The overall reaction
Photosynthesis is the process by which green plants use light energy to make carbohydrates from carbon dioxide and water, releasing oxygen:
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂ (in the presence of light and chlorophyll)
Experiments with heavy oxygen (¹⁸O) showed that the oxygen released comes from water, not from CO₂.
Where it happens: the chloroplast
Photosynthesis takes place in chloroplasts, mainly in the mesophyll cells of leaves.
The thylakoid membranes (stacked into grana) hold the pigments and carry out the light reactions.
The fluid stroma contains the enzymes of the dark reactions (the Calvin cycle).
Photosynthetic pigments
Pigment | Colour | Role |
|---|---|---|
Chlorophyll a | blue-green | the main pigment; forms the reaction centres |
Chlorophyll b | yellow-green | accessory pigment |
Carotenoids (xanthophylls, carotenes) | yellow to orange | accessory pigments; also protect chlorophyll from damage by strong light |
Chlorophyll absorbs mainly blue and red light and reflects green, which is why leaves look green. Accessory pigments widen the range of wavelengths a plant can use.
The light reactions (photochemical phase)
Pigments are organised into photosystems in the thylakoid membrane:
Photosystem I (PS I): reaction centre P700.
Photosystem II (PS II): reaction centre P680.
Light excites electrons in PS II. These high-energy electrons pass along an electron transport chain.
The electrons lost by PS II are replaced by splitting water (photolysis): 2H₂O → 4H⁺ + O₂ + 4e⁻. This is the source of the oxygen released.
The electrons reach PS I, are excited again by light, and are finally used to reduce NADP⁺ to NADPH.
Plotted by energy level, this path looks like the letter Z, so it is called the Z-scheme.
Making ATP: chemiosmosis
As electrons move along the chain, protons (H⁺) build up inside the thylakoid lumen. They flow back out through the enzyme ATP synthase, which uses this flow to make ATP from ADP and phosphate. Making ATP with the help of light is called photophosphorylation.
Non-cyclic photophosphorylation uses both photosystems and produces ATP, NADPH and O₂.
Cyclic photophosphorylation uses only PS I. Electrons cycle back, producing ATP only (no NADPH and no O₂).
Products of the light reactions: ATP and NADPH (used in the stroma), and O₂ (released).
The Calvin cycle (biosynthetic phase)
In the stroma, the energy of ATP and NADPH is used to fix CO₂ into sugar. The cycle has three stages:
Carboxylation: CO₂ combines with the 5-carbon acceptor RuBP (ribulose bisphosphate). The enzyme is RuBisCO, the most abundant enzyme on Earth. The unstable 6-carbon product splits into two molecules of the 3-carbon 3-PGA. Because the first stable product has 3 carbons, this is the C₃ pathway.
Reduction: 3-PGA is reduced to a 3-carbon sugar (triose phosphate), using ATP and NADPH.
Regeneration: RuBP is regenerated so that the cycle can continue. This also uses ATP.
To make one glucose molecule, the cycle must turn six times, using 18 ATP and 12 NADPH.
Photorespiration and C₄ plants
RuBisCO can also bind O₂ instead of CO₂, especially when it is hot and CO₂ levels inside the leaf are low. This wasteful process, photorespiration, releases CO₂ and consumes energy without making sugar.
C₄ plants such as maize, sugarcane and sorghum avoid this problem:
They have a special leaf anatomy, called Kranz anatomy, with large bundle-sheath cells around the veins.
In the mesophyll, CO₂ is first fixed by PEP carboxylase into a 4-carbon acid (oxaloacetic acid), hence the name "C₄".
This acid is carried to the bundle-sheath cells and releases CO₂ there at a high concentration. The Calvin cycle runs in these cells, where RuBisCO is surrounded by CO₂, so photorespiration hardly occurs.
C₄ plants are therefore more productive in hot, dry and bright conditions.
Factors affecting photosynthesis
Light intensity, CO₂ concentration, temperature and water availability all affect the rate. Blackman's law of limiting factors states that when a process depends on several factors, its rate is limited by the factor that is in shortest supply. Increasing other factors has no effect until the limiting one is increased.