Botany MCQs for NEET — Practice Questions with Answers

Anoxygenic photosynthesis (in which O₂ is not released) is seen in Rhodospirillum, a purple non-sulphur bacteria. It helps an organism to trap light energy and store it as chemical energy.

Other than this anoxygenic photosynthesis commonly occurs in purple non-sulphur bacteria, green sulphur/non-sulphur bacteria heliobacteria etc. 

Photorespiration required three cell organelles in sequence of chloroplast, peroxisome and mitochondria.

The use of radioactive ¹⁴C by Melvin Calvin in algal (Chlorella) photosynthesis studies led to the discovery that the first CO₂ fixation product was a 3-carbon organic acid. The first product indentified was 3-phosphoglyceric acid (PGA).

C₄-plants are more efficient in photosynthesis than C₃ -plants but use more energy. They possess the larger number of chloroplasts in the leaf cells. In the leaves of C₄-plants, the vascular bundles are surrounded by bundle sheath of larger parenchymatous cells, which in turn are surrounded by mesophyll cells. Chloroplast in bundle sheath cells are larger and always contain grana, whereas chloroplasts in mesophyll cells are smaller. 

Cyclic-photophosphorylation involves only pigment system-I. When the photons activate PS-I, a pair of electrons are raised to a higher energy level. They are captured by primary acceptor, which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS-I, i.e., P₇₀₀. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pump H⁺ across the thylakoid membrane. The proton gradient thus established is responsible for forming ATP (2 molecules). No reduction of NADP to NADPH + H⁺.

C₄ -plants show Kranz anatomy. In the leaves of C₄ -plants, the vascular bundles are surrounded by bundle sheath of parenchymatous cells, which in turn are surrounded by mesophyll cells. Chloroplasts in bundle sheath cells are larger and always contain grana whereas chloroplasts in mesophyll cells are smaller. The C₄ -plants are photosynthetically more efficient than C₃ -plants because they have more chloroplasts. 

Photorespiration takes place only in C₃ -plants. It occurs in chlorophyllous tissues. At high temperature and high oxygen concentration the affinity of RuBP carboxylase for CO₂ decrease and for O₂ increases. 

Photosynthesis takes CO₂ and release O₂, while respiration utilizes O₂ and release CO₂. If the light factor is saturating, there will be certain concentraton of CO₂ in the atmosphere at which the rate of photosynthesis will be equal to rate of the respiration. This is called carbon dioxide compensation point. C₄ -plants have very low CO₂ compensation point. 

Electrons from excited chlorphyll molecule of photosystem II are accepted first by Quinone. Photosystem II is a photosynthetic pigment system along with some electron carriers that is located in the appressed part of the grana thylakoids. Photosystem II has chlorophyll a, b and carotenoids. Other components of PS II are phaeophytin, plastoquinone (PQ), cytochrome complex and blue coloured copper containing plastocyanin.

C₄ -plants show Kranz anatomy. In the leaves of C₄ -plants the vascular bundles are surrounded by bundle sheath of parenchymatous cells, which in turn are surrounded by mesophyll cells. In C₄ -plants the Hatch and Slack pathway involves two carboxylation reaction one taking place in chloroplast of mesophyll cells and other in chloroplast of bundle sheath cells. The initial CO₂ fixation occurs in mesophyll cells where primary CO₂ acceptor is phosphoenol pyruvate and formed oxaloacetate. The oxaloacetate is reduced to malic acid. From chloroplasts of mesophyll cells the malic acid is transferred to chloroplast of bundle sheath cells and decarboxylated to form CO₂ and pyruvic acid. The CO₂ is again fixed by C₃ cycle. 

In plants, stoma is surrounded by two small specialized, green epidermal cells called guard cells. The guard cells are small in size and therefore, rapidly influenced by turgor changes.

Starch is a high molecular weight polymer of D-glucose in α 1 → 4 linkage. It is synthesized in chloroplasts as it is one of the stable end product of photosynthesis. It is most abundant and common storage polysaccharide in plants hence, most staple food for man and herbivores.

Carbohydrates are commonly found as starch in plant storage organs. It is chemically non-reactive and osmotically inactive polysaccharides of much greater molecules weight. Carbohydrates performs a vital role in living organisms. Starch and other polysaccharides serve as energy storage in plants, particularly in seeds, tubers, etc. which provide a major energy source for animals, including humans.

When P_r absorbs red light (660-665 nm) it is converted into P_fr form and when P_fr absorbs far red light (730-735 nm) it is converted into P_r form.