Photosynthesis: pigments and light-dependent reactions

Biology · Bioenergetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Photosynthesis has two stages. The light-dependent reactions take place on the thylakoid membranes and turn light energy into ATP and NADPH. These two products then drive the Calvin cycle in the stroma.

Pigments are grouped into photosystems. Light excites an electron in the reaction centre chlorophyll a, and the electron passes down a chain of carriers. In the non-cyclic path, electrons travel from water through photosystem II and photosystem I to NADP⁺. Water is split, so the oxygen released comes from water, not from carbon dioxide.

As electrons move, protons are pumped into the thylakoid space. They flow back through ATP synthase and ATP is made. This is chemiosmosis.

Rules to remember

Overall equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O (light and chlorophyll needed).
Chlorophyll a has a –CH₃ group where chlorophyll b has –CHO. Both have a porphyrin head with magnesium at the centre and a phytol tail.
Chlorophylls absorb mainly violet-blue and orange-red light and reflect green. Carotenoids absorb other wavelengths and protect chlorophyll.
Absorption spectrum: light absorbed by a pigment at each wavelength. Action spectrum: rate of photosynthesis at each wavelength.
Photosystem I has reaction centre P700. Photosystem II has P680 and is linked to the photolysis of water.
Non-cyclic phosphorylation: both photosystems; makes ATP, NADPH and O₂.
Cyclic phosphorylation: photosystem I only; makes ATP only, no NADPH and no O₂.

Common mistake

Students say the oxygen comes from carbon dioxide. All the oxygen gas released comes from the splitting of water at photosystem II. The oxygen atoms of CO₂ end up in the sugar and in the new water.

Worked example

A plant is given water containing the heavy isotope ¹⁸O and normal CO₂. After a time in the light, where is the ¹⁸O found: in the oxygen gas released or in the glucose made?

Water is split at photosystem II: 2H₂O → 4H⁺ + 4e⁻ + O₂.
So the oxygen atoms of water leave the leaf as oxygen gas.
The oxygen in glucose comes from CO₂, which is fixed in the Calvin cycle, and the CO₂ here is not labelled.

Answer: The ¹⁸O appears in the oxygen gas released, not in the glucose.

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Chapter 11: Bioenergetic

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