1. Chapter 3 - Photosynthesis: The Details

2. Photosynthesis Using the sun to make useful forms of energy
Sunlight plays a much larger role in our sustenance than we may expect: all the food we eat and all the fossil fuel we use is a product of photosynthesis, which is the process that converts energy in sunlight to chemical forms of energy that can be used by biological systems

3. How does this occur
Various forms of radiation surround us, from the sun and other sources.
Some are visible and some are invisible.

4. Wave model of light Electromagnetic radiation travels at 300000000m/s
Frequencies of visible radiation (light) are perceived as different colours
We can remember the visible spectrum with ROYGBIV!

5. Frequencies of visible radiation (light) are perceived as different colours.
Highest frequency, smallest wavelength = violet
Lowest frequency, largest wavelength = red
All frequencies and wavelengths = white

6. Properties of Light

7. Photon model of light Light travels in energy packets called photons
Photons travel at m/s
The amount of energy in a photon depends on the frequency of the light. The higher the frequency of light, the more energy the photon carries
Light can transmitted reflected or absorbed
Air, water ….mirror…plants

8. How Does a Plant Capture Light?
Light can be
transmitted (light passes through an object.
Reflected (light bounces off object)
Absorbed (light goes into object)

9. How Does a Plant Capture Light?
Plants have chlorophyll PIGMENTS (molecules that can absorb specific wavelengths of light)
Plant leaves appear green. Therefore, what colours must the chlorophyll pigments absorb? reflect?
Everything but Green
GREEN

10. Chlorophyll Pigments There are 2 types of cholorophyll Yellow - green
Blue – green
Found in the highly folded plant organelle

11. Chlorophyll Absorption
Whatever chlorophyll a does not absorb, chlorophyll b will absorb – they are good complements to each other.
As you can see, the region between nm is not absorbed very well, thus plants appear green.
As the amount of chlorophyll in autumn decays, the green colour fades and is replaced with oranges and reds of carotenoids (other plant pigments).

12. Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells.

13. Photosynthesis in plants
Light energy is used to transform carbon dioxide and water to energy rich food molecules composed of glucose monomers
There are 2 stages in this process

14. Photosynthesis The Light Reactions The Calvin Cycle
Divided into two steps:
The Light Reactions
Noncyclic electron flow
The Calvin Cycle
Cyclic electron flow

15. The Light Reactions Photoexcitation Electron Transport Chain
Divided into three steps:
Photoexcitation
Electron Transport Chain
3. Chemiosmosis

16. The Light Reactions
Photosystems are embedded in the thylakoid membrane.
They contain chlorophyll and accessory pigments that are associated with proteins.
A photosystem consists of an antenna complex and a reaction centre.

17. The Light Reactions
The antenna complex absorbs a photon and transfers energy to the reaction centre.
The reaction centre contains chlorophyll a, whose electrons absorb energy and begin photosynthesis.

19. The Light Reactions Photosystem II (P680)
Two photons strike photosystem II and excite 2 electrons from chlorophyll P680.
The excited electrons are captured by a primary electron acceptor and are then transferred to plastoquinone (PQ) and the ETC.

20. The Light Reactions Photosystem II (P680)
In the ETC, the 2 electrons pass through a proton pump (Q cycle).
The Q cycle transports 4 protons from the stroma into the thylakoid lumen to create a proton gradient.

21. The Light Reactions Photosystem II (P680)
The electrochemical gradient drives the photophosphorylation of ADP to ATP.
1 ATP forms for every 4 protons that pass through ATPase from the thylakoid lumen into the stroma.

22. The Light Reactions Photosystem II (P680)
A Z protein splits water into 2 protons, 2 electrons and 1 oxygen atom.
The electrons replace those lost from chlorophyll P680.
The protons remain in the thylakoid space to add to the proton gradient.
Oxygen leaves as a byproduct.

24. The Light Reactions Photosystem I (P700)
Two photons strike photosystem I and excite 2 electrons from chlorophyll P700 (replaced by electrons from P680).
These electrons pass through another ETC.
The enzyme NADP reductase uses the 2 electrons and a proton from the stroma to reduce 1 NADP+ to 1 NADPH.

27. The Calvin Cycle Occurs in the stroma of chloroplasts.
Cyclical reactions similar to the Krebs Cycle.
Divided into three phases:
Carbon Fixation
Reduction Reactions
Regeneration of RuBP

28. The Calvin Cycle Phase 1: Carbon Fixation
3 CO2 are added to RuBP to form 3 unstable 6-carbon intermediates.
The intermediates split into six 3-carbon molecules called PGA.
These reactions are catalyzed by rubisco.

29. The Calvin Cycle Phase 2: Reduction Reactions
6 PGAs are phosphorylated by 6 ATPs to form 6 molecules of 1, 3-BPG.
6 NADPH molecules reduce the six 1,3-BPG to 6 G3P or PGAL.
One molecule of G3P exits the cycle as a final product.

30. The Calvin Cycle Phase 3: Regeneration of RuBP
3 ATP are used to rearrange the remaining 5 G3P into 3 molecules of RuBP.
The cycle continues with the RuBP fixing more CO2.

32. To Produce One G3P…
3 RuBP + 3 CO2 + 9 ATP + 6 NADPH + 5 H2O  9 ADP + 8 Pi + 6 NADP+ + G3P + 3 RuBP