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825  Explain the light-dependent reactions

Page history last edited by ohmichael12@... 13 years, 2 months ago




Light Absroption

Chlorophyll absorbs light and the energy from the light raises an electron in the chlorophyll molecule to a higher energy level. The electron at a higher energy is an excited electron and the chlorophyll is photoactivated. In single down to its original level, re-emitting the energy. Chlorophyll is located in thylakoid membranes and is arranged in groups of hundred of molecules, called photosystems. There are two types of phtosystem - photosystem I and II. Excited electrons from absorption of photons of light anywhere in the photosystem are passed from molecule to molecule until they reach a special chlorophyll molecule at the reaction centre of the photosystem. This chlorophyll passes the excited electron to a chain of electron carriers.


Production of ATP

An excited electron from the reaction centre of photosytem II is passed along a chain of carriers in the thylakoid membrane. It gives up some of its energy each time that it passes from one carrier to the next. At one stage, enough neergy is released to ATP synthesis is by chemiosmosis, as in the mitochondrion. Electron flow causes a proton to be pumped across the thylakoid membrane into the fluid space inside the thylakoid. A proton gradient is created. ATP syntahse, located in the thylakoid membranes, lets the protons across the membrane down the concentration gradient and uses the energy released to synthesize ATP. The production of ATP using the energy from an excited electron from Photosystem II is called non-cyclic photophosphorylation.


Production of NADP

After releasing the energy needed to make ATP, the electron that was given away by photosystem II is accepted by photosystem I. The electron replaces one previously given away by photosystem I. With its electron replaced, photosystem I can be photoactivated by absorbing light and then give away another excited electron. This high-enerfgy electron passes along a short chain of carriers to NADP+ in the stroma. NADP+ accepts two high-energy electrons from the electron transport chain and one H+ ion from the stroma, to form NADPH.


Production of oxygen

Photosystem II needs to replace the excited ele ctrons that it gives away. The special chlorophyll molecule at the reaction centre is positively charged after giving away an electron. With the help of an enzyme at the reaction centre, water molecules in the thylakoid space are split and electrons from them are given to chlorophyll. Oxygen and H+ ions are formed as by-products. The splitting of water molecules only happens in the light, so is called photolysis. The oxygen produced in photosynthesis is all the result of photolysis of water. Oxygen is a waste product and is excreted.







Comments (1)

jeonelizabeth12@gsiscommunity.kr said

at 8:39 pm on Mar 16, 2011

Michael; I'm sorry to say but isn't this page for the assessment statement 8.2.5 'explain light-independent reaction'?
Click 4 Biology has a clear summary about this topic. Also, the diagrams http://bcs.whfreeman.com/thelifewire/content/chp08/f08013.gif http://bcs.whfreeman.com/thelifewire/content/chp08/f08014.gif are very great :)
Don't forget to include the key terms /points (the roles of ribulose bisphosphate (RuBP) carboxylase, reduction of glycerate 3-phosphate (GP) to triose phosphate (TP), NADPH + H+, ATP, regeneration of RuBP, and subsequent synthesis of more complex carbohydrates.) and organize in bullet points
Thank you :D

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