Photosynthesis: Using Light to Make Food Plants are autotrophs (“self-feeders”, producers) capable of making their own organic material from inorganic nutrients Since they do with through photosynthesis, they are photoautotrophs Plants require inorganic nutrients, including: 1. CO2 from the air 2. Water and minerals from the soil Plants can make all their own organic molecules Carbohydrates, lipids, proteins, nucleic acids Photosynthesis and cellular respiration are intimately tied to one another There is an exchange of molecules and energy Basics of photosynthesis Photosynthesis takes place in chloroplasts that are found mainly in mesophyll cells in the interior of leaves Stomata are microscopic pores on the undersides of leaves that open and close to allow CO2 in and H2O vapor and O2 out Structure of chloroplasts The chloroplast’s inner membrane encloses a compartment filled with stroma, a thick fluid where sugars are made Within the stroma are disk-like membranous sacs called thylakoids Thylakoids are found in stacks called grana These contain the chlorophyll The two major sets of reactions of photosynthesis Light reactions convert solar energy into chemical energy ATP is produced as a source of chemical energy; NADPH as a source of high energy electrons The Calvin cycle makes sugars from carbon dioxide The Nature of Sunlight Sunlight is a type of energy called radiation, or electromagnetic energy The distance between the crests of two adjacent waves is called a wavelength The full range of radiation is called the electromagnetic spectrum Electrons derived from water are energetically boosted by sunlight energy The energized electrons are passed through a series of electron carriers, losing energy along the way Chloroplasts contain several kinds of pigments Chlorophyll a, chlorophyll b, carotenoids All are present in the thylakoid membranes (grana) All are part of the light-harvesting complexes called photosystems Light carries energy Behaves as both waves and Discreet packets of energy called photons, a fixed quantity of light energy What happens in a photosystem? A cluster of pigment molecules form a light-gathering antenna When a photon strikes one pigment molecule, the energy is transferred from one pigment to another until it reaches the reaction center chlorophyll a molecule The reaction center of the photosystem contains a special chlorophyll a molecule As the electron is excited in the chlorophyll a molecule, it is transferred to the primary electron acceptor When this occurs, the photosystem has just lost electrons To continue, the electrons have to be replenished That’s where the water comes in The light reactions generate ATP and NADPH Two types of photosystems work together First Photosystem - water-splitting Second Photosystem - NADPH-producing Energy from the electrons moving through the electron transport chain is used to pump H+ (protons) into the thylakoids This generates a gradient As the H+ move down the gradient, they drive the production of ATP The photosystems and ATP synthases are localized in the thylakoid membrane Calvin cycle Produces sugars from carbon dioxide The cycle uses Carbon from CO2 Energy from ATP High-energy electrons from NADPH Produces Glyceraldehyde 3-phosphate (G3P) - a 3C molecule CO2 joins with the 5-C sugar RuBP (ribulose-1,5-bisphosphate) catalyzed by the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) How many ATP and NADPH are needed per G3P produced? How many ATP and NADPH are needed per glucose produced?