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Light reaction is taught in the note for Photosynthesis. Read it below and look for the section on this topic.

Photosynthesis

14 min read

Making food from light

Photosynthesis is the process by which green plants use light energy to make glucose from carbon dioxide and water, releasing oxygen. It takes place in the chloroplasts.

The overall equation for photosynthesis.
A chloroplastA chloroplast has a double outer membrane. Inside are stacks of flat discs called grana, whose membranes, the thylakoids, hold chlorophyll and carry out the light reactions. The grana lie in a fluid called the stroma, where the dark reactions that make sugar from carbon dioxide take place.double membranegranum (stack of thylakoids)stroma (dark reactions)starch grain

The light reaction

The light reaction takes place in the thylakoid membranes of the grana. Chlorophyll absorbs light. The energy is used to split water (photolysis), releasing oxygen, and to make ATP and reduced NADP.

The dark reaction

The dark reaction does not need light directly but uses the ATP and NADPH made in the light reaction. It takes place in the stroma and is called the Calvin cycle. Carbon dioxide combines with a 5-carbon sugar, RuBP, with the help of the enzyme rubisco. The product is converted to glucose.

The light and dark reactionsTwo linked stages in a chloroplast. In the light reactions in the thylakoids, light splits water, releasing oxygen, and makes ATP and reduced NADP. In the dark reactions in the stroma, called the Calvin cycle, carbon dioxide is fixed by RuBP, using the ATP and NADPH from the light reactions, to make sugar. The cycle then regenerates RuBP.Light reactions (thylakoid)light + H₂O→ O₂ + H⁺ + e⁻→ ATP + NADPHATP, NADPHCalvin cycle (stroma)CO₂ + RuBP (5C)2 × 3C compound (GP)triose phosphate (TP)→ glucose, starchRuBPO₂ releasedATP and NADPH used

C3 and C4 plants

Most plants are C3 plants: the first product is a 3-carbon compound. In hot, dry places C4 plants such as maize and sugarcane first fix carbon dioxide into a 4-carbon acid in mesophyll cells. The acid moves to the bundle sheath cells, where it releases CO₂ to the Calvin cycle. This keeps CO₂ high around rubisco, so C4 plants waste less energy through photorespiration and do better in hot weather.

FeatureC3 plantC4 plant
First product3-carbon (GP)4-carbon acid
Calvin cycle inMesophyll cellsBundle sheath cells
PhotorespirationHigh in hot weatherVery low
ExamplesRice, wheat, beanMaize, sugarcane, sorghum

Limiting factors

The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration or temperature.

The rate rises with light until another factor, such as CO₂, becomes limiting.
Try itLimiting factors

Choose the light intensity and the CO₂ level. The rate is limited by the smaller of the two effects.

Check yourself Where does the dark reaction take place?
Check yourself What is released when water is split in the light reaction?
Check yourself Which plant is a C4 plant?

Key terms

Photolysis
Splitting of water by light.
Calvin cycle
The cycle of reactions that makes sugar from CO₂.
RuBP
The 5-carbon sugar that fixes CO₂.
Limiting factor
The factor in shortest supply which limits the rate.

Summary

  • Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ with light and chlorophyll.
  • The light reaction in the thylakoids makes ATP, NADPH and oxygen.
  • The dark reaction (Calvin cycle) in the stroma makes sugar.
  • C4 plants concentrate CO₂ and do better in hot, dry places.
Dark reaction in C3 and C4 plants