Photosynthesis Notes

The Light-Dependent Stage:The main aim of the light-dependent stage of photosynthesis is to produce the products required for the light-independent stage, such as ATP and reduced NADP. There are two types of photophosphorylation, cyclic and non-cyclic. Both take place in the thylakoid membranes. Cyclic Photophosphorylation- Cyclic Photophosphorylation only uses photosystem I. When a photon of light hits the chlorophyll molecule the energy is transferred to two electrons which become excited. These electrons pass to an electron acceptor and back to the chlorophyll molecule from which they were lost. As the electrons pass through the electron carriers, the energy within them is released which pumps protons (hydrogen ions) across the thylakoid membrane, into the thylakoid space, where these protons accumulate. This forms a proton gradient, which the protons flow down., back across the thylakoid membrane through channels associated with ATP synthase enzymes. This flow of protons is called chemiosmosis and produces a force which joins ADP and Pi together, forming ATP. Kinetic energy from the proton flow is converted to chemical energy within the ATP molecules, used in the light independent stage of photosynthesis. Alternatively the ATP is used in guard cells to move potassium ions into the cell, lowering the water potential. This causes water to move into the cells by osmosis, causing the cells to swell, opening the stomata.Non-cyclic Photophosphorylation-Non-cyclic photophosphorylation involves both photosystem I and II. The light first strikes photosystem II which excites a pair of electrons causing them to leave the chlorophyll molecule from the primary pigment reaction center. Also when the light strikes photosystem II photolysis occurs- the splitting of water molecules into protons, electrons and oxygen. The full equation for photolysis is two water molecules produce four hydrogen ions, four protons and a water molecule. The electrons pass along a chain of electron carriers and the energy released is used to synthesise ATP (as described above). The light will have also struck photosystem I and a pair of electrons will become excited and leave the chlorophyll. The electrons will join NADP (along with the protons produced by photolysis- after they've taken part in chemiosmosis) to become reduced NADP. The electrons initially lost from photosystem II replace those lost by photosystem I. The electrons produced from photolysis replace those lost by photosystem II.

The Light-Independent Stage:It is also known as the Calvin cycle and takes place in the stroma.Carbon dioxide diffuses into the leaf through the open stroma (which is opened through cyclic photophosphorylation), where it eventually reaches the stroma. Once in the stroma, the carbon dioxide binds to the ribulose bisphosphate (RuBP) and the reaction is catalysed by ribulose bisphosphate carboxylase-oxygenase (rubisco) and this forms carboxylated RuBP. This then forms two molecules of glycerate 3-phosphate (GP). GP is reduced by the reduced NADP donating its hydrogen (becoming just NADP), and phosphorlyated by by the Pi formed from the splitting of ATP, forming triose phosphate (TP). Five of every six TP molecules are recycled into three molecules of RuBP. This is then reused and combined with more carbon dioxide and the cycle continues. There are various products of the Calvin cycle. TP can produce hexose sugars such as glucose, fructose, sucrose and starch or cellulose. It can also make glycerol. GP can form amino acids and fatty acids. Combining glycerol and fatty acids can form lipids.