Photosynthesis in Higher Plants

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to synthesise food (carbohydrate) from carbon dioxide and water, releasing oxygen. It is the ultimate source of food and oxygen for almost all life. The overall reaction can be summarised as: $6\,\text{CO}_2 + 12\,\text{H}_2\text{O} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 + 6\,\text{H}_2\text{O}$. In higher plants it takes place in the chloroplast of mesophyll cells.

The chloroplast is organised into membranous thylakoids (stacked into grana) and a surrounding fluid stroma. This compartmentalisation is functionally important and frequently tested: the light reaction (the photochemical phase) occurs on the thylakoid membranes, while the dark reaction (the biosynthetic Calvin cycle) occurs in the stroma.

Photosynthetic pigments capture light. The principal pigment is chlorophyll a, which forms the reaction centre and chiefly drives photosynthesis; it absorbs mainly in the blue and red regions. The other pigments — chlorophyll b, xanthophylls and carotenoids — are accessory pigments that absorb light at other wavelengths and hand the energy to chlorophyll a, while carotenoids also protect chlorophyll a from photo-oxidation. Chlorophyll a is the most abundant pigment.

A set of classic experiments built our understanding and are reliable NEET facts. Joseph Priestley showed that plants restore to air the component used up by burning candles or breathing animals (i.e. release O₂). Jan Ingenhousz demonstrated that light is essential and only the green parts release oxygen. Julius von Sachs showed glucose (stored as starch) is made in the green parts, and T.W. Engelmann used a prism, the alga Cladophora and aerobic bacteria to plot the first action spectrum, finding blue and red light most effective. Finally, Cornelius van Niel established that the O₂ released comes from water, not from CO₂ — perhaps the single most examined experimental fact of this chapter.

The light reaction (photochemical phase) happens on the thylakoid membranes and converts light energy into the chemical energy of ATP and NADPH, releasing oxygen. It uses two pigment systems, Photosystem II (PS II) with its reaction centre P680 and Photosystem I (PS I) with reaction centre P700 (named for the wavelength each absorbs best). Each photosystem has a light-harvesting antenna that funnels energy to the reaction centre.

In non-cyclic photophosphorylation, light excites P680 in PS II; the high-energy electrons travel down an electron transport chain (plastoquinone → cytochrome complex → plastocyanin) to PS I, and onward from excited P700 through ferredoxin to reduce NADP⁺ to NADPH. The electrons lost by PS II are replaced by the splitting (photolysis) of water at PS II, which releases O₂, protons and electrons. Because the electron path traced on an energy diagram looks like a 'Z', this is called the Z-scheme. Its products are ATP, NADPH and O₂.

In cyclic photophosphorylation, only PS I is active: the excited electron from P700 is passed back to P700 through the electron transport chain instead of going to NADP⁺. This produces only ATP — no NADPH and no O₂ — and occurs in the stroma lamellae or when only light of wavelength beyond 680 nm is available. NEET frequently contrasts the two: non-cyclic gives ATP + NADPH + O₂; cyclic gives ATP alone.

How exactly is ATP made? By the chemiosmotic hypothesis. As water is split inside the thylakoid lumen and electrons move through the chain, protons (H⁺) accumulate inside the lumen, creating a proton gradient across the thylakoid membrane. These protons then flow down their gradient back into the stroma through the enzyme ATP synthase (the F₀–F₁ particle), and this flow drives the synthesis of ATP. So the gradient — built up by photolysis and electron transport — is the immediate energy source for ATP formation, a mechanism shared in principle with respiration.

The dark reaction (biosynthetic phase) uses the ATP and NADPH made in the light reaction to fix CO₂ into sugar; it occurs in the stroma and is called 'dark' only because it does not directly need light. In most plants it follows the Calvin cycle, which has three stages. Carboxylation: CO₂ is added to the 5-carbon acceptor RuBP by the enzyme RuBisCO, producing two molecules of the 3-carbon 3-phosphoglyceric acid (3-PGA) — the first stable product, which is why this is the C3 pathway. Reduction: 3-PGA is converted to sugar (G3P) using ATP and NADPH. Regeneration: RuBP is regenerated using ATP so the cycle can continue. To make one glucose, the cycle turns six times and uses 18 ATP and 12 NADPH.

RuBisCO (ribulose bisphosphate carboxylase-oxygenase) is the most abundant enzyme on Earth and has a dual nature: it can act as a carboxylase (fixing CO₂) or as an oxygenase (fixing O₂), depending on the relative concentrations of CO₂ and O₂.

Some tropical plants such as maize, sugarcane and sorghum use the more efficient C4 pathway (the Hatch–Slack pathway). They show a special leaf anatomy called Kranz anatomy, with large bundle-sheath cells around the veins. Here the first step of CO₂ fixation occurs in the mesophyll, where the enzyme PEP carboxylase (PEPcase) adds CO₂ to PEP to form the 4-carbon oxaloacetic acid (OAA) — hence 'C4'. OAA (as malate) is shuttled to the bundle-sheath cells, where it releases CO₂ for the normal Calvin cycle. Because this concentrates CO₂ around RuBisCO, C4 plants are highly productive.

The C4 trick also solves a problem called photorespiration, which afflicts C3 plants. When O₂ is high and CO₂ is low, RuBisCO acts as an oxygenase: it adds O₂ to RuBP, yielding one 3-PGA and one 2-carbon phosphoglycolate, with no sugar, no ATP and no NADPH produced — a wasteful process. In C4 plants the high CO₂ in the bundle sheath keeps RuBisCO working as a carboxylase, so they avoid photorespiration. The high-yield NEET points are: first product 3-PGA (C3) vs OAA (C4), PEPcase + Kranz anatomy in C4, RuBisCO's dual nature, and photorespiration occurring in C3 plants only.

The rate of photosynthesis is governed by several external and internal factors. The guiding principle, frequently tested, is Blackman's law of limiting factors: when a process depends on several factors, its rate is set by the factor that is in shortest supply (the limiting factor). For example, if light is abundant but CO₂ is scarce, increasing light further will not help — CO₂ is limiting and only adding CO₂ will raise the rate. The main factors are light, carbon dioxide, temperature and water.

Light matters in three ways — quality (wavelength), intensity and duration. At low intensity the rate rises with light, but at higher intensities it reaches a light saturation point (around 10% of full sunlight), beyond which light is no longer limiting and very high intensity may even cause damage (photo-oxidation, photoinhibition). Both blue and red light are most effective, matching the absorption peaks of chlorophyll.

Carbon dioxide is often the major limiting factor under natural conditions because its concentration in air is low (about 0.04%). C3 and C4 plants respond differently: C4 plants saturate at much lower CO₂ levels and are more efficient, while C3 plants benefit from higher CO₂ and show increased photosynthesis as CO₂ rises toward saturation, which is why greenhouses are sometimes enriched with CO₂.

Temperature affects the enzyme-driven dark reactions most. Each plant has an optimum temperature; C4 plants generally have a higher temperature optimum than C3 plants, reflecting their tropical origins. Water influences photosynthesis mainly indirectly: water stress closes the stomata, reducing CO₂ entry, and also causes leaves to wilt, lowering the surface available for light capture. The practical takeaway for NEET is to identify, in a given situation, which single factor is limiting — and to remember that under field conditions CO₂ and light are the usual limiters, with C4 plants being the more efficient performers.