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Photosynthesis

Photosynthesis. Photosynthesis in Overview. Process by which plants and other autotrophs store the energy of sunlight into sugars. Requires sunlight, water, and carbon dioxide. Overall equation: 6 CO 2 + 12 H 2 0  C 6 H 12 O 6 + 6 O 2 + 6 H 2 0

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Photosynthesis

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  1. Photosynthesis

  2. Photosynthesis in Overview • Process by which plants and other autotrophs store the energy of sunlight into sugars. • Requires sunlight, water, and carbon dioxide. • Overall equation: 6 CO2 + 12 H20  C6H12O6 + 6 O2 + 6 H20 • Occurs in the leaves of plants in organelles called chloroplasts.

  3. Leaf Structure • Most photosynthesis occurs in the palisade layer. • Gas exchange of CO2 and O2 occurs at openings called stomata surrounded by guard cells on the lower leaf surface. Palisade Spongy

  4. Stomata Turgid Plasmolysed

  5. Pigments • Chlorophyll A is the most important photosynthetic pigment. • Other pigments called accessory pigments are also present in the leaf. • Chlorophyll B • Carotenoids (orange / red) • Xanthophylls (yellow / brown) • These pigments are embedded in the membranes of the chloroplast in groups called photosystems.

  6. Chlorophyll absorbs blue and red light

  7. Chlorophyll A & B

  8. Chloroplast Structure • Thickened regions called thylakoids. A stack of thylakoids is called a granum. (Plural – grana) • Stroma is a liquid surrounding the thylakoids.

  9. Answer key1. outer membrane2. inner membrane space3. inner membrane 4. stroma (aqueous fluid)5. thylakoid lumen (inside of thylakoid)6. thylakoid membrane7. granum (stack of thylakoids)8. thylakoid (lamella)9. starch10. ribosome11. DNA12. lipid drops

  10. Photosynthesis: The Chemical Process • Occurs in two main phases. • Light reactions • Dark reactions (aka – the Calvin Cycle) • Light reactions are the “photo” part of photosynthesis. Light is absorbed by pigments. • Dark reactions are the “synthesis” part of photosynthesis. Trapped energy from the sun is converted to the chemical energy of sugars.

  11. . • Light-dependent reactions occur on the thylakoid membranes. • INPUT • - Light and water are required for this process. -ADP needed to conserve the excess energy -NADP needed to mop up the H+ and electrons OUTPUT • -Energy storage molecule ATP is formed. • -Loaded molecules NADPH formed • -Oxygen gas is made as a waste product

  12. Light Dependent Reaction INPUT OUTPUT H+ NADPH H+ e ATP O2

  13. The light-dependent portion of photosynthesis is carried out by two consecutive photosystems (photosystem I and photosystem II) in the thylakoid membrane of the chloroplasts.

  14. Photosystems are clusters of light-absorbing pigments

  15. Dark Reaction/Calvin-Benson cycle • Dark reactions (light-independent) occur in the stroma. • Carbon dioxide is “fixed” into the sugar glucose. • ATP and NADPH molecules created during the light reactions power the production of this glucose.

  16. DARK REACTION-Carbon Fixation INPUT OUTPUT NADPH ADP ATP NADP CALVIN CYCLE PGAL or G3P PGAL

  17. Rubisco Most abundant enzyme in leaves and hence on earth. Made up of 16 polypeptide chains.

  18. RuBP + CO2  2 copies of 3-PGA PGAL The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or "triose phosphates," namely, glyceraldehyde-3-phosphate (G3P)/PGAL- phosphoglyceraldehyde.

  19. Photorespiration • Photorespirationoccurs when the CO2 levels inside a leaf become low. This happens on hot dry days when a plant is forced to close its stomata to prevent excess water loss. • When the CO2 levels inside the leaf drops to certain level, Rubisco starts to combine O2 with RuBP instead of CO2. • The net result of this is that instead of producing 2 3C PGA molecules, only one molecule of PGA is produced and a toxic 2C molecule called phosphoglycolateis produced. • However, unlike cellular respiration, photorespiration consumes ATP.

  20. Photorespiration • The plant must get rid of the phosphoglycolate . • The peroxisome converts it into non poisnous state.

  21. C4Plants In hot and dry conditions, plants close their stomata to prevent the loss of water. Under these conditions, CO2 will decrease. Some plants have evolved mechanisms to increase the CO2 concentration in the leaves under these conditions.C4 plants chemically fix carbon dioxide in the cells of the mesophyll by adding it to the three-carbon molecule . Carbon dioxide is drawn out of malate and into this reaction rather than directly from the air. C4 plants can produce more sugar than C3 plants in conditions of high light and temperature.

  22. Light and Dark Reaction

  23. The effect of light intensity • Low light intensity lowers the rate of photosynthesis. • There is a shortage of the products of the light dependent reactions, NADPH and ATP. • Very high intensity may, in fact, slow down the rate as it bleaches the chlorophyll.

  24. Temperature Since both the stages of photosynthesis require enzyme activity, the temperature has an affect on the rate of photosynthesis. Enzyme activity increases with moderate increase of temperature.

  25. Carbon Dioxide Concentration • Increased amount of CO2 will increase the rate of photosynthesis to a certain limit, after which a further increase in its amount will no longer increase the rate any further. This is when the other factors necessary for photosynthesis, such as light, become "limiting reactants." Those other factors also need to increase to bring about a further increase in the rate.

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