Biology EDEXCEL

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Photosynthesis

Light-dependent reaction 1. Light energy is absorbed by PSII which excited e- in the chlorophyll moving them to a higher energy level

2. The e- then move along the electron transport chain to PSI so need to be replaced

3. Light energy splits water into protons (H+ ions), electrons and oxygen: H2O ----> 2H+ 1/2O via The e- 2

4. Excited e- lose energy as they move along the electron transport chain which is used to transport protons into the thylakoid so it has a higher conc. of protons than the stroma forming a proton gradient across the membrane.

5. Protons move down the conc. gradient into the stroma via an enzyme called ATP synthase producing energy which is used to combine an inorganic phosphate and ADP to form ATP.

6. Light energy is absorbed by PSI which excited e- again to an even higher energy level.

7. The e- are finally transferred to NADP along with a proton (H+ ion) from the stroma to form reduced NADP

light-independent reaction 1. CO2 enters leaf via stromata and diffuses into the stroma

2. It then combines with RuBP (5C), catalysed by RUBISCO this gives an unstable 6C compound which quickly breaks down into 2 molecules of a 3C compound called GP

3. ATP from the light-dependent reaction provides energy to reduce the 3C compound, GP to a different 3C compound called GALP

4. This reduction reaction also requires H+ ions coming from reduced NADP which is recycled to NADP

5. 2/10 GALP is then converted into useful organic compounds e.g. glucose

6. 10/12 GALP produced are used to regenerate RuBP

7. Regenerating RuBP uses the rest of ATP produced by the light-independant reaction

Cyclic photophosphorylation This only uses PSI and the e- from the chlorophyll molecule aren't passed onto NADP but are passed back to PSI via electron carriers so e- are recycled and can repeatedly flow through PSI. Which only produces small amounts of ATP.

GP & GALP ARE CONVERTED INTO... GP & GALP are used to make carbohydrates, lipids, proteins and nucleic acids:

Carbohydrates: simple sugars (e.g. glucose) made by joining 4 GALP molecules together and polysaccharides (e.g. starch and cellulose) made by joined hexose sugars together in different ways

Lipids: made using glycerol which is synthesised from GALP and fatty acids which are synthesised from GP

Amino acids: some made from GP

Nucleic acids: sugar in RNA made using GALP

Calvin cycle needs to turn 6 times to make 1 glucose molecule 1. 3 turns of the cycle=6 molecules 12 molecules of GALP because 4 molecules of GALP are made for every CO2 molecule used.

2. 10 out of 12 GALP molecules are used to regenerate RuBP.

3. 3 turns of the cycle only produce 2 GALP that's used to make glucose.

4. Glucose has 6 carbons though so 4 GALP molecules are needed to form 1 glucose molecule.

5. Thus the cycle has to turn 6 times to produce 4 molecules of GALP that can be used to make 1 glucose molecule.

6. 6 turns of the cycle need 18 ATP and 12 reduced NADP from the light-dependant reaction.

7. This makes sure there's always enough RuBP to combine with CO2 taken in from the atmosphere.

how chloroplast structure is adapted for photosynthesis 1. Chloroplast envelope keeps the reactants for photosynthesis close to their reaction sites.

2. The thylakoids have a large surface area to allow as much light energy to be absorbed as possible

3. Lots of ATP synthase molecules are present in the thylakoid membranes to produce ATP in the light dependent reaction

4. Stroma contains all the enzymes, sugars and organic acids for the light-independent reaction to take place

COENZYMES 1. Photosynthesis involves coenzymes that aid the function of an enzyme by transferring a chemical group from one molecule to another

2. The one used in photosynthesis is NADP which transfers hydrogen - thus can reduce or oxidise

Biology EDEXCEL

Biology A2