All Free Biology MCQs with Answers

3481. A graph plotting the absorption of light of different wavelengths by a pigment is called:

• A. Absorption spectrum
• B. Spectrophotometer
• C. Carotenoids
• D. Xanthophylls

Explanation: The correct answer is absorption spectrum because it is a graph that shows how much light of different wavelengths is absorbed by a pigment. This graph is crucial in understanding which wavelengths of light are most effectively used by a pigment in processes like photosynthesis.The spectrophotometer is incorrect because it refers to an instrument, not a graph. Carotenoids and xanthophylls are specific types of pigments, not graphs or plots, and are therefore incorrect in this context.

3482. The water-splitting step of photosynthesis that releases oxygen is called

• A. Plastoquinone
• B. Photolysis
• C. Plastocyanin
• D. Ferredoxin

Explanation: The correct answer is B: photolysis. During the light-dependent reactions of photosynthesis, photolysis refers to the splitting of water molecules into oxygen, protons, and electrons using light energy. This process occurs in the thylakoid membranes of chloroplasts and is crucial for providing electrons to the photosynthetic electron transport chain. Options A, C, and D are incorrect because they refer to molecules involved in electron transfer, but not in the process of water splitting that releases oxygen.

3483. Of the following, which one is light-gathering?

• A. Antenna complex
• B. Reaction centre
• C. Photosystem
• D. ATP synthases

Explanation: The antenna complex is a light-harvesting structure made up of proteins and pigments such as chlorophyll and carotenoids that capture light energy and transfer it to the reaction center. The reaction center, located at the core of the photosystem, contains chlorophyll a and associated proteins responsible for initiating the primary electron transfer reactions of photosynthesis. However, the arrangement of pigments into functional clusters is called a photosystem. The thylakoid system refers to the network of interconnected thylakoid membranes within the chloroplast where the light-dependent reactions take place.ATP synthase is an enzyme which converts ADP to ATP.

3484. Carotenoids perform a protective function in:

• A. Animals
• B. Plants
• C. Both A and B
• D. Synthesizing ATP during chemiosmosis

Explanation: Carotenoids perform a protective function in both plants and animals. In plants, they prevent photooxidative damage by dissipating excess energy from chlorophyll and neutralizing reactive oxygen species during photosynthesis. In animals, they act as powerful antioxidants that protect cells from oxidative stress and contribute to health by supporting the immune system and vision. Therefore, while plants produce carotenoids for photoprotection, animals use them (obtained through diet) for antioxidant defence, making option C (both in plants and animals) the correct and most comprehensive choice.

3485. About _% of photosynthesis is carried out by terrestrial plants, while the rest occurs in oceans, lakes, and ponds.

• A. 10%
• B. 20%
• C. 30%
• D. 40%

Explanation: The correct answer is 10%. Terrestrial plants, such as trees, grasses, and other land-based vegetation, contribute to about 10% of the total photosynthesis on Earth. This process involves capturing sunlight and converting it into chemical energy. However, the majority of photosynthesis occurs in aquatic environments, primarily by phytoplankton in oceans, lakes, and ponds, which account for the remaining 90%. The other options (20%, 30%, and 40%) are incorrect, as they overestimate the contribution of terrestrial plants.

3486. The percentage of light absorbed by the leaf:

• A. 10%
• B. 30%
• C. 80%
• D. 1%

Explanation: Although a leaf absorbs a large portion of sunlight-typically around 80-90%-only a very small fraction of that light is actually converted into chemical energy through photosynthesis. In fact, the efficiency of this conversion is roughly 1%, meaning that only about 1% of the sunlight that strikes a leaf is transformed into glucose and other chemical compounds that the plant can use for growth and energy. The rest of the absorbed light is lost as heat or reflected as fluorescence. Therefore, while leaves appear green because they absorb most wavelengths except green, the actual percentage of light energy harnessed for photosynthesis is surprisingly low.

3487. What would happen if green plants disappeared from the Earth?

• A. Only herbivores will die
• B. Only carnivores will die
• C. No species will die
• D. Both herbivores and carnivores will die

Explanation: If green plants disappeared from the Earth, herbivores would be the first to suffer because they rely directly on plants as their primary source of food. Without plants, herbivore populations would quickly decline and eventually face extinction due to starvation. This collapse would then have a cascading effect on carnivores, which depend on herbivores for food. With no prey available, carnivore populations would also decline sharply, leading to widespread disruptions in ecosystems. Ultimately, the disappearance of plants would trigger a chain reaction in the food chain, causing the collapse of both herbivore and carnivore populations and destabilizing life across the planet.

3488. The most abundant protein in the chloroplasts is

• A. Rubisco
• B. Ribulose
• C. Ribulose bisphosphate
• D. Cytochrome

Explanation: The correct answer is Rubisco, which stands for ribulose bisphosphate carboxylase/oxygenase. This enzyme catalyzes the first major step of carbon fixation in the Calvin cycle, making it the most abundant protein in chloroplasts and likely on Earth.Option B, ribulose, is a sugar, not a protein. Option C, ribulose bisphosphate, is also a sugar and not a protein. Option D, Cytochrome, refers to a group of proteins involved in the electron transport chain, but they are not as abundant as Rubisco in chloroplasts.

3489. The incorrect statement for photosynthesis is

• A. It occurs during daytime
• B. It uses water with CO2
• C. It requires chlorophyll
• D. It uses oxygen

Explanation: The statement "photosynthesis uses oxygen" is incorrect because, in reality, photosynthesis produces oxygen rather than consuming it. During this process, green plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into glucose, releasing oxygen as a byproduct. Oxygen is generated when water molecules are split in the light-dependent reactions, and it is essential for aerobic respiration but not for photosynthesis itself. Therefore, claiming that photosynthesis uses oxygen reverses the actual role of oxygen in this vital biological process.

3490. Conversion of light energy into chemical energy is a function of:

• A. Mitochondria
• B. Cytoplasm
• C. Stroma of chloroplast
• D. Grana

Explanation: The conversion of light energy into chemical energy occurs in the grana of chloroplasts. Grana are composed of thylakoids, which contain chlorophyll, the pigment responsible for capturing light energy. This energy is used to produce ATP and NADPH, which are essential for the photosynthesis process. The other options, such as mitochondria, cytoplasm, and stroma, do not perform this specific function. Mitochondria generate ATP through cellular respiration. The cytoplasm supports cellular processes but does not convert light energy. The stroma facilitates the Calvin cycle but does not capture light energy directly.