Photosynthesis in Higher Plants

Photosynthesis is the process by which green plants make their own food (glucose) from carbon dioxide and water, using light energy, and release oxygen. It is the source of nearly all the food and oxygen on Earth. The overall equation is:

6CO₂ + 12H₂O →_{(light, chlorophyll)} C₆H₁₂O₆ + 6O₂ + 6H₂O

Photosynthesis takes place in the chloroplasts, mostly in the leaf mesophyll. A chloroplast has stacks of flattened sacs called grana (made of thylakoids) sitting in a fluid called the stroma. The light-trapping pigments are in the thylakoid membranes; the main pigment is chlorophyll a, helped by accessory pigments (chlorophyll b, carotenoids) that absorb light and pass the energy to chlorophyll a.

Photosynthesis has two stages. The first is the light reaction (photochemical phase), which happens in the thylakoid membranes and needs light. Here:

• Light energy is absorbed and used to split water — photolysis — releasing oxygen (the O₂ we breathe comes from water, not CO₂).
• The energy is stored in the molecules ATP and NADPH. Making ATP using light is called photophosphorylation.

So the light reaction produces the "assimilatory power" (ATP + NADPH) and oxygen.

The second stage is the dark reaction (biosynthetic phase), also called the Calvin cycle. Despite the name, it does not require darkness — it simply does not directly use light. It takes place in the stroma of the chloroplast and uses the ATP and NADPH made in the light reaction to fix carbon dioxide into sugar.

In the Calvin cycle, CO₂ is added to a 5-carbon acceptor molecule (RuBP) by the enzyme RuBisCO — the most abundant enzyme on Earth. Through a series of steps using ATP and NADPH, this builds up glucose, and the acceptor RuBP is regenerated to keep the cycle going. Because the first stable product here is a 3-carbon compound (3-PGA), plants that fix carbon only this way are called C3 plants (e.g. wheat, rice).

Some plants in hot, dry climates use a clever extra step. They first fix CO₂ into a 4-carbon acid in their mesophyll cells, then release the CO₂ to the Calvin cycle inside special bundle-sheath cells. These are C4 plants (e.g. maize, sugarcane). C4 plants are more efficient in strong sunlight and high temperature, lose less water, and largely avoid the wasteful side reaction of RuBisCO called photorespiration. This special leaf structure in C4 plants is called Kranz anatomy.

The rate of photosynthesis depends on conditions inside and outside the plant. The external factors are light, carbon dioxide, temperature and water; the internal factors include the amount of chlorophyll and the number and state of the leaves.

• Light — the rate rises as light increases, up to a point, then levels off (at very high light it can even fall slightly).
• Carbon dioxide — usually the most important limiting factor in nature; raising CO₂ increases the rate up to a saturation point. (This is why greenhouses sometimes add extra CO₂.)
• Temperature — the reactions are enzyme-controlled, so the rate rises with temperature to an optimum and then falls as enzymes are denatured.
• Water — a shortage closes stomata (reducing CO₂ intake) and slows growth, indirectly lowering the rate.

A key idea here 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, on a bright warm day with plenty of light and warmth, the rate may still be low simply because CO₂ is limited; adding more light will not help until the CO₂ supply is increased. Understanding limiting factors helps farmers and greenhouse growers boost crop yields by improving whatever is most lacking.