Chapter overview: photosynthesis
Photosynthesis is the physico-chemical process by which green plants use light energy to synthesise organic compounds. It is the primary source of food on Earth and releases oxygen into the atmosphere. Chlorophyll, light and carbon dioxide are essential requirements.
Overall equation: 6CO2 + 12H2O -> C6H12O6 + 6H2O + 6O2. The oxygen evolved comes from water, not carbon dioxide.
11.1 Early experiments and requirements
Starch tests on variegated or partially covered leaves show that photosynthesis occurs only in green parts exposed to light. A leaf region enclosed with KOH-soaked cotton does not form starch because KOH absorbs CO2; this proves the requirement of carbon dioxide.
| Scientist | Key contribution |
|---|---|
| Joseph Priestley | Plants restore air damaged by breathing animals and burning candles. |
| Jan Ingenhousz | Sunlight and green parts are necessary; green aquatic parts release oxygen bubbles in light. |
| Julius von Sachs | Green parts form glucose, usually stored as starch; chlorophyll occurs in special bodies later called chloroplasts. |
| T.W. Engelmann | Aerobic bacteria accumulated in blue and red regions around illuminated alga, showing action spectrum of photosynthesis. |
| Cornelius van Niel | Showed that the hydrogen donor is oxidised; in green plants water supplies hydrogen, so released O2 comes from water. |
11.2 Site of photosynthesis: chloroplast
Photosynthesis occurs in green leaves and other green plant parts. Mesophyll cells contain numerous chloroplasts, which can orient along cell walls to optimise incident light. Chloroplasts have a membrane system of grana and stromal lamellae, plus the surrounding stroma.
| Chloroplast region | Major role |
|---|---|
| Thylakoid/granal membrane system | Traps light energy and synthesises ATP and NADPH through light reactions. |
| Stroma | Enzyme-driven carbon reactions synthesise sugars that can form starch. |
11.3 Photosynthetic pigments
Paper chromatography separates four leaf pigments: chlorophyll a (blue-green), chlorophyll b (yellow-green), xanthophylls (yellow) and carotenoids (yellow to yellow-orange). Pigments absorb light at specific wavelengths; most photosynthesis occurs in blue and red regions.
| Pigment group | Role |
|---|---|
| Chlorophyll a | Chief photosynthetic pigment and reaction-centre pigment. |
| Chlorophyll b, xanthophylls and carotenoids | Accessory pigments: absorb additional wavelengths, transfer energy to chlorophyll a and protect it from photo-oxidation. |
11.4 Light reaction and photosystems
Light reactions include absorption of light, splitting of water, oxygen evolution, electron transport, and formation of ATP and NADPH. Pigment molecules in the light-harvesting complex/antenna funnel energy to a reaction-centre chlorophyll a molecule.
| Photosystem | Reaction centre | Key role |
|---|---|---|
| Photosystem II (PS II) | P680, absorbs red light at 680 nm | Receives electrons from photolysis of water and initiates non-cyclic electron flow. |
| Photosystem I (PS I) | P700, absorbs red light at 700 nm | Re-excites electrons and reduces NADP+ to NADPH in non-cyclic flow. |
Water splitting is associated with PS II: 2H2O -> 4H+ + O2 + 4e-. The electrons replace those lost by PS II, protons contribute to the gradient, and oxygen is released.
Electron transport and photophosphorylation
In non-cyclic photophosphorylation, electrons move from PS II through carriers to PS I, are re-excited and finally reduce NADP+ to NADPH. This route produces ATP, NADPH and O2.
In cyclic photophosphorylation, electrons excited from PS I cycle back through the electron transport chain to PS I. It produces ATP only: no NADPH is formed and no O2 is evolved. It is useful when the chloroplast needs additional ATP relative to NADPH.
Electron transport builds a proton gradient across the thylakoid membrane. High H+ concentration develops in the lumen through water splitting, proton translocation via electron carriers, and use of stromal H+ in NADPH formation. H+ flows back through CF0-CF1 ATP synthase; this chemiosmotic flow powers ATP formation.
