Biology Notes

PHOTOSYNTHESIS (90-93)

There are TWO main stages of photosynthesis:- Light-dependent stage, in which water is broken down into hydrogen and oxygen using light

energy- Light-independent stage in which the hydrogen reacts with carbon dioxide to form a

carbohydrate.

Takes place in leaf green parts chloroplasts contain all machinery needed for LDS and LIS.

Light-independent takes place Stroma

Light-dependent takes place Thylakoid membranes

Equation:6CO2 + 12H2O C6H12O6 + 6O2 + 6H20

Light-Dependent Stage: Photophosphorylation Photolysis Z-scheme

Photophosphorylation- Chlorophyll a absorbs light- Energy level of one of its electrons = raised- In this excited stage electron can leave the chlorophyll molecule and pass to another

molecule- Electrons in excited state are passed along a chain of electron carrier molecules called an

electron transport system- Each molecule along the chain wants the electron more than the previous one and so its

passed on through the chain- As electrons pass down the system in a series of redox reactions they release energy which

is used to make ATP.

Basically light energy is used to add a phosphate group to ADP.

Photolysis This occurs when water molecules are split with the aid of light energy absorbed by

chlorophyll a.- Chlorophyll a absorbs light- It then looses an electron = positively charged- In this oxidized state the chlorophyll becomes electron hungry, it needs to replace the

electron before reacting again- The enzyme extracts electrons from water to replace each electron lost from chlorophyll

a

= water molecules split= electrons, protons and oxygen producedOxygen is given up as waste gas/respirationProtons used to reduce NADP+ NADPH

Z-scheme Photophosphorylation and photolysis = not isolated events but are linked

- The Z shaped pathway (N) show the fact that the energy levels of the electrons is twice boosted by the light absorbed by the photosystems and lowered as they pass through the electron transport system

- The final acceptor of electrons = NAD+ which is reduced to NADPH which stores the electrons and protons until they can be transferred to carbon dioxide (LIS)

Photosystem 1 P700Photosystem 2 P680 (first)

Non-cyclic Photophosphorylation involves both photosystems

Cyclic Photophosphorylation involves only photosystem 1

SummaryThe light-dependent stage of photosynthesis uses energy from sunlight to make ATP and splits water to release protons and electron which reduce NADP+ to make NADPH. The ATP and NADPH are used in the next stage to synthesis carbohydrates from CO2

Light-Independent stage The Calvin CycleDuring this stage CO2 is fixed. It is taken up into a plant by being converted into biochemical products in the plant.Overall Carbon dioxide reduced to glucoseEnzymes convert the carbon dioxide to glucose in a series of reaction – CALVIN CYCLE – these reactions use ATP and NADPH which are the products of the LDS.

Calvin Cycle Cyclic chain of reactions converting CO2 glucose

- CO2 fixation involves a 5-carbon compound called ribulose biphosphate (RuBP) and an enzyme called rubisco (same as above + carboxylase)

- With the help of rubisco, RuBP combines with CO2 = unstable six-carbon compound formed which splits immediately into two molecules of a three-carbon compound; glycerate 2-phosphate (GP)

rubiscoRuBP + CO2 2GP5C 1C 2 times 3C

- This GP is then reduced to a triose phosphate called GALP. This reduction requires hydrogen from NADPH and energy from ATP.

ATP --- ADP + PiGP triose phosphate (GALP)NADPH --- NADP+

- About 1/6 of the GALP is used to make glucose (which can be converted into other carbohydrates e.g. sucrose, starch and cellulose) amino acids, fatty acids or glycerol.

- The remaining 5/6 = converted back to RuBP = requires phosphate from ATP.

SummaryBasically CO2 is converted to glucose, amino acids, fatty acids or glycerol. This conversion is explained in the Calvin cycle and uses RuBP and rubisco.

GALP glyceraldehyde 3-phosphateGP glycerate 3-phosphate