Living Cells

1. Living Cells Photosynthesis Mr G Davidson

2. Light To make Photosynthesis • Green plants are capable of making their own food using energy from sunlight. • This process is called photosynthesis. Photosynthesis Mr G Davidson

3. 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. Mr G Davidson

4. Xylem H2O Light Leaf Cross Section O2 CO2 CO2 O2 Mr G Davidson

5. 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. Mr G Davidson

6. 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. Mr G Davidson

7. 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. Mr G Davidson

8. Light energy Hydrogen acceptor Released as a waste product Oxygen Hydrogen Reduced hydrogen acceptor Photolysis WATER (H2O) Next stage ATP ADP + Pi Mr G Davidson

9. 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. Mr G Davidson

10. From photolysis From photolysis From the air Carbon Fixation Reduced hydrogen acceptor ATP ADP + Pi Glucose Carbon dioxide Mr G Davidson

11. 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. Mr G Davidson

12. 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 Mr G Davidson

13. 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. Mr G Davidson

14. Oxygen bubbles Dilute sodium bicarbonate (to provide carbon dioxide) Filter funnel Lamp Heat shield Elodea Light Intensity Mr G Davidson

15. 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. Mr G Davidson

16. Light Intensity And from these results we can then plot a line graph. Mr G Davidson

17. Light Intensity Mr G Davidson

18. 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. Mr G Davidson

19. 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. Mr G Davidson

20. 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. Mr G Davidson

21. Temperature Mr G Davidson

22. 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. Mr G Davidson

23. 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. Mr G Davidson

24. 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. Mr G Davidson