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Light Dependent Process

Photosynthesis is frequently described in two stages– the light dependent process, which requires light, and the light independent process, which does not require light.


The light dependent process occurs in the thylakoid membranes of the chloroplast. The process, illustrated below, begins when light strikes a molecule of chlorophyll and excites electrons to a higher energy state. Water is split to replace the electrons, releasing oxygen as a by-product. In a series of reactions within a redox chain, or electron transport chain, the energy from the electron is converted into ATP and NADPH. The ATP and NADPH are used to create carbohydrate during the light independent process.


Photosynthetic eukaryotes and cyanobacteria have two photosystems – Photosystem II and Photosystem I. Light energy causes the excitation and loss of an electron from a PSII reaction center chlorophyll (P680). Water is oxidized to replace the lost electron, generating H+ ions and oxygen (O-2) ions. These O-2 ions combine to form the diatomic O2. The excited electron is boosted to a higher energy state. Electrons are passed from a photosystem into a redox or electron transport chain, eventually attaching to a chlorophyll molecule in Photosystem I (P700).

Light acts on chlorophyll in Photosystem I, causing an electron to be boosted to a still higher potential. The electron is attached to a primary electron acceptor (different from the one associated with Photosystem II). The electron is passed again through a series of redox reactions, eventually joining NADP+ and H+ to form NADPH. Electrons flow continuously from water to NADPH. In Photosystem II, the pumping of H+ ions into the interior space of the thylakoid membranes creates a proton gradient, which is coupled to ATP production.

Animation (111 K)

Overview | Food Web | Leaf Structure | The Chloroplast | Pigments | Oxygen

Light Dependent Process | Light Independent Process

Problem Set

The Biology Project > Biochemistry > Introduction to Photosynthesis >