1. Photosynthesis
Honors Biology

2. What you will learn… 1. How do plants get food?
2. Photosynthesis overview
3. Leaf structure
4. Chloroplast structure
5. Pigments
6. Overview of Two Stages
6a. Overview of Light Reactions
6b. Overview of Dark Reactions
7. Light Reactions
8. Dark Reactions (Calvin Cycle)
9. Relationship Between the 2 Stages
10. Factors Affecting the Photosynthetic Rate
11. Alternate Pathways

3. 1.How do plants get food?
Plants are autotrophs (meaning “self-feeders” in Greek)
Often referred to as the producers of the biosphere because they produce its food supply
All organisms that produce organic molecules from inorganic molecules using the energy of light are called photoautotrophs.

4. 2.Photosynthesis Overview
Photo, from the Greek word for light, refers to the first stage.
Synthesis, meaning “putting together” refers to the sugar construction in the second stage

5. 2.Photosynthesis: Overview
The main purpose of photosynthesis is to make organic molecules (carbohydrates).
Overall equation:
6 CO H20  C6H12O O2
Occurs in the leaves of plants in the chloroplasts.
Oxygen is also produced in this process.

6. 3.Leaf Structure
Most photosynthesis occurs in the mesophyll layer of the leaf.
Gas exchange of CO2 and O2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface.
Stomata are able to open and close because water is also evaporated through them into the atmosphere from the plant.

7. 3.Leaf Structure

8. 4.Chloroplast Structure
Similar to mitochondria, chloroplast has an outer membrane and an inner membrane, with an intermembrane space between them.
Inner membrane is filled with a thick fluid called stroma
*Stroma is where sugars are made from carbon dioxide and water

9. 4.Chloroplast Structure
Within stroma is a system of interconnected membranous sacs called thylakoids
contains thylakoid space
Built into thylakoid membranes are the chlorophyll molecules that capture light energy
concentrated in stacks called grana.

10. 5. Pigments Pigment is any molecule that is able to absorb light .
Only light that is absorbed by pigments is useful for photosynthesis.
Chlorophyll a is the most important photosynthetic pigment.
Other pigments called antenna or accessory pigments are also present in the leaf.
Chlorophyll b
Carotenoids (orange / red)
Xanthophylls (yellow / brown)
These pigments are embedded in the membranes of the thylakoid in groups called photosystems.

12. 6.Two Stages of Photosynthesis
1. Light Reactions
2. Dark Reactins (Calvin Cycle)

13. 6a.Light Reactions overview
Include the steps that convert light energy to chemical energy stored in ATP and NADPH and produce O2 gas as a waste product.
Occur in thylakoid membranes
Light energy absorbed by chlorophyll is used to make ATP from ADP and phophate.
Also used to drive a transfer of electrons from water to NADP+, an electron carrier similar to NAD+ that carries electrons in cellular respiration.
NADP+ gets reduced to NADPH via enzymes by adding a pair of light-excited electrons along with an H+
Reaction temporarily stores energized electrons which originally came form water that is split and O2 is released.

14. 6b.Dark Reactions overview
Dark reactions, or Calvin Cycle
Occurs in the stroma
Does not require light directly
Cyclic series of reactions that assembles sugar molecules using CO2 and the energy-containing products (NADPH and ATP) of the light reactions.
Incorporation of carbon from CO2 into organic compounds is called carbon fixation.
After carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds.

15. 7. Light Reactions

16. 7. Light Reactions
Light energy is transformed into the chemical energy of ATP and NADPH
In this process, electrons removed from water molecules pass from photosystem II to photosystem I to NADP+
Between the two photosystems, the electrons move down an electron transport chain and provide energy for ATP production.

17. 7. Light Reactions Consists of two Photosystems:
Contain clusters of chlorophyll molecules along with other pigments and proteins in the thylakoid membrane
Consists of a number of light-harvesting complexes surrounding a reaction center.
Have chlorophyll a, chlorophyll b, and carotenoid pigments that function collectively as a light-gathering antenna.
Pigments absorb photons and pass the energy from molecule to molecule until it reaches the reaction center.
A protein complex that contains a chlorophyll a molecule and a molecule called the primary electron acceptor:
Captures a light-excited electron from the reaction-center chlorophyll molecule and passes it to an electron transport chain

18. 7. Light Reactions Two types: Photosystem I and Photosytem II:
Occurs second in light reactions
Reaction center is called P700 because the wavelength of light it absorbs best is 700 nm
Photosystem II:
Occurs first in light reactions
Chlorophyll a molecule in reaction center is called P680 because the light it absorbs best is red light with a wavelength of 680nm

19. 7. Light Reactions Flow of electrons in light reactions (Figure 7.8A):
A pigment molecule in a light-harvesting complex absorbs a photon of light. The energy is passed to other pigment molecules and finally to the reaction center of Photosystem II, where it excites an electron of chlorophyll P680 to a higher energy level.
The electron is captured by the primary electron acceptor.
Water is split, and its electrons are supplied one by one to P680, replacign those lost to the primary electron acceptor. The oxygen atom compbines with an oxygen from another split water molecule to form a molecule of O2.

