyribose belongs to DNA. The extra hydroxyl is also why RNA is chemically less stable than DNA.

14. In a double stranded DNA molecule, adenine pairs with thymine through

• A. one hydrogen bond
• B. two hydrogen bonds
• C. three hydrogen bonds
• D. a covalent bond

Explanation: Adenine and thymine are held together by two hydrogen bonds, while guanine and cytosine are held by three. That difference is why a DNA region rich in G and C needs a higher temperature to separate the strands, a fact used directly when designing PCR conditions. The covalent bonds in DNA run along each backbone, not across the strands.

15. If one strand of a DNA molecule contains 22% adenine, what percentage of the whole molecule is guanine?

• A. 22%
• B. 28%
• C. 44%
• D. 56%

Explanation: Chargaff's rule applies to the double stranded molecule, so A equals T and G equals C. With A at 22%, T is also 22%, leaving 100 minus 44 which is 56% shared equally between G and C, giving 28% guanine. The common error is to stop at 56% and forget that it is split between two bases.

16. The two strands of a DNA double helix are described as antiparallel, which means that

• A. they carry identical base sequences
• B. one runs 5' to 3' while the other runs 3' to 5'
• C. they are held together by covalent bonds
• D. they coil in opposite directions around each other

Explanation: Antiparallel refers to the direction of the sugar-phosphate backbones: one strand runs 5' to 3' while its partner runs 3' to 5'. This is why replication is continuous on the leading strand and discontinuous, in Okazaki fragments, on the lagging strand. The strands are complementary rather than identical, and they are held together by hydrogen bonds.

17. A conjugated molecule such as a glycoprotein consists of

• A. two identical monomers joined together
• B. a protein combined with a non-protein group
• C. a lipid combined with another lipid
• D. a polysaccharide broken into monosaccharides

Explanation: Conjugated molecules are built from two different classes of biological molecule joined together, so a glycoprotein is protein plus carbohydrate and a lipoprotein is protein plus lipid. The cell surface is coated with such molecules, and they are what allow cells to recognise one another. Two identical monomers joined together would simply be a dimer of a single class.

18. Which structure is present in a plant cell but absent from an animal cell?

• A. Mitochondrion
• B. Cellulose cell wall
• C. Ribosome
• D. Golgi apparatus

Explanation: A cellulose cell wall lies outside the plasma membrane of every plant cell and gives it a fixed shape and resistance to bursting when turgid. Mitochondria, ribosomes and the Golgi apparatus are found in both plant and animal cells. Note that fungi also have a wall, but it is chitin rather than cellulose.

19. The main difference between a prokaryotic and a eukaryotic cell is that a prokaryotic cell has

• A. no ribosomes
• B. no membrane bound nucleus or organelles
• C. no genetic material
• D. no plasma membrane

Explanation: Prokaryotes keep their DNA in a nucleoid region with no surrounding envelope and have no mitochondria, endoplasmic reticulum or Golgi apparatus. They do have ribosomes, though the 70S type rather than the 80S type of the eukaryotic cytoplasm, and that difference is exactly what several antibiotics exploit. Every cell has genetic material and a plasma membrane.

20. Ribosomes in the cytoplasm of a bacterium are of which type?

• A. 70S
• B. 80S
• C. 90S
• D. 60S

Explanation: Prokaryotic ribosomes are 70S, made of a 50S and a 30S subunit, while eukaryotic cytoplasmic ribosomes are 80S. Antibiotics such as streptomycin and tetracycline bind the 70S ribosome and leave the host's 80S ribosomes alone, which is why they kill bacteria without killing the patient. Mitochondria and chloroplasts also contain 70S ribosomes, evidence for the endosymbiotic theory.