All Free Biology MCQs with Answers

21. Which organelle is the site of aerobic respiration and ATP synthesis?

• A. Lysosome
• B. Mitochondrion
• C. Golgi apparatus
• D. Smooth endoplasmic reticulum

Explanation: The Krebs cycle runs in the mitochondrial matrix and the electron transport chain sits in the inner membrane, where the proton gradient drives ATP synthase. The inner membrane is folded into cristae to increase that surface area, which is why heart muscle cells are packed with mitochondria. Lysosomes digest material, and the Golgi and smooth ER handle processing and lipid synthesis.

22. Lysosomes are formed by which organelle?

• A. Nucleolus
• B. Golgi apparatus
• C. Rough endoplasmic reticulum
• D. Centriole

Explanation: Hydrolytic enzymes are made on the rough endoplasmic reticulum but the Golgi apparatus packages and buds them off as lysosomes, after tagging them so they are sorted correctly. This is why the Golgi is described as the packaging and dispatch organelle. The nucleolus assembles ribosomal subunits and centrioles organise the spindle.

23. Rough endoplasmic reticulum is distinguished from smooth endoplasmic reticulum by the presence of

• A. cristae
• B. ribosomes on its surface
• C. thylakoids
• D. a double nuclear envelope

Explanation: Ribosomes bound to the outer face of the membrane give rough ER its appearance and its function, which is synthesising proteins destined for secretion or for membranes. Smooth ER has no ribosomes and handles lipid and steroid synthesis and detoxification, which is why liver cells are rich in it. Cristae belong to mitochondria and thylakoids to chloroplasts.

24. The organelle known as the suicidal bag of the cell is the

• A. ribosome
• B. lysosome
• C. peroxisome
• D. vacuole

Explanation: Lysosomes hold hydrolytic enzymes at an acidic pH, and if the membrane ruptures those enzymes digest the cell itself, a process called autolysis. This is deliberate during development, for example when the tail of a tadpole is removed. Peroxisomes also contain enzymes but they handle hydrogen peroxide and fatty acid oxidation rather than wholesale digestion.

25. Which of the following organelles is NOT bounded by a membrane?

• A. Ribosome
• B. Lysosome
• C. Mitochondrion
• D. Golgi apparatus

Explanation: A ribosome is a complex of ribosomal RNA and protein with no surrounding membrane at all, which is why it is found in prokaryotes as well as eukaryotes. Lysosomes and the Golgi apparatus each have a single membrane, and mitochondria have two. Nucleoli and centrioles are the other common non-membranous structures.

26. The fluid mosaic model describes the plasma membrane as

• A. a rigid protein sheet with lipids attached
• B. a phospholipid bilayer with proteins embedded in and moving through it
• C. a single layer of phospholipids
• D. a continuous layer of cholesterol

Explanation: Singer and Nicolson described a bilayer of phospholipids in which proteins float like icebergs, free to move laterally. Fluid refers to that lateral movement and mosaic to the scattered pattern of proteins. Cholesterol is present between the phospholipids and modulates fluidity, but it does not form a layer of its own.

27. A human somatic cell contains how many chromosomes?

• A. 23
• B. 44
• C. 46
• D. 92

Explanation: Human body cells are diploid with 46 chromosomes, which is 23 homologous pairs made up of 22 pairs of autosomes and one pair of sex chromosomes. Gametes are haploid and carry 23. The count of 92 would only appear briefly after replication if chromatids were miscounted as separate chromosomes, which they are not.

28. The region of a chromosome where the two sister chromatids are joined and to which spindle fibres attach is the

• A. telomere
• B. centromere
• C. centriole
• D. chromomere

Explanation: The centromere holds sister chromatids together and carries the kinetochore, the protein structure the spindle microtubules attach to during division. Telomeres are the protective caps at the chromosome ends that shorten with each replication. Centrioles are separate cytoplasmic structures that organise the spindle rather than part of the chromosome.

29. In a eukaryotic chromosome, DNA is packaged by winding around proteins called

• A. histones
• B. collagens
• C. tubulins
• D. keratins

Explanation: Histones are small positively charged proteins that bind the negatively charged phosphate backbone of DNA, forming nucleosomes that condense roughly two metres of DNA into a nucleus a few micrometres across. Tubulin builds microtubules, collagen is a structural protein of connective tissue, and keratin is found in hair and nails.

30. Enzymes increase the rate of a reaction by

• A. raising the temperature of the reaction mixture
• B. lowering the activation energy of the reaction
• C. changing the equilibrium position in favour of the products
• D. supplying the energy the reaction needs

Explanation: An enzyme provides an alternative route with a lower activation energy, so a much larger fraction of molecules has enough energy to react at body temperature. It does not change the free energy difference between reactants and products, which means the equilibrium position stays exactly where it was and only the time taken to reach it falls. Enzymes supply no energy of their own.