Presentation on theme: "Photosynthesis Chapter 10. What is photosynthesis… Photosynthesis transforms light energy into chemical bond energy stored in sugar and other organic."— Presentation transcript:
Photosynthesis Chapter 10
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, CO 2 and H 2 O. Directly or indirectly supplies energy to most living organisms. 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.
The Nature if Light and Pigments Sun emits electromagnetic radiation, the energy of which depends on the wavelength of light. A wavelength is the distance between the crests of electromagnetic waves. Visible light is only a small portion of the electromagnetic spectrum and ranges from about 380 to 750 nanometers in wavelength. 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.
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.
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.
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 CO 2 enters and O 2 exits. Vascular bundles (veins) – transport water absorbed by the roots to leaves; also export sugar from leaves to other parts of the plant.
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.
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.
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 H 2 O in the thylakoid space split; 4 H+ are pumped into the membrane; 4 e- are transferred to the chlorophyll; O 2 is produced as a by-product.
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.
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.
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.