Light Dependent Reactions

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Role of Pigments in Photosynthesis

Pigments: Pigments are substances that absorb light at particular wavelengths and generally reflect light at other wavelengths. Pigments are important for photosynthesis because in order to convert light energy to chemical energy, the organism must first absorb light energy with the pigments invlved in photosynthesis. When light is absorbed its energy is taken up by some of the electrons in the outer most energy level of the pigment molecules. The electrons jump up to a higher energy level. This is very similar to what happens in fluorecence, only in fluorecence, the electrons radiate the stored energy as light and drop back to their original energy level. Iin photosynthesis the energy stored by these electrons can harvested in the next steps of photosynthesis.

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Light and Color.

One elementary concept is the relationship between the visable spectrum and the colors you see. White light is a mixture of all wavelengths of visable light. Certain wavelengths of light are absorbed by the leaf while others are reflected. When the reflected light reaches your eye, the eye and brain interpret the mixture of reflected wavelengths as color.

We discuss and measure light in terms of wavelength rather than color because we can objectively measure wavelength. Color in subjective and not every one perceives color in the same way. For example some people are unable to distinguish between red and green, seeing both as shades of grey.

In basic biology laboratories a standard exercise involves using an instrument called a spectrophotometer to determine the percentage of light at particular wavelenghts of light are absorbed or transmitted(reflected) from pigments extracted from plants or other materials.

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Basic processes in Light Reactions:

Capture of Light Energy by Photosystems

Light energy represented by the arrow on the left hand side of the panel is absorbed by electrons in the photosystem. There are two photosystems involved in most photosynthesis. Photosystem II (Hexagonal shape) is shown here. When the elecrons gain energy from the light, they are boosted up to a high energy level where they can be harvested, the extra energy used to do work. In plants there is also another photosystem called photosystem I

Photosystem: A photosystem is a molecular assembly consisting of several hundred pigment molecules and associated proteins. Photosystems are an adaptation involved in the absorbtion of light energy and the production of high energy electrons in photosynthesis. Each photosystem consists of the following parts:

Sometimes the reaction center in photosystem II is designated P680, which stands for the pigment that has its maximum light absorbance at 680nm. The reaction center in photosystem I is often called P700 since its pigment absorbs best at 700nm.

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Transfer of Electrons and Hydrogen Ions to Electron Acceptor.

Photosynthesis involves a series of specialised nucleotide and protens which serve to capture high energy electrons from the photosystems. Typically these electron acceptors (yellow sphere) also carry hydrogen ions along with the electrons.

 

 

 

 

 

 

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Transfer of High Energy Electrons to Electron Transport Systems.

The electron acceptors typically pass the electrons to a series of specialised electron transport systems(ETS) which use energy in electrons to do work.

Two types of ETS: In plants there are two types of ETS.

The ETS receiving electrons and hydrogen ions from photosysterm II uses the energy from the electrons to make ATP by a process called electron transport phosphorylation. Electron transport phoshorylation also happens in aerobic cellular respiration as discussed on this page.

The ETS receiving electrons from photosystem I transfers the electrons and hydrogen ions to an electron acceptor called NADP+ yielding a molecule called NADPH.

Note that NADPH is a high energy molecule.

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Photolysis:

Photolysis refers to the use of light energy to break water molecules or some other electron donor, to obtain electrons for photosynthesis. The function of these electrons is to replace electrons lost by the photosystems to the electron transport systems. Photolysis involves a poorly understood proteins associated with photosystem II. Typically in photolysis water is the electron donor and it is split to yield hydrogen ions, oxygen and electrons. The situation is a bit similar to what happens in solar cells only there the electrons are recycled in the electrical circuit which returns electrons to the solar cell.

Photolysis is the source of most oxygen gas found in the earth's atmosphere. While oxygen is important for aerobic respiration, it is a waste product of photosyhtesis!

 

 

 

Summary diagragm for the light dependent reactions:

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pgd Created: 07/05/02