Photosynthesis: Organelles, Pores, and Reactants

2. Chapter 10 RQ • What organelles are the site for photosynthesis? • What are the pores in a leaf called through which gases exchange? • From which reactant molecule does the oxygen we breathe come from? • What is “photophosphorylation”? • What component of light is actually ABSORBED by chlorophyll?

3. Heterotrophic Nutritional mode of acquiring organic molecules from compounds produced by other organisms Autotrophic Nutritional mode of synthesizing organic molecules from inorganic raw materials  1. Distinguish between autotrophic and heterotrophic nutrition.

4. Photoautotrophs Autotrophic organisms that use light as their energy source Ex: plants, algae, some prokaryotes Chemoautotrophs Autotrophic organisms that use oxidation of inorganic substances, such as sulfur or ammonia, as their energy source Ex: unique to some bacteria (generally archaebacteria)  2. Distinguish between photosynthetic autotrophs and chemosynthetic autotrophs.

5. 3. Describe the location and structure of the chloroplast. • Chloroplasts are found within the mesophyll (the green tissue) of a leaf • Lens-shaped organelles measuring 2-4 µm by 4-7µm and is divided into 3 compartments by membranes • Intermembrane space – separates the double membranes that encase the chloroplast • Thylakoid – chlorophyll is here  helps to initially convert light energy to chemical energy - composed of grana, which are stacks of thylakoids 3. Stroma – the viscous fluid outside the thylakoids where carbon dioxide is made to sugar

7. 4. Write a summary equation for photosynthesis. Explain how this equation relates to the equation for cellular respiration. Photosynthesis 6 CO2 + 6H2O + light energy  C6H12O6 + 6O2 Cellular Respiration C6H12O6 + 6O2  6CO2 + 6H2O + energy 

8. 5. Explain van Niel’s hypothesis and describe how it contributed to our current understanding of photosynthesis. • Van Niel hypothesized that plants split water as a source of hydrogen and release oxygen as a by-product • For the experiment, 18O was used as a tracer CO2 + 2H2O*  CH2O + H2O + O2* OR CO2* + 2H2O  CH2O* + H2O* + O2 

9. 6. Explain the role of redox reactions in photosynthesis. • Photosynthesis is an endergonic redox process energy is required to reduce carbon dioxide • Light is the energy source that boosts potential energy of electrons as they are moved from water to sugar • When water is split, electrons are transferred from the water to carbon dioxide, reducing it to sugar 

10. Light Reactions - to convert light energy to chemical energy (ATP) Dark Reactions also known as the Calvin Cycle To convert carbon dioxide to sugar, using chemical energy  7. Briefly describe the purpose of the light reactions and the Calvin Cycle.

12. Wavelike Wavelength distance between the crests of electromagnetic waves Rhythmic waves Visible light is the part that drives photosynthesis Particlelike Behaves as if it consists of discrete particles or quanta called ‘photons’ The sun radiates the full spectrum  8. Describe the wavelike and particlelike behaviors of light.

14. 9. Describe the relationship between and action spectrum and an absorption spectrum. • Each pigment has a characteristic absorption spectrum; or wavelengths is absorbs • The graph of wavelength versus rate of photsynthesis is the action spectrum and profiles the effectiveness of different wavelengths

15. 10. Explain why the absorption spectrum for chlorophyll differs from the action spectrum for photosynthesis. • Chlorophyll ‘a’ is not the only pigment in chloroplasts that absorbs light - the absorption spectrum for chlorophyll ‘a’ underestimates the effectiveness of some wavelengths • Even though only special chlorophyll ‘a’ molecules can participate directly in light reactions, other pigments called ‘accessory pigments’ can absorb light and transfer the energy to chlorophyll ‘a’ 

17. 11. List the wavelengths of light that are most effective for photosynthesis. • In Photosystem I the wavelength absorbed is 700nm (red color) • In Photosystem II the wavelength absorbed is 680nm 

18. 12. Explain what happens when chlorophyll or accessory pigments absorb photons (photoexcitation). • Colors of absorbed wavelengths disappear from the spectrum of transmitted and reflected light • The absorbed photon boosts one of the pigment molecules’ electrons in its lowest energy state to an orbital of higher potential energy (ground state  excited state) 

20. 13. List the components of a photosystem and explain their function. • Antenna complex: lots of chlorophyll ‘a’, ‘b’, and carotenoid molecules absorb photons and pass energy - different pigments absorb wider photons 2. Reaction center chlorophyll: one of the chlorophyll ‘a’ molecules transfers the excited electrons that starts the light reactions 3. Primary electron acceptor: traps the excited electron (which is the first step of the light reactions) 

