Photosynthesis in Higher Plants — Class 11 Biology
Biology · Chapter 11
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Chapter summary
This chapter explains how green plants synthesise food using light energy and why photosynthesis is the basis of life and of atmospheric oxygen. Early experiments by Priestley, Ingenhousz, Sachs, Engelmann and van Niel established that plants use light, chlorophyll and carbon dioxide, that oxygen is released only by green parts, and that the released oxygen comes from water. Photosynthesis takes place in chloroplasts: the thylakoid membranes carry out the light reaction and the stroma carries out the carbon-fixing reactions. Four pigments (chlorophyll a, chlorophyll b, xanthophylls and carotenoids) absorb light, with chlorophyll a the chief pigment. In the light reaction, pigments organised into Photosystem II (P680) and Photosystem I (P700) drive electron transport along the Z scheme, splitting water to release oxygen and producing ATP and NADPH; ATP is made by chemiosmosis across the thylakoid membrane. In the biosynthetic phase the Calvin cycle fixes carbon dioxide using the enzyme RuBisCO through carboxylation, reduction and regeneration, requiring 18 ATP and 12 NADPH per glucose. C4 plants with Kranz anatomy use the Hatch and Slack pathway to concentrate carbon dioxide and avoid photorespiration, a wasteful oxygenation of RuBP that occurs in C3 plants. The rate of photosynthesis is governed by light, carbon dioxide, temperature and water, following Blackman's law of limiting factors.
Early experiments on photosynthesisSite of photosynthesis and pigmentsLight reaction, photosystems and the Z schemeChemiosmosis and ATP synthesisThe Calvin cycle and the C4 pathwayPhotorespiration and limiting factors
Key ideas & terms
- Photosynthesis
- The physico-chemical process by which green plants use light energy to synthesise organic compounds from carbon dioxide and water.
- Photosystem
- A light-harvesting complex of pigments with a reaction-centre chlorophyll a (P700 in PS I, P680 in PS II).
- Z scheme
- The path of electrons from PS II through the electron transport chain to PS I and finally to NADP+, shaped like a Z on a redox scale.
- Calvin cycle
- The C3 carbon-fixation pathway of carboxylation, reduction and regeneration that synthesises sugar in the stroma.
- RuBisCO
- Ribulose bisphosphate carboxylase-oxygenase, the enzyme that fixes carbon dioxide and can also carry out oxygenation.
- Photorespiration
- The wasteful oxygenation of RuBP in C3 plants that consumes ATP and releases carbon dioxide without producing sugar.
Extra questions & answers
Where in the chloroplast do the light and dark reactions occur?
The light reactions occur in the thylakoid membranes (grana), while the carbon-fixing (dark) reactions occur in the stroma.
Name the pigments involved in photosynthesis.
Chlorophyll a, chlorophyll b, xanthophylls and carotenoids, of which chlorophyll a is the chief pigment.
What is the role of water splitting in the light reaction?
Splitting of water associated with PS II provides electrons to replace those lost from the reaction centre, and releases protons and oxygen.
Differentiate between cyclic and non-cyclic photophosphorylation.
Non-cyclic photophosphorylation involves both PS II and PS I and produces ATP and NADPH, while cyclic photophosphorylation involves only PS I and produces only ATP.
Outline the three stages of the Calvin cycle.
Carboxylation, in which carbon dioxide combines with RuBP to form PGA; reduction, in which PGA is converted to sugar using ATP and NADPH; and regeneration, in which RuBP is reformed.
How many ATP and NADPH are required to make one glucose in the Calvin cycle?
Six turns of the cycle require 18 ATP and 12 NADPH to make one molecule of glucose.
What is Kranz anatomy and in which plants is it found?
It is the leaf anatomy of C4 plants in which large bundle sheath cells with many chloroplasts surround the vascular bundles, as in maize and sorghum.
Why are C4 plants more productive than C3 plants?
Because they concentrate carbon dioxide in the bundle sheath cells and lack photorespiration, so RuBisCO acts mainly as a carboxylase, giving higher productivity even at high temperatures.
State Blackman's law of limiting factors.
If a chemical process is affected by more than one factor, its rate is determined by the factor that is nearest to its minimal value.
Which factors affect the rate of photosynthesis?
Light, carbon dioxide concentration, temperature and water are the external factors, along with internal plant factors such as chlorophyll content and leaf area.