PHOTOSYNTHESIS Photosynthesis changes light energy into chemical energy. Plants can then use the energy in metabolism. Because animals feed on plants, the chemical energy is also passed to them. Thus photosynthesis provides the world with energy. The chemical energy is trapped in the chemical bonds inside molecules of glucose. These are made by joining together molecules of carbon dioxide and water and releasing oxygen gas. 6CO2 + 6H2O + light C6H12O6 Role of Photosynthesis 1. makes food for plants 2. makes food for animals 3. forms oxygen for plants and animals 4. formed fossil fuels Location of Photosynthesis Photosynthesis depends on chlorophyll, a green pigment. In most cells chlorophyll is found in structures called chloroplasts and so photosynthesis takes places in the chloroplast. Chloroplasts are heavily concentrated in cells (called palisade cells) in upper surfaces of leaves. They are sometimes also present in the green stems of some plants. The chlorophyll captures light energy and converts it into a flow of electrons. Instead of the electrons flowing along wires, they flow from chemical to chemical, called electron carriers. As the electron carriers pass the electrons from one to another, their energy is harnessed to cause a series of chemical changes, which result in carbon dioxide and water being converted to glucose and oxygen. What happens to the glucose? The glucose is converted into starch and stored fro later use; or it can be moved to another part of the plant to give it energy (respiration); or it can be changed to lipid or to protein. It can also be used to make cellulose for new cell walls for growth. Energy Carriers involved in Photosynthesis NADP+: is a low energy molecule, it can combine with 2 high energy electrons and a single proton (also called a hydrogen ion of H+) to form NADPH. NADP+ + 2 e- + H+ NADPH NADPH: an energy rich molecule used to carry electrons and hydrogen ions in photosynthesis. When NADPH breaks down, it releases high energy electrons and hydrogen. NADPH NADP+ + 2e- + H+ ADP: Adensosine diphosphate is a low energy molecule. It is made up of a molecule called adenine that is attached to a carbohydrate called ribose. These 2 molecules form adenosine which is attached to 2 phosphate molecules. Adenine Ribose P P The bond between the 2 phosphates is an unstable bond and if energy is added to ADP in the form of another unstable phosphate bone, ATP will be formed. ATP: Adenosine triphosphate is an energy rich molecule. It is the molecule most often used by cells to provide energy for cell reactions. Adenine Ribose P P P Active cells break down millions of ATP every second. The bond between the last 2 phosphates in ATP contains the energy that is released to allow other cell reactions to take place. The Process of Photosynthesis Photosynthesis takes place in 2 stages: the Light Stage and the Dark stage. Both stages take place in the chloroplast but the light stage requires light and can only take place in the light, whereas the dark stage does not require light and can take place in light or dark. The dark stage however, uses molecules that are made in the light stage and so the dark stage requires the light stage to have taken place. The Light Stage Takes place in the grana of the chloroplasts. Light is absorbed by chlorophyll molecules and some of its electrons are excited to higher energy levels. The excited electrons are picked up by an electron acceptor (a specially placed chlorophyll molecule) that sends them through electron carriers in either of 2 pathways. Pathway 1. Cyclic Photophosphorylation • The electrons from chlorophyll flow along a sequence of electron acceptors. The electrons return or are recycled back to the chlorophyll molecule. The electrons lose energy as they move back along the pathway. • This energy is used to convert ADP to ATP: ADP + energy + P ATP + water Pathway 2. Non-cyclic Photophosphorylation A pair of electrons leaves the specially placed chlorophyll molecule (the electron acceptor) and passes along a sequence of electron carriers. At one point the pair of electrons joins with NADP+ to form NADP- NADP+ + 2 electrons NADP- The chlorophyll molecule is now short of 2 electrons but will regain electrons when water is spilt using light energy. 2H2O 4H+ + 4 electrons + O2 Hydrogen ions (H+ or protons) formed when the water is split are stored in the chloroplasts. When necessary, one of them combines with NADP- to form NADPH. NADP- + H+ NADPH AS the electrons flow from acceptor to acceptor, they lose energy. Some of this energy is used to convert ADP to ATP. ADP + energy + P ATP + water The End Products 1. ATP is made in both pathways 1 and 2 and is used to provide the energy required for the dark stage. 2. NADPH is made in pathway 2 and this will supply protons and high energy electrons for the dark stage. 3. Oxygen is released when water is split. Most of the oxygen passes out of the leaf into the air and some of it is used in respiration. The Dark Stage The dark stage takes place in the stroma, the enzyme containing liquid around the grana. This stage is not directly affected by light, but because enzymes control it, its rate is directly affected by temperature. Its rate is also affected by the concentration of carbon dioxide in the atmosphere. The dark stage uses NADPH to supply H atoms and joins them to CO2 to produce glucose, C6H12O6. This process uses the energy of ATP. The NADP+, the ADP and P return to the light phase to reform NADPH and ATP for the dark phase. Events: • Hydrogen ions (H+) released from NADPH combine with carbon dioxide to from glucose. The addition of hydrogen to a molecule is called reduction. Carbon dioxide is said to be reduced to form glucose. • The energy for this is supplied by the breakdown of ATP to form ADP and phosphate. ATP + water ADP + P NADPH NADP+ CO2 C6H1206 The End Products 1. Glucose 2. ADP 3. Phosphate 4. NADP+ - Glucose may be used to supply energy or it may be converted into starch for storage, cellulose for cell walls or a range of other substances needed by the plant. - ADP, phosphate and NADP+ are reused in the light phase to form ATP and NADPH. The Application of Photosynthesis The method of growing crops in greenhouses can be promoted or increased by 2 methods; 1. Artificial light can be used to increase the intensity of the light and/or to increase the length of time the plant is exposed to light. 2. The air in the greenhouse can be enriched with carbon dioxide. This can be produces by burning or can be released from cylinders. Mandatory Experiment One of the following is expected.