Photosynthesis in Higher PlantsNEET Botany · Class 11 · NCERT Chapter 13

Photosynthesis in Higher Plants is a High Weightage chapter with 4 to 6 questions in most NEET exams. Questions focus on photosynthetic pigments and absorption peaks, Z-scheme electron flow, Calvin cycle intermediates (RuBP, 3-PGA, G3P), the C4 Hatch-Slack pathway (PEP, OAA, Kranz anatomy), photorespiration, and Blackman's law of limiting factors. Mastering all these sub-topics is essential for a strong NEET score.

The absorption spectrum shows which wavelengths of light a pigment absorbs. You measure this in a lab using a spectrophotometer. The action spectrum shows which wavelengths actually drive photosynthesis (measured by the rate of O2 release or CO2 uptake). The action spectrum closely matches the absorption spectrum of chlorophyll a, which confirms that chlorophyll a is the main pigment responsible for photosynthesis. Engelmann's experiment with Spirogyra and bacteria was the first to demonstrate the action spectrum.

The Z-scheme describes the non-cyclic flow of electrons during light reactions. It gets its name from the zigzag shape of the energy diagram. Electrons start at PS II (P680), which absorbs light and gets excited. The electrons pass through plastoquinone, the cytochrome b6-f complex, and plastocyanin to PS I (P700). PS I absorbs more light, re-energises the electrons, and passes them via ferredoxin to NADP+ reductase, which reduces NADP+ to NADPH. Water is split at PS II to replace the lost electrons, and O2 is released as a byproduct.

In C3 plants, CO2 is fixed directly by RuBisCO in mesophyll cells to form 3-PGA (a 3-carbon compound) as the first stable product. In C4 plants, CO2 is first fixed in mesophyll cells using PEP carboxylase to form OAA (a 4-carbon compound). OAA is converted to malate, which moves to bundle sheath cells where CO2 is released and then fixed again by RuBisCO in the Calvin cycle. C4 plants (like maize and sugarcane) have Kranz anatomy (a ring of bundle sheath cells around vascular bundles), do not show photorespiration, and are more efficient in hot, dry, and high-light conditions.

Blackman's Law (1905) states that when a process depends on multiple factors, its rate is limited by the factor present at the least favourable value. For photosynthesis, the main limiting factors are light intensity, CO2 concentration, and temperature. For example, even if light intensity is high, increasing CO2 will increase the photosynthesis rate until another factor (such as light or temperature) becomes limiting. This explains why rate-vs-factor graphs show a plateau where a different factor becomes the new limit.

Photorespiration occurs when RuBisCO uses O2 instead of CO2 as its substrate (oxygenase activity) in conditions of high O2 and low CO2. This produces glycolate, which is processed in peroxisomes and mitochondria, releasing CO2 and consuming ATP without producing sugar. It is a wasteful process. C4 plants avoid photorespiration because their CO2 pump (using PEP carboxylase in mesophyll cells) concentrates CO2 in bundle sheath cells, where the Calvin cycle runs. The high CO2 concentration in bundle sheath cells suppresses the oxygenase activity of RuBisCO.

Light reactions (in thylakoid membranes) produce: ATP, NADPH, and O2. These reactions need light. The Calvin cycle (in the stroma) uses ATP and NADPH from light reactions to fix CO2 and produce G3P (glyceraldehyde-3-phosphate), which is the precursor for glucose. The Calvin cycle does not directly need light, which is why it was formerly called the "dark reaction," but this name is misleading because it runs most actively in the light when ATP and NADPH are available.