recap

10.2 recap

Conversion of light energy into chemical energy occurs when photons are absorbed by chlorophylls and accessory pigments in the light-harvesting complex within the chloroplast. Light energy is used to drive a series of protein-associated redox reactions in the thylakoid membranes of the chloroplast. In the process of photophosphorylation, two linked photosystems establish a proton gradient across the membrane that drives ATP synthesis.

learning outcomes

You should be able to:

  • Describe the transfer of energy within the reaction center of a light-harvesting complex.

  • 202

    Compare and contrast a pigment’s absorption spectrum and its action spectrum.

  • Explain how ATP and NADPH are produced in a chloroplast.

Question 1

How can a pigment molecule lose the energy of an absorbed photon?

A pigment molecule tends to lose the absorbed energy of a photon by returning to ground state and emitting the energy as light or heat. Alternatively, the energy can be transferred as an excited electron from the pigment to another molecule, reducing that molecule.

Question 2

What is the difference between an absorption spectrum and an action spectrum?

An absorption spectrum plots the extent of absorption by pigments (y axis) versus the wavelength of light to which the pigments are exposed (x axis). An action spectrum also plots wavelengths of light, but in this case the y axis is a biological activity (e.g., photosynthesis).

Question 3

How does cyclic electron transport in photosystem I result in the production of ATP?

See Figure 10.8. In cyclic electron transport, ATP is produced chemiosmotically by electron transport in the thylakoid membrane.

You have seen how light energy drives the synthesis of ATP and NADPH in the stroma of chloroplasts. We will now turn to the light-independent reactions of photosynthesis, which use energy-rich ATP and NADPH to reduce CO2 and form carbohydrates.