Living Cells Photosynthesis Mr G Davidson

Photosynthesis Photosynthesis Green plants are capable of making their own food using energy from sunlight. This process is called photosynthesis. Photosynthesis Light To make Tuesday, December 04, 2018 Mr G Davidson

Photosynthesis Light is trapped by a green pigment called chlorophyll. Chlorophyll is found inside the chloroplasts, which are disc shaped organelles found in the leaf cells. Light is used to combine water and carbon dioxide to make a carbohydrate called glucose, with oxygen being released as a waste product. Tuesday, December 04, 2018 Mr G Davidson

Leaf Cross Section Xylem O2 H2O CO2 CO2 O2 Light Tuesday, December 04, 2018 Mr G Davidson

Chemistry of Photosynthesis Photosynthesis is a series of enzyme-controlled reactions. Photosynthesis occurs in 2 stages: Photolysis which is a light dependent stage. Carbon fixation which is a temperature dependent stage. Tuesday, December 04, 2018 Mr G Davidson

Photolysis It is in photolysis that light energy is trapped by the chlorophyll and converted into chemical energy. Light energy is used to split water molecules into hydrogen and oxygen. The oxygen is released as a waste product. Tuesday, December 04, 2018 Mr G Davidson

Photolysis The hydrogen joins with a hydrogen acceptor and this “reduces” it. The reduced hydrogen acceptor carries the hydrogen to the carbon fixation stage. ATP is also made during photolysis and this is called PHOTOPHOSPHORYLATION. This ATP will also be used in the next stage. Tuesday, December 04, 2018 Mr G Davidson

Photolysis WATER (H2O) Oxygen Hydrogen Light energy Next stage acceptor Oxygen Hydrogen Released as a waste product Reduced hydrogen acceptor Next stage ADP + Pi ATP Tuesday, December 04, 2018 Mr G Davidson

Carbon Fixation Carbon fixation is the 2nd stage of photosynthesis and also occurs in the chloroplasts. It is a series of enzyme controlled reactions using the hydrogen and ATP from photolysis, and carbon dioxide from the air. The carbon dioxide and the hydrogen are combined to produce glucose. Tuesday, December 04, 2018 Mr G Davidson

Carbon Fixation From From photolysis photolysis Glucose Carbon dioxide Reduced hydrogen acceptor ATP ADP + Pi Glucose Carbon dioxide From the air Tuesday, December 04, 2018 Mr G Davidson

Photosynthesis The glucose made in photosynthesis can be used by the plant in a number of ways. It can be used by the plant as an immediate energy source for such things as reproduction, growth, etc.. It can be stored in plants as a complex carbohydrate called starch and then converted back to glucose as and when it is required. It can be used by the plant as a structural carbohydrate called cellulose which is used to make cell walls. Tuesday, December 04, 2018 Mr G Davidson

Factors affecting photosynthesis There are a number of factors which can affect the rate at which a plant can carry out photosynthesis, and these are called limiting factors. They include: Light intensity Temperature Carbon dioxide concentration Tuesday, December 04, 2018 Mr G Davidson

Light intensity In dim conditions, plants will photosynthesise more slowly than in bright conditions. This is because the more light there is, the more energy is available for photosynthesis to happen. We can see this using a pondweed called Elodea in the following investigation. Tuesday, December 04, 2018 Mr G Davidson

Light Intensity Oxygen bubbles Dilute sodium bicarbonate (to provide carbon dioxide) Filter funnel Lamp Heat shield Elodea Tuesday, December 04, 2018 Mr G Davidson

Light Intensity The lamp is moved a certain distance away from the elodea, and the number of bubbles released per minute is counted. The lamp can then be moved farther away and the bubbles counted again. The results would look something like the following. Tuesday, December 04, 2018 Mr G Davidson

Light Intensity And from these results we can then plot a line graph. Tuesday, December 04, 2018 Mr G Davidson

Light Intensity Tuesday, December 04, 2018 Mr G Davidson

Light Intensity As we can see from the graph, the line levels off indicating that further increases in light intensity will not increase the rate of photosynthesis. Some other limiting factor must be taking effect at this stage. Tuesday, December 04, 2018 Mr G Davidson

Carbon Dioxide Concentration This investigation also uses the Elodea bubbler apparatus. This time the lamp is kept in the same place and the mass of sodium bicarbonate in the water is gradually increased. The results would produce a similar line graph, i.e. it would increase then level off. Tuesday, December 04, 2018 Mr G Davidson

Temperature Temperature is also a limiting factor for photosynthesis. However, the line graph is not the same if we keep increasing the temperature because the enzymes are eventually denatured by excessive heat. This means the graph would look like the following. Tuesday, December 04, 2018 Mr G Davidson

Temperature Tuesday, December 04, 2018 Mr G Davidson

Temperature We can see from the graph that as the temperature increases, so does the rate of photosynthesis. This happens until it reaches the optimum temperature, when photosynthesis cannot happen any quicker. After this, the rate of photosynthesis slows down as the enzymes which control it are denatured. Tuesday, December 04, 2018 Mr G Davidson

Horticulture If we can reduce the limiting factors in photosynthesis, then we may be able to increase the yield of plants. Obviously this can’t be done by a farmer in his fields, but it can be done in controlled environments such as greenhouses. Tuesday, December 04, 2018 Mr G Davidson

Horticulture Extra lighting can be supplied to greenhouses to extend the number of daylight hours. The temperature in winter and spring is often increased using paraffin heaters, and these have the added bonus of releasing extra carbon dioxide which may also be limiting photosynthesis. This also allows, not only an increased yield, but earlier crops. Tuesday, December 04, 2018 Mr G Davidson