Chapter 10 Photosynthesis

1. Chapter 10 Photosynthesis

2. Review • This reaction uses light energy to produce sugars and other food molecules from carbon dioxide and water. • Organisms that perform this reaction are called AUTOTROPHS, therefore are considered PRODUCERS.

4. Chloroplast • Membrane bound organelle that contains two inner areas for reactions: • GRANA (THYLAKOID): A suspended system of disk like membranes sacs, interconnected. Contain chlorophyll molecules and much of the machinery to convert light into chemical energy. • STROMA: Thick fluid found within the chloroplast responsible for the conversion of carbon dioxide and water into sugars.

9. General Reaction 6CO2 + 12H2O  C6H12O6 + 6H2O + 6O2 Determined that oxygen came from the splitting of water through the use of O18 in 1950’s. First proposed by C.B. Van Niel of Stanford University.

10. Visible Light Radiation • The visible spectrum of light is the most important to life on this planet (380-750nm). • Inverse relationship between wavelength and energy. • Chlorophyll a absorbs strong in the blue-violet and red light and reflects mainly green light. • Chlorophyll b absorbs blue and orange and reflects mainly yellow-green. • Carotenoids absorb mainly blue-green light.

16. Stages of Photosynthesis • 1st stage – Light reaction • 2nd Stage – Dark reaction (Calvin Cycle) or carbon fixation. • Redox involved using the molecule NADPH – nicotinamide adenine dinucleotide phosphate.

17. Excitation of Chlorophyll by Light • When a molecule absorbs a photon, one of the molecule’s electrons is elevated to a higher energy level excited state • The only photons absorbed are those whose energy is exactly equal to the energy difference between ground and excited, this varies between molecules. • Therefore a particular compound absorbs photons corresponding to specific wavelengths, therefore unique absorption spectrum.

18. Photosystems • Each photosystem contains a region called a reaction center surrounded by a number of pigment complexes containing chlorophyll a, b and carotenoids bound by other proteins • The number and variety of pigment proteins allows the photosytem to harvest light over a larger surface area than one pigment alone.

19. Together these light harvesting complexes act as an “antenna” by funneling the photon energy into the reaction center. • Each reaction center contains 2 chlorophyll a molecules and a primary electron accepter molecule. • These chlorophyll a molecules are special because their location and environment enables them to use energy from light to boost their electrons to an excited state.

22. Photosystem (con.) Two types of photosystems • Photosystem II: chlorophyll a of the reaction center absorbs red light (680nm), therefore called p680. • Photosystem I: chlorophyll a of this reaction center absorbs far red light (700nm), therefore called p700.

23. Light Reactions 3 key events take place in the grana: • Absorption of light energy • Excitation of electrons by that energy • Formation of ATP and NADPH

27. Comparison of Chemiosmosis in Chloroplasts and Mitochondria 1.In mitochondria, high energy electrons are extracted from food molecules. In photosynthesis photosystems capture light energy and use it to drive electrons to the top of the transport chain. • Mitochondria pump protons from the matrix out to the inner membrane space. • Chloroplasts pump protons from the stroma into the thylakoid space.

29. Calvin Cycle (dark rxn.) • Occurs in the stroma • Carbon enters as CO2 and leaves as sugar. • The cycle requires the ATP made during the light rxn. and consumes the NADPH as reducing power fro adding electrons to make sugar.

32. Alternative Methods of Carbon- fixaton There are other ways of carbon fixation that certain species of plants perform as an adaption to their environment. They are: • C3 • C4 • CAM

33. C3 Method • These plants utilize RUBISCO as the enzyme to initially fix CO2 to Ribulosebisphosphate. A 3 carbon compound (3-phosphoglycerate) is formed. ie. Rice, wheat and soybeans

34. C3 Issues: • In the case of hot, dry weather, RUBISCO incorporates O2 instead of CO2 when concentrations are high. This reaction produces a 2 carbon molecule (phosphoglycolate). • Phosphoglycolate’s phosphate is hydrolyzed in the chloroplast and glycolate is transported to peroxisomes where the H electrons reduce O2 into H2O2 and converted to H2O and O2.

35. The remaining portions of the glycolate are transported to the mitochondria where the Kreb’s cycle releases CO2. As a result of this, carbons are siphoned away from carbon fixation without the production of sugar or ATP.

36. Photorespiration When weather conditions are hot and dry, the plant responds by closing the stomates. Stomates are the openings into the mesophyll (middle layer, air spaces) of the leaf. This reduces the loss of transpirational water vapor but also decreases the concentration of incoming CO2 and increases the production of O2.

38. C4 • These plants modify their carbon fixation by forming a 4 carbon oxaloacetate in the mesophyll cell, through the use of the enzyme PEP carboxylase. • This enzyme has a high affinity for CO2 therefore preventing O2 bonding, even though CO2 concentration is low. • CO2 is released into a bundle sheath cell where the Calvin cycle is isolated

40. CAM • These plants utilize the night time as a time to open their stomates for CO2 instead of the day. This helps to prevent water loss when the sunlight is at its peak. • The CO2 taken in during the night store it as organic acids. In the day time the leaves use the light reaction to provide ATP and NADPH and the cells release the CO2 from the organic acids to become incorporated into sugar.