1. Chapter 10 – Photosynthesis

2. Photosynthesis
Conversion process of captured light energy to chemical energy stored in sugar and other organic materials.
Autotrophs – organisms which can sustain themselves without eating anything derived from other organisms
Heterotrophs – organisms which obtain their organic material from compounds produced by other organisms
Photoautotrophs use light

3. Chloroplasts The site of photosynthesis Primary pigment: chlorophyll
Located in: mesophyll, in each cell
Gas exchange: stomata
Double membrane
Stroma: dense fluid
Thylakoid: membranous sac, contains pigment
Grana: stacks of thylakoids
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4. Overview 6CO2 + 6H2O + light  C6H12O6 + 6O2 A REDOX reaction
Actually produce 3C compounds that are linked together to make glucose
Actually have 12 water on left and 6 water on right, but equation is simplified
Occurs in 2 stages:
Light Reactions – NADP+ reduction and photophosphorylation
Calvin Cycle – carbon fixation

6. Photosystems
A reaction center surrounded by a number of light-harvesting complexes
These complexes consist of a variety of pigment molecules
When absorbs a photon, energy is transferred to reaction center – a protein complex that contains 2 special chlorophyll a molecules and a primary electron acceptor
High energy electrons transferred from these chlorophyll a molecules to primary electron acceptor
Step 1 of light reactions

7. Photosystems
There are 2 types of photosystems: photosystem I (PS I) and photosystem II (PS II)
Each have a primary electron acceptor next to a special chlorophyll a molecules.
The reaction center of PS II is known as P680 (best at absorbing 680nm wavelength light)
PS I is called P700 (absorbs 700nm best)
Named in the order of their discovery but PS II functions first

8. Light Reactions Light driving the synthesis of NADPH and ATP animation
Two different pathways:
Noncyclic electron flow
Cyclic electron flow
Both are required for the Calvin cycle to function properly

9. Noncyclic Electron Flow

10. Noncyclic Electron Flow
Photon of light strikes pigment in harvesting complex of PS II. Excites an electron in P680.
Excited electron captured by primary electron acceptor
Enzyme splits water into 2 electrons, 2 hydrogen ions and an oxygen atom. Electrons supply P680.
Photoexcited electron passes from P680 to P700 through an electron transport chain.
Energetic fall of electron drives ATP synthesis
Meanwhile, photon of light strikes pigment in harvesting complex of PS I, excites an electron in P700. Captured by primary electron acceptor and electron replaced by electron from P680.
Photoexcited electron passes from P700 down a second electron transport chain
Enzyme NADP+ reductase transfers electrons from the final protein in the chain to reduce NADP+ to NADPH (takes 2 electrons)
movie

11. Cyclic Electron Flow

12. Cyclic Electron Flow
On occasion, the excited electrons from the P700 will not fall to NADP+, but will instead fall back through the 1st electron transport chain.
This generates additional ATP and recycles the excited electrons back to P700
Why?
Calvin cycle consumes more ATP than NADPH and the noncyclic pathway produces an equal amount of ATP to NADPH.
Could be driven by the concentration of NADPH in the chloroplast.

13. Chemiosmosis
Essentially the same process in both photosynthesis and cellular respiration
Different electron donors
Different electron acceptors
But chemiosmosis is the same.

14. Light Reactions Summary
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15. Calvin Cycle ..\ppt lectures cd\animations\10_18CalvinCycle_A.swf
Animation - experiment
animation2

16. Calvin Cycle
An anabolic pathway, building sugar from smaller molecules and consuming energy
Occurs in 3 phases:
Phase 1 – carbon fixation
Attaches CO2 one at a time to ribulose bisphosphate (RuBP) with rubisco
Creates 6 carbon compound that is unstable
Splits into 2, 3 carbon 3-phosphoglycerate molecules
Phase 2 – reduction
Phosphate from ATP added to 3-phosphoglycerate to make 1,3-phosphoglycerate
Electrons from NADPH reduces 1,3-phosphoglycerate to G3P
1 G3P molecule generated for every 3 CO2 molecules used
Phase 3 – regeneration of CO2 acceptor
With use of energy from ATP, convert 5 G3P molecules into 3 RuBP molecules

17. Review

18. Alternative Forms of Photosynthesis
When it is hot/dry, stomata close, CO2 scarce, O2 abundant, binds to rubisco, produces a 2 carbon compound instead of a 3 carbon compound, consumes ATP
So what do plants do that live in hot/dry climates?
Use an alternative form of the Calvin cycle

19. C4 Plants Common in members of the grass family
Have different anatomy – bundle-sheath cells and mesophyll cells
CO2 fixed into a 4 carbon intermediate before entering the Calvin cycle
Forces CO2 into the bundle-sheath causing rubisco to bind CO2 instead of O2

20. CAM plants CAM (crassulacean acid metabolim)
Occurs in water storing plants like cacti and pineapples.
Close stomata during day, open at night (opposite of C3 plants)
Helps prevent water loss
Store organic acids during the night, Calvin cycle occurs during the day

21. Question
Using your knowledge of photosynthesis, explain what would happen to the photosynthetic process in the following situations:
1. There is no available water
2. NADP+ is not efficiently regenerated
3. Something prevents cyclic-electron flow