Photosynthesis in Higher Plants notes

Photosynthesis is the anabolic process by which green plants use light energy to synthesize carbohydrates from carbon dioxide and water. In higher plants, it occurs mainly in chloroplasts of mesophyll cells. The chapter begins with photosynthetic pigments and chloroplast structure, then explains light reactions in thylakoid membranes where ATP, NADPH and oxygen are produced. The Calvin cycle in the stroma uses ATP and NADPH to reduce CO2 into sugars. C4 plants show special adaptations such as Kranz anatomy to reduce photorespiration. The rate of photosynthesis depends on light, CO2, temperature and water, according to Blackman’s law of limiting factors. For NEET, focus on sites, products, pigment spectra, Z-scheme, chemiosmosis, ATP/NADPH requirements, C3 vs C4 differences and limiting-factor graphs.

Photosynthesis in higher plants occurs mainly in chloroplasts of mesophyll cells. A chloroplast has a double membrane, stroma, grana and thylakoids. The light reactions occur on thylakoid membranes because they contain photosynthetic pigments and electron carriers. The Calvin cycle occurs in the stroma. Pigments include chlorophyll a, chlorophyll b, xanthophylls and carotenoids. Chlorophyll a is the chief pigment, while accessory pigments transfer absorbed light energy to it and protect chlorophyll from photo-oxidation. The absorption spectrum shows wavelengths absorbed by pigments, while the action spectrum shows wavelengths most effective for photosynthesis. Red and blue light are most effective, while green light is least effective because it is mostly reflected.

The light reaction is the photochemical phase of photosynthesis and occurs in thylakoid membranes. It begins when light excites reaction centre chlorophylls in photosystems. PS II has reaction centre P680 and absorbs light first in non-cyclic electron flow. Electrons lost by PS II are replaced by photolysis of water, releasing oxygen and protons. Electrons move through carriers, helping pump protons into the thylakoid lumen. PS I has reaction centre P700; it re-excites electrons and transfers them to NADP+, forming NADPH. The proton gradient drives ATP synthase to produce ATP by photophosphorylation. Non-cyclic photophosphorylation gives ATP, NADPH and O2, while cyclic photophosphorylation around PS I produces only ATP.

Electron transport in photosynthesis occurs through carriers embedded in the thylakoid membrane. In the Z-scheme, electrons from water enter PS II, are excited by light, pass through plastoquinone, cytochrome b6f and plastocyanin to PS I, are excited again and finally reduce NADP+ through ferredoxin and NADP+ reductase. As electrons move through the chain, protons accumulate inside the thylakoid lumen due to water splitting and proton pumping. This creates a proton gradient: high H+ inside the lumen and lower H+ in the stroma. ATP synthase uses this gradient to synthesize ATP, a process called chemiosmosis. The CF0 part forms the channel and CF1 catalytic head faces the stroma.

The Calvin cycle, also called the C3 pathway, is the carbon fixation phase of photosynthesis and occurs in the stroma of chloroplasts. It uses ATP and NADPH produced in the light reaction to convert CO2 into carbohydrate. The first stable product is 3-phosphoglyceric acid, a three-carbon compound, so it is called the C3 pathway. The cycle has three phases: carboxylation, reduction and regeneration. In carboxylation, RuBisCO fixes CO2 to RuBP, forming 3-PGA. In reduction, ATP and NADPH convert 3-PGA into triose phosphate. Some triose phosphate exits to form sugars, while most is used to regenerate RuBP. For 3 CO2, the cycle uses 9 ATP and 6 NADPH and produces one net triose phosphate.

The C4 pathway, also called Hatch–Slack cycle, is an adaptation that concentrates CO2 around RuBisCO and minimizes photorespiration. It occurs in plants such as maize, sugarcane and sorghum. C4 leaves show Kranz anatomy, where bundle sheath cells form a wreath-like arrangement around vascular bundles and contain abundant chloroplasts. In mesophyll cells, PEP carboxylase fixes CO2 into oxaloacetic acid, a four-carbon compound, which is converted to malate or aspartate and transported to bundle sheath cells. There, CO2 is released and enters the Calvin cycle. Photorespiration occurs when RuBisCO binds O2 instead of CO2, wasting energy and releasing CO2. CAM plants, such as cactus, separate CO2 fixation and Calvin cycle by time, opening stomata at night.

The rate of photosynthesis depends on internal factors such as chlorophyll content, leaf anatomy and enzyme activity, and external factors such as light, carbon dioxide, temperature and water. Light affects intensity, quality and duration; rate increases with light intensity until saturation. CO2 concentration is often limiting in nature because atmospheric CO2 is low. Temperature affects enzyme-controlled dark reactions more than photochemical light reactions. Water indirectly affects photosynthesis by causing stomatal closure, reducing CO2 entry and decreasing leaf hydration. Blackman’s law of limiting factors states that when a process is controlled by multiple factors, the rate is governed by the factor nearest to its minimum. NEET frequently asks graph interpretation: a plateau means another factor has become limiting.