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Light dependent and independent reactions (AQA A2 Biology)

Mark Gavartin

on 28 September 2012

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Transcript of Photosynthesis

Light dependent reaction Photosynthesis Cyclic vs. Acyclic Election Flow Photosynthesis has used: H2O, NADP+ and ADP +Pi So far... Ribulose Biphosphate (5C)
RuBP The Calvin Cycle Cyclic flow In cyclic flow, the high-energy electron pair that is released from the reaction centre chlorophyll travels along the E.T.C and the energy that is lost is used to pump 2 hydrogen ions into the thylakoid space. Those electrons are then 'recycled'. This process involves only Photosystem I. Photoexcitation Photons entering the light harvesting centre strike molecules of
pigments such as chlorophyll. This raises a pair of electrons in energy that are released and strike other pigment molecules in the antenna complex. This chain reaction continues until the electrons reach the reaction centre chlorophyll, where a pair of high energy electrons are released onto the E.T.C. Acyclic flow In acyclic flow, the photons strike not only PSI, but also PSII. As a result of the photoexcitation in PSII, photolysis of water takes place. An electron pair from the water is sent into the E.T.C, while the hydrogen ions are left in the thylakoid space. The oxygen diffuses across the membrane into the stroma. The electrons travelling along the E.T.C can then enter into cyclic flow, or continue to NADPH + H+ synthesis. ATP synthesis Part of the function of the light dependent reaction is to synthesise ATP for use in the light independent reaction. This is done via chemiosmosis. The H+
ions that are pumped into the thylakoid space
are released in a controlled manner, through
an ATP synthase enzyme. and has created: O2, NADPH + H+ and ATP Onwards to the light independent reaction! Light independent reaction 6 carbon compound CO2 2x Glycerate Phosphate (2x3C)
GP ATP ADP + Pi NADPH+H+ NADP+ 2x Triose Phosphate (2x3C)
TP or GALP 5 carbon compound
Ribulose Phosphate (5C) Carbon atom into glucose
(1C) ATP ADP The Calvin Cycle then continues, building glucose molecules as it does so. Summarising photosynthesis Light dependent: 6H2O 6O2 Depends on light intensity Does not depend on CO2 concentration Does not depend on temperature - not enzyme controlled Light independent: 6CO2 C6H12O6 Does not depend on light intensity Depends on CO2 concentration Depends on temperature - enzyme controlled This process is called carbon fixation. It is a reduction reaction, which in this case involves the addition of hydrogen. In addition, it is an anabolic (synthesis) reaction, and so requires ATP. So, photosynthesis... ... is the origin of all energy in the ecosystem
... is the process of converting light energy to
chemical energy using chlorophyll
... can be summarised by the equation: 6CO2 + 6H2O C6H12O6 + 6O2 Takes place in the thylakoid space and membrane Takes place in the stroma So now... Photosynthesis has used: CO2, NADPH + H+ and ATP and has created CHO (carbohydrate), NADP+ and ADP + Pi
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