1. Light-dependent Reactions
Photosynthesis
Light-dependent Reactions

2. video

3. Overview
Photosynthesis transforms the radiant energy from the sun into the chemical energy of high-energy compounds
Much of the glucose produced by plants is used to make cellulose, other sugars and starches as well as other essential cellular components such as amino acids
Other organisms use the substances made by plants for their own use

4. Overview
The process of photosynthesis can be summarized by the following equation:
6CO2(g) + 6H2O(l) + energy C6H12O6(s) + 6O2(g)
There are more than 100 reactions that lead to the end products

5. Photo (light) Synthesis (the reactions that produce the carbohydrate)
There are two distinct sets of reactions:
The light–dependent reactions
The light–independent reactions
In the light-dependent reactions light energy is trapped to generate ATP and NADPH (similar to NADH)
The light-independent reactions use the energy of ATP and the reducing power of NADPH to make glucose

6. Cross-section of a dicot leaf

7. Chloroplasts
Photosynthesis occurs in the 40 – 200 chloroplasts that are contained in each photosynthetic cell
Thylakoids are interconnected disks that are formed by a membrane system within the chloroplast
Thylakoids are stacked to form grana (singular, granum)

8. Chloroplasts
Molecules that absorb the sun’s energy are embedded in the thylakoid membranes
Surrounding the grana is a fluid filled interior called the stroma
The stroma contains the enzymes that catalyse the conversion of CO2 into carbohydrates

9. Chloroplasts

10. Absorption of Light energy
When any matter absorbs light energy, the light is absorbed in packets of energy called photons
Photons carry specific amounts of energy
Each wavelength of light is associated with photons of one distinct amount of energy
Longer-wavelength photons have smaller amounts of energy

11. Light wavelengths

12. Light absorption
The wavelength of the photon and thus the colour of the light, that an atom or molecule absorbs is determined by the energy levels of the electrons in the atom or molecule.
An atom or molecule can absorb a photon only if they have an amount of energy exactly equal to the difference between two energy levels

14. Photosynthetic pigments
Pigments are compounds that absorb certain wavelengths
Chlorophyll is the main photosynthetic pigment
Chloroplasts contain two types: chlorophyll a and chlorophyll b
They absorb photons from different wavelengths

15. http://ghadvisor. blogspot

16. Photosystems capture energy
In the thylakoid, pigments are arranged into clusters called photosystems. 
Each photosystem has about pigment molecules
Energy absorbed by a pigment is passed to a neighbouring pigment until it reaches the reaction centre (chlorophyll a)
All the surrounding pigment molecules that gather the light energy are called the antenna complex

17. Image from http://kvhs.nbed.nb.ca/gallant/biology/photosystem.jpg

18. Photosystems
In green plants and algae, there are 2 photosystems (I and II) named in order of their discovery
Photosystem I: P700 is the chlorophyll a that is associated with its reaction centre
Photosystem II: P680 is the chlorophyll a that is associated with its reaction centre.
The names are based on the wavelength of light that these molecules absorb
There are thousands of photosystems in the thylakoid membranes of just one chloroplast

19. How does photosynthetic pigment bind light energy?
Pigment molecules have conjugated double bonds, this means that every second bond is a double bond
The electrons in these bonds are easily "detachable", so when a light photon excites them, the energy level of one of the electrons in the bond increases
The energy is quickly given off as heat, light, phosphorescence or it can be transferred to another pigment molecule.
Image from

20. Light-dependent Reactions
Image from

21. Light-dependent Reactions
Step 1
The P680 molecule in the reaction centre of photosystem II absorbs a photon, exciting an electron
The excited electron is picked up by an electron carrier leaving a “hole” in the P680 molecule
The P680+ pulls an electron from a water molecule

22. Light-dependent Reactions
Step 2
The energized electrons are passed from the electron acceptor along an electron transport system
With each transfer, a small amount of energy is released and used by the b6f complex to pump hydrogen ions from the stroma into the thylakoid space
This, along with the water splitting, creates a hydrogen ion gradient

23. Light-dependent Reactions
Step 3
While steps 1 and 2 are happening, light energy is absorbed by photosystem I and is transferred to the reactions centre P700 molecule
Excited electrons are passed to an electron acceptor
Lost electrons are replaced by those that have reached the end of the transport system

24. Light-dependent Reactions
Step 4
Electrons are used by enzyme NADP reductase to reduce NADP+ to NADPH
The reducing power of NADPH will be used in the light-independent reactions

25. Making ATP ATP is formed using the energy of chemiosmosis
The H+ ion gradient formed by the b6-f complex of the electron transport chain is used by the ATP synthase molecule to make ATP from ADP and Pi
This is photophosphorylation

26. Cyclic and noncyclic Photophosphorylation
The production of ATP by the passing of electrons through the Z–scheme is often called noncyclic photophosphorylation
The flow of electrons is unidirectional
The passage of one electron pair produces one NADPH and slightly more than one ATP
Light-independent reactions require two NADPH and three ATP

27. Cyclic and noncyclic Photophosphorylation
Cyclic photophosphorylation produces more ATP
Excited electrons leave photosystem I and are passed to a electron acceptor, then to b6-f complex and back to photosystem I
ATP is formed by chemiosmosis as the proton gradient is generated
No NADPH is made

28. Cyclic Photophosphorylation

30. animations
and Noncyclic Photophosphorylation

31. The end of part 1
The light-independent reactions are next