ATP and the light-dependent reactionsEdexcel A-Level Biology A: Revision notes

ATP (adenosine triphosphate) is a nucleotide made of adenine, the pentose sugar ribose and three phosphate groups. It is the immediate source of energy for processes such as active transport, muscle contraction, DNA replication, protein synthesis and nerve impulses.

When ATP is hydrolysed, the terminal phosphate bond is broken by the enzyme ATP hydrolase:

ATP + H₂O → ADP + Pi

This is an exergonic reaction that releases about 30.6 kJ mol⁻¹. The energy is released in a single step, in small, manageable amounts, so little is wasted as heat. ATP is soluble and small, so it moves easily around the cell.

ATP is not stored in large quantities: the body holds only about 50 g at any time, so ATP must be resynthesised continually from ADP and Pi. This is why it is called an immediate energy source and not a long-term store, for which cells use lipids and starch or glycogen.

ATP is made by phosphorylation: adding an inorganic phosphate (Pi) to ADP in a condensation reaction:

ADP + Pi → ATP + H₂O

This needs energy, which is supplied by respiration or by light, and is catalysed by ATP synthase.

In both mitochondria and chloroplasts the energy is used to build up a proton (H⁺) gradient across a membrane. The protons then flow back down the gradient through ATP synthase, and the energy released is used to join ADP and Pi. This is chemiosmosis.

In respiration the process is oxidative phosphorylation. In photosynthesis the energy comes from light, and the process is photophosphorylation.

The light-dependent reactions take place in the thylakoid membranes of the chloroplast, where pigments are arranged in photosystems. A photosystem is a group of chlorophyll and accessory pigment molecules held by proteins. The pigments absorb different wavelengths of light, mainly red and blue, and pass the energy to a special chlorophyll molecule at the reaction centre.

When this chlorophyll absorbs light energy, a pair of electrons is raised to a higher energy level and leaves the chlorophyll molecule. This is photoionisation, and the electrons are said to be excited. They are accepted by the first carrier in an electron transport chain. Chlorophyll has been oxidised and has a positive charge, so it needs to gain replacement electrons (see photolysis).

The excited electrons pass along a chain of electron carriers in the thylakoid membrane, moving from carrier to carrier in a series of redox reactions. The electrons lose energy at each step.

This energy is used to pump protons (H⁺) from the stroma into the thylakoid space. The thylakoid space therefore has a higher H⁺ concentration (lower pH) than the stroma, so a proton gradient exists across the thylakoid membrane.

Protons diffuse back into the stroma through ATP synthase, which is a channel protein and an enzyme. The energy released drives the synthesis of ATP from ADP and Pi. This is photophosphorylation, and the ATP is used in the light-independent reactions.

At the end of the chain, the electrons are accepted by the coenzyme NADP. Together with a proton (H⁺) from the stroma, they reduce NADP to form reduced NADP:

NADP + 2e⁻ + 2H⁺ → reduced NADP

The reaction is catalysed by an enzyme on the stroma side of the membrane. Reduced NADP carries hydrogen (the electrons and protons) to the light-independent reactions, where it is the reducing agent that reduces carbon dioxide to sugars.

In the light-dependent reactions, therefore, light energy is converted into chemical energy held in ATP and reduced NADP.

The chlorophyll in the photosystem that has lost electrons must be supplied with replacements. These come from water. An enzyme in the thylakoid, using light energy, splits water in a process called photolysis:

2H₂O → 4H⁺ + 4e⁻ + O₂

• The electrons replace those lost from chlorophyll.
• The protons add to the H⁺ in the thylakoid space (building the gradient) and are used to reduce NADP.
• Oxygen is a by-product that diffuses out of the chloroplast and is released from the leaf, or used in respiration.

The products of the light-dependent reactions are ATP, reduced NADP and oxygen, and the net inputs are light, water, ADP and Pi, and NADP.