Lecture 6 - Metabolism I - Photosynthesis

LECTURE 6 – METABOLISM I: PHOTOSYNTHESIS  Autotrophs – make food from sunlight and some other energy sources  Heterotrophs – use the food to do work  Photosynthesis – most important biological process on earth  Every spring, photosynthesis begins and when carbon dioxide is being consumed, there is a fall and plants respire, causing carbon dioxide concentrations to increase Pigments  Molecules that absorb photons o Generally colored (the reflected wavelengths)  Absorption of the photon increases the energy level of the molecule  Color of pigment is not indicative of what they are absorbing, it is indicative of what they are reflecting  Blue photon – high excited state, emits some heat, and falls back down to lower excited state  Red photon – low excited state, not gaining as much energy as blue photon, which returns to ground state by fluorescence (loss of energy by the emission of light of longer wavelength)  Absorption spectrum of a plant (or leaf pigment extract) describes how much a pigment is absorbing a particular wavelength of light  Action spectrum describes O 2volution rate (relative rate of photosynthesis) at each wavelength of light Catching Light  Light capture occurs in chloroplasts  Two membranes – outer envelope and inner envelope  Inside the inner envelope is the stroma, which contain thylakoids (stacks known as grana)  Thylakoids are filled with thylakoid lumen, and thylakoids are connected by lamella  Too much light can cause photo-inhibition or damage to the cells  Under weak blue light, chloroplasts will try to maximize their exposure to photons, while under strong blue light, chloroplasts will try to minimize their exposure to photons (hiding behind cell walls) Energy from Harvested Photons is Transferred to the Reaction Centre  In photosynthesis, there is no absorption of a single photon by a single chlorophyll  Instead a group of photons absorb light which are then funneled to a single chlorophyll molecule  Energy released by this is transferred from molecule to molecule until reaction center is reached  A group of pigment molecules will form an antenna complex with the reaction center denoting a special pair of chlorophyll molecules – this reaction center (comprised of either P680 or P700) will receive the transfer of energy (light) by other chlorophylls in the antenna complex in order to accept and then donate electrons (electron transfer)  Antenna complex is known as light harvesting complex  Reaction center drives electron transport from donor to acceptor  Combination of reaction center and its antenna complex – known as photosystem Two Sets of Reactions:  Light-dependent  Light-independent o Uses water, produces oxygen o Uses CO2, produces sugars o Uses ADP, NADP+ o Uses ATP, NADPH o Produces ATP, NADPH o Produces ADP, NADP+ o Energy input from photons o Energy input from light-dependent o Can use O 2roduction to measure reactions rate of photosynthesis o Can use net CO2consumption to measure rate of photosynthesis Photosynthetic Electron Transport Chain  As red light hits PSII and is transferred to the P680 (strong oxidant) reaction center, two water molecules are split and the electrons are transferred from the high energy state P680* (weak reductan