All Free Biology MCQs with Answers

3731. The substances that absorb visible light are called

• A. Radioactive substances
• B. Bioluminescent substances
• C. Pigments
• D. Fluorescent substances

Explanation: Pigments are substances that absorb visible light, which is crucial in processes such as photosynthesis, where they capture light energy. Unlike pigments, radioactive substances do not interact with light in this way; they emit radiation due to nuclear instability. Bioluminescent substances produce light through biochemical reactions, not by absorption. Fluorescent substances absorb light but re-emit it, rather than absorbing it for processes like photosynthesis.

3732. Different pigments absorb light of:

• A. Same wavelength
• B. Single wavelength
• C. Different wavelengths
• D. Broad range of wavelengths

Explanation: Different pigments absorb light of different wavelengths due to variations in their molecular structure, which allows them to capture energy from different parts of the light spectrum. This is crucial in processes like photosynthesis, where specific wavelengths are required for optimal energy capture. Options A, B, and D do not account for the unique absorption properties of each pigment, which is why they are incorrect.

3733. An instrument that is used to measure the relative abilities of different pigments to absorb different wavelengths of light is called

• A. Spectrometer
• B. Photometer
• C. Barometer
• D. Spectrophotometer

Explanation: A spectrometer separates and measures the spectral components of a physical phenomenon. A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared, including the visible spectrum. A barometer is an instrument used to measure atmospheric pressureA spectrophotometer performs quantitative measurements of reflectance or transmission properties of a material as a function of wavelength.

3734. Carotenes are mostly:

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

Explanation: Carotenes are mostly orange pigments that play an important role in capturing light energy for photosynthesis. They absorb light mainly in the blue and blue-green regions of the spectrum and reflect orange light, which gives them their characteristic color. This is why fruits and vegetables such as carrots, pumpkins, and sweet potatoes appear orange. Although other pigments like xanthophylls are yellow and chlorophylls are green, carotenes stand out for their distinct orange hue and hydrocarbon structure. Unlike chlorophyll, which directly participates in the light reactions of photosynthesis, carotenes function as accessory pigments that extend the range of light absorbed by plants and also protect chlorophyll from damage caused by excessive light.

3735. These broaden the absorption and utilization of light:

• A. Yellow pigments
• B. Orange pigments
• C. Red pigments
• D. Yellow and red to orange pigments

Explanation: Carotenoids, which include carotenes and xanthophylls, are accessory pigments found in plants. They play a crucial role in broadening the range of light absorption beyond the wavelengths absorbed by chlorophyll alone. By capturing additional light energy, they enhance the plant's ability to perform photosynthesis efficiently. The incorrect options suggest individual pigments, which alone do not provide the broad spectrum of light absorption that is achieved by the combined effect of yellow and red-to-orange pigments.

3736. Chlorophylls found in photosynthetic bacteria are called

• A. Chlorophyll a and b
• B. Chlorophy c and d
• C. Chlorophyll b and c
• D. Bacteriochlorophylls

Explanation: The correct answer is bacteriochlorophylls. These pigments are found in photosynthetic bacteria that perform anoxygenic photosynthesis, a process that does not produce oxygen as a byproduct. This differentiates them from chlorophyll a and b, which are found in plants, algae, and cyanobacteria involved in oxygenic photosynthesis. Chlorophylls c and d are found in some algae and cyanobacteria, but not in the photosynthetic bacteria that utilize bacteriochlorophylls for their light-harvesting processes.

3737. Green, yellow, and indigo wavelengths of light are least absorbed by:

• A. Carotenes
• B. Xanthophylls
• C. Chlorophylls
• D. Carotenoids

Explanation: Chlorophylls are the primary pigments in plants responsible for photosynthesis. They absorb light most effectively in the blue (around 430 nm) and red (around 680 nm) wavelengths but are less efficient in absorbing green, yellow, and indigo light. This is why chlorophyll-containing tissues, like leaves, appear green to our eyes. On the other hand, carotenes and xanthophylls are types of carotenoids that absorb different spectra of light, complementing chlorophylls by capturing energy that chlorophylls do not absorb efficiently. Carotenoids generally absorb blue light and reflect red, orange, and yellow, contributing to the colours of fruits and flowers, but are not as involved in the primary absorption for photosynthesis compared to chlorophylls.

3738. Plants appear green because:

• A. They absorb green light
• B. They reflect green light
• C. They transmit all colours equally
• D. They absorb only yellow and green wavelengths

Explanation: Plants appear green because chlorophyll, the main pigment involved in photosynthesis, reflects green light while absorbing red and blue wavelengths from sunlight. The reflected green light is what reaches our eyes, giving plants their characteristic green color. Other pigments like carotenoids absorb different wavelengths, but chlorophyll's reflection of green remains dominant in most plants.

3739. The light-absorbing hydrophilic part of chlorophyll is

• A. Porphyrin head
• B. Phytol tail
• C. Magnesium ion
• D. Carbon chain

Explanation: The light-absorbing hydrophilic part of chlorophyll is the porphyrin head, which is composed of four connected pyrrole rings, forming a flat and square-shaped structure. This configuration allows it to efficiently absorb light. The other options are either incomplete or incorrect, as they do not fully describe this specific part of chlorophyll.

3740. The porphyrin ring represents the:

• A. Light-absorbing part of chlorophyll
• B. Hydrophilic tail of chlorophyll
• C. Metal ion center
• D. Electron transport chain component

Explanation: The porphyrin ring represents the light-absorbing part of chlorophyll. It contains a magnesium ion at its center and a cyclic structure of carbon and nitrogen atoms that absorbs red and blue light for photosynthesis. Its hydrophilic nature allows it to interact with the aqueous environment of the chloroplast, while the phytol tail anchors the molecule into the membrane. This arrangement ensures efficient capture of light energy and proper positioning within the photosynthetic machinery.