Photosynthesis is the process by which plants convert light energy into chemical energy stored in glucose. CBSE Class 11 and NEET test it heavily — expect three to four questions a year. Topics include chloroplast structure, light and dark reactions, C3/C4/CAM variants and photorespiration.
Every molecule of glucose you eat, every breath of oxygen you take — it all traces back to photosynthesis. Plants capture a tiny fraction of sunlight and use it to stitch carbon dioxide and water into sugar, releasing oxygen as a by-product. This is the single most important chemical reaction on Earth.
Core Concepts
Overall equation
Light splits water (photolysis), carbon dioxide is fixed into organic molecules, glucose forms, and oxygen is released. Notice the 12 water molecules on the left — this is the correct balanced equation. The oxygen comes entirely from water, not from CO.
Chloroplast structure
Double membrane enclosing a gel-like stroma. Stacked disc-like structures called thylakoids form columns called grana (singular: granum). Adjacent grana are connected by stroma lamellae.
- Thylakoid membrane — contains chlorophyll, photosystems (PS I and PS II), electron carriers, and ATP synthase. This is where the light reactions occur.
- Stroma — contains enzymes of the Calvin cycle, ribosomes, and small circular DNA. This is where the dark reactions occur.
Chlorophyll a (blue-green) is the primary photosynthetic pigment. Chlorophyll b (yellow-green), carotenoids (yellow-orange) and xanthophylls are accessory pigments that absorb light at different wavelengths and pass energy to chlorophyll a.
Light reactions (thylakoid membrane)
The purpose: convert light energy into chemical energy (ATP and NADPH) and split water to release O.
PS II (P680) absorbs photons and excites electrons to a higher energy level. These electrons are passed to the electron transport chain. The electrons lost by PS II are replaced by splitting water: . This photolysis of water is the source of all oxygen in photosynthesis.
Electrons flow from PS II through plastoquinone, cytochrome b6f complex, and plastocyanin to PS I. As they flow, protons are pumped from the stroma into the thylakoid lumen, building a proton gradient.
The proton gradient drives H back through ATP synthase into the stroma, generating ATP. This is non-cyclic photophosphorylation (involves both PS I and PS II). In cyclic photophosphorylation, electrons from PS I cycle back through the ETC without involving PS II — this produces ATP but no NADPH and no O.
PS I (P700) absorbs light, excites electrons further. These electrons pass through ferredoxin to NADP reductase, which reduces NADP to NADPH using the electrons and H from the stroma.
Per glucose molecule (requires 6 turns of the Calvin cycle).
Dark reactions — Calvin cycle (stroma)
The purpose: use ATP and NADPH from light reactions to fix CO into glucose. Does not directly require light but depends on products of light reactions.
Three phases:
RuBisCO (ribulose bisphosphate carboxylase-oxygenase) — the most abundant enzyme on Earth — catalyses the fixation of CO onto a 5-carbon sugar RuBP. The product is two molecules of 3-PGA (3-phosphoglycerate) — a 3-carbon compound. This is why it is called the C3 pathway.
ATP and NADPH from the light reactions reduce 3-PGA to G3P (glyceraldehyde-3-phosphate). This is where light energy (now in chemical form) enters the sugar molecule.
Most of the G3P is used to regenerate RuBP so the cycle can continue. For every 6 CO fixed, 12 G3P are made, 2 leave the cycle as net product (used to make glucose), and 10 are recycled to regenerate 6 RuBP. This regeneration phase consumes ATP.
The Calvin cycle must turn 6 times (fixing one CO per turn) to produce one glucose molecule.
C4 pathway (Hatch-Slack pathway)
In plants like maize, sugarcane and sorghum that have Kranz anatomy — two distinct cell types around the vascular bundle:
- Mesophyll cells — CO is first fixed by PEP carboxylase (not RuBisCO) onto PEP to form OAA (oxaloacetate), a 4-carbon compound (hence C4). OAA is converted to malate.
- Malate is transported to bundle sheath cells, where it is decarboxylated, releasing CO and pyruvate. The released CO enters the Calvin cycle (using RuBisCO) at a much higher concentration than atmospheric.
- Pyruvate returns to mesophyll cells and is converted back to PEP using ATP.
The advantage: PEP carboxylase has a very high affinity for CO and no affinity for O. By concentrating CO around RuBisCO in the bundle sheath, C4 plants virtually eliminate photorespiration. The cost is extra ATP (2 ATP per CO for the PEP regeneration).
CAM pathway
CAM (Crassulacean Acid Metabolism) plants — succulents like cacti, pineapple, Bryophyllum. They open stomata at night (to avoid water loss) and fix CO into malate using PEP carboxylase. During the day, stomata close, malate is decarboxylated, and CO enters the Calvin cycle. Same biochemistry as C4, but temporally separated instead of spatially separated.
Photorespiration
RuBisCO can bind O instead of CO (oxygenase activity), leading to a wasteful pathway that consumes ATP and releases CO without producing useful energy. This is photorespiration — it wastes 20-50% of the carbon fixed by the Calvin cycle in C3 plants.
Photorespiration is worse at: high temperature (RuBisCO’s affinity for O increases relative to CO), high O, and low CO. C4 plants avoid it by concentrating CO around RuBisCO.
Photorespiration is a NEET favourite. Key facts: (1) occurs only in C3 plants, (2) involves chloroplast, peroxisome and mitochondrion, (3) wastes carbon and energy, (4) RuBisCO’s oxygenase activity is the cause, (5) C4 plants avoid it by using PEP carboxylase as the initial CO fixer.
Factors affecting photosynthesis
- Light intensity — rate increases linearly at low light, then plateaus (light saturation). C4 plants saturate at higher light than C3.
