Photosynthesis in Higher Plants: common doubts, answered
The questions students ask most often about Photosynthesis in Higher Plants, each with a short answer. For the full chapter, read the Photosynthesis in Higher Plants notes.
Early experiments
Read this section in the notes →Where does the oxygen released in photosynthesis come from?
It comes from water, not from carbon dioxide. Van Niel, studying photosynthetic bacteria, inferred that the hydrogen donor is split and its oxygen released, and later isotope work confirmed it for green plants. In the light reaction, water is split to give oxygen, protons and electrons, so oxygen is a product of the light reaction.
What did Priestley, Ingenhousz and Sachs each show?
Priestley showed in 1770 that plants restore air spoilt by a burning candle or a breathing mouse. Ingenhousz showed that sunlight is essential and that only the green parts release oxygen. Sachs showed that glucose is made in the green parts and usually stored as starch. Engelmann later produced the first action spectrum using Cladophora.
Where photosynthesis takes place
Read this section in the notes →Where exactly do the light reaction and the carbon reaction occur in a chloroplast?
The light reaction occurs in the thylakoid membranes, in the grana and the stroma lamellae. The carbon reaction, or synthesis of sugars, occurs in the stroma. The two are linked, as the light reaction supplies ATP and NADPH to the stroma, where they are used to reduce carbon dioxide to sugar.
Pigments involved in photosynthesis
Read this section in the notes →Why are there accessory pigments if chlorophyll a is the main pigment?
Chlorophyll b, xanthophylls and carotenoids absorb light of other wavelengths and pass the energy to chlorophyll a, which widens the range of light that can be used. They also protect chlorophyll a from photo-oxidation. Chlorophyll a is the chief pigment, since only it takes part directly in the photochemical reaction.
What is an action spectrum and how does it differ from an absorption spectrum?
An absorption spectrum shows how much light a pigment takes up at each wavelength, while an action spectrum shows how effective each wavelength is in driving photosynthesis. The two are similar but not identical. Engelmann's experiment with the alga Cladophora and aerobic bacteria showed the activity peaks in the blue and red regions.
The light reaction and the photosystems
Read this section in the notes →Why is PS II called PS II if it works first?
The photosystems were numbered in the order of their discovery, not in the order of their working. In the non-cyclic Z scheme, PS II with its reaction centre P680 acts first and passes electrons to PS I, whose reaction centre is P700. So PS I is not the first in the sequence even though it has the smaller number.
Electron transport, splitting of water and photophosphorylation
Read this section in the notes →What is the difference between cyclic and non-cyclic photophosphorylation?
Non-cyclic photophosphorylation uses both photosystems, produces ATP and NADPH, and releases oxygen from the splitting of water. Cyclic photophosphorylation uses only PS I, in which the electrons return to the photosystem, so only ATP is made, with no NADPH and no oxygen. It is thought to occur mostly in the stroma lamellae.
Where is water split in the light reaction?
Water is split in association with PS II, on the inner (lumen) side of the thylakoid membrane, giving oxygen, protons and electrons: 2H₂O → 4H⁺ + O₂ + 4e⁻. The electrons replace those lost by PS II, and the protons add to the gradient inside the lumen, which later powers ATP synthesis.
Chemiosmotic hypothesis of ATP synthesis
Read this section in the notes →Where do protons accumulate during ATP synthesis in chloroplasts?
Protons accumulate in the thylakoid lumen, which is why the lumen becomes more acidic than the stroma. They then flow down their gradient through the CF₀ channel of ATP synthase to the stroma, where the CF₁ part catalyses ATP formation. Many students wrongly say they accumulate in the stroma.
Use of ATP and NADPH: the Calvin cycle (C₃ pathway)
Read this section in the notes →What is the first stable product and the primary acceptor in the Calvin cycle?
