NCERT Solutions for Class 11 Biology Chapter 13 Photosynthesis in higher plants

Ans : One cannot distinguish whether a plant is \(C_{3}\) or \(C_{4}\) by observing its leaves and other morphological features externally. Unlike \(C_{3}\) plants, the leaves of \(C_{4}\) plants have a special anatomy called Kranz anatomy and this difference can only be observed at the cellular level. For example, although wheat and maize are grasses, wheat is a \(C_{3}\) plant, while maize is a \(C_{4}\) plant. 

Ans : The leaves of \(C_{4}\) plants have a special anatomy caned Kranz anatomy. This makes them different from \(C_{3}\) plants. Special cells, known as bundle-sheath cells, surround the vascular bundles. These cells have a large number of chloroplasts. They are thick-walled and have no intercellular spaces. They are also impervious to gaseous exchange. All these anatomical features help prevent photorespiration in \(C_{4}\) Plants, thereby increasing their ability to photosynthesise. 

Ans : The productivity of a plant is measured by the rate at which it photosynthesises. The amount of carbon dioxide present in a plant is directly proportional to the rate of photosynthesis. \(C_{4}\) plants have a mechanism for increasing the concentration of carbon dioxide. In \(C_{4}\) plants, the Calvin cycle occurs in the bundle-sheath cells. The \(C_{4}\) compound (malic acid) from the mesophyll cells is broken down in the bundle- sheath cells. As a result, \(CO_{2}\) is released. The increase in \(CO_{2}\) ensures that the enzyme RuBisCo does not act as an oxygenase, but as a carboxylase. This prevents photorespiration and increases the rate of photosynthesis. Thus, \(C_{4}\) plants are highly productive. 

Ans : The enzyme RuBisCo is absent from the mesophyll cells of \(C_{4}\) plants. It is present in the bundle-sheath cells surrounding the vascular bundles. In \(C_{4}\) plants, the Calvin cycle occurs in the bundle-sheath cells. The primary \(CO_{2}\) acceptor in the mesophyll cells is phosphoenolpyruvate — a three-carbon compound. It is converted into the four-carbon compound oxaloacetic acid (OAA). 
OAA is further converted into malic acid. Malic acid is transported to the bundle-sheath cells, where it undergoes decarboxylation and \(CO_{2}\)fixation occurs by the Calvin cycle. This prevents the enzyme RuBisCo from acting as an oxygenase. 

Ans : Chlorophyll-a molecules act as antenna molecules. They get excited by absorbing light and emit electrons during cyclic and non-cyclic photophosphorylations. They form the reaction centres for both photosystems I and II. Chlorophyll-b and other photosynthetic pigments such as carotenoids and xanthophylls act as accessory pigments. Their role is to absorb energy and transfer it to chlorophyll-a. Carotenoids and xanthophylls also protect the chlorophyll molecule from photo-oxidation. Therefore, chlorophyll-a is essential for photosynthesis. If any plant were to lack chlorophyll-a and contain a high concentration of chlorophyll-b, then this plant would not undergo photosynthesis.