Main events of the Light Stage
1. Light is absorbed -
White light is composed of different wavelengths of light. Chloroplasts contain a range of pigments, including chlorophyll. Each of the chlorophyll pigments absorbs a different colour of light. Plants absorb all the colours of white light except green. This is why most plants are green.
2. Light energy is transferred to electrons -
Chloroplast pigments are arranged in clusters in the chloroplast. Each cluster consists of a variety of pigments - these include a stragetically placed chlorophyll molecule and an electron acceptor. In each cluster, the different pigments absorb light energy of different wavelengths. The function of the cluster is to absorb as much light as possible. The different pigments transfer the absorbed energy from one to another until it reaches the chlorophyll associated with the electron accpeotr. The energy is then transferred to electrons, causing it to become energised. These electrons are known as 'High-Energy' electrons. these are passed from the chlorophyll to the electron accpeot. They flow the here alone one of two different pathways.
3. Electron Flow: Pathway 1 - 'Cyclic PhotoPhosphorylation'
The high energy electrons pass from chlorophyll to the electron acceptor and along a series of electron acceptors and back to the chlorophyll molecule. All these molecules are located in the chloroplast. The electrons are recycled to the chlorophyll. When they return to the chlorophyll, they lose energy. The energy they release is trapped by ADP and a phosphate group. ADP stands for Adenosine DiPhosphate. ADP and Phosphate form ATP and Water. The addition of the phosphate to a molecule is known as phosphorylation. In this pathway, the source of energy is light. The production of ATP is known as photophosphorylation. This is a cyclic pathway because the electrons flow from the chlorophyll to the electron acceptor and back to the chlorophyll. Therefore this pathway is referred to as 'Cyclic PhotoPhosphorylation'.
4. Electron Flow: Pathway 2 - 'Non-Cyclic PhotoPhosphorylation'
Two high-energy electrons at a time are passed from chlorophyll to the electron acceptor and along a series of electron acceptors. This occurs within the chloroplast. The energised electrons do not return to the chlorophyll. Instead, they lose some energy as they pass from acceptor to acceptor. The energy they release is used to make ATP - ATP + energy + P ---> ATP + Water.
Eventually the two electrons combine with NADP+ to temporarily form NADP-:
NADP+ + 2 electrons ----> NADP-
The chlorophyll molecule is now short of electrons. It gains new electrons from the splitting of water. This reaction is called photolysis. When two water molecules are split by light, they form 4 protons, 4 electrons and oxygen (O2). Two of these electrons replace the electrons lost by the chlorophyll. The protons formed by photolysis are stored in a pool of protons in the chloroplast. These are attracted to the NADP- and combine with it to reduce it to NADPH:
NADP- + proton ---> NADPH.
As a result of this pathway, two electrons pass from a water molecules to chlorophyll. They are energised by light and flow to the electron acceptor. From here, they move along the series of electron acccpeots and are used to form NADPH. As they flow, some of their energy is used to convery ADP to ATP. In this pathway the electrons start with water and end up in NADPH. They do not recycle. This pathway is therefore referred to as 'Non-Cyclic PhotoPhosphorylation'.

	Created by jill.examtime almost 12 years ago

	Copied by daniel.praecox almost 12 years ago

Process by which plants convert solar energy to light energy