Photosynthesis

Photosynthesis is a reaction in which light energy is used to break the strong hydrogen bonds in water molecules through the process of photolysis.

Light dependant reaction:

  1. Light energy from the sun hits PSII (in the thylakoid membrane) which is used to break the hydrogen bonds between water molecules to separate them, electrons are released and are excited to a higher energy level. = Photolysis

  2. Electrons are released, carried by the electron carriers from one electron carrier to the next, they get excited and pass down an electron transport chain. This process is called photo-ionisation.

  3. These electrons eventually lose energy as they move along the electron transport chain. They arrive at PSI, this process produces ATP from ADP and Pi in a process called photophosphorylation, which can be either cyclic or non-cyclic.

  4. Reduced NADP is also generated as the electrons are transferred to NADP (NADPH2) along with a proton. Both ATP and reduced NADP (NADPH2) are then used in the light-dependant stage of photosynthesis.

  5. The light energy splits water molecules into protons (H+), oxygen (waste) and electrons. The electrons are used to replace those lost from chlorophyll in the first step through photolysis. Protons get pumped across the membrane using the ATP created in the previous step in a process known as chemiosmosis, creating a chemical potential gradient.

  6. ATP is synthesised. The proton passes back through the membrane with an ATP synthase enzyme, creating a molecule of ATP. It takes around 4 protons to make 1 ATP molecule.


These electron transport chains play an important role in ATP synthesis.

Chlorphyll is found attached to the thylakoid membrane. Chlorophyll is mostly made up of chlorophyll a (most abundant), chlorophyll b and carotenoids (xanthophylls, carotenes).

Carotenoids absorb light energy for use in photosynthesis, they also provide photoprotection. They are used as accessory pigments to chlorophyll in the light-harvesting part of photosynthesis. Carotenoids also participate in cell signalling to signal the production of abscisic acid (a plant hormone) that stimulates plant growth, germination and cell division.

Carotenes make up the orange colour in carrots, they absorb blue light and reflect the longer yellow, red wavelengths . They also absorb light energy for photosynthesis.

Xanthophylls are yellow pigments that absorb blue light