The light-dependent reaction in such detail as to show that: chlorophyll…

The idea

The light-dependent reaction takes place on the thylakoid membranes of the chloroplast. Chlorophyll absorbs light, and the energy raises an electron to a higher energy level so that it leaves the molecule, which is photoionisation. The electrons pass along a chain of electron carriers embedded in the membrane, losing energy at each transfer, and that energy actively moves protons from the stroma into the thylakoid space. A proton gradient builds across the membrane, and protons pass back into the stroma through ATP synthase; their movement drives the synthesis of ATP from ADP and inorganic phosphate, which is chemiosmosis, called photophosphorylation here. At the end of the chain, electrons and protons combine with NADP to form reduced NADP. The electrons lost from chlorophyll are replaced by photolysis of water, which splits into protons, electrons and oxygen.

What earns the marks

• Defining photoionisation as chlorophyll absorbing light and losing an electron, and stating that the electron is raised to a higher energy level before leaving the molecule.
• Describing electrons passing down the electron transfer chain in the thylakoid membrane, releasing energy that moves protons across the membrane.
• Explaining that protons pass back through ATP synthase so that ATP is made from ADP and inorganic phosphate, by chemiosmosis.
• Stating that electrons and protons reduce NADP to form reduced NADP, and that two electrons are needed overall to reduce one NADP.
• Stating that photolysis of water produces protons, electrons and oxygen, and that the electrons replace those lost from chlorophyll.

Common mistakes and how to put them right

• Writing NADP where reduced NADP is meant, or using NAD and reduced NAD, which examiners reject in a photosynthesis answer.

  Put it right by naming reduced NADP (or NADPH) as the product.

• Saying light energy is converted to chemical energy without naming photoionisation, the electron transfer chain or ATP synthase.

  Put it right by naming each stage explicitly.

• Placing the light-dependent reaction in the stroma instead of on the thylakoid membranes.

  Put it right by locating it on the thylakoid membranes, with the stroma as the site of the light-independent reaction.

• Saying the oxygen released comes from carbon dioxide rather than from the photolysis of water.

  Put it right by stating that oxygen is released when water is split.

• Describing protons as being pumped out of the thylakoid space instead of into it.

  Put it right by stating that protons are moved from the stroma into the thylakoid space, creating the gradient.

• Omitting photolysis of water, or failing to say that it replaces the electrons lost from chlorophyll.

  Put it right by stating that photolysis supplies the replacement electrons.

Exam tips

• Use the oxygen released as a measure of the rate of the light-dependent reaction: the faster oxygen appears, the faster the reaction.
• Name all three products in a description, ATP, reduced NADP and oxygen, and say where each one goes.
• Write reduced NADP in full, or NADPH; the mark scheme rejects reduced NAD in this context.