1. The light reactions Chapter 10
Photosynthesis
The light reactions
Chapter 10

2. Photosynthesis
One of the most important biochemical process in plants.
Let’s not forget cell wall biosynthesis and adaptation during plant development, growth, interaction with the environment, and disease defense.
Among the most expensive biochemical processes in plant in terms of investment
The biochemical process that has driven plant form and function

3. General overall reaction
6 CO2 + 6 H2O C6H12O O2
Carbon dioxide Water Carbohydrate Oxygen
Photosynthetic organisms use solar energy to synthesize carbon compounds that cannot be formed without the input of energy.
More specifically, light energy drives the synthesis of carbohydrates from carbon dioxide and water with the generation of oxygen.

4. Overall Perspective Light reactions: Harvest light energy
Convert light energy to chemical energy
Carbon (Dark) Reactions:
Expend chemical energy
Fix Carbon [convert CO2 to organic form]

5. The sun emits a tremendous amount of energy, only some of which is useable

6. But only a small fraction of the sun’s energy reaches the surface
Solar spectrum and its relation to the absorption spectrum of chlorophyll
Very little of the Sun’s energy gets to the ground (red line).
gets absorbed by water vapor in the atmosphere
The absorbance spectra of chlorophyll (green line).
Absorbs strongly in the blue and red portion of the spectrum
Green light is reflected and gives plants their color.

7. Photosynthetic pigments
Two types in plants:
Chlorophyll- a
Chlorophyll –b
Structure almost identical,
Differ in the composition of a sidechain
In a it is -CH3, in b it is CHO
The different sidegroups 'tune' the absorption spectrum to slightly different wavelengths
light that is not significantly absorbed by chlorophyll a, will instead be captured by chlorophyll b

8. Photosynthetic pigments
Chlorophyll has a complex ring structure
The basic structure is a porphyrin ring, co-coordinated to a central atom.
This is very similar to the heme group of hemoglobin
Ring contains loosely bound electrons
It is the part of the molecule involved in electron transitions and redox reactions of photosynthesis

9. Photosynthetic pigments
When chlorophyll absorbs a light particle (Proton)
Enters a higher excitation state
Becomes unstable, gives up energy as heat
Enters lower excited state
can be stable for a few nanoseconds
This energy causes chemical reactions to occur
These reactions are the fastest known to science!!!!

10. Photosynthesis
Takes place in complexes containing light-harvesting antennas & photochemical reaction centers
Antenna complex
Chemical oxidation & reduction reactions leading to long term energy storage take place
Antenna collects light and transfers its energy to the reaction center
Chemical reactions store some of the energy by transferring electrons from chlorophyll to an electron acceptor molecule

11. Photosynthesis An electron donor then reduces the chlorophyll again
The transfer of energy to the antenna is a purely physical phenomenon and involves no chemical changes
Even in bright sunlight, a chlorophyll molecule absorbs only a few photons each second
Therefore, many chlorophyll molecules send energy into a common reaction center
The whole system is kept active most of the time

12. The chemical reaction of photosynthesis is driven by light
The initial reaction of photosynthesis is:
CO2 +H2O (CHOH) + O2
Under optimal conditions (red light at 680 nm), the photochemical yield is almost 100 %
However, the efficiency of converting light energy to chemical energy is about 27 %
Very high for an energy conversion system
Quantum efficiency: Measure of the fraction of absorbed photons that take part in photosynthesis
Energy efficiency: Measure of how much energy in the absorbed photons is stored as chemical products
¼ energy from photons stored – the rest is converted to heat

13. Light drives the reduction of NADP & the formation of ATP
Overall process of photosynthesis is a redox chemical reaction
Electrons are removed from one chemical species (oxidation) and added to another (reduction)
Light reduces NADP, which serves as the reducing agent for carbon fixation during the dark reactions
ATP also formed during electron flow from water to NADH and is also used during carbon fixation
Thylakoid reactions: water oxidized to oxygen, NADP reduced and ATP is formed
Stroma reactions: carbon fixation and reduction reactions

14. Oxygen-evolving organisms have two photosystems that operate in series
Two photochemical complexes operate in series to carry out the early energy storage reactions of photosynthesis
Photosystem II: Absorbs red light of 680 nm
Produces a strong oxidant, capable of oxidizing water
Also produces a weak reductant
Photosystem I: Absorbs far-red light (700 nm)
Produces a strong reductant, capable of reducing NADP
Also produces a weak oxidant
These two photosystems are linked by an electron transport chain

