PHOTOSYNTHESIS 6.1 • The main form of energy from the sun is in the form of electromagnetic radiation • Visible radiation (white light) used for photosynthesis • Remember : ROY G. BIV? The electromagnetic spectrum • A Red Object absorbs the blue and green wavelengths and reflects the red wavelengths Why are plants green? • pigment a compound that absorbs light – different pigments absorb different wavelengths of white light. • chlorophyll is a pigment that absorbs red & blue light (photons) so green is reflected or transmitted. • Chlorophyll is located in the chloroplasts. So, Plants are green because the green wavelength is reflected, not absorbed. 2 types of chlorophyll • Chlorophyll a – involved in light reactions • Chlorophyll b – assists in capturing light energy – accessory pigment • Carotenoids – accessory pigments – captures more light energy – Red, orange & yellow The electromagnetic wavelengths and the wavelengths that are absorbed by the chlorophyll During the fall, what causes the leaves to change colors? Fall Colors • In addition to the chlorophyll pigments, there are other pigments present • During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments • Carotenoids are pigments that are either red, orange, or yellow Photosynthesis is • conversion of light energy into chemical energy that is stored in organic compounds (carbohydrates > glucose) • Used by autotrophs such as: – Plants – Algae – Some bacteria (prokaryotes) • glucose - energy-rich chemical produced through photosynthesis (carbohydrate) – C6H12O6 • Biochemical pathway – series of reactions where the product of one reaction is consumed in the next – E.g. photosynthesis product is glucose which is used in cellular respiration to make ATP Remember Redox Reaction reduction/oxidation The transfer of electrons from another Two types: 1. Oxidation is 2. Reduction is one or more one reactant to the loss of ethe gain of e- Oxidation Reaction The loss of electrons from a substance or the gain of oxygen. Oxidation 6CO2 + 6H2O C6H12O6 + 6O2 glucose Carbon dioxide Water Oxygen Reduction Reaction The gain of electrons to a substance or the loss of oxygen. Reduction 6CO2 + 6H2O C6H12O6 + 6O2 glucose Photosynthesis equation Light energy 6CO2 + 6H2O C6H12O6 + 6O2 Chlorophyll Reactants: Carbon dioxide and water Products: glucose and oxygen which is a byproduct Where does photosynthesis take place? Plants • Mainly occurs in the leaves: a. stoma - pores b.mesophyll cells Mesophyll Cell Chloroplast Stoma Mesophyll Cell of Leaf Nucleus Cell Wall Chloroplast Central Vacuole Photosynthesis occurs in these cells! Stomata (stoma) Pores in a plant’s cuticle through which water vapor and gases (CO2 & O2) are exchanged between the plant and the atmosphere. Stoma Carbon Dioxide (CO2) Guard Cell Oxygen (O2) Guard Cell Found on the underside of leaves Chloroplast Organelle where photosynthesis takes place. Stroma Outer Membrane Inner Membrane Thylakoid Granum Thylakoid stacks are connected together Parts Chloroplasts double membrane organelle that absorbs light energy Thylakoids – flattened sacs contain pigment - chlorophyll Grana (pl: granum) – layered thylakoids (like pancakes) Stroma – solution around thylakoids Stomata – pore on underside of leaf where O2 is released and CO2 enters Stroma : chloroplast :: cytosol : cytoplasm Thylakoid Thylakoid Membrane Granum Thylakoid Space Grana make up the inner membrane What do cells use for energy? Energy for Life on Earth • Sunlight is the ULTIMATE energy for all life on Earth • Plants store energy in the chemical bonds of sugars • Chemical energy is released as ATP during cellular respiration Structure of ATP • ATP stands for adenosine triphosphate • It is composed of the nitrogen base ADENINE, the pentose (5C) sugar RIBOSE, and three PHOSPHATE groups • The LAST phosphate group is bonded with a HIGH ENERGY chemical bond • This bond can be BROKEN to release ENERGY for CELLS to use Removing a Phosphate from ATP Breaking the LAST PHOSPHATE bond from ATP, will --– Release ENERGY for cells to use – Form ADP (adenosine diphosphate) – Produce a FREE PHOSPHATE GROUP High Energy Phosphate Bond FREE PHOSPHATE can be re-attached to ADP reforming ATP Process called Phosphorylation Phosphorylation Photosynthesis SUN 1. Light Reaction Produces energy from solar power (photons) in the form of ATP and NADPH. 