All Free Biology MCQs with Answers

3251. In chemiosmosis, the proton (H+) moves from _.

• A. Stroma to Lumen
• B. Cytoplasm to Stroma
• C. Lumen to Stroma
• D. Stroma to cytoplasm

Explanation: Chemiosmosis is the process whereby protons move down their electrochemical gradient, i.e., from a region of higher concentration to a region of lower concentration. During this process, they move through ATP synthase, and their movement results in the coupling of ADP and inorganic phosphate to form ATP. In chloroplasts, the concentration of protons is greater in the thylakoid lumen relative to the stroma; hence, protons move across the semipermeable membrane from the lumen into the stroma.

3252. How many molecules of ATP would be utilized for phosphorylation of one glucose molecule during glycolysis?

• A. Three
• B. Two
• C. Four
• D. One

Explanation: Two ATP molecules are used to phosphorylate glucose; the first is used to convert glucose into glucose-6-phosphate, while the second is used to convert fructose-6-phosphate into fructose-1,6-bisphosphate.

3253. The following flowchart depicts the steps of the Calvin Cycle. Which option, according to you, fits in as the correct answer to the missing step?

• A. Pyruvate
• B. Hydrogenase
• C. Ribulose bisphosphate
• D. Oxaloacetate

Explanation: The first step involves the fixing of carbon (derived from CO2) into molecules of Ribulose Bisphosphate (RuBP) via the action of the enzyme 'rubisco', resulting in the formation of 3-phosphoglycerate. This is further processed into 1,3-BPG and eventually, G-3-P, before the cycle starts again. The chart above only has RuBP missing.

3254. Which of the following wavelength is absorbed maximum for photosynthesis?

• A. 660 nm
• B. 680 nm
• C. 440 nm
• D. 700 nm

Explanation: Since chlorophyll absorbs maximum energy from the blue and red wavelengths, blue light is absorbed more for photosynthesis than any other light, and this color also comes first in the spectrum. A, B, and D are not the answers because of the given reason. The wavelength of light that is absorbed most efficiently for photosynthesis is primarily in the blue and red regions of the electromagnetic spectrum. These are the wavelengths absorbed most by the chlorophyll pigments in plant cells, which are responsible for capturing light energy during photosynthesis. The two main types of chlorophyll found in plants are chlorophyll a and chlorophyll b. Chlorophyll-a absorbs light most efficiently in the blue (around 430-450 nm) and red (around 640-680 nm) regions of the spectrum. Chlorophyll b complements chlorophyll a by absorbing light in the blue (around 450-500 nm) and red-orange (around 600-650 nm) regions. Green light, which has a wavelength of around 500-570 nm, is not absorbed as effectively by chlorophyll. Instead, it is reflected, giving leaves their green color. So, in summary, light with wavelengths in the blue and red regions is absorbed most efficiently for photosynthesis, while green light is not absorbed as much and is instead reflected.

3255. The part of the chloroplast where carbon dioxide is fixed to manufacture sugar is:

• A. Stroma
• B. Grana
• C. Thylakoid
• D. Outer membrane

Explanation: The correct answer is the stroma. The stroma is the site of the Calvin cycle, the series of light-independent reactions where carbon dioxide is fixed into sugars. This process occurs after the light-dependent reactions in the thylakoids which produce ATP and NADPH, which are used in the stroma to drive the synthesis of glucose. The other options, such as grana and thylakoids, are involved in the light-dependent reactions, not carbon fixation. The outer membrane functions as a protective boundary and does not participate directly in photosynthesis.

3256. It is the most energy rich compound:

• A. FADH2
• B. ATP
• C. NADH
• D. GTP

Explanation: ATP is indeed the most energy-rich compound in cellular metabolism. ATP (adenosine triphosphate) is the cell's primary energy currency. It stores and transfers energy for various cellular processes. The energy stored in ATP is primarily in the form of the high-energy phosphate bonds between its three phosphate groups. When ATP is hydrolyzed by the enzyme ATPase, it undergoes a reaction in which one of the phosphate groups is cleaved, forming adenosine diphosphate (ADP) and inorganic phosphate (Pi). This hydrolysis reaction releases a significant amount of energy that can be utilized by cells to perform work. The energy released from the hydrolysis of ATP powers a wide range of cellular processes, including muscle contraction, active transport of molecules across membranes, synthesis of macromolecules, nerve signal transmission, and many other energy-requiring reactions in cells.

3257. How much net gain of G3P is obtained after one calvin cycle?

• A. 3
• B. 6
• C. 2
• D. 1

Explanation: The Calvin cycle, also referred to as the C3 cycle, processes CO2 into carbohydrates in the stroma of chloroplasts. Each cycle fixes three CO2 molecules to produce six G3P molecules. However, five G3P molecules are recycled to regenerate RuBP, the molecule that captures CO2, thus allowing the cycle to proceed. As a result, only one G3P is a net gain for further carbohydrate synthesis. This is why the net gain of G3P after one Calvin cycle is one. The other options incorrectly assume a higher net gain due to a misunderstanding of the cycle's regeneration process.

3258. What is the color of the Xanthophyll pigment?

• A. Yellow
• B. Red
• C. Orange
• D. Green

Explanation: Xanthophylls are a class of oxygen-containing carotenoid pigments primarily responsible for the yellow color observed in many flowers, fruits, and vegetables. While some Xanthophylls can appear orange, the characteristic color is yellow. Red and green are not associated with Xanthophyll pigments; red is typically due to anthocyanins, and green is due to chlorophyll.

3259. Where does the dark reaction of photosynthesis takes place?

• A. Thylakoid
• B. Grana
• C. Stroma
• D. All of these options are correct

Explanation: The dark reaction or light-independent reaction occurs in the stroma of the chloroplast.

3260. Chlorophyll b is found in which organisms?

• A. Green plants
• B. Green algae
• C. Animals
• D. Both Options A and B are correct

Explanation: There are four types of chlorophyll: chlorophyll a, found in all higher plants, algae, and cyanobacteria; chlorophyll b, found in higher plants and green algae; chlorophyll c, found in diatoms, dinoflagellates, and brown algae; and chlorophyll d, found only in red algae.