1. GCSE revision Click the hyperlink to see the movie Photosynthesis.wmv
You should take notes of the important concepts
If you do not know all that you see in the movie already, you should go back to the text books of GCSE and revise.

2. Energy Transfer
Organisms need energy to undertake the various forms of biological work including:
Movement
Active transport
Synthesis of organic compounds
Cell division
But where does this energy come from?
Autotrophic Nutrition
Heterotrophic Nutrition

3. Heterotrophic Nutrition
Heterotrophs are unable to make their own food like autotrophs and rely on the compounds produced by autotrophs
Heterotrophs obtain organic food molecules by consuming other organisms or their by-products

4. Autotrophic Nutrition
Autotrophs are ‘Producers’ and are able to synthesise organic molecules from inorganic raw materials obtained from the environment
In particular PHOTOAUTOTROPHS use light as a source of energy to synthesise carbohydrates.
This process is PHOTOSYNTHESIS
Photosynthesis nourishes almost all the living world, either directly or indirectly

5. ATP Adenosine Triphosphate ATP acts like a rechargeable battery
Adenine
Phosphate groups
Ribose
ATP acts like a rechargeable battery
Adenosine

6. ATP as an energy source ATP + H2O ADP + Pi + 30kJ
ATP breaks down to form ADP and a phosphate ion
This involves the hydrolysis of ATP and releases a large amount of energy
This energy can be used for other energy-requiring reactions
ATP + H2O ADP + Pi + 30kJ
ATP is formed by adding a 3rd phosphate ion (Pi ) to ADP
This reaction uses a large amount of energy
This energy comes from chemical reactions like those that occur in photosynthesis and respiration

8. Why is ATP better than glucose as an energy source?
The breakdown of ATP to ADP is a single step reaction whereas the breakdown of glucose to CO2 and H20 is a complex multi-step process:
Energy is instantly available with ATP
The breakdown of ATP releases small amounts of energy, ideal for fuelling the energy requirements of the body but glucose produces more energy than is required
ATP is a more efficient energy source

9. Photosynthesis Definition:
The conversion of light energy into chemical energy that is stored in glucose or other organic compounds
It involves:
Trapping of light energy to form ATP
Fixation of CO2 (ie converting it into organic compounds)
The products can be used directly by the plants or can be transferred to other organisms which feed on the plants or break down dead plant material

10. Photosynthesis Light-dependent reactions Light-independent reactions
Light energy
Water
Oxygen
Light-dependent reactions
ADP + Pi
ATP
NADP
reduced NADP
Light-independent reactions
Carbon Dioxide
Carbohydrate

11. Photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 Light energy
Used in the light independent reaction
Used in the light dependent reaction
Product of the light independent reaction
Product of the light dependent reaction

14. Leaf organization 3

15. The site of photosynthesis
Chloroplast
The site of photosynthesis

16. Chloroplast The photograph shown below details chloroplast structure
as viewed with a transmission electron microscope
Courtesy of Dr. Julian
Thorpe – EM & FACS Lab,
Biological Sciences
University Of Sussex
Chloroplast envelope visible as two membranes
Lipid
droplets
A single granum
Chloroplast molecules are
Located here for the light
dependent stage of photosynthesis
Stroma
Enzymes for the light
independent stage of
photosynthesis located here

17. Photosystems
The light harvesting units of photosynthesis, Photosystem I and Photosystem II
They are made of clusters of chlorophyll and other light harvesting pigments
There are 2 in the chloroplast located in the thylakoid membrane
Consists of an antenna complex and reaction centre

18. Light absorption Diagram of the Antenna system The absorbed energy can
Be passed onto other pigment
molecules
This is the antenna system
The organization of pigments
in a photosynthetic organism
Eventually one pigment molecule takes on all photon-absorbed energy
This molecule is the reaction center
Then it releases the energy by
Giving up an electron which can in turn reduce a suitable molecule
This is what starts the
“light” reactions

19. Chlorophyll

20. Other pigments contribute to light absorption:
Carotenes (absorb in the blue: appear orange-yellow)
Xanthophylls (absorb in the blue-green: appear reddish)
These pigments pass their energy to chlorophyll

21. Light-Dependent Reactions
Electron transport
makes NADPH + H+ and ATP
involves 2 separate reaction centers or photosystems:
photosystem II and photosystem I
Photosystem II
The chlorophyll receives electrons from water
2 H2O H+ + O electrons
This “splitting” reaction provides constant source of electrons to the antenna system in the presence of light
The electrons are passed from the reaction center to an electron transport chain which sets up a proton gradient and this drives ATP synthesis
3. Photosystem I – receives an electron from the transport chain
Here the electron is passed to an electron carrier and eventually to ferredoxin which gives the electrons to NADP+ reductase to make NADPH + H+

22. Light-dependent reactions7
ADP + Pi
reduced NADP 4
Electron carriers 3
ATP 8
electrons
Electron carriers
NADP
Phytolysis 9
Oxygen
water 6 2
electrons
electrons
Light
PS I 5
Hydrogen ions
Light
PS II 1

23. Light-Independent (Dark) Reactions
This is the Calvin Cycle
Occurs in the chloroplast stroma
Uses the ATP and NADPH + H+ from the light reactions to make carbohydrates
Phase 1:
Fixation of CO2
Phase 2:
Reduction phase
Phase 3: regeneration of RuBP
Requires ATP consumption and electrons from NADPH
Converts 3-phosphoglycerate to glyceraldehyde-3-phosphate
Some of this G3P goes into making glucose
Ribulose-1,5- bisphosphate (RuBP) has a new carbon (C from CO2) covalently attached by the enzyme “rubisco”
Requires ATP consumption and allows the cycle to continue by resupplying the CO2 acceptor

24. Light-Independent (Dark) Reactions
2 x
x 2 2
x 2
x 2 1
x 2
2 x 4 3