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 ( ) 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.