1. The Light-Dependent Reactions
Photosynthesis Step 1:
The Light-Dependent Reactions

2. Recall the forms of energy found in each location for the light reactions of photosynthesis:
Where does the energy for this system come from?

3. Big Idea:
The light reactions capture the sun’s energy in molecular bonds.
Today’s goals:
Know the structure of the chloroplast
Understand how pigments capture light
Understand how light energy is converted to molecular energy

4. Chloroplast The chloroplast is the site of photosynthesis.
Chloroplasts are concentrated in the mesophyll tissue found in the leaf.
Where is the mesophyll?

5. Chloroplast – the Photosynthetic Organelle in Plant Cells

6. Chloroplast Structure
Chloroplasts have two membranes.
The space inside the inner membrane is called the stroma.

7. Chloroplast Structure
In the stroma they have a set of membraneous sacs called thylakoids.
Thylakoids have an inner lumen called the thylakoid space.
Stacks of thylakoids are called grana (granum is singular).

8. SEM of a Chloroplast

9. Identify Chloroplast Partsb. d. c.

10. Photosynthetic Processes
The processes of photosynthesis take place in various parts of the chloroplast.
Before we talk about those, we need to take a look at the overall photosynthesis process and its inputs and outputs.

11. Two Stages of Photosynthesis
Light-dependent reactions (during day-time only, occur in the thylakoid membrane)
Light-independent reactions, or the Calvin Cycle (could occur both day and night, in the stroma),
These two stages are linked together by NADPH and ATP.

12. The Light Reactions
In order for the light reactions to be successful, light must be absorbed.
We need to understand how pigments absorb light.
Scale of the universe
Cosmos: A Spacetime Odyssey – Episode 2 “The Things That Molecules Do.”

13. Understanding Light Energy
Electromagnetic energy from the sun is composed of a range of wavelengths

14. Understanding Light Energy
The visible light spectrum are the wavelengths between about 400nm in length to about 740nm in length
Different colors correspond to different wavelengths of light.

15. Understanding Light Energy
Light waves travel as photons
Photon = a discrete packet of light energy.
A photon is a fixed quantity of light energy

16. Big Idea: Connecting Light Energy to Energy in Electrons
Plant cells contain pigments
Pigments are molecules that can absorb photons of light
Different pigments absorb different wavelengths of light.

17. Big Idea: Connecting Light Energy to Energy in Electrons
When a pigment absorbs a photon, one of the pigment’s electrons gains energy
The electrons bump up to a higher electron shell
When pigments absorb light photons, the energy from the light transfers to the pigment’s electrons – that’s what gives them energy to go up to a higher energy level.
This idea is important! –
When pigments absorb light energy,
their electrons move to a higher energy level.

18. Overview: Light-dependent Reactions
Require light and occur only during the day in nature.
They take place in the thylakoid membrane of the chloroplast.
Light reactions involve
a) Splitting of water to produce oxygen (photolysis)
b) Energy production (ATP)
c) Reduction of NADP+ to NADPH.
Light reactions produce NO organic molecules! They furnish the energy that eventually powers the food-making machinery of photosynthesis.

19. Light-dependent Reactions: Video
We’ll watch this 5-min video clip to see how photosystems and electron transport chains work together.

20. Photosystems Capture Solar Power
When a pigment absorbs a photon, one of the pigment’s electrons gains energy.
The electron gains enough energy to bump up into a higher shell.
Normally, the electron then drops back into its original shell, releasing the energy it had gained.

21. Photosystems Capture Solar Power
In the light-dependent reactions, pigments are arranged into photosystems.
In the photosystems, excited electrons get transferred to other molecules, instead of dropping down to their lower energy level.
As we talked about in respiration, transfer of electrons to other molecules also transfers their energy.

22. Photosystems Capture Solar Power
The pigments in photosystems transfer electrons to a primary electron acceptor.
This is the first step in the light-dependent reactions.

23. Basic steps of a Light Reaction
Light reaction involves the following basic steps:
1) Absorption of light
2) Excitation of Chlorophyll and emission of electron
3) Formation of ATP and NADPH via the electron transport chain

24. Photosystem I
There are two types of Photosystems, (Photosystem I and Photosystem II) occuring within the thylakoid membrane.
In Photosystem I, a chlorophyll molecule is the reaction center.
When it is excited by a photon, it emits electrons which are accepted by the primary electron acceptor and passed down an electron transport chain, eventually reducing NADP+ to NADPH.

25. Photosystem II
In Photosystem II, another chlorophyll molecule is the reaction center.
It absorbs light and emits electrons that are accepted by a primary electron acceptor and passed down an electron transport chain, eventually replenishing the lost electrons from Photosystem I.

26. Photosystem II
The electron transport chain in Photosystem II enables the chloroplast to make ATP by pumping H+ to create a proton gradient.
ATP synthesis here occurs just like it does in the electron transport chain of cellular respiration.

27. Photosystem II
Before Photosystem II, H20 is split into H+ (protons), electrons and Oxygen gas. These electrons replenish the lost electrons from the chlorophyll molecule

28. Photosystems I and II together

29. Summary of Light Rxns
Every molecule of NADPH formed in the light reactions requires 2 electrons from photosystem 1.
NADPH, ATP, and O2 are the products of the light reactions!

30. Electron Transport Powers ATP Synthesis in Light Reactions
DOES THIS SOUND FAMILIAR?
Energy released during electron flow drives the transport of hydrogen ions (H+) across the thylakoid membrane
Electron transport in the chloroplast drives ATP production the same way it does in the mitochondria

31. Electron Transport Powers ATP Synthesis in Light Reactions
The two photosystems and electron transport chains are located within the thylakoid membrane of a chloroplast.
The photosystems are arranged in such a way that energy released during electron flow drives the transport of hydrogen ions (H+) across the thylakoid membrane, into the thylakoid lumen.

33. Electron Transport Powers ATP Synthesis in Light Reactions
The pumping of H+ creates a huge concentration gradient of H+ across the thylakoid membrane.
The only way for the H+ to go down its concentration gradient is through ATP synthase
ATP synthase uses the kinetic energy of the H+ flow to create ATP from ADP and Pi.

35. Light-dependent Reactions: Video
We’ll watch this 5-min video clip again (if we have time) to see how the molecules of the photosystems and electron transport chains work together.

36. Comparison of Chemiosmosis
At its heart, the chemiosmosis of both cellular respiration and photosynthesis is very similar.

38. Review
How, specifically, does light energy drive the light- dependent reactions?
What are the products of the light-dependent reactions?