- CO concentration — rate increases with CO up to a saturation point. Current atmospheric CO (~420 ppm) is below saturation for C3 plants.
- Temperature — rate increases up to an optimum (~25-35°C for C3, higher for C4), then declines sharply as enzymes denature.
- Water — water stress causes stomatal closure, reducing CO entry.
The law of limiting factors (Blackman, 1905): the rate of photosynthesis is determined by the factor that is most limiting at any given time.
Worked Examples
At high temperature, RuBisCO’s oxygenase activity increases, so photorespiration rises sharply in C3 plants. C4 plants use PEP carboxylase (which has no oxygenase activity) to initially fix CO in mesophyll cells, then concentrate it in bundle sheath cells where RuBisCO operates. With CO concentration 10-20 times higher than atmospheric around RuBisCO, the oxygenase reaction is suppressed and photorespiration is negligible.
Robert Hill (1937) showed that isolated chloroplasts, given a suitable electron acceptor (like ferricyanide) and light, could split water and evolve O even without CO. This proved that O in photosynthesis comes from water, not from CO. The experiment was confirmed using -labelled water — the isotope appeared in O, not in glucose.
A plant fixes 5000 kJ/m/year through photosynthesis (GPP). It uses 2000 kJ/m/year in respiration. NPP = GPP - Respiration = 5000 - 2000 = 3000 kJ/m/year. This NPP is what is available to herbivores.
Common Mistakes
Saying light reactions happen in the stroma. They happen in thylakoid membranes. The Calvin cycle (dark reactions) happens in the stroma. Getting the compartment wrong is a very common NEET error.
Confusing C3 and C4 first products. C3 pathway: first stable product is 3-PGA (3-carbon). C4 pathway: first product is OAA (4-carbon). The naming convention itself tells you the carbon number.
Writing that oxygen comes from CO. It comes from splitting water (photolysis). This was proved by the Hill reaction and isotope tracing experiments.
Saying dark reactions happen only at night. They are called ‘dark’ because they do not directly require light — but they depend on ATP and NADPH from light reactions, so they usually happen during the day alongside light reactions.
Thinking all plants are either C3 or C4. Most plants are C3. C4 plants are a minority (~3% of species but many important crops). CAM plants are a third category with temporal separation of CO fixation.
Exam Weightage and Strategy
Photosynthesis is one of the highest-weightage chapters in NEET biology — expect 3-4 questions per year. CBSE boards give 5-7 marks. The PYQs cluster around: (1) light vs dark reactions location and products, (2) C3 vs C4 differences, (3) photorespiration mechanism, (4) factors affecting rate. Master these four areas and you cover the chapter.
Memorise three steps — light reactions in thylakoid (ATP + NADPH + O), Calvin cycle in stroma (CO fixation to glucose), C4 adds an extra CO pump via PEP carboxylase. That structure captures most PYQs. For the Calvin cycle, remember the three phases: fixation, reduction, regeneration.
Practice Questions
Q1. Why is RuBisCO considered the most abundant enzyme on Earth?
RuBisCO makes up about 25-50% of total leaf protein in C3 plants. Because it is relatively slow (only 3-10 CO molecules per second), plants compensate by producing enormous quantities of it. With billions of plants on Earth, RuBisCO is estimated to be the most abundant protein, totalling about 700 million tonnes globally.
Q2. Compare cyclic and non-cyclic photophosphorylation.
Non-cyclic: involves both PS I and PS II, produces ATP + NADPH + O, electrons flow linearly from water to NADP. Cyclic: involves only PS I, produces only ATP, electrons cycle from PS I back through the ETC to PS I, no NADPH or O produced. Cyclic phosphorylation supplements ATP production when the Calvin cycle demands more ATP than NADPH.
Q3. What is Kranz anatomy? Why is it important for C4 photosynthesis?
Kranz anatomy refers to the wreath-like arrangement of two types of cells around vascular bundles in C4 plants: (1) mesophyll cells (loosely arranged, where initial CO fixation by PEP carboxylase occurs) and (2) bundle sheath cells (tightly packed, thick-walled, where the Calvin cycle operates). The spatial separation allows C4 plants to concentrate CO around RuBisCO in the bundle sheath, suppressing photorespiration.
Q4. An experiment shows that increasing CO from 300 to 600 ppm increases photosynthesis rate in wheat but not in maize. Explain.
Wheat is a C3 plant — at 300 ppm CO, RuBisCO is not saturated, so increasing CO increases fixation rate. Maize is a C4 plant — PEP carboxylase already concentrates CO to high levels in the bundle sheath, so RuBisCO is already operating near saturation. Additional atmospheric CO provides little benefit to maize.
FAQs
Why do leaves appear green?
Chlorophyll absorbs red and blue light most efficiently and reflects green light. The reflected green light is what our eyes detect. In autumn, as chlorophyll breaks down, carotenoids (yellow/orange) become visible, causing leaf colour change.
What is the difference between photophosphorylation and oxidative phosphorylation?
Both produce ATP using a proton gradient and ATP synthase. Photophosphorylation occurs in thylakoid membranes using light energy. Oxidative phosphorylation occurs in the inner mitochondrial membrane using energy from electron transport (from NADH/FADH). One is powered by sunlight, the other by food breakdown.
Can photosynthesis occur in artificial light?
Yes. Photosynthesis requires photons of the right wavelength (mainly red and blue), not specifically sunlight. Greenhouses and indoor farms use LED lights tuned to the absorption peaks of chlorophyll for efficient year-round production.
Photosynthesis is the reason there is oxygen to breathe and food to eat. Study it with that sense of scale — the chapter deserves it.