Photosynthesis in Higher Plants Test - 28

Plants using the C$$_4$$ (Hatch-Slack) photosynthetic pathway are key for global food production and account for 25% of terrestrial primary productivity, mostly in relatively warm, dry regions.

In $$C_3$$ plants, $$CO_2$$ is pumped directly into the Calvin cycle, leaving rubisco exposed to $$O_2$$.  Rubisco in a $$C_3$$ will take up $$O_2$$ rather than $$CO_2$$ to undergo photorespiration.  Photorespiration occurs under low concentrations of $$CO_2$$ and high concentrations of $$O_2$$. This process reduces efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in $$C_3$$ plants. $$C_4$$ plants capture carbon dioxide in their mesophyll cells forming oxaloacetate. This oxaloacetate is then converted to malate and is transported into the bundle sheath cells, where oxygen concentration is low to avoid photorespiration. So, $$C_4$$ plants are more efficient in photosynthesis than $$C_3$$ plants. Thus, option A is correct.

Brown algae contain chlorophyll a and c. 
Red algae contain Chlorophyll a and d. 
Green algae contain chlorophyll a and b. 

Kranz anatomy is a special structure in the leaves of plants that have a $$C_4$$ pathway of carbon dioxide fixation. The leaves contain a ring of mesophyll cells, containing a few small chloroplasts concerned with the initial fixing of carbon dioxide, surrounding a sheath of parenchyma cells (the bundle sheath) which has large chloroplasts involved in the Calvin cycle. Bundle sheath cells, contains starch-rich chloroplasts (agranal) lacking grana in a large amount. These bundle sheath cells have no inter cellular spaces. Thus, option D is correct and other options are incorrect.

The given picture denotes $$C_4$$ cycle, which is also known as Hatch and Slack pathway. In the given pathway:
A denotes 'fixation'. 
B denotes 'decarboxylation'.
C denotes 'regeneration'.
The $$CO_2$$ acceptor in $$C_4$$ plants in phosphoenolpyruvate (PEP). PEP reacts with $$CO_2$$ to form oxaloacetic acid, which is reduced by NADPH to form malic acid. The malic acid then reacts with RUBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as $$C_3$$ plants. These reactions are called as the Hatch and Slack pathway.

Chemiosmosis theory relates to the generation of ATP by the movement of hydrogen ions across the membrane of the thylakoid. The accumulation of positively-charged ions inside the lumen creates a proton gradient which is then used to drive hydrogen ions out of the ATP synthase complex to phosphorylate ADP, making ATP.

Chlorophyll pigments trap sunlight during the process of photosynthesis. The chlorophylls are cyclic tetrapyrrole structures having a porphyrin head and a long phytol tail attached to the head. One atom of magnesium is coordinated to the cyclic tetrapyrrole head. The chemical formula of chlorophyll a is $$C_{55}H_{72}O_5N_4Mg$$.

Chlorophyll e has been reported from Xanthophyta members like Vaucheria and Tribonema. It absorbs blue and red wavelengths giving peaks at 415 and 654 nm.

• The important photosynthetic pigments are chlorophyll, carotenoids, and phycobilins.
• As many as 8 major types of chlorophylls are known to exist in the plant kingdom- chlorophyll a, b, c, d, e, bacteriochlorophylls a and b and chlorobium chlorophyll.
• The chlorophyll a is the primary photosynthetic pigment and is found in all photosynthetic organisms except bacteria.

Usually, the process of photosynthesis is faster than respiration. This difference is crucial in making food available to all the living organisms in the food chain as all the organisms either directly or indirectly depend on photosynthesizing plants for food supply. Gross production of sugars or fixation of carbon dioxide minus respiratory losses in producers equals the net production. The rate of respiration in green plants has been reduced to suit the sedentary life style. The green plants have evolved for a sedentary life and most of the plant body is dead. In fact, most of the big woody trunk and brown areas of a plant body are dead and only the green leaves are living. This reduces the demand for food for respiration so that green plants are able to provide food to other more active organisms. The rate of respiration depends on the physiological demands of organisms. In active organisms energy demands are more and rate of respiration will be more. But in general usually, overall it is highly essential to have photosynthesis proceed at rates higher than respiration.