31 Years NEET Previous Year Questions: Photosynthesis in Higher Plants

• You may like to cut a section of the leaves of C₄ plants – maize or sorghum – to observe the Kranz anatomy and the distribution of mesophyll cells.
• The C₄ acid OAA is formed in the mesophyll cells.

Photorespiration is the light dependent process. At high temperature, RuBP carboxylase functions as oxygenase and instead of fixing carbon dioxide (C3 cycle), oxidises ribulose 1, 5-biphosphate to produce a 3-carbon phosphoglyceric acid and a 2-carbon phosphoglycolate.

In C₄ plants, bundle sheath cells are rich in RuBisCO, but lack PEPcase. The CO₂ released in the bundle sheath cells enters the C₃ or the Calvin pathway, common to all plants. Thus, the basic pathway that results in the formation of sugars, the calvin pathway is common to the C₃ and C₄ plants.

Photorespiration is absent is C₄ plants. Photorespiration does not produce energy or reducing power. Rather, it consumes energy.  Further, it undoes the work of photosynthesis. There is 25% loss of fixed CO_2. Therefore, photorespiration is a highly wasteful process. This happens only in case of C₃ plants. 

CAM plants are most common in arid environments. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evapotranspiration.

In C3​ plants, CO2​ is pumped directly into the Calvin cycle, leaving rubisco exposed to O2​.  Rubisco in a C3​ will take up O2​ rather than CO2​ to undergo photorespiration.  Photorespiration occurs under low concentrations of CO2​ and high concentrations of O2​. This process reduces efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C3​ plants. C4​ 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, C4​ plants are more efficient in photosynthesis than C3​ plants.

Phosphoglyceric and (PGA) is the first stable product of photosynthesis. It was first discovered by Calvin, Benson and their colleagues in Chlorella, algae.

Cyclic photophosphorylation results in the formation of ATP. This process is called photophosphorylation, which occurs in two different ways. Adenosine triphosphate (ATP) is considered by biologists to be the energy currency of life. It is the high-energy molecule that stores the energy we need to do just about everything we do. It is present in the cytoplasm and nucleoplasm of every cell, and essentially all the physiological mechanisms that require energy for operation obtain it directly from the stored ATP. 

In leaves of C₄ plants, malic acid synthesis during CO₂ fixation occurs in mesophyll cells. The mesophyll cells perform C₄ cycle and the cells of bundle sheath perform C₃ cycle. CO₂ taken from the atmosphere is accepted by phosphoenol pyruvic acid (PEP) present in the chloroplasts of mesophyll cell of these leaves, leading to the formation of a 4-C compound, oxaloacetic acid (OAA). This acid is converted to another 4-C acid, the malic acid which enters into the chloroplast of bundle  sheath cells and these undergoes oxidative  decarboxylation yielding pyruvic acid  (3C - compound) and CO₂. 

The Pr form absorbs light between 660 to 680 nm and absorbs at a peak of 666 nm. It is the form synthesized in darkgrown seedlings. When Pr absorbs red light, it is converted to the Pfr form.

The first reaction of photorespiration occur in stroma of chloroplast. In this reaction the RuBP(Ribulose 1-5 biphosphate) consume one oxygen molecule in presence of enzyme Rubisco.
In peroxisome the glycolate transferred  from chloroplast takes up O₂ and formed the glyoxylate whereas the H₂O₂ release as byproduct.

In photosystem I or photoact I electrons released by P-700 are accepted by a substance of iron-sulphur-protein complex denoted as A(FeS). A(FeS) is thus reduced and it passes these high electrons to oxidised ferredoxin and it gets reduced. From reduced Fd these electrons are passed to FAD which now reduced to FADH2 and then electrons and hydrogen are accepted by NADP which makes NADPH2. Here photosystem I end.

The fixation of CO₂ in C₄ -plants takes place in two places and by two different organic compounds. Phosphoenol Pyruvate (PEP) is found in mesophyll cells which primarily fixes atmospheric CO₂ into oxalo acetic acid (4C).

RuBisCO is present in bundle sheath cells where final fixation of CO₂ in hexose sugars takes place. CO₂ is primarily fixed by PEP carboxylase because this enzyme has greater affinity to CO₂ than RuBisCO.

Internally a chloroplast contains a matrix or stroma which is similar to cytoplasm in its constitution. It contains DNA, RNA, ribsomes, enzymes for CO₂ assimilation, proteins, starch grains and fat droplets or plastoglobuli.
In the matrix or stroma embedded a number of flattened membranous sacs called thylakoids or lamellae. At places the thylakoids are aggregated to form stacks called grana.

Equation for C₄ pathway 6 PEP + 6 RuBP + 6CO2 + 30 ATP + 12 NADPH 6PEP + 6 RuBP + C6H12O6 + 30 ADP + 30 H3PO4 + 12 NADP+
The net reaction of C₃ dark fixation is 6RuBP + 6CO2 + 18ATP + 12 NADPH  6 RuBP + C6H12O6 + 18 ADP + 18 P + 12 NADP+

The Proteinaceous Photosynthetic unit (PSU) size increases as light levels decline. At lower light levels, chloroplasts have larger PSU to increase the probability that a photon will strike the chlorophyll antenna. So, plants adapted to low light intensity have larger photosynthetic unit size than the sun plants. Plants adapted to low light intensity does not have a higher rate of CO2​ fixation, more extended root system and leaves are not modified to spines.

3-phosphoglyceric acid, 3 carbon compound is the first product formed in the C₃ cycle. It is then converted to glyceraldehyde 3-phosphate. Oxaloacetic acid, 4 carbon compound is the first product formed in C₄ cycle. It is then converted to malic acid.

A non ionic Mg atom is held in the centre of porphyrin (of chlorophyll) held by N atom of pyrrole ring.

Carotenoids are a group of yellow, red and orange pigments which function as acessary pigments and protect chlorophyll molecules from destruction by intensive light rays. Carotenoids have three absorption peaks in the blue-violet range of the spectrum.

In C3 plants, when the level of carbon dioxide falls below about 50 ppm, the catalyst RuBisCO that helps to fix carbon begins to fix oxygen. C4 plants have the enzyme phosphoenolpyruvate carboxylase. This enzyme catalyzes the conversion of 3 carbon phosphoenolpyruvate (PEP) to oxaloacetete which is quickly converted to malate. This malate is actively pumped across the cell membrane into the bundle sheath cell and is split into carbon dioxide and a 3-carbon compound. This carbon dioxide then enters the Calvin cycle. Thus the correct answer is option D.