15. Oxygen-evolving organisms have two photosystems (PS) that operate in series
Z (zigzag) scheme – the basis of understanding O2-evolution
light at 680 nm absorbed by PS-II makes strong oxidant (& weak reductant)
Light at 700nm absorbed by PS-I makes strong reductant (& weak oxidant)
PS-II oxidizes water PS-I reduces NADP

16. The chloroplast
A quick recap

17. The Chloroplast
Contain their own DNA and protein-synthesizing machinery
Ribosomes, transfer RNAs, nucleotides.
Thought to have evolved from endosymbiotic bacteria.
Divide by fusion
The DNA is in the form of circular chromosomes, like bacteria
DNA replication is independent from DNA replication in the nucleus

18. The Chloroplast
Membranes contain chlophyll and it’s associated proteins
Site of photosynthesis
Have inner & outer membranes
3rd membrane system
Thylakoids
Stack of Thylakoids = Granum
Surrounded by Stroma
Works like mitochondria
During photosynthesis, ATP from stroma provide the energy for the production of sugar molecules

19. The Chloroplast

20. PS-I and PS-II are spatially separated in the thylakoid membrane
The PS-II reaction center is located mostly in Granum.
Stack of Thylakoids
The PS-I reaction center is located in the Stroma & the edges of the Granum.
There is a cytochrome b6f complex that connects the two photosystems that is evenly distributed between Granum and Stroma
One or more of the electron carriers that function between the photosystems diffuses from the from the Granum to the Stroma.

21. Electron transfer
A step by step look

22. Mechanisms of electron transfer
The energy changes of electrons as they flow through the light reactions are analogous to the cartoon.
The light reactions use solar power to generate ATP and NADPH which provide chemical energy and reducing power to the sugar making reactions

23. Z (zigzag) scheme – the basis of understanding O2-evolution
The transport chain
Z (zigzag) scheme – the basis of understanding O2-evolution

24. Oxygen-evolving organisms have two photosystems (PS) that operate in series
What is an electron transport chain?
A series of coupled oxidation/reduction reactions where electrons are passed from one membrane-bound protein/enzyme to another before being finally attached to a terminal electron acceptor (usually oxygen or NADPH).

25. The transport chain PS-II oxidizes water to O2 in the thylakoid lumen
Releases protons into the lumen
Cytochrome b6f receives electrons from PS-II & delivers them to PS-I
Also transports additional protons into lumen from stroma

26. The transport chain PS-I reduces NADP to NADPH in the stroma
Uses the action of:
Ferredoxin (Fd)
Ferredoxin-NADP reductase (FNR)
ATP synthesis produces ATP as protons diffuse back through it from the lumen into the stroma

27. Summary of light reactions
Photosynthesis (light reactions):
Storage of solar energy carried out by plants
Absorbed photons excite chlorophyll molecules
can dispose of this energy as:
Heat
Fluorescence
Energy transfer
Photochemistry – the light reactions of photosynthesis
Absorption of light occurs in the thylakoid membranes of the chloroplast by chlorophyll a & b

28. Summary of light reactions
Plants have two reaction centers:
PS-II
Absorbs Red light – 680mn
makes strong oxidant
oxidizes 2 H2O molecules to 4 electrons, 4 protons & 1 O2 molecule
Mostly found in Granum
PS-I
Absorbs Far-Red light – 700nm
strong reductant
PS-I reduces NADP to NADPH
Mostly found in Stroma

29. Summary of light reactions
Excess light energy can damage photosynthetic systems
Several mechanisms occur to minimize such damage
Some proteins made in chloroplasts act as photoprotective agents to control excited state of chlorophyll molecules
Chloroplasts contain DNA and encode and synthesize most of the proteins essential for photosynthesis
Others encoded by nuclear DNA

30. Summary of light reactions
Chlorophylls made in a biosynthetic pathway involving more than 12 steps.
Once synthesized, pigment proteins are assembled in the thylakoid membrane.
Lastly:
The initial reaction of the light reaction is:
CO2 +H2O (CHOH) + O2

31. ANY QUESTIONS?