23. 14. Trace noncyclic electron flow through photosystems II and I. • Light excites electrons from P700, which are ultimately stored in NADPH, which will become the electron donor in the Calvin Cycle • Excited electron is transferred from P700 to the primary electron acceptor in photosystem I • They are then passed to ferredoxin (Fd), an iron protein - NADP+ reductase catalyzes the redox reaction that transfers Fd 3. The oxidized P700 chlorophyll becomes an oxidizing agent as the ‘hole’ is filled and photosystem II supplies the electrons 

26. 15. Compare cyclic and noncyclic electron flow and explain the relationship between these components of the light reactions. • Electrons from P680 flow to P700 during noncyclic electron flow, restoring missing electrons in P700 • Cyclic  because excited electrons that leave chlorophyll ‘a’ return to that reaction center - involves only photosystem I - produces additional ATPs, without producing NADPH or O2 • [NADPH] might influence whether electron flow will go through cyclic or noncyclic pathways 

28. 16. Summarize the light reactions with an equation and describe where they occur. • Light reactions occur in the chloroplast’s thylakoid membranes Light energy  chemical energy (in NADPH & ATP) • Noncyclic  produces ATP, NADPH, & O2 as a byproduct • Cyclic  produces ATP 

29. 17. Describe important differences in chemiosmosis between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. • Electron transport chain - mitochondria transfer chemical energy from food  ATP - high energy electrons pass down and are extracted by the oxidation of food molecules - chloroplasts transform light energy to chemical energy; use light to drive electrons to the top of the chain • Spatial organization - mitochondria’s inner membrane pumps protons from the matrix out to the intermembrane space where there is lots of protons - chloroplasts pump protons from stroma to thylakoid 

31. 18. Summarize the carbon-fixing reactions of the Calvin Cycle and describe changes that occur in the carbon skeleton of the intermediates. • Molecule of sugar  G3P (glyceraldehyde-3-phosphate) • Each molecule of CO2 is attached to a five carbon sugar (ribulose biphosphate – RuBP) • Coupling of ATP hydrolysis with the reduction of 3-phosphoglycerate to glyceraldehyde phosphate • Rearrangement of the 5 G3P molecules into 3 RuBP molecules - requires 3 ATP molecules - regeneration of RuBP 

35. 19. Describe the role of ATP and NADPH in the Calvin cycle. • Converts CO2 to sugar • ATP is the energy source to do this • NADPH is the reducing agent that adds high-energy electrons to form sugar 

36. 20. Describe what happens to rubisco when the oxygen concentration is much higher that carbon dioxide. • Rubisco accepts O2 and transfers it to RuBP “photorespiration” • Occurs because the active site of rubisco can accept O2 as well as CO2 • Produces no ATP molecules • Decreases photosynthetic output by reducing organic molecules used in the Calvin Cycle 

37. 21. Describe the major consequences of photorespiration. • Photorespiration is a metabolic pathway that consumes O2, evolves CO2, produces no ATP and decreases photosynthetic output • Fostered on hot dry bright days (close stomata) • In the desert, some species have evolved alternate modes of carbon fixation that minimize photorespiration (C4 and CAM) 

38. 22. Describe two important photosynthetic adaptations that minimize photorespiration. CAM and C4 Plants are similar…. • CO2 is first incorporated into organic intermediates before it enters the Calvin Cycle Differ…. • The initial steps of carbon fixation in C4 plants are structurally separate from the Calvin Cycle; CAM the 2 steps occur at different times 

39. C4 plants… • Incorporate CO2 into 4-carbon compounds • Used by 1000’s of species (corn, sugarcane, grasses) • Enhances carbon fixation under conditions for photorespiration • Leaf anatomy segregates the Calvin Cycle from the 1st incorporation of CO2 into organic compounds • Calvin Cycle of these plants is preceded by the incorporation of CO2 into organic compounds in the mesophyll 

40. CAM plants… • Generally are succulent plants in very dry regions • Open stomata primarily at night and close them during the day (opposite of most plants) • Conserves water during the day, but prevents CO2 from entering the leaves • When the stomata are open at night CO2 is taken up and incorporated into a variety of organic acids (called crassulacean acid metabolism) • Acids are stored until morning - during the daytime the light reactions supply ATP and NADPH for the Calvin Cycle; here is where the CO2 is released from the organic acids from nighttime  sugar 

42. 23. Describe the fate of photosynthetic products. O2 & H2O & sugars • Supply the entire plant with chemical energy and carbon skeletons to synthesize organic molecules

43. The End 

44. The only reason people get lost in thought is because it's unfamiliar territory.