University:
Cambridge College
Course:
SCI 225W | Foundations in Biology
Academic year:

2023
Views:

111

Pages:

1
Author:

M Theodore

Light reactions of photosynthesis The light reaction occurs in the thylakoid membrane, specifically in the chloroplasts. The light collecting unit in the thylakoid membrane is called a photosystem. There are two types of photosystems found in the thylakoid membrane, namely photosystem I and photosystem II. The light reaction occurs when photosystem I and photosystem II are exposed to sunlight. The light reaction is divided into 2 stages, namely: 1) Cyclic photophosphorylation Taking place in photosystem I, in photosystem I there is chlorophyll a which is sensitive to a wavelength of 700 nm so it is called p700. Light that hits chlorophyll will cause the chlorophyll to be activated, thereby releasing its electrons. The electrons released by chlorophyll will be transferred from one enzyme to another, and some of the energy will be absorbed by ADP to bind phosphate to form ATP. 2) Noncyclic photophosphorylation Photosystem I when exposed to sunlight will release its electrons, which then immediately follow the electron transfer chain. Some of the energy in this electron transfer reaction is used to form ATP from ADP. Simultaneously with this event, water molecules are broken down into O2 , hydrogen ions and electrons. NADP will take electrons from photosystem I to bind hydrogen ions to form NADPH.Photosystem I which has lost its electrons will immediately absorb electrons from photosystem II (p680) which is exposed to light. Photosystem II which lacks electrons will immediately take the electrons produced by the decomposition of water. The ATP and NADPH produced in the light reaction will be used to form glucose in the dark reaction, while the O2 produced will immediately be released as a by-product of photosynthesis.

Light Reactions of Photosynthesis

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