All Free Biology MCQs with Answers

3491. In plants, Chlorophyll contains which central element?

• A. Iron
• B. Magnesium
• C. Phosphorus
• D. Nitrogen

Explanation: The molecular formulae for chlorophyll a and b are:Chlorophyll a : C55 H72 05 N4 Mg Chlorophyll b : C55 H70 06 N4 Mgchlorophyll a is the-most abundant and the most important photosynthetic pigment as it takes part directly in the light-dependent reactions which convert solar energy into chemical energy. It is found in all photosynthetic organisms except photosynthetic bacteria. Chlorophyll a itself exists in several forms difering slightly in their red absorbing peaks e.g. at 670, 680, 690, 700 nm. Chlorophyll b is found alongwith chlorophyll a in all green plants (embryophytes) and green algae.

3492. In the light-independent stage of photosynthesis, the CO₂ combines with _ to form an unstable 6-carbon intermediate.

• A. Ribulose bisphosphate
• B. Hexose sugar
• C. Glycerate-3-phosphate
• D. Glyceraldehyde-9-phosphate

Explanation: In the light-independent stage of photosynthesis, also known as the Calvin cycle or dark reactions, carbon dioxide (CO2) combines with a five-carbon sugar called ribulose-1,5-bisphosphate (RuBP) to form an unstable 6-carbon intermediate. The enzyme responsible for this initial reaction is ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly abbreviated as Rubisco.

3493. Thylakoid membranes contain:

• A. Several kinds of pigments
• B. Only chlorophylls
• C. Only carotenoids
• D. Only xanthophylls

Explanation: Thylakoid membranes are integral to photosynthesis, as they house a variety of pigments. These include chlorophylls, which are primarily responsible for capturing light energy, and accessory pigments like carotenoids and xanthophylls, which expand the range of light wavelengths that can be absorbed. This diversity of pigments allows plants to maximize their efficiency in capturing light energy. The incorrect options highlight only one type of pigment, failing to recognize the full complement of pigments present in the thylakoid membranes.

3494. The second ATP is consumed in the _ step of glycolysis.

• A. First
• B. Second
• C. Third
• D. Fourth

Explanation: In glycolysis, the first step involves the phosphorylation of glucose to glucose 6-phosphate, consuming the first ATP. The second step converts glucose 6-phosphate into fructose 6-phosphate through an isomerization reaction and does not use any ATP. The third step is catalyzed by phosphofructokinase-1, which phosphorylates fructose 6-phosphate to fructose 1,6-bisphosphate, consuming the second ATP and serving as a key regulatory step. The fourth step involves the cleavage of fructose 1,6-bisphosphate into two three-carbon molecules, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, and does not require ATP. Therefore, the second ATP is specifically consumed in the third step.

3495. ATP formation during photosynthesis is known as

• A. Phosphorylation
• B. Dephosphorylation
• C. Oxidative phosphorylation
• D. Substrate-level phosphorylation

Explanation: Photophosphorylation is the correct term for ATP formation during photosynthesis because it specifically refers to the process in which light energy is used to add a phosphate group to ADP, forming ATP. This process occurs in the chloroplasts and is directly driven by light, which excites electrons in the photosystems and creates a proton gradient to power ATP synthase. Dephosphorylation is the removal of a phosphate group from a molecule, which releases energy rather than storing it as ATP. Oxidative phosphorylation occurs in mitochondria during cellular respiration, where electrons from nutrients pass through an electron transport chain to generate ATP using oxygen, not light. Substrate-level phosphorylation involves the direct transfer of a phosphate group from a high-energy substrate molecule to ADP, typically during glycolysis or the Krebs cycle, and does not require light energy. Therefore, only photophosphorylation correctly describes ATP formation in photosynthesis.

3496. Carbon dioxide joins the photosynthetic pathway at:

• A. PS I
• B. PS II
• C. Light reaction
• D. Dark reaction

Explanation: The dark reaction, also known as the Calvin Cycle, is the second stage of photosynthesis and takes place in the stroma of the chloroplasts. During this phase, carbon dioxide is utilized to synthesize glucose and other carbohydrates.

3497. Water vapor exits and CO2 enters a leaf through the _.

• A. Stomata
• B. Grana
• C. Porphyrin ring
• D. Photons

Explanation: Option A is correct. Water vapor exits and carbon dioxide (CO2) enters a leaf through tiny pores called stomata. Stomata are small openings primarily found on the undersides of leaves, although they can also be present on stems and other plant organs. Each stoma is surrounded by two specialized cells known as guard cells, which control the opening and closing of the pore.

3498. Which component enters into mitochondria after glycolysis?

• A. Pyruvate
• B. Acetate
• C. Oxaloacetate
• D. Acetyl - CoA

Explanation: Pyruvate is produced by glycolysis in the cytoplasm, but pyruvate oxidation takes place in the mitochondrial matrix (in eukaryotes). So, before the chemical reactions can begin, pyruvate must enter the mitochondrion, crossing its inner membrane and arriving at the matrix. Each pyruvate from glycolysis goes into the mitochondrial matrix-the innermost compartment of mitochondria. There, it's converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA.

3499. Unlike eukaryotes, prokaryotes have no membrane-bound organelles, How then are prokaryotes able to generate energy?

• A. Prokaryotes do not generate energy
• B. Prokaryotes produce energy via photosynthesis
• C. Prokaryotes have specialized mitochondria
• D. Prokaryotes generate proton gradients across their plasma membranes

Explanation: Prokaryotes generate energy through chemiosmosis, where proton gradients are formed across their plasma membranes, driving ATP synthesis. Unlike eukaryotes, which use mitochondria for this process, prokaryotes rely on their cell membranes. Option D is correct because it accurately describes this energy generation mechanism. Option A is incorrect because prokaryotes do generate energy. Option B is partially correct but doesn't address how energy is generated from photosynthetic processes. Option C is incorrect as prokaryotes lack mitochondria and instead use their plasma membranes for energy-related processes.

3500. Enzymes for light dependent reaction are present in:

• A. Thylakoid membrane of chloroplast
• B. Lumen of grana
• C. Stroma
• D. Outer membrane

Explanation: The correct answer is the thylakoid membrane of chloroplast. This location is integral for the light-dependent reactions of photosynthesis, where enzymes and components like the electron transport chain are embedded. These reactions result in the conversion of light energy into chemical energy in the form of ATP and NADPH. The lumen of grana is involved in creating a proton gradient but lacks the necessary enzymes. The stroma is the site for the Calvin cycle (light-independent reactions), while the outer membrane serves as a protective barrier and is not involved in these specific reactions.