1. Plants making their own food
Photosynthesis
Plants making their own food

2. Plants actually grow by making their own food, not by eating soil!
How do plants grow?
Plants actually grow by making their own food, not by eating soil!

5. Balanced Equation for Photosynthesis
6CO2 + 6H2O + light +Chlorophyll C6H 12O6 + 6O2

6. Sources of light for plants
Sunlight is the natural source of light for plants
plants can use artificial light for photosynthesis and this is often used in greenhouses stimulate growth 6

7. Chlorophyll chlorophyll
The green pigment chlorophyll is found in chloroplasts.

8. Sources of carbon dioxide
From the atmosphere through stoma in their leaves
From their own cellular respiration
Artificial sources of carbon dioxide eg. burning gas in a green house 8

9. Sources of water for plants
Water is absorbed from the soil by the roots of plants
This water passes up the stem via xylem cells 9

10. By the end of today’s class you should be able to:
Explain the main events in photosynthesis

11. The main stages in Photosynthesis
Light is absorbed
Water is split
Products are produced (4 protons, 4 electrons and oxygen)
Light energises electrons
Glucose is formed

12. 1. Light is Absorbed
Energy from the sun is absorbed by chlorophyll in chloroplasts.

13. 2. Water is Split
The trapped light energy is used to split water into oxygen gas (O2) protons (H+) and electrons (e-)
Summarised as
2H2O H+ and 4e- and O2

14. 3. The products of water splitting seperate
The electrons are passed to chlorophyll
The protons are stored in a proton pool for later use
The oxygen may pass out of the leaf into the atmosphere or else may be used for plant respiration

15. Light Energises Electrons
The electrons that were passed to the chlorophyll gain light energy and this changes them into high energy electrons

16. Glucose is formed
The high energy electrons along with protons from the proton pool are combined with carbon dioxide to form glucose (C6H 12O6 )

17. Learning Check What are the main stages in photosynthesis?
What is water split into?
What happens to each of these products?
What is the trapped light energy used for?

18. Mandatory experiment Investigating the effect of the
Investigate the effect of light intensity on the rate of photosynthesis.
Mandatory experiment
Investigating the effect of the
Light could heat up water so be careful of this .
Cip plant down with a paperclip.
For light source at 100cm away from Elodea: Allow 5 minutes for plant to stabilise, then count air bubbles in one minute, Repeat for 2 different minutes to get the average.
Repeat for plant at 80cm, 60cm, 40cm,20cm, from plant.

19. (No. of bubbles/ minute)
Table of results
Distance [d] from
light source
(cm)
Light Intensity or
1/d2
(No. of bubbles/ minute)
Average
Trial 1
Trial 2
Trial 3

20. Graph of results
With increasing light intensity photosynthesis will increase up to a point where photosynthesis will not increase any more (called the saturation point)
It does not increase any further as there is something else which the plant needs – maybe more Carbon dioxide or water!!

21. Photosynthesis Extended Study (Higher Level only)

22. Light Stage (HL)
It involves the very fast movement of electrons and is not controlled by enzymes
Light is absorbed by pigment molecules, which pass light to the chlorophyll (almost all colours of light are absorbed but green is normally reflected)

23. Light Stage (HL)
Chlorophyll absorbs as much light energy as possible and passes which it passes to an electron
The energised electron is passed to the electron acceptor which can the send it on one of two pathways

24. Electron Flow Pathway 1 (HL)
In pathway 1 the electrons pass from the first electron acceptor to a series of other electron acceptors and back again to the chlorophyll
As the electrons are passed around they lose energy
This energy is used to join a phosphate to ADP to form high energy ATP (Water is also formed in this process)

25. Electron Pathway 1 (HL) ADP + Energy + P ATP + Water
The addition of phosphate to ADP is called phosphorylation
Because the electron travel in a cycle and returns to its original chlorophyll this process is called Cyclic Electron Transport or Cyclic Electron Phosphorylation

26. Electron Pathway 2 (HL)
2 high energy electrons are passed from chlorophyll to the electron acceptor
In this case the electrons do not return to the original chlorophyll
Eventually the 2 electrons are passed to combine with NADP+ to form NADP-

27. Electron Pathway 2 (HL) The splitting of water happens (Photolysis)
The electrons pass along electron tranport chain and release energy, which is used to make ATP
Oxygen is released from the plant or used in respiration
The protons that were stored in the proton pool are attracted to NADP- and combine with it to form NADPH
Known as Non cyclic electron transport

28. End Products of the Light Stage (HL)
There are 3 end products of the light stage
ATP this will provide energy for the dark stage
NADPH this will provide protons + energised electrons for the dark stage
Oxygen is made when water is split

29. Dark Stage (HL)
This may also be called the light independent stage as it can occur in the light but does not need to use it
It is controlled by enzymes and therefore can be affected by temperature

30. Dark Stage (HL)
Carbon dioxide from the air enters the chloroplast where they combine with protons + electrons to form glucose
This needs energy and protons + electrons

31. Dark Stage (HL) It uses energy got from breaking down ATP into ADP + P
It uses hydrogen ions and electrons got from breaking down
NADPH into NADP+ + 2 electrons + H+

32. Dark Stage
ADP + P and also NADP+ return to the light phase where they an be reused.