Photosynthesis in Higher Plants

Photosynthesis is the process by which green plants capture light energy and store it as chemical energy in carbohydrates. In higher plants, the main site is the chloroplast. Inside it, thylakoid membranes form grana, while the stroma contains enzymes of carbon fixation.

The chief pigment is chlorophyll a. Chlorophyll b, carotenoids, and xanthophylls act as accessory pigments that widen the absorption range and transfer energy to the reaction center pigment.

The light reaction occurs on the thylakoid membrane. Its job is to absorb light, excite electrons, split water, release oxygen, and form ATP and NADPH.

In non-cyclic photophosphorylation, PS II (P680) acts first, water is split, electrons pass through carriers, PS I (P700) re-excites them, and finally NADP+ is reduced to NADPH. Because water donates electrons, oxygen is released.

In cyclic photophosphorylation, only PS I participates. ATP is formed, but oxygen is not evolved and NADPH is not produced.

ATP formation follows chemiosmosis. Protons accumulate in the thylakoid lumen and return through ATP synthase, driving phosphorylation of ADP.

The Calvin cycle, or C3 pathway, occurs in the chloroplast stroma. Here, RuBisCO fixes carbon dioxide onto RuBP, producing 3-phosphoglycerate as the first stable product.

The cycle is best understood in three phases: carboxylation, reduction, and regeneration of RuBP. This approach is much stronger than memorizing disconnected labels.

A rank-level understanding always links this with the light reaction: ATP and NADPH are not extras, they are the exact resources the Calvin cycle consumes to reduce fixed carbon.

C4 plants reduce photorespiration by first fixing CO2 with PEP carboxylase in mesophyll cells. The first stable product is oxaloacetate. Later, CO2 is concentrated in bundle sheath cells, where the Calvin cycle runs.

Kranz anatomy is the anatomical signature of C4 leaves and a repeated NEET favorite. It explains why C4 plants perform well under high temperature and intense light.

In CAM plants, stomata open at night and close during the day. This improves water-use efficiency in dry habitats.

The rate of photosynthesis depends on light intensity, CO2 concentration, temperature, and water availability. According to Blackman's law of limiting factors, the factor nearest its minimum limits the process.

This means increasing one factor helps only until another factor becomes limiting. For example, even strong light will not rescue photosynthesis if CO2 is low or stomata remain closed because of water stress.

If you can explain why ATP and NADPH are needed, why C4 plants are efficient, and why limiting factors matter, you are working at an advanced level in this chapter.