The first stable product is 3-phosphoglyceric acid, a three-carbon compound, which is why the pathway is called C₃. The primary acceptor of carbon dioxide is ribulose bisphosphate, a five-carbon compound. Do not mix up the two: the acceptor is RuBP, while the first product is 3-PGA.
How many ATP and NADPH are needed to make one glucose in the Calvin cycle?
Fixing one carbon dioxide needs 3 ATP and 2 NADPH, so making one glucose, which takes six turns of the cycle, needs 18 ATP and 12 NADPH. The cycle has three stages: carboxylation, reduction and regeneration of the acceptor. Regeneration of RuBP needs ATP too.
What decides whether RuBisCO fixes carbon dioxide or oxygen?
The relative concentrations of the two gases decide it. RuBisCO, ribulose bisphosphate carboxylase-oxygenase, the most abundant enzyme in the world, has an active site that can bind either carbon dioxide or oxygen, so the two compete. Its affinity for carbon dioxide is much higher, but when carbon dioxide is low relative to oxygen, some RuBP binds oxygen instead and photorespiration follows.
Is the dark reaction really a reaction that takes place in the dark?
No. The carbon reactions do not need light directly but depend on the ATP and NADPH produced by the light reaction, so they stop soon after light is withdrawn. The name dark reaction is therefore misleading, and the term carbon reaction or biosynthetic phase is preferred.
The C₄ pathway
Read this section in the notes →Do C₄ plants have a Calvin cycle?
Yes. All photosynthetic plants use the Calvin cycle for making sugar. In C₄ plants it runs in the bundle sheath cells, while the mesophyll cells first fix carbon dioxide into oxaloacetic acid through PEP carboxylase. The C₄ pathway is a carbon concentrating step added to it, not a replacement for it.
What is the difference between C₃ and C₄ plants?
In C₃ plants carbon dioxide is fixed directly by RuBisCO to give a three-carbon product, 3-PGA. In C₄ plants carbon dioxide is first fixed by PEP carboxylase in the mesophyll to give a four-carbon product, oxaloacetic acid, and the Calvin cycle then runs in the bundle sheath. C₄ plants show higher productivity and no photorespiration.
Which cells of a C₄ leaf contain RuBisCO and PEPcase?
Mesophyll cells contain PEP carboxylase and lack RuBisCO, while the bundle sheath cells contain RuBisCO and lack PEP carboxylase. The leaf shows Kranz anatomy, with large bundle sheath cells arranged in a wreath around the vascular bundles. Swapping these two enzymes between cell types is a frequent error.
Photorespiration
Read this section in the notes →What is photorespiration and why is it wasteful?
Photorespiration occurs when RuBisCO binds oxygen instead of carbon dioxide, so RuBP is split into phosphoglycerate and phosphoglycolate. It is wasteful because no sugar and no ATP are made, and no NADPH either, in the process. It happens in C₃ plants and is absent in C₄ plants, since the bundle sheath has high carbon dioxide.
Why do C₄ plants have no photorespiration?
C₄ plants have a mechanism that raises the carbon dioxide concentration around RuBisCO in the bundle sheath cells, so RuBisCO acts as a carboxylase and does not bind oxygen. With oxygen kept from competing, the wasteful process does not occur. This is a reason C₄ plants are more productive in warm, bright conditions.
Factors affecting photosynthesis
Read this section in the notes →What is Blackman's law of limiting factors?
When a process depends on several factors, its rate is limited by the one that is nearest its minimum value. Blackman proposed this in 1905. For example, if light is plentiful but carbon dioxide is scarce, then increasing light will not raise photosynthesis, but increasing carbon dioxide will.
Why is CO₂ usually the limiting factor rather than light?
Light saturates photosynthesis at about 10 per cent of full sunlight, so except in shade or dense forests, light is rarely limiting. Carbon dioxide in the air is low, and the rate rises when its concentration is increased. C₄ plants saturate at about 360 microlitres per litre, whereas C₃ plants respond to levels beyond 450.
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