Chapter 10 - Photosynthesis (ML) PDF

Chapter 10: Photosynthesis  Outline An overview of photosynthesis. Light absorption by pigments. The photosynthetic electron transport chain. – Light reaction - reaction centre. The Calvin cycle. Re...

Chapter 10: Photosynthesis  Outline An overview of photosynthesis. Light absorption by pigments. The photosynthetic electron transport chain. – Light reaction - reaction centre. The Calvin cycle. Readings: Chapter 10, p212 - p232.  Learning outcomes By the end of this lesson, you should be able to 1. describe the biological function of photosynthesis. 2. describe the two major stages of photosynthesis. 3. describe the structure of chloroplasts, mesophyll cells, and leaves. 4. characterize the nature of light. 5. explain the function of photosynthetic pigments. 6. describe the structure and function of a photosystem. 7. describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. 8. describe the three phases of the Calvin cycle.  Plants are photoautotrophic organisms An autotroph is an organism that produces organic molecules using CO2 as a carbon source. Autotrophs produce their own food to sustain themselves without eating other organisms. Plants, some protists, algae and cyanobacteria are photoautotrophs.  Photosynthesis Photoautotrophs use the energy of sunlight and carbon from CO2 to make carbohydrates by the process of photosynthesis. What does CO2 contribute to photosynthesis? 6 6 6 Which is the reducing agent? CO2 or H2O? Is photosynthesis an endergonic or exergonic reaction? Is photosynthesis a catabolic process or an anabolic process?  Photosynthesis is a series of redox reactions During photosynthesis, CO2 molecules are reduced to form carbohydrate molecules. ─ The electron donor (reducing agent) is water. The oxidation of water results in the production of electrons, protons, and O2. The electrons and protons are incorporated into carbohydrates, and O2 is a by-product. Where does CO2 come from? So which molecule produced the O2 in the reaction?  Photosynthesis occurs in the chloroplasts Photosynthesis occurs mostly in leaves of plants which contain the chloroplasts. Which plant cells perform photosynthesis? What are the functions of the phloem and xylem tissue? What are the functions of the guard cells?  The chloroplast Chloroplasts are enclosed by a double membrane. Inside the chloroplast is a third, highly folded membrane known as the thylakoid membrane. ─ The photosynthetic electron transport chain is located at the thylakoid membrane.  Photosynthesis occurs in two stages linked by ATP and NADPH – Photosynthesis occurs in two metabolic stages: 1. The light harvesting reactions 2. The Calvin cycle The light harvesting reaction occurs at the thylakoid membranes. The Calvin cycle occurs in the stroma.  The light harvesting reactions H 2O Light Light is absorbed by protein-pigment NADP+ complexes known as ADP photosystems to drive + Pi the transfer of electrons Light from water to NADP+ Reactions reducing it to NADPH. ATP ATP is synthesized by NADPH using an electron transport chain. Chloroplast O2 is a byproduct. O2 The Calvin cycle H2O CO2 Light Return ADP, P and NADP to The Calvin the light reaction cycle uses ATP NADP+ and NADPH ADP produced from + Pi the light Calvin Light Cycle reactions to Reactions convert CO2 ATP into sugars. NADPH Chloroplast O2 [CH2O] (sugar)  Photosynthesis uses light energy to make food molecules H2O CO2 About half of the Chloroplast sugars made by Light photosynthesis are consumed as fuel for NADP+ cellular respiration in ADP plant cells. P Light Reactions RuBP Photosystem II Calvin Cycle 3-PGA Electron (in stroma) transport chain Thylakoids Photosystem I ATP Stroma NADPH Triose phosphate Cellular respiration Cellulose Starch O2 Sugars Other organic compounds Why do plants need mitochondria for ATP production? Starch granules in a chloroplast Excess carbohydrates are converted to starch and stored temporarily in the chloroplasts.  Light absorption Light is a type of electromagnetic energy with wave-like properties. Wavelength is the distance between crests of waves. – Wavelength determines the type of electromagnetic energy. – The amount of energy is inversely related to the wavelength of the light. Visible radiation absorbed by pigments drives the light reactions Pigments are molecules that absorb wavelengths of visible light. Pigments look colored because they reflect or transmit light wavelengths that they do not absorb. Explain why leaves appear green? Light Reflected light Chloroplast Absorbed Thylakoid light Transmitted light The absorption spectra of pigments in chloroplasts An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength. – The absorption spectrum of chlorophyll suggests that violet-blue and red wavelengths of light work best for photosynthesis. What wavelength of light to chlorophyll a and chlorophyll b reflect? Carotenoids are able to 1. absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll. 2. It can also protect chlorophyll from high-energy photons, which can be damaging.  Chlorophyll Chlorophyll has a large, light-absorbing head. The light-absorbing head absorbs violet-blue light and red light while reflecting green light. a long hydrocarbon tail. The hydrocarbon tail allows the chlorophyll pigment to be anchored in the lipid membrane.  