All Free Biology MCQs with Answers

3641. Which of the following is used by green plants for the manufacture of food?

• A. Carbon dioxide
• B. Water
• C. Sunlight
• D. All of these

Explanation: The correct answer is Option: All of these. Photosynthesis in green plants requires carbon dioxide, water, and sunlight. Carbon dioxide is taken from the air, water is absorbed through the roots, and sunlight provides the energy needed for the process. Together, these components allow plants to produce glucose, a form of food, and oxygen as a byproduct. Options A, B, and C individually represent essential components, but none alone can complete the process of photosynthesis.

3642. The products of photosynthesis in green plants are:

• A. Glucose and oxygen
• B. Carbon dioxide and water
• C. ATP and NADPH
• D. Proteins and lipids

Explanation: The main products of photosynthesis in green plants are glucose and oxygen, which provide energy for the plant and oxygen for other organisms. Other options like carbon dioxide and water, ATP and NADPH, or proteins and lipids are either reactants, intermediates, or secondary products derived from glucose, not the direct products of photosynthesis.

3643. A spectrophotometer is used to measure:

• A. Absorption of CO2
• B. Absorption of O2
• C. Reflection of pigments
• D. Absorption of different wavelengths

Explanation: A spectrophotometer measures the absorption of various wavelengths of light. This device shines light through a sample and assesses how much of each wavelength is absorbed. The resulting absorption pattern can provide valuable information about the sample's composition, concentration, and other properties. The other options are incorrect because they do not relate to the primary function of a spectrophotometer, which is to measure light absorption, not gas absorption or light reflection.

3644. Which shows the effectiveness of wavelengths of light in driving photosynthesis?

• A. Absorption spectrum
• B. Action spectrum
• C. Broad spectrum
• D. Narrow spectrum

Explanation: The action spectrum is the correct answer because it directly measures how effective different wavelengths of light are in facilitating photosynthesis. It helps identify which wavelengths are most efficient at driving the process. The absorption spectrum, while related, only shows which wavelengths are absorbed, not their effectiveness in photosynthesis. "Broad spectrum" and "narrow spectrum" are unrelated terms, about UV protection and antibiotic specificity, respectively.

3645. The source of oxygen released during photosynthesis is

• A. Water
• B. CO2
• C. Glucose
• D. Oxidation of chlorophyll

Explanation: The oxygen released during photosynthesis originates from water molecules. In the light-dependent reactions, water is split into oxygen, hydrogen ions, and electrons through a process called photolysis. The oxygen is released as a byproduct and contributes to the breathable air we rely on. The other options, such as carbon dioxide, glucose, and the oxidation of chlorophyll, do not produce oxygen. Carbon dioxide is fixed into glucose in the Calvin cycle, glucose is an energy source for the plant, and chlorophyll oxidation is involved in electron transfer but not oxygen production.

3646. Wavelength of blue light that is absorbed maximally by pigments:

• A. 670 nm
• B. 540 nm
• C. 430 nm
• D. 750 nm

Explanation: In the visible light spectrum, chlorophyll pigments in plants absorb light most effectively in the blue (around 430 nm) and red (around 670 nm) regions. The correct answer is 430 nm, as this is the peak absorption for blue light by chlorophyll. Other wavelengths, such as 670 nm, represent red light absorption, and 540 nm represents green light, which is typically reflected rather than absorbed. 750 nm is near-infrared and not relevant to the absorption of blue light.

3647. Which of the following groups does not use water as a proton donor?

• A. Cyanobacteria
• B. Purple sulphur bacteria
• C. Plants
• D. Algae

Explanation: Purple sulfur bacteria use hydrogen sulfide (H₂S) as their proton and electron donor, distinguishing them from other groups like cyanobacteria, plants, and algae, which all use water (H₂O) in their photosynthetic processes. This unique characteristic of purple sulfur bacteria allows them to thrive in environments where hydrogen sulfide is more abundant than water.

3648. Carotenoids absorb strongly:

• A. Red to orange
• B. Yellow green
• C. Yellow red
• D. Blue violet

Explanation: Carotenoids are pigments that appear yellow, orange, or red because they reflect those colors. However, they absorb light strongly in the blue-violet range, which complements chlorophyll's absorption of red and blue light, thus broadening the spectrum of light available for photosynthesis. The other options describe colors that carotenoids reflect rather than absorb.

3649. Magnesium is a central part of:

• A. Porphyrin ring
• B. Phytol tail
• C. Pyrrole ring
• D. Hydrophobic tail

Explanation: The correct answer is the porphyrin ring. In chlorophyll, the porphyrin ring is a crucial component where magnesium acts as the central metal ion. It plays an essential role in capturing light energy. The metal ion is coordinated by nitrogen atoms from the pyrrole subunits, forming a stable complex known as a metalloporphyrin. Other options are incorrect: the phytol tail is a hydrocarbon chain used for anchoring the chlorophyll molecule, the pyrrole ring is a subunit of the porphyrin ring and does not itself contain magnesium, and the hydrophobic part refers to structures like the phytol tail, which do not involve magnesium.

3650. Chlorophyll consists of:

• A. A head of phytol and a tail of four pyrrole rings
• B. A head of linked carbons and a tail of four pyrrole rings
• C. A head of four pyrrole rings and a tail of linked nitrogen
• D. It has a head of four pyrrole rings and a phytol tail

Explanation: The chlorophyll molecule consists of two main parts: a hydrophilic head and a hydrophobic tail. The head, which is responsible for absorbing light, is a porphyrin ring made up of four pyrrole rings. This structure is crucial for the molecule's function in photosynthesis. The tail, which is hydrophobic and helps anchor the chlorophyll within the thylakoid membrane of the chloroplast, is a phytol chain. The incorrect options either misidentify the components or their arrangement, failing to describe the classic chlorophyll structure.