Describe the process of light-dependent reactions in photosynthesis.

Light-dependent reactions in photosynthesis convert light energy into chemical energy, producing ATP and NADPH.

In more detail, light-dependent reactions are the first stage of photosynthesis, occurring in the thylakoid membranes of the chloroplasts in plant cells. These reactions are dependent on light, hence the name, and they convert light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).

The process begins when light photons are absorbed by the pigment molecules (chlorophyll and carotenoids) in the photosystems (Photosystem II and Photosystem I) located in the thylakoid membranes. The absorption of light energy excites the electrons in the pigment molecules, raising them to a higher energy level. This is the photoactivation stage.

In Photosystem II, the excited electrons are passed along an electron transport chain, losing energy as they go. This energy is used to pump hydrogen ions (protons) from the stroma into the thylakoid space, creating a proton gradient. The loss of electrons from Photosystem II is compensated by the splitting of water molecules, a process known as photolysis, which releases electrons, protons and oxygen.

The excited electrons from Photosystem I are used to reduce NADP+ to NADPH. The proton gradient created earlier drives the synthesis of ATP from ADP and inorganic phosphate via ATP synthase, a process known as photophosphorylation.

In summary, light-dependent reactions harness light energy to produce ATP and NADPH, which are essential for the subsequent light-independent reactions (Calvin cycle) where carbon dioxide is fixed into glucose. The by-product of these reactions, oxygen, is released into the atmosphere.