Photosynthesis

1. Photosynthesis Chapter 10

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 if 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. Photoexcitation of Pigments • When light is absorbed, electrons in the pigment molecule are boosted from its lowest-energy state (ground state) to a higher energy level (excited state). • The light energy absorbed is converted to potential energy of an electron elevated to the excited state. • This state is unstable, so electrons quickly fall back to the ground state, releasing energy. This energy may: • 1. Be lost as heat. • 2. Be re-emitted as light of lower energy (longer wavelength) -- fluorescence. • 3. Trigger another reaction if nearby electron acceptor molecules trap excited electrons.

9. 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.

12. 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.

14. Photosystems: Light-Harvesters of the Thylakoid Membrane • Chlorophyll a, chlorophyll b and the carotenoids are assembled into photosystems located within the thylakoid membrane. Each photosystem is composed of: • 1. antenna complex -- Pigment molecules (200-300) absorb photons of light and pass the energy from molecule to molecule to the reaction center. • 2. reaction-center chlorophyll -- One of the many chlorophyll a molecules transfers an excited electron to initiate the light reactions. • 3. primary electron acceptor -- Molecule traps excited state electrons released from the reaction center chlorophyll; powers the synthesis of ATP and NADPH later. • Two types of photosystems: • • Photosystem I has a specialized chlorophyll a molecule known as P700, which absorbs best at 700 nm. • • Photosystem II has a specialized chlorophyll a molecule known as P680, which absorbs best at a wavelength of 680 nm.

16. Part 1: The light-dependent reactions • Light excites electrons from P680 (reaction center chlorophyll in photosystem II). • Electrons ejected from P680 are trapped by the photosystem II primary electron acceptor. • The electrons are then transferred to an electron transport chain embedded in the thylakoid membrane. • Carriers: plastoquinone(Pq)  2 cytochromes  plastocyanin (Pc) to P700 of photosystem I (non-cyclic electron flow). • Electrons lost from the P680 reaction center must be replaced; 2 H2O in the thylakoid space split; 4 H+ are pumped into the membrane; 4 e- are transferred to the chlorophyll; O2 is produced as a by-product.

18. Non-cyclic Photosynthetic Phosphorylation • Excited electrons lose potential energy along the transport chain as they fall back to P700. • 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.

20. Part 1: The light-dependent reactions continued • Light excites electrons from P700 (reaction center chlorophyll in photosystem I). • Excited electrons are transferred to the primary electron acceptor for photosystem I, then passed to ferredoxin (Fd), an iron-containing protein. • An enzyme catalyzes the reduction of NADP+, transferring electrons from ferredoxin and producing NADPH (electron carrier for the second part of photosynthesis, the Calvin Cycle). • The electron "holes" in P700 are filled by electrons supplied by photosystem II.

22. Cyclic Photo-phosphorylation • Involves only photosystem I and generates ATP without producing NADPH or evolving oxygen; this system probably evolved first. • Called cyclic because excited electrons that leave from chlorophyll a at the P700 reaction center return to the same place. • Photons are absorbed by Photosystem I; P700 chlorophyll releases electrons to the primary electron acceptor, which passes them to ferredoxin. • Electrons them move down the electron transport chain (same one from P680 to P700). • H+ are pumped across the membrane, setting up the proton gradient for ATP production by chemiosmosis. • This cyclic pathway supplements the ATP required for the Calvin cycle and other metabolic pathways. The noncyclic pathway does not produce enough ATP to meet demand. • NADPH concentration may influence whether electrons flow through cyclic or noncyclic pathways.