1. Photosynthesis
Chapter 10
CO2 + H2O  C6H12O6 + O2

2. What is photosynthesis…
Photosynthesis transforms light energy into chemical bond energy stored in sugar and other organic molecules.
Energy-rich organic molecules made from energy-poor molecules, CO2 and H2O.
Autotrophic organisms require an energy from light (photoautotrophs) or from the oxidation of inorganic substances (chemoautotrophs).
Photoautotrophs -- plants, algae and some bacteria.
Chemoautotrophs -- some bacteria.

3. The Nature of Light and Pigments
Sun emits electromagnetic radiation, the energy of which depends on the wavelength of light.
Visible light is only a small portion of the electromagnetic spectrum.
Blue and red are the colors (wavelengths) most useful as energy for photosynthesis.
Pigments -- Substances that absorb visible light.
Different pigments absorb different wavelengths of light.
Color you see is the color most reflected or transmitted by the pigment.
A leaf appears green because it reflects green light.

5. Chlorophyll and other pigments
Chlorophyll a – blue-green pigment that participates directly in the light reactions.
Other accessory pigments can absorb light and transfer the energy to chlorophyll a, expanding the range of wavelengths available for photosynthesis.
Chlorophyll b -- yellow-green pigment with a minor structural difference that gives the pigment slightly different absorption spectra.
Carotenoids -- yellow and orange pigments that can transfer energy to chlorophyll a.
We see these in the fall as chlorophyll breaks down.

7. Leaf Structure
Leaves are the major organs of photosynthesis in most plants.
Photosynthetic pigments are found in chloroplasts which are concentrated in leaf’s interior.
Mesophyll -- green tissue inside the leaf.
Stomata – microscopic pores in the leaf through which CO2 enters and O2 exits.
Vascular bundles (veins) – transport water absorbed by the roots to leaves; also export sugar from leaves to other parts of the plant.

10. Chloroplasts
Intermembrane Space – narrow space which separates the two membranes of the chloroplast.
Thylakoids -- Flattened membranous sacs inside the chloroplast; Chlorophyll is found in the thylakoid membranes.
Grana -- (Singular = granum) Stacks of thylakoids.
Thylakoid Space – space inside the thylakoid
Stroma -- viscous fluid outside the thylakoids.
Photosynthetic prokaryotes lack chloroplasts, but have chlorophyll built into the plasma membrane or into membranes of vesicles within the cell.

12. Part 1: The light-dependent reactions
Light excites electrons in photosystem II (cluster of pigment molecules).
The electrons are then transferred to an electron transport chain embedded in the thylakoid membrane and end up in photosystem I.
Electrons lost from the photosystem II must be replaced; H2O in the thylakoid space split; H+ are pumped into the membrane; e- are transferred to the chlorophyll; O2 is produced as a by-product.

14. Light-dependent reactions cont.
This flow of electrons is coupled to reactions that phosphorylate ADP to ATP (another example of chemiosmosis).
Protons are pumped from the stroma to the thylakoid space as the electrons move along the transport chain, creating a proton gradient.
ATP synthase enzyme in the thylakoid membrane uses this proton-motive force to make ATP as H+ flows back across the membrane.

16. Light-dependent reactions cont
Next, light excites electrons in photosystem I chlorophyll.
Excited electrons are transferred to another electron acceptor.
An enzyme catalyzes the reduction of NADP+, transferring the electrons and producing NADPH (electron carrier for the second part of photosynthesis, the Calvin-Benson Cycle).
The electron "holes" in photosystem I are filled by electrons from photosystem II.

19. Light reactions animation (don’t worry about electron carrier names)
Another clip…