Photosynthesis in Higher Plants Test - 25

Plants usually convert light into chemical energy with a photosynthetic efficiency of 3 to 6%. The absorbed light that is unconverted is dissipated primarily as heat, with a small fraction, which accounts nearly 1 to 2 %, re-emitted as chlorophyll fluorescence at longer wavelengths. When 1 mol (180 g) of glucose reacts with oxygen under standard conditions, 686 kcal of energy is released.

Carboxy dismutase is a dimerizing carboxylase enzyme that catalyses the addition of carbon dioxide to D-ribulose-1,5-bisphosphate and the hydrolysis of the addition product to two molecules of 3-D-phosphoglyceric acid, a key reaction in the fixation of carbon dioxide in the photosynthesis process.

•  In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS) or rate-limiting step.
• The rate-limiting step in photosynthesis is a dark reaction.
• In dark reaction, two types of cyclic reaction occur.
• The carbon dioxide is fixed to energy-rich carbohydrates using ATP and the assimilatory power NADPH2 of light reaction. 
• Hence, The rate-limiting step in photosynthesis is Dark reaction.
• So, the correct answer is 'Dark reaction'.

Chlorophyll and carotenoids are the major pigments in plants, while there are dozens of carotenoids, there are only five important chlorophylls, a, b, c, d, and bacteriochlorophyll. Chlorophyll a absorbs light in the blue-violet region and reflects green colour.

C carbon fixation is a metabolic pathway for carbon fixation in photosynthesis, along with C and CAM metabolic pathways. In C$$_{4}$$ plants, for example maize, PEP carboxylase fixes bicarbonate in the mesophyll cells of the leaf and the resulting four-carbon compound, malate, is shuttled into the bundle sheath cells where it releases CO for fixation by RuBisCo. So, first carbon fixation in C$$_{4}$$ plants occurs in spongy mesophyll cells. Thus, option B is correct and other options are incorrect.

In Calvin cycle, carbon dioxide along with water gets converts into phosphoglyceric acid. Then this phosphoglyceric acid converts into phosphoglyceraldehyde or glyceraldehyde 3 phosphate (G3P) with the use of ATP and NADPH. In this reaction NADPH gets oxidised to NADP+. Thus, this reaction is called as reduction reaction. Thus option B is correct. The other options are incorrect.

Photorespiration involves conversion of the phosphorylated phosphoglycolate to the amino acid glycine. This is followed by the conversion of two glycine molecules (two carbons each) to the amino acid serine (three carbons) plus one molecule of CO$$_{2}$$ and one of ammonia. Thus, option C is correct.
Dark respiration is a form of respiration in plants where carbon dioxide is released without the aid of sunlight. Thus, option B is incorrect.
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO$$_{2}$$). The CO$$_{2}$$ is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected CO$$_{2}$$ is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency. In CAM photosynthesis ammonia is not released. Thus, option A is also incorrect.

The most distinct feature is presence of bundle sheath with chloroplasts in the leaves. It is like a wreath around the vascular bundle. Hence, such anatomy is called as Kranz anatomy. Bundle sheath may be single layered as in Maize or double layered as in sugarcane. If two layered, the outer one is green and the inner one is thick walled and called as mestome sheath. Mesophyll cell chloroplasts are granal and starch lacking but the bundle sheath cell chloroplasts are grana - lacking and starch - rich. Such feature is known as chloroplast dimorphism. Dimorphism of chloroplast is found in C$$_{4}$$ plants and absent in $$C_{3}$$ and CAM plants. Thus, option A is correct and other options are incorrect.

Bundle sheath cells are a layer of cells in plant leaves and stems, that forms a sheath surrounding the vascular bundles. In $$C_4$$ plants the bundle sheath cells contain chloroplasts and are the site of the Calvin cycle. The initial fixation of carbon dioxide to form malic acid takes place in the palisade mesophyll cells, which in $$C_4$$ plants form a circle around the bundle sheath. This arrangement, known as Kranz anatomy or structure (after the German Kranz, wreath), ensures that the palisade cells are in close contact with the bundle sheath cells, so that the malic acid can easily pass to the bundle sheath. It also means that the products of photosynthesis can be quickly transferred from the bundle sheath to the adjacent phloem tissue for transport to other parts of the plant. Thus, option C is correct and other options are incorrect.

• Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities. 
• Photosynthesis involves the photochemical phase where light energy converts into chemical energy. 
• Carboxylation and carbon reduction phase takes place in the Calvin cycle.