20. 7. Light Reactions
4. each photoexcited electron passes from photosystem II to photosystem I via an electron transport chain. The exergonic “fall” of electrons provides energy for the synthesis of ATP.
5. Meanwhile, light energy excites an electron of chlorophyll P700 in the reaction center of photosystem I. The primary electron acceptor captures the excited electron and an electron from the bottom of the electron transport chain replaces the lost electron in P700.
6. The excited electrons of photosystem I is passed through a short electron transport chain to NADP+, reducing it to NADPH

21. 7. Light Reactions Chemiosmosis
Drives ATP synthesis using the potential energy of a concentration gradient of hydrogen ions across a membrane
Gradient is created when an electron transport chain pumps hydrogen ions across a membrane as it passes electrons down the chain.
Relationship between chloroplast structure and function in light reactions:
The two photosystems and e.t.c. are all located in the thylakoid membrane of a chloroplast.
As photoexcited electrons are passed down the e.t.c. connecting the two photosystems, H+ are pumped across the membrane from the stroma into the thylakoid space. This generates a concentration gradient across the membrane.

22. 7. Light Reactions Chemiosmosis (continued):
Similar ATP synthase complex in mitochondria
Energy of concentration gradient drives H+ back across the membrane through ATP synthase
ATP synthase couples the flow of H+ to the phosphorylation of ADP: called photophosphorylation

23. 7. Light Reactions Photosynthesis vs. Cell Respiration:
In photosynthesis, light energy is used to drive electrons to the top of the transport chain (whereas, cell respiration, high-energy electrons pass down the e.t.c. coming from oxidation of food molecules)
Chloroplasts transform light energy into the chemical energy of ATP (whereas, mitochondria transfer chemical energy from food to ATP)
In photosynthesis, the final electron acceptor is NADP+ (whereas, in cell respiration, O2 is)
In photosynthesis, electrons are stored in at a high state of potential energy in NADPH (whereas, in cell respiration, they are at a low energy level in H20)

24. 8. Dark Reactions
During this process, carbohydrates are formed.
This is the only process on the earth that can form organic molecules from inorganic ones. All other organic molecules (big 4) form from carbohydrates!
This cycle requires ATP, NADPH and CO2 to take place in the stroma of the chloroplast.
ATP, NADPH are from the light reaction, while CO2 has to be taken in from the atmosphere through the stomata of the leaves.

25. 8. Dark Reactions Figure 7.10A: Overview of Calvin Cycle
CO2 (from air), energy from ATP and high energy electrons from NADPH (both generated by light reactions) , the Calvin Cycle constructs an energy-rich, three-carbon sugar, glyceraldehyde-3-phosphate (G3P).
A plant cell uses G3P to make glucose and other organic molecules as needed.

26. 8. Dark Reactions Figure 7.10B: Details of the Calvin Cycle
Carbon fixation: the enzyme rubisco attaches CO2 to RuBP (5-C). The unstable 6-C product splits into two molecules called 3-PGA.
For three CO2, six 3-PGA result
Reduction: NADPH reduces the organic acid six 3-PGA to six molecules G3P with the assistance of ATP

27. 8. Dark Reactions 3. Release of one molecule of G3P:
Five G3Ps remain in the cycle, and one G3P will leave. Plant cells use two G3P molecules to make one molecule of glucose.
4. Regeneration of RuBP
energy from ATP drives a series of chemical reactions to rearrange the atoms in the five G3P molecules to form three RuBP molecules. These can start another turn of the cycle.

28. 8. Dark Reactions

29. 9. Relationship between the 2 Stages

30. 10. Abiotic Factors Affecting Photosynthetic Rate
Photosynthetic rate is depended on environmental factors:
Amount of light available
Level of carbon dioxide
temperature

31. 10. Abiotic Factors Affecting Photosynthetic Rate
Light intensity
Up to a certain intensity, photosynthesis increases as more light is available to the chlorophyll.
When all the chlorophyll molecules are activated (saturated) by the light, more light has no further effect.

32. 10. Abiotic Factors Affecting Photosynthetic Rate
Temperature:
Increased temperature increases photosynthetic rate until an optimal temperature is reached.
Above the optimal temperature, enzymes cannot function properly and photosynthesis will decrease.

33. 10. Abiotic Factors Affecting Photosynthetic Rate
Carbon Dioxide Levels:
Increased carbon dioxide levels increases photosynthesis, unless limited by another factor, then levels off.

35. 11. Alternate Pathways
These pathways adapt to perform photosynthesis in dry and hot environment
They are more efficient than the traditional C3 pathway which use CO2 directly from the air
Plants with alternative pathways have a slightly different Calvin cycle.
In C4 plants the location of the Calvin cycle is different
In CAM plants the timing is different

36. 11. Alternate Pathways C4 Pathway:
CO2 fixation and the Calvin cycle take place in two separate location.
CO2 fixation is in the mesophyll cells of the leaf, even when CO2 levels are low, producing a C4 product used for the Calvin Cycle
C4 product acts as a carbon shuttle to…
The Calvin cycle takes place in the bundle sheath cells (around the veins of the leaf) where sugars are made
Examples of C4 plants: corn, sugar cane

37. 11. Alternate Pathway CAM Occurs in succulent plants (cacti)
Carbon fixation (trapping CO2) takes place at night when the stomata are open
Calvin cycle takes place during the day, when stomata are closed
This way plants do not lose much water during hot and dry days.

38. 11. Alternate Pathway