Overview of photosynthesis and ATPEdexcel International A Level Biology: Revision notes

Photosynthesis is the process in which plants, algae and some bacteria use light energy to make organic compounds from carbon dioxide and water. The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

Light energy is used to split water (photolysis). The oxygen from the water is released as a by-product, and the hydrogen is used to reduce carbon dioxide, so that it is stored in glucose. This converts light energy into chemical energy in glucose.

Evidence that the oxygen comes from water, not from carbon dioxide, was obtained using the heavy isotope ¹⁸O: when algae were given H₂¹⁸O the oxygen released contained ¹⁸O, but when they were given C¹⁸O₂ it did not.

Worked example: if a plant releases 0.30 mol of oxygen, then 0.30 ÷ 6 = 0.050 mol of glucose is made, which is 0.050 × 180 = 9.0 g.

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

ATP is a good immediate energy source because:

• its hydrolysis releases a small, manageable amount of energy (about 30 kJ per mole), which is enough for one process and is not wasted as heat
• it is hydrolysed in a single-step reaction, so energy is available quickly
• it can be resynthesised from ADP and Pi, so it is recycled
• the same molecule is used in many different reactions

ATP cannot be stored in large amounts, so it is made continually.

In hydrolysis, ATP is broken down using water to form ADP and inorganic phosphate (Pi), and energy is released. The reaction is catalysed by the enzyme ATP hydrolase (ATPase):

ATP + H₂O → ADP + Pi + energy

The energy released is used directly to drive a process. For example, in active transport the energy changes the shape of a carrier protein, and in muscle contraction it allows myosin heads to move along actin. The phosphate is often transferred to another molecule, making that molecule more reactive.

The reverse reaction is a condensation: ADP + Pi + energy → ATP + H₂O. It needs energy and is catalysed by ATP synthase.

Phosphorylation is the addition of a phosphate group to a molecule. Photophosphorylation is the formation of ATP from ADP and inorganic phosphate using light energy:

ADP + Pi + light energy → ATP

It takes place in the chloroplasts. Chlorophyll absorbs light energy, and this is used to make ATP. The ATP provides the immediate energy supply for the reactions that reduce carbon dioxide to glucose.

In contrast, in respiration, ATP is made using energy released from the oxidation of glucose and other organic molecules. Plant cells use both processes: photophosphorylation provides the ATP for making glucose, and respiration provides ATP for other cell processes, including at night.

• Overall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, using light energy.
• Light splits water: oxygen is released and the hydrogen is stored in glucose.
• Photophosphorylation: ADP + Pi + light energy → ATP.
• ATP + H₂O → ADP + Pi + energy; ATP is the immediate source of energy.
• Glucose is a store of energy; ATP is the immediate supply.
• Use the ratio 6 mol O₂ to 1 mol glucose in calculations.