1. The Light-Dependent & Light-Independent Reactions
Photosynthesis
The Light-Dependent & Light-Independent Reactions

2. Recall the forms of energy found in each location:

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.

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 a shell.
This idea is important! We’ll return to it tomorrow when we talk about the light reactions of photosynthesis.

18. Absorption Spectrum of Plant Pigments
Photosynthetic pigments are found in the chloroplast (an organelle in plant cells)
There are several plant pigments
Chlorophyll is the main photosynthetic pigment.

19. Absorption Spectrum of Plant Pigments
Each pigment has a different absorption spectrum
The absorption spectrum shows the wavelengths of light that are absorbed by the pigment.

20. Absorption Spectrum of Plant Pigments
What do you notice about the absorption spectrum for these pigments?
Describe what you see.

21. Absorption Spectrum of Plant Pigments
How do you think photosynthesis in plant cells might operate differently in different colors of light?

22. The Action Spectrum
Photosynthesis happens at different rates depending on the color of light (the wavelength) to which it is exposed.
We can measure the rate of photosynthesis in different light conditions.
These measurements give us an action spectrum for photosynthesis.

23. The Absorption Spectrum & the Action Spectrum
Compare the absorption spectrum of the pigments with the photosynthesis action spectrum.
What do you notice about the relationship between these two?
Why do you think that relationship exists?
Explain your answers.

24. 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.

25. Overview: Light-independent Reactions
Are independent of light and may occur during the day or the night.
Take place in the stroma of the chloroplast.
Involves the production of ‘food’ (stored energy) as glucose and other organic molecules by using CO2, ATP, and NADPH.

26. Overview of the Overviews
NADPH is produced by the light-dependent reactions and provides high-energy electrons for reduction in the light-independent reactions.
ATP from the light-dependent reactions provides chemical energy that powers several of the steps of the light-independent reactions.
The light-independent reactions usually run in the daytime when light reactions power the cycle’s sugar assembly.

27. Light-dependent Reactions Details:

28. Photosystems Capture Solar Power
We said in the previous presentation that 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.

29. 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.

30. Photosystems Capture Solar Power
The pigments in photosystems transfer electrons to a primary electron acceptor.
The solar-powered electron transfer from chlorophyll (pigments) to the primary electron acceptor is the first step in the light-dependent reactions.

31. Visible Light Drives the Light Reactions
As we saw in the absorbance spectrum, pigments can only absorb certain wavelengths of light
Therefore, the light reactions of photosynthesis only use certain wavelengths of visible light.
Chlorophyll a absorbs mainly blue-violet and red light.
Chlorophyll b absorbs mainly blue and orange light and reflects (appears) yellow-green.

32. Pigments of the Chloroplast
Chlorophyll a directly participates in the light- dependent reactions.
Chlorophyll b and carotenoids (other pigments) do not directly participate in light-dependent reactions.
They broaden the range of light the plant can use
Then, these pigments convey the light energy they absorb (their excited electrons) to chlorophyll a, which puts the energy to work.

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

34. Photosystem I
There are two types of Photosystems, (Photosystem I and Photosystem II) occuring within the thylakoid membrane.
In Photosystem I, chlorophyll a molecule, P700, is the reaction center. This molecule absorbs light of 700 nanometers wavelength (red light).
Its excited state emits electrons which are accepted by the primary electron acceptor and passed down an electron transport chain, eventually reducing NADP+ to NADPH.

35. Photosystem II
In Photosystem II, chlorophyll a molecule, P680, is the reaction center. This molecule absorbs light of 680 nanometers wavelength (orange-red light) – which has a higher energy level.
Its excited state (due to light absorption) emits electrons that are accepted by a primary electron acceptor and passed down an electron transport chain, eventually replenishing the lost electrons from P700.

36. Photosystem II
The electron transport chain in Photosystem II enables the chloroplast to make ATP by chemiosmosis.
H20 is split into H+ (protons), electrons and Oxygen gas. These electrons replenish the lost electrons from P680

37. Photosystems I and II

38. Electron Transport Chains: Noncyclic Electron Flow
Key events in the light reactions:
The absorption of light energy
The excitation of electrons by that energy
The formation of ATP and NADPH using energy made available by the cascade of energized electrons down electron transport chains

39. Noncyclic Electron Flow and the Light Reactions
In the diagram, we can see that both photosystems absorb light energy and excited electrons pass from the reaction-center chlorophylls to the primary electron acceptors
Then each primary electron acceptor is oxidized as it donates high energy electrons to the first electron carrier of an electron transport chain.

40. Photosystems I and II

41. Noncyclic e- Flow and the Light Rxns
Additional redox reactions then shuttle the electrons from one electron carrier molecule to the next down an energy cascade.
At each step in the cascade, the electrons lose energy, some of which ends up being captured and temporarily stored in either ATP or NADPH molecules.

42. Photosystems I and II

43. 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!
ATP is made by chemiosmosis - which we will look at next!

44. Chemiosmosis 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 chemiosmosis the same way it does in the mitochondria

45. Chemiosmosis
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.

47. Chemiosmosis
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.
In photosynthesis, the chemisomotic production of ATP is called photophosphorylation because the initial energy input is light energy.

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

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

52. Cyclic Electron Flow
Noncyclic Electron Flow provides BOTH NADPH and ATP for the Calvin cycle
The light-independent reactions need more ATP than Noncyclic e- Flow can provide, SO –
Cyclic Electron Flow occurs to provide extra ATP to the light-independent reactions.

53. Cyclic Electron Flow

54. Cyclic e- Flow In cyclic e- flow, only Photosystem I is used.
Photoexcited electrons from photosynthesis can be returned from ferredoxin (Fd) to P700 chlorophyll through the cytochrome complex and plastocyanin (Pc)
This provides additional ATP for the Calvin cycle but does not produce NADPH
This production of ATP is called cyclic photophosphorylation, to differentiate from noncyclic photophosphorylation.

55. Review
How, specifically, does light energy drive the light- dependent reactions?
What are the products of the light-dependent reactions?
Describe how chemiosmosis works in the light- dependent reactions.

56. Light-independent Reactions Details

57. Light-independent Reactions: The Calvin Cycle
Take place during the day and sometimes the night in the stroma of the chloroplast.
The inputs are:
CO2 (from the air – an oxidized form of carbon)
ATP and NADPH ( both generated by the light reactions)
Using carbon from CO2 , energy from ATP, and high energy electrons from NADPH, the light- independent reactions construct an energy rich sugar molecule, glyceraldehyde 3-phosphate (G3P).
G3P is a reduced form of carbon.

58. The Calvin Cycle
The plant cell can use G3P to make glucose or other organic molecules as needed. Any excess of glucose is converted into starch – stored in roots, tubers and fruits.
To make a molecule of G3P, the cycle must incorporate the carbon atoms from 3 molecules of CO2.

59. glycerate-3-phosphate
triose phosphate (TP)

60. The Calvin Cycle: Summary
For a net gain of 1 G3P molecule:
9 ATPs were consumed
6 NADPHs were oxidized
The G3P that left the cycle is the starting material for synthesis of other compounds (e.g. glucose, starch and other more complex carbohydrates)

61. Review:
Think back to the light-dependent and light-independent reactions.
What are the big ideas from each section?
How are these two reaction series connected to each other?

62. Adaptations of the Mitochondrion and the Chloroplast for Function
Both mitochondria and chloroplasts’ structures are adapted for their functions.
Explain three ways each is structurally suited to its purpose.