1. Chapter 6
Photosynthesis

2. Section 1: The Light Reactions
Autotrophs: use energy from sunlight or chemical bonds of inorganic molecules to make organic substances
MOST autotrophs use PHOTOSYNTHESIS
Other autotrophs use energy from broken chemical bonds to make organic molecules
Heterotrophs: get energy from food instead of from sunlight or inorganic substances

3. Autotrophs
Heterotrophs

4. PHOTOSYNTHESIS 6CO2 + 6H2O + light energy C6H12O6 + 6O2
Plants can use sunlight convert carbon dioxide and water into glucose and oxygen
Autotrophs and heterotrophs will then use the oxygen released from photosynthesis for cellular respiration (Chapter 7)
Photosynthesis is broken into: THE LIGHT REACTION and THE CALVIN CYCLE

6. THE LIGHT REACTION Requires light in order to occur Light is
absorbed in
chloroplasts:
thylakoids
grana
stroma

7. Light and Pigments
Sunlight is made of a variety of colors, even though it looks white
If we pass white light through a prism, we can separate it into its component colors (red, orange, yellow, green, blue, indigo and violet)
We call this the visible light spectrum and each color can be reflected, transmitted or absorbed
Pigments: compounds that absorb light; some pigments absorb certain colors more strongly than other colors

8. If a color is absorbed, it takes that color OUT of the spectrum; if blue and red are absorbed, orange, yellow, green, indigo and violet will be reflected

9. Chloroplast Pigments
Located in the thylakoid membranes of chloroplasts are several pigments; the most important of these pigments are called chlorophylls
The most common types are chlorophyll a (absorbs more red light) and chlorophyll b (absorbs more blue light)
Neither a or b absorbs much green light, so it is usually reflected (that’s why leaves LOOK green)

10. Chlorophyll a is directly involved in the light reaction, but chlorophyll b aids chlorophyll a in capturing light (called an accessory pigment);
Other accessory pigments found in the thylakoid membrane: yellow, orange and brown carotenoids; accessory pigments allow plants to capture much more of the light spectrum

11. In leaves, chlorophyll is the most abundance pigment, but other plant parts the colors of OTHER pigments are visible (petals and fruits); During fall, plant leaves being to lose their chlorophyll and the pigments of the carotenoids are shown in the leaves

12. Light Energy to Chemical Energy
Once the pigments in chloroplasts have captured the light energy, it has to be converted to chemical energy
This energy is TEMPORARILY stored in ATP and NADPH; O2 is given off during this reaction
Chlorophylls and carotenoids are clustered in the thylakoid membrane; These clusters of pigments and proteins are known as photosystems: photosystem I and photosystem II
Start here Wednesday Oct 19

14. Photosystem II
The light reaction begins when a photosystem absorbs light and that energy is bounced around until it reaches a pair of chlorophyll a pigments
The light reaction starts in photosystem II where light “excites” electrons and forces them to enter a higher energy level
After the chlorophyll a molecules lose electrons (oxidation), the electrons MUST go to a primary electron acceptor (reduction)

15. Photosystem II (cont)
The primary electron acceptor will donate the electrons to molecules called the electron transport chain (they transfer electrons from one molecule to the next)
As electrons move down the ETC, they lose energy while moving hydrogen protons (H+) into the thylakoid
The electron is then moved into photosystem I

17. Photosystem I
Light is absorbed by chloroplast pigments in photosystem I
This happens at the same time that light is absorbed in photosystem II
Electrons move from a part of chlorophyll a molecules in PS1 to another primary electron acceptor
The electrons lost by the chlorophyll a pair to the electron acceptor are replaced by the electrons that have passed through the photosystem II complex

18. Photosystem I (cont)
The electrons are then donated by the primary electron acceptor to ANOTHER electron transport chain
This ETC brings electrons to the side of the thylakoid membrane that faces the stroma
The electrons then combine with a proton and NADP+ (an organic electron acceptor)
This causes NADP+ to be reduced to NADPH

21. Replacing Electrons
Electrons from photosystem II replace the electrons lost in photosystem I
If this did not occur, the BOTH electron transport chains would stop and photosynthesis would not occur
Electrons in photosystem II are replaced by the splitting of water
2H2O 4H+ + 4e- +O2
The protons are left inside of the thylakoid membrane, the oxygen diffuses out of the chloroplasts and out of the plant

23. Making ATP Synthesis of ATP is VERY important in the light reaction
This occurs through a process called chemiosmosis: this is reliant on the concentration gradient of protons across the thylakoid membrane
Some protons are created from splitting of water, but others are “pumped” from the stroma to inside of the thylakoid membrane
This creates a concentration gradient and ATP synthase uses the diffusion of protons to make ATP from ADP and a phosphate group

25. Section 2: The Calvin Cycle
During the Calvin cycle, plants use products of the light reaction to make sugars from CO2
The Calvin cycle produces a three-carbon sugar through carbon fixation: incorporation of CO2 into organic compounds
A total of 3CO2 must enter the Calvin cycle to produce a three-carbon sugar

27. Alternative Pathways
There are plant species that have evolved in hot, dry climates
These plants MUST fix carbon, but they can lose water very rapidly through stomata (small pores in leaves)
Stomata allow CO2 in and O2 out so if the stomata are closed, the level of CO2 will be too low and O2 will be too high
How do they avoid this?

28. C4 Pathway This allows plants to fix CO2 into four carbon compounds
Plants that use this are called C4 plants
During the hottest part of the day, the stomata are partially closed, CO2 is fixed into the four carbon compound
The CO2 is then released in other cells and it enters the Calvin cycle
Ex: corn, sugar cane, and crab grass

30. The CAM Pathway Also adapted in a hot, dry climate
These plants fix carbon through a CAM pathway (crassulacean acid metabolism)
Plants that use the CAM pathway keep their stomata open at night and close them during the day
At night, CAM plants take in CO2 and fix it into a variety of organic compounds
During the day (when there is SUNLIGHT), CO2 is released from these organic compounds and put into the Calvin cycle

32. Summary of Photosynthesis
Photosynthesis happens in two stages:
The light reactions: energy is absorbed from sunlight and converted to chemical energy
The Calvin cycle: CO2 and the chemical energy stored in ATP and NADPH are used to form organic compounds
Photosynthesis is an ongoing cycle: some of the products of the light cycle are used in the Calvin cycle and some of the products of the Calvin cycle are used in the light cycle

34. What affects photosynthesis?
Light Intensity: the rate of photosynthesis increases as the intensity of light increases (more electrons are excited)
CO2 levels: increasing carbon dioxide levels will stimulate photosynthesis until the rate levels off
Temperature: Increasing temperature will accelerate photosynthesis until a certain point; past this point the enzymes used in photosynthesis start to denature and will not work; the stomata also close in hot temperatures to prevent water loss