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Photosynthesis Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Oxygenic photosynthesis is carried out by: cyanobacteria, 7 groups of algae, all land plants 2 Photosynthesis Overview • Photosynthesis takes place in 3 stages: – Capturing energy from sunlight – Using the energy to make ATP and reduce NADP+ to NADPH • (nicotinamide adenine dinucleotide phosphate) – Using the ATP and NADPH to synthesize organic molecules (glucose) from CO2 3 Photosynthesis Overview Photosynthesis is divided into: light-dependent reactions -capture energy from sunlight -make ATP and reduce NADP+ to NADPH carbon fixation reactions (lightindependent reactions) -use ATP and NADPH to synthesize organic molecules from CO2 4 5 Photosynthesis Overview • Photosynthesis takes place in chloroplasts. • thylakoid membrane – internal membrane arranged in flattened sacs – contain chlorophyll and other pigments – Organized into photosystems • Capture light and transfer energy (to pigment molecules) • grana – stacks of thylakoid membranes • stroma – semiliquid substance surrounding thylakoid membranes (houses the enzymes to make organic molecules) 6 Photosynthesis Overview • Photosynthesis takes place in the green portions of plants – Leaf of flowering plant contains mesophyll tissue – Cells containing chloroplasts – Specialized to carry on photosynthesis • CO2 enters leaf through stomata – Diffuses into chloroplasts in mesophyll cells – In stroma, CO2 fixed to C6H12O6 (sugar) – Energy supplied by light 7 8 Discovery of Photosynthesis The work of many scientists led to the discovery of how photosynthesis works. Jan Baptista van Helmont (1580-1644) Joseph Priestly (1733-1804) Jan Ingen-Housz (1730-1799) F. F. Blackman (1866-1947) 9 Pigments photon: a particle of light -acts as a discrete bundle of energy -energy content of a photon is inversely proportional to the wavelength of the light photoelectric effect: removal of an electron from a molecule by light -occurs when photons transfer energy to electrons 10 Electromagnetic Spectrum 11 Pigments Pigments: molecules that absorb visible light Each pigment has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing. 12 Pigments chlorophyll a – primary pigment in plants and cyanobacteria -absorbs violet-blue and red light chlorophyll b – secondary pigment absorbing light wavelengths that chlorophyll a does not absorb 13 Pigments accessory pigments: secondary pigments absorbing light wavelengths other than those absorbed by chlorophyll a -increase the range of light wavelengths that can be used in photosynthesis -include: chlorophyll b, carotenoids, phycobiloproteins -carotenoids also act as antioxidants 14 Photosystem Organization A photosystem consists of 1. an antenna complex (light harvesting complex) of hundreds of accessory pigment molecules that gather photons and feeds energy to reaaction center 2. a reaction center of one or more chlorophyll a molecules pass electrons out of photosystem (photochemical reactions) In summary, energy of electrons is transferred through the antenna complex to the reaction center. 15 Photosystem Organization At the reaction center (transmembrane protein complex), the energy from the antenna complex is transferred to chlorophyll a. This energy causes an electron from chlorophyll to become excited. The excited electron is transferred from chlorophyll a to an electron acceptor. Water donates an electron to chlorophyll a to replace the excited electron. 16 Photosynthesis Overview •Light Reactions •Two electron pathways operate in the thylakoid membrane: the noncyclic pathway and the cyclic pathway. •Both pathways produce ATP; only the noncyclic pathway also produces NADPH. •ATP production during photosynthesis is called photophosphorylation; therefore these pathways are also known as cyclic and noncyclic photophosphorylation. Calvin Cycle Reactions: Carbon Dioxide Fixation • CO2 is attached to 5-carbon RuBP molecule – Result in a 6-carbon molecule – This splits into two 3-carbon molecules (3PG) – Reaction accelerated by RuBP Carboxylase (Rubisco) • CO2 now “fixed” because it is part of a carbohydrate 20 Calvin Cycle Reactions: Carbon Dioxide Reduction • 3PG reduced to BPG • BPG then reduced to G3P • Utilizes NADPH and some ATP produced in light reactions 21 Calvin Cycle Reactions: Regeneration of RuBP • RuBP used in CO2 fixation must be replaced • Every three turns of Calvin Cycle, – Five G3P (a 3-carbon molecule) used To remake three RuBP (a 5-carbon molecule) 22 The Calvin Cycle: Fixation of CO2 23 Importance of Calvin Cycle • G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules • The hydrocarbon skeleton of G3P can form – Fatty acids and glycerol to make plant oils – Glucose phosphate (simple sugar) – Fructose (which with glucose = sucrose) – Starch and cellulose 24 – Amino acids