Chlorophyll light absorption in the lab When chlorophyll molecules absorb light, one of its electrons goes from a ground state to an excited state. For isolated chlorophyll in the lab, most of the light energy absorbed by the electron is rapidly released as heat and as fluorescent light. Why is heat released when the excited electron returns to the ground state? Chlorophyll light absorption in a plant cell When electrons in a chlorophyll molecule of a plant cell absorbs light and becomes excited, the electron can transfer the absorbed energy to another electron in an adjacent chlorophyll molecule. The excited electron in the second chlorophyll molecule can then transfer the energy to another electron in another adjacent chlorophyll molecule. The chlorophyll molecules that absorb and transfer the energy from light are called antenna chlorophylls. The reaction centre converts light to chemical energy The energy from the absorbed light is passed from antenna chlorophyll to antenna chlorophyll molecules until it reaches a specially configured pair of chlorophyll molecules known as the reaction centre. The reaction centre chlorophylls have a distinct configuration and can transfer the high-energy electron to an electron acceptor. When the electron transfer takes place, the reaction centre becomes oxidized, and the electron acceptor is reduced. The electron acceptor now enters the photosynthetic electron transport chain.  The reaction centre converts light to chemical energy In order for the photosystem II to continue capturing light energy, another electron must be delivered to the reaction center to replace the one that was lost. Which molecule is the electron donor in photosynthesis? A O 2. B CO2. C H2O. H2O  The photosynthetic electron transport chain contains two photosystems The photosynthetic electron transport chain connects photosystem II to photosystem I. Two photosystems are necessary to provide enough energy to pull electrons from water and then use them to reduce NADP+ to form NADPH.  Photosystem II and photosystem I Photosystem II supplies electrons to the beginning of the photosynthetic electron transport chain. When photosystem II loses an electron, it can pull another electron from water. Photosystem I energizes the electrons with a second input of light energy, so they have enough energy to reduce NADP+ to form NADPH. What do you think this structure is?  Proton accumulate in the thylakoid lumen 1. The oxidation of H2O in the thylakoid lumen form O2 and H+, increasing the H+ concentration. 2. Cytochrome b6f complex works as a H+ pump to move H+ from the stroma to the lumen as electrons pass through the complex, releasing energy. 2 1 Protons flow back into the stroma During electron through the ATP synthase, driving the transport, protons synthesis of ATP. Stroma accumulate in the ATP thylakoid lumen. ADP + Pi Light Light H+ ATP synthase H+ PS II Cyt-b6f PS I Pq H+ O2 H+ H2 O H+ H+ Lumen This type of ATP production is called photophosphorylation.  Photosystem I Photosystem II The Z Scheme As an electron is transferred from water (1) in photosystem II, its energy level is initially H2O high (2), but as the electron NADP+ + H+ travels through the electron e- e- e- e- transport chain, its energy NADPH level decreases (3). 1/2 O2 + 2 H+ 4 The Z energy trajectory 5 It takes a second input of light 2 energy in photosystem I to ET Energy of electrons C 3 raise the electron energy level 1 high (4) enough so that it can be used to reduce NADP+ (5). Absorption of light energy A second input... so that by PS II energizes electrons of light energy they can pulled from water, allowing by PS I raises the be used to them to enter the electrons reduce photosynthetic electron to an even NADP+. transport chain. higher level...  Mitochondrion Chloroplast H+ c. a. d. b. e. Compare chemiosmosis and electron transport in mitochondria and chloroplast. In each case, answer the following questions below. What do each of the labels represent in the mitochondrion vs. the chloroplast? Where do the electrons come from? How do the electrons get their high potential energy? What picks up the electrons at the end of the chain? How is the energy from the electrons used to do work?  The Calvin cycle: reducing CO2 to sugar The Calvin cycle makes sugar in the chloroplast. The Calvin cycle occurs in the stroma of the CO2 chloroplast. Input ATP NADPH To produce sugar, the key molecules are – CO2. – ATP and NADPH generated by the light reactions. Calvin The Calvin cycle uses these three molecules to Cycle produce a three-carbon sugar called glyceraldehyde 3-phosphate (G3P)/triose phosphate. A plant cell may then use G3P to make glucose and other carbohydrates. Output: G3P Can the Calvin cycle occur in the dark?  The 3 main steps of the Calvin cycle The Calvin cycle consists of three main steps: 1. Carboxylation: CO2 is added to a 5-carbon molecule, ribulose-1,5- bisphosphate (RUBP). I. This reaction is catalyzed by the enzyme rubisco. II. The 6-carbon molecule formed is immediately broken down into two 3-carbon molecules of 3-phosphoglycerate (3-PGA). 2. Reduction: The 3-PGA is then reduced by the electron carrier NAPDH to form G3P. I. First, ATP is used to phosphorylate 3-PGA. II. Then NADPH transfers electrons resulting in the formation of the high-energy molecule glyceraldehyde 3-phosphate (G3P)/triose phosphate. G3P is a precursor for glucose. 3. Regeneration: The RuBP needed for carboxylation is regenerated which involves 12 steps. ATP is also required for this step.  The Calvin cycle Rubisco G3P For every 6 glyceraldehyde The 5 remaining 3-carbon 3-phosphate/triose triose phosphates are phosphate molecules that reshuffled to produce three are produced, only one can 5-carbon RuBP molecules. be withdrawn from the Calvin cycle.