All Free Biology MCQs with Answers

3291. Photosystem 2 absorbs maximum wavelength of light?

• A. 750
• B. 650
• C. 680
• D. 670

Explanation: The chlorophyll, a special pair of the two photosystems, absorb different wavelengths of light. The PSII special pair absorbs best at 680 nm, while the PSI special absorbs best at 700 nm.

3292. How many molecules of carbon dioxide enter the Calvin cycle to produce one molecule of carbohydrate?

• A. 2
• B. 3
• C. 4
• D. 1

Explanation: During the Calvin cycle, three molecules of carbon dioxide (CO₂) combine with three molecules of ribulose-1,5-bisphosphate (RuBP) through the enzyme RuBisCO. This produces six molecules of 3-phosphoglycerate (3-PGA), which are then reduced using ATP and NADPH to form one molecule of glyceraldehyde-3-phosphate (G3P) - a 3-carbon carbohydrate. Therefore, three CO₂ molecules are required to produce one molecule of a 3-carbon carbohydrate (G3P).

3293. ATP is used when _

• A. To synthesize the macromolecules
• B. To transport molecules and ions
• C. To perform mechanical work
• D. All of the above

Explanation: ATP (Adenosine Triphosphate) is a molecule that serves as the primary energy currency of the cell. It is used in various cellular processes to provide energy, for synthesizing macromolecules, ATP provides the energy needed for biosynthetic reactions that build complex molecules such as proteins, nucleic acids, and lipids.

3294. The part of chlorophyll molecule is embedded in the core of thylakoid membrane which is?

• A. Hydrophilic
• B. Hydrophobic
• C. Both of these
• D. None of these

Explanation: Phytol tail of chlorophyll is a long hydrocarbon chain that is hydrophobic in nature. It anchors the pigment to membranes of thylakoids.It is embedded in the core of the thylakoid membrane.they also help to anchor the chlorophyll molecule( As word hydrophobic means water repelling )

3295. The stomata arc closes at which of the following temperature? (In centigrade)

• A. 45°C
• B. 35°C
• C. 15°C
• D. 25°C

Explanation: Despite the fact that the stomata open in response to light, they may close on a hot day in order to conserve water. This is because the heat may cause the water to evaporate out through the stomata, causing the plant to lose water, reducing the water potential inside the leaf. Stomata are closed at 45°C.

3296. Hans Krebs discovered _

• A. Glycolysis
• B. Fermentation
• C. Oxidation of pyruvate
• D. The citric acid cycle

Explanation: Hans Krebs (1900-1981) was a distinguished biochemist who did discovery of the "Krebs cycle" also known as the citric acid cycle and the tricarboxylic acid (TCA) cycle.

3297. The stage of photosynthesis that actually produces sugar is_.

• A. The Calvin cycle
• B. Photosystem I
• C. Photosystem II
• D. The light reaction

Explanation: The Calvin cycle is a process that plants and algae use to turn carbon dioxide from the air into sugar, it is a light independent process. In the Calvin cycle, carbon atoms are fixed (incorporated into organic molecules) and used to build three-carbon sugars. This process is fueled by, and dependent on, ATP and NADPH from the light reactions. G3P formed during it is basically a sugar molecule

3298. The G3P is end product of this process

• A. Kreb's cycle
• B. Calvin cycle
• C. Chemiosmosis
• D. Electron transport chain

Explanation: The Calvin cycle is the correct answer because it is the phase of photosynthesis where carbon fixation occurs, leading to the production of G3P as a primary end product. This cycle uses ATP and NADPH generated in the light-dependent reactions to convert carbon dioxide into glucose precursors, including G3P. The Kreb's cycle, chemiosmosis, and the electron transport chain are parts of cellular respiration and other photosynthetic processes, but they do not produce G3P as an end product.

3299. Cooperation of the two photosystems of the chloroplast is required for_

• A. ATP synthesis
• B. Reduction of NADP
• C. Cyclic photophosphorylation
• D. Oxidation of the reaction center of photosystem I

Explanation: For reduction of NADP to take place, non-cyclic electron flow should occur. Electrons excited from PS-II travel to PS-I, while those excited from PS-I finally reach NADP and reduce it to NADPH.

3300. Find out the correct sequence for movement of electrons during the light-dependent reaction:

• A. P680, P700, water, NADP
• B. Water, P700, NADP, P680
• C. P700, P680, NADP, water
• D. Water, P680, P700, NADP

Explanation: In the presence of light, the "photolysis (breaking in the presence of light ) of water occurs in the water splitting center within photosystem II," the electrons released are used to replace the ones lost by photosystem II in non-cyclic photophosphorylation. The first system in the chain is photosystem II, whose primary pigment can absorb at an optimal wavelength of 680mm hence it is termed as P680. Excitation of electrons occurs causing the electrons to move down the chain to photosystem I also known as P700, as its primary pigments absorb at an optimum wavelength of 700mm.