2. Calvin Cycle • Also called Carbon Fixation or Carbon Cycle, Uses energy (ATP and NADPH) from light reaction to make sugar (glucose). • 3 stages of photosynthesis- Stages: • STAGE 1 - LIGHT REACTIONS - energy from sun is used to split water into H+ an O2 • STAGE 2 – energy is converted to chemical energy & stored in ATP & NADPH in stroma • STAGE 3 - CALVIN CYCLE where carbon is fixed into glucose Light Reaction (Electron Flow) • Occurs in the Thylakoid membranes • 2 possible routes for electron flow: Use Photosystem I and Electron Transport Chain (ETC) and generate ATP only • OR use Photosystem II and Photosystem I with ETC and generate O2, ATP and NADPH Photosynthesis animation • http://www.mhhe.com/biosci/genbio/biolink/ j_explorations/ch09expl.htm ELECTRON TRANSPORT - LIGHT REACTIONS in 5 steps Photosystem I and II • Step 1 – light excites e- in photosystem II • Step 2 – e- move to primary e- acceptor Step 3 – e- move along electron transport chain (etc) • Step 4 – light excites e- in photosystem I • Step 5 – e- move along 2nd (etc) • End – NADP+ combine H+ to make NADPH Light reaction animation • http://www.science.smith.edu/departments /Biology/Bio231/ltrxn.html Electron transport chain song • Play the "Come On Down (The Electron Transport Chain)" song performed by Sam Reid. Photolysis –photo-chemical splitting of water (restoring photosystem II) Chemiosmosis – synthesis of ATP • Powers ATP synthesis • Takes place across the thylakoid membrane • Uses ETC and ATP synthase • H+ move down their concentration gradient forming ATP from ADP • Concentration of protons is greater in thylakoid than stroma Chemiosmosis SUN H+ H+ Thylakoid (Proton Pumping) E T PS II PS I C H+ H+ H+ H+ H+ H+ ADP + P H+ H+ high H+ concentration ATP Synthase ATP Thylakoid Space low H+ concentration The Calvin Cycle 6.2 Calvin Cycle • Biochemical pathway in photosynthesis that produces organic compounds using ATP & NADPH • Carbon fixation – carbon atoms from CO2 are bonded or ‘fixed’ into carbohydrates • occurs in stroma Calvin Cycle Carbon Fixation C3 plants (80% of plants on earth) Occurs in the stroma Uses ATP and NADPH from light reaction as energy • Uses CO2 • To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH. • • • • Chloroplast Outer Membrane Inner Membrane STROMA– where Calvin Cycle occurs Thylakoid Granum Calvin Cycle (C3 fixation) (36C) 6C-C-C-C-C-C (6C) 6CO2 (unstable) (30C) 6C-C-C-C-C RuBP (30C) glucose 6C-C-C 12PGA (36C) 6ATP 6ATP 6NADPH 6NADPH 6C-C-C 6ATP C3 6C-C-C (36C) 6C-C-C 12G3P (6C) C-C-C-C-C-C Glucose Calvin Cycle Remember: C3 = Calvin Cycle C3 Glucose Step 1 • CO2 diffuses fr cytosol & combines with RuBP which splits into pair of PGA Step 2 • PGA gets phosphate gr fr ATP gets proton fr NADPH to become PGAL • Reaction produces: ADP, NADP+ & phosphate to be used again Step 3 • PGAL converts back to RuBP • Allows Calvin cycle to continue Alternates: • C3 plants – use Calvin cycle exclusively – Form 3-carbon compounds • C4 pathway – evolved in hot, dry climate – Form 4-carbon compounds – Partially close stomata – E.g. Corn, sugar cane, crabgrass • CAM – open stomata at night, close in day – Grow slow, lose less water – E.g. cactus, pineapple C4 Plants • Hot, moist environments • 15% of plants (grasses, corn, sugarcane) • Photosynthesis occurs in 2 places: – Light reaction mesophyll cells – Calvin cycle - bundle sheath cells C4 Plants Malate-4C sugar C-C-C-C Malate C-C-C-C Transported CO2 CO2 C3 glucose C-C-C PEP ATP Mesophyll Cell C-C-C Pyruvic Acid Bundle Sheath Cell Vascul Tissu CAM Plants • Hot, dry environments • 5% of plants (cactus and ice plants) • Stomates closed during day • Stomates open during the night • Light reaction - occurs during the day • Calvin Cycle - occurs when CO2 is present CAM Plants Night (Stomates Open) Day (Stomates Closed) Vacuole CO2 C-C-C-C Malate C-C-C-C Malate C-C-C-C Malate CO2 C3 C-C-C PEP ATP C-C-C Pyruvic acid glucose Rate of photosynthesis is effected by light intensity, CO2 or temperature • High intensity or high CO2 = high rate – Growth graph levels off (plateau) • High temp = initial high rate but peaks – Rate drops when stomata closes Recap • Photosynthesis converts light energy into chemical energy thru series of biochemical pathways • Electrons excite in photosystem II – move along ETC to photosystem I • electrons are replaced when water is split • oxygen is byproduct • ATP synthesized across thylakoid • Calvin cycle – carbon is fixed & sugar produced • 3 turns produce PGAL (PGAL keeps cycle going • Other pathways – C3, C4, CAM References • www.biologyjunction.com
