Photosynthesis in Higher Plants — NEET Notes

Early experiments — who showed what?

The understanding that plants make their own food was built piece by piece across two centuries. Joseph Priestley discovered in the 1770s that plants restore air "injured" by a burning candle or a breathing animal. Jan Ingenhousz then showed that this restoration happens only in sunlight and only in green parts. By 1854, Julius von Sachs had provided evidence that glucose is the product, stored as starch in chloroplasts. T. W. Engelmann split light with a prism and exposed filamentous algae to it — aerobic bacteria clustered in the blue and red regions, giving photosynthesis its first action spectrum.

Cornelius van Niel later proposed the general equation using purple and green sulphur bacteria, which use H₂S instead of H₂O — establishing that the O₂ released in green-plant photosynthesis comes from water, not from CO₂.

6 CO₂ + 12 H₂O  →  C₆H₁₂O₆ + 6 H₂O + 6 O₂

The overall equation — note 12 H₂O in, 6 H₂O out

The asymmetry matters: 12 water molecules enter, only 6 leave. The difference is the water that gets split by the light reactions to liberate oxygen.

Site of photosynthesis

Photosynthesis takes place in the chloroplast — a double-membrane plastid found mainly in the mesophyll cells of leaves. Three compartments inside the chloroplast each play a distinct role: the thylakoid membrane houses the light reactions, the stroma hosts the Calvin cycle, and the thylakoid lumen is where the proton gradient builds up to power ATP synthesis.

Photosynthetic pigments & action spectrum

Three pigment classes harvest light in higher plants. Chlorophyll a is the only pigment that directly converts light energy into chemical energy in the reaction centres; everything else funnels absorbed energy to it. The action spectrum of photosynthesis (rate vs wavelength) overlaps closely with the absorption spectrum of these pigments — confirming that the same pigments that absorb light are the ones doing the work. Leaves appear green because chlorophyll absorbs strongly in blue and red but reflects green wavelengths.

Light reaction & the Z-scheme

The light reactions occur on the thylakoid membrane and generate ATP, NADPH, and O₂. Two pigment systems work in series. The path of electrons begins at Photosystem II (PS II): a photon excites P680, the energised electron is captured by a primary acceptor and shuttled down an electron transport chain — plastoquinone → cytochrome b₆-f complex → plastocyanin — to Photosystem I (PS I). The electron lost by P680 is replaced by water-splitting, which liberates O₂ on the lumen side of the membrane. At PS I, a second photon re-energises the electron from P700, which is then ferried by ferredoxin to NADP⁺ reductase to reduce NADP⁺ to NADPH. Plotted against redox potential, the journey from PS II to PS I traces a sideways "Z" — hence the name.

The water-splitting reaction itself is the source of every oxygen molecule we breathe:

2 H₂O  →  4 H⁺ + 4 e⁻ + O₂

Water-splitting at the oxygen-evolving complex (PS II, lumen side)

Cyclic vs non-cyclic photophosphorylation

The Z-scheme described above is non-cyclic photophosphorylation — both photosystems involved, water split, products ATP + NADPH + O₂. Sometimes the cell needs only ATP. In that case, the electron from P700 loops back via ferredoxin to the cytochrome b₆-f complex and returns to PS I — never reaching NADP⁺. Only PS I is involved, no water is split, the only product is ATP. This is cyclic photophosphorylation, observed in stroma lamellae (which lack PS II) and at lower light intensities.

Chemiosmosis in the chloroplast

The proton gradient that drives ATP synthesis is built by three processes during the light reactions: water-splitting releases protons into the lumen, plastoquinone shuttles protons from stroma to lumen as it carries electrons, and NADP⁺ reductase removes protons from the stroma. Protons then flow back down their gradient through ATP synthase, embedded in the thylakoid membrane. Each crossing phosphorylates ADP to ATP — the same Mitchell mechanism that operates in mitochondria. A working chemiosmotic system needs four ingredients, and NEET has tested them in this exact combination.

Calvin cycle (C3 pathway)

The ATP and NADPH from the light reactions are spent immediately in the Calvin cycle in the stroma. Melvin Calvin worked it out in the 1950s by radiolabelling carbon in Chlorella. CO₂ enters through stomata, is fixed onto the 5-carbon acceptor RuBP, and the cycle moves through three phases — carboxylation, reduction, and regeneration. The first stable product is a 3-carbon compound (3-PGA), which is why this is called the C3 pathway.

C4 pathway & Kranz anatomy

Plants like sugarcane, maize, sorghum, and amaranth — growing in hot, sunny, arid environments — use a variant called the C4 or Hatch-Slack pathway. C4 leaves show Kranz anatomy: vascular bundles wrapped in chloroplast-rich bundle sheath cells, themselves surrounded by mesophyll cells. ("Kranz" is German for "wreath.") In mesophyll cells, PEP carboxylase fixes CO₂ onto a 3-carbon acceptor (PEP) to make a 4-carbon product (OAA). OAA is shuttled to bundle sheath cells, where it releases CO₂ near RuBisCO — concentrating CO₂ far above atmospheric levels and shutting down RuBisCO's wasteful oxygenase activity. The released CO₂ then enters the Calvin cycle normally in the bundle sheath.

Photorespiration

RuBisCO's active site can bind either CO₂ or O₂. When O₂ wins — which happens more often at high temperatures and low intracellular CO₂ — RuBisCO acts as an oxygenase, converting RuBP into one molecule of 3-PGA plus one molecule of 2-phosphoglycolate, a 2-carbon compound. The phosphoglycolate is processed in peroxisomes and mitochondria, releasing CO₂ without producing any ATP, NADPH, or sugar. This is photorespiration — wasted carbon, wasted energy. C3 plants suffer from it; C4 plants do not, because their CO₂-concentrating mechanism keeps O₂ from competing for RuBisCO's active site. This is the molecular reason why C4 plants outperform C3 in hot, sunny climates.

Factors affecting photosynthesis

Four external factors govern photosynthetic rate. They do not act independently — at any given moment, one of them is the bottleneck, and only that one matters. This is Blackman's Law of Limiting Factors, formulated in 1905.