11.5 Carbon reactions: Calvin cycle
The Calvin cycle occurs in the stroma of all photosynthetic plants, including C3 and C4 plants. Its primary CO2 acceptor is the 5-carbon ketose ribulose-1,5-bisphosphate (RuBP). RuBisCO carboxylates RuBP, producing two molecules of 3-phosphoglycerate (3-PGA).
| Stage | Core event |
|---|---|
| Carboxylation | CO2 is fixed to RuBP by RuBisCO, yielding two 3-PGA molecules. |
| Reduction | ATP and NADPH convert intermediates to carbohydrate/triose phosphate. |
| Regeneration | ATP regenerates RuBP so the cycle can continue. |
For each CO2 fixed, 3 ATP and 2 NADPH are used. Six turns fix six CO2 to make one glucose, consuming 18 ATP and 12 NADPH.
11.6 C3 pathway vs C4 pathway
All photosynthetic plants use the Calvin cycle to form sugars. C4 plants first concentrate CO2 through the Hatch-Slack pathway, an adaptation of dry tropical plants.
| Feature | C3 plants | C4 plants |
|---|---|---|
| First stable product of CO2 fixation | 3-PGA, a 3-carbon acid | Oxaloacetate (OAA), a 4-carbon acid |
| Primary acceptor | RuBP, 5-carbon | PEP, 3-carbon |
| Initial carboxylation | Mesophyll cells by RuBisCO | Mesophyll cells by PEP carboxylase (PEPcase) |
| Calvin cycle | Mesophyll cells | Bundle sheath cells |
| Leaf anatomy | No Kranz anatomy | Kranz anatomy: chloroplast-rich, thick-walled bundle sheath cells with no intercellular spaces |
| Response | Lower temperature optimum and lower high-light CO2 fixation | Tolerates high temperature, responds to high light and has greater biomass productivity |
| Photorespiration | Present | Absent/negligible |
In C4 plants, OAA becomes malate or aspartate in mesophyll cells and is transported to bundle sheath cells. Decarboxylation releases CO2 for Calvin cycle; the 3-carbon compound returns to mesophyll to regenerate PEP.
11.7 Photorespiration
RuBisCO acts as both carboxylase and oxygenase. In C3 plants, O2 can compete with CO2 at the active site. Oxygenation of RuBP produces one phosphoglycerate and one 2-carbon phosphoglycolate, initiating photorespiration.
C4 plants avoid photorespiration because decarboxylation of C4 acids elevates CO2 around RuBisCO in bundle sheath cells, favouring carboxylase activity over oxygenase activity.
11.8 Factors affecting photosynthesis
Internal factors include leaf number, size, age and orientation; mesophyll cells and chloroplasts; internal CO2; and chlorophyll quantity. External factors are light, temperature, CO2 concentration and water. Blackman's law of limiting factors states that when several factors affect a process, its rate is determined by the factor nearest its minimum value.
| Factor | NCERT effect |
|---|---|
| Light | CO2 fixation rises linearly at low intensity, then plateaus when other factors limit. Saturation occurs at about 10% of full sunlight; excessive light can break down chlorophyll. |
| CO2 | Major limiting factor. Atmospheric level is 0.03-0.04%; increase up to 0.05% raises fixation, but higher long-term levels may be damaging. C4 saturates near 360 microlitres L-1; C3 beyond 450 microlitres L-1. |
| Temperature | Carbon reactions are strongly temperature-controlled. C4 plants have higher optimum temperature than C3 plants; adaptation to habitat matters. |
| Water | Water stress closes stomata, reducing CO2; it also causes wilting, reducing leaf area and metabolism. |
NCERT revision prompts
- State the experiments proving the need for chlorophyll, light and CO2.
- Which experiment established that oxygen released during photosynthesis comes from water?
- Differentiate the sites and products of light and carbon reactions.
- Compare cyclic and non-cyclic photophosphorylation.
- Explain the chemiosmotic mechanism of ATP synthesis in chloroplasts.
- State the three phases and energy cost of Calvin cycle.
- Compare C3 and C4 plants, including Kranz anatomy and the primary CO2 acceptor.
- Why is photorespiration negligible in C4 plants?
- Apply Blackman's law to light, CO2, temperature and water.