1. Cells & Photosynthesis
Section 4
Cells & Photosynthesis

2. Light
Carbon dioxide + Water Glucose + Oxygen

3. Sunlight & Starch Production
Cells in green plants make their own food in a process known as……….
photosynthesis

4. Carbohydrates Carbohydrate Carbon (C) Hydrogen (H) Oxygen (O)
Plants make glucose. Some of this is used immediately as an energy source and the rest is converted to starch for storage.
Starch is a large molecule so it cannot leave the plant cell.
Glucose and starch are carbohydrates made up of the following elements:
These are:
Carbon (C)
Hydrogen (H)
Oxygen (O)
Carbohydrate

5. Carbohydrates
You now know that glucose is converted to starch for storage
Presence of starch shows that plants have first made glucose by photosynthesis

6. How is starch made? Small, soluble glucose molecules
Large, insoluble starch molecule

7. Chloroplasts
Light energy from the sun is trapped by the green pigment chlorophyll.
Chlorophyll is found in disc-shaped structures called chloroplasts in green leaves.
chloroplasts

8. Variegated leaf White area: No chlorophyll Green area:
Contains chlorophyll
Variegated leaf

9. carbon dioxide absorbed by the air
Raw Materials
sunlight
carbon dioxide absorbed by the air
water absorbed
from soil

10. Carbon Dioxide + Water Glucose + Oxygen
Word equation for photosynthesis
Oxygen is a waste product and given off as a gas
Light energy
Carbon Dioxide + Water Glucose + Oxygen
Raw Materials
Products

11. The Two Stages of Photosynthesis
Light energy
Carbon Dioxide + Water Glucose + Oxygen
Photosynthesis occurs in two stages
Photolysis - The splitting of water
Carbon Fixation

12. Light energy from the sun trapped by chlorophyll
Light energy converted to chemical energy
Passed to second stage
Released to the air as oxygen gas

13. Carbon Dioxide + Water Glucose + Oxygen
Light energy
Carbon Dioxide + Water Glucose + Oxygen
Light energy
CO2 + H2O C6H12O6 + O2

14. Stage 1: Photolysis
Water is split up into the two elements that it is made from – hydrogen and oxygen
Oxygen is a by-product of plants and is given off as a gas
Hydrogen is carried to the second stage by a hydrogen carrier
Light energy from the sun used to build up ADP + Pi into ATP for use in the second stage
Photolysis is controlled by a series of enzymes

15. Stage 1: Photolysis Water is split into hydrogen and oxygen.
Hydrogen is carried to the second stage (carbon fixation) by a hydrogen carrier and is used in this stage.
Oxygen is a by-product that is given off as a gas.
ATP is built up using light energy. This ATP is used in carbon fixation.

16. Stage 2: Carbon Fixation
Hydrogen that is produced in photolysis combines with carbon dioxide to form the carbohydrate, glucose
Hydrogen is carried to the second stage by a hydrogen acceptor molecule
Light energy
Carbon Dioxide + Water Glucose + Oxygen
CO2 + H2O C6H12O6 + O2
Light energy

17. Stage 2: Carbon Fixation
Carbon dioxide and hydrogen are able to combine using the energy from the ATP made in the first stage photolysis
ATP
(from stage 1)
Hydrogen (from stage 1)
ADP +Pi
Stage 2: Carbon Fixation. This reaction is controlled by a series of enzymes
Carbon Dioxide
(enters by diffusion from the air)
Glucose
(C + H + O)

18. Diffusion Carbon dioxide gas diffuses into the leaf from the air
Oxygen gas that is produced inside the leaf diffuses out
Carbon dioxide
Oxygen

19. Storage of Carbohydrates
Glucose molecules can link to each other in different ways
This results in different types of carbohydrates being formed
E.g. Starch and cellulose

20. Starch How plants store excess glucose
Large and insoluble
Chains of glucose units that form are coiled up and becomes tangled to form a dense starch grain
Easily broken down to be used as a source of energy

22. Cellulose Used as building materials Forms parallel chains
Cell walls in plants
Not soluble in water
Forms parallel chains
Not easily broken down (tough fibres!)

23. Cellulose Fibres
Fibres arranged in sheets that run at right angles to each other
Criss-cross arrangement adds strength to cell wall
Small molecules are soluble, e.g. glucose, water and carbon dioxide and can pass into cell

25. Measuring rate of photosynthesis
The rate of photosynthesis can be measured in the following ways:
Measuring the increase in dry mass of a plant over a period of time
Measuring the volume of oxygen given off over a period of time
Measuring the volume of carbon dioxide taken in over a period of time

26. Measuring rate of photosynthesis with Elodea (pondweed)
Upside down test tube
(to collect oxygen gas)
Thermometer
Solution of sodium carbonate (a source of CO2)
funnel
elodea
lamp
How could the light intensity be altered?
How could the temperature be altered (and kept constant)?
How could the CO2 concentration be altered?
How can the rate of photosynthesis be measured?

27. Elodea

28. Limiting Factors
A limiting factor is a factor which slows down the rate of photosynthesis when it is in short supply. There are three limiting factors that affect the rate of photosynthesis:
Light intensity
Carbon dioxide concentration
Temperature (because photosynthesis is an enzyme-controlled reaction and enzymes are affected by temperatures)

29. Light intensity as a limiting factor
At A, low light intensity is limiting the rate of photosynthesis.
At B, light intensity is high, so another factor must be in short supply (carbon dioxide concentration or temperature). B A
Light intensity

30. Carbon dioxide concentration as a limiting factor
At A, low carbon dioxide concentration is limiting the rate of photosynthesis.
At B, carbon dioxide concentration is high, so another factor must be in short supply (light intensity or temperature). B A
Carbon dioxide concentration

31. Temperature as a limiting factor
At A, low temperature is limiting the rate of photosynthesis.
At B, temperature is high, so another factor must be in short supply (carbon dioxide concentration or light intensity). B A 20 40
Temperature (°C)

32. Limiting Factors Light Intensity
At this point, something other than light intensity is the limiting factor
As a general rule, whatever is on the x-axis is the limiting factor on the slope of the graph
At this point, light intensity is the limiting factor
Light Intensity

33. What’s the limiting factor?
At A, low ________________ is the limiting factor.
At B, ___________ or ______________ is the limiting factor. B A

34. Making the most of photosynthesis – Reducing limiting factors
By reducing the limiting factors for photosynthesis, plant growers can grow larger plants with higher yield of fruit, flowers or grain.
By covering plants with polythene tunnels, or growing them in greenhouses, the temperature is raised and the plants are protected from damaging wind and pests
If a paraffin stove is burned, this increases the temperature and has the added benefit of providing them with extra carbon dioxide.
Artificial lights provide extra light and warmth for plants