Sunday, March 20, 2011

Photosynthesis: Light Reactions

What is it?
Photosynthesis is the process by which autotrophs convert light energy into potential chemical energy which can then be used by both themselves and heterotrophs.

Where does it take place?
Photosynthesis occurs in the chloroplasts of plants which are usually located in the mesophyll tissues of their leaves.
To the right is an image of a chloroplast. Refer to it as we walk through the entire process.
Chlorophyll is the green pigment that absorbs the light energy.

Below is the electromagnetic spectrum. Visible light (from 380-750) is what powers photosynthesis. More specifically, the pigments responsible for capturing light absorb violet, blue and red light best. Green is reflected.

Embedded in the thylakoid membrane are numerous photosystems. These consist of light harvesting complexes containing chlorophyll-b,and a reaction-center complex containing 2 chlorophyll-a's and an electron acceptor.
When light is absorbed by electrons in the chlorophyll are excited. The energy is transferred from chlorophyll-b molecules to the chlorophyll-a molecule from which the excited electron can be ejected and captured by the electron acceptor.

In photosystem 2, the electron that is lost from the a-chlorophyll is replaced by an electron obtained from the splitting of water into oxygen and hydrogen ions. The hydrogen ions remain in the lumen and contribute to the charge buildup.

From the primary electron acceptor, the electron proceeds through a transport chain consisting of plastoquinone, a cytosome complex, and plastocyanin. In this process, more H+ are pumped into the lumen allowing for ATP synthesis using an ATP synthase structure similar to the one used in cellular respiration.
Note: 4H+ yield 1ATP

At the end, this electron is passed to photosystem 1 where it replaces an excited electron that has been used to create NADPH.

This process is summarized well in the diagram below.

Net ATP: 24
Location: Thylakoid Membrane
Reactants : 48 Photons + 12 water + 24ADP + 12NADP+
Products: 6O2 + 24ATP + 12NADPH

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