Photosynthesis Folens PDF

PHOTOSYNTHESIS 13 UNIT 2 THE CELL Learning outcomes After studying this topic you should be able to: 1 Define photosynthesis. 2 State the role of photosynthesis. 3 Give a balanced equation for the overall reaction of photosynthesis. 4 Identify the location of chlor...

PHOTOSYNTHESIS 13 UNIT 2 THE CELL Learning outcomes After studying this topic you should be able to: 1 Define photosynthesis. 2 State the role of photosynthesis. 3 Give a balanced equation for the overall reaction of photosynthesis. 4 Identify the location of chlorophyll. 5 Describe how light energy causes electrons to be released by chlorophyll, which combine with protons and carbon dioxide to form carbohydrate. The protons come from the splitting of water, which also releases oxygen. UNIT 2 THE CELL Learning outcomes After studying this topic you should be able to: 6 Identify the source of light, water and carbon dioxide for photosynthesis. 7 Carry out an experiment to show the effect of light intensity or carbon dioxide concentration on photosynthesis. 8 Refer to the methods by which humans can artificially modify the rates of photosynthesis in horticulture. 9 Describe the two stages of photosynthesis: the light- dependent stage producing ATP, NADPH and oxygen; and the light-independent stage, which uses CO2, ATP and NADPH and produces carbohydrate. UNIT 2 THE CELL Energy for cells All living organisms need energy to survive. UNIT 2 THE CELL Energy for cells Green plants can produce carbohydrates from simple chemicals using the sun’s energy. This reaction is called photosynthesis. Photosynthesis takes place in the chloroplasts of cells. light 6H2O + 6CO2 C6H12O6 + 6O2 chlorophyll UNIT 2 THE CELL Photosynthesis overview light 6H2O + 6CO2 C6H12O6 + 6O2 chlorophyll Photosynthesis requires: Water Carbon dioxide Light energy Chlorophyll UNIT 2 THE CELL Photosynthesis Photosynthesis requires the presence of a green pigment, chlorophyll, in the plant cell. UNIT 2 THE CELL Photosynthesis This pigment is found in a special cellular organelle called a chloroplast. UNIT 2 THE CELL Photosynthesis Some light energy is used to release high-energy electrons from chlorophyll. Light energy is also used to split water into H+ ions (protons), electrons and oxygen. UNIT 2 THE CELL What happens to the products when water is split? The H+ ions go into a common pool of protons in the chloroplast. The electrons from water go to the chlorophyll to replace those lost. Most of the oxygen gas is released as a waste product out of the leaf through the stomata. Some of the oxygen produced will be used in the cells of the leaf for respiration. UNIT 2 THE CELL The source of raw materials for photosynthesis Carbon dioxide enters the leaf through the stomata, found mainly on the underside of the leaf. The water for photosynthesis comes up to the leaf from the roots via the xylem vessels in the veins. UNIT 2 THE CELL To show the effect of light intensity on the rate of photosynthesis UNIT 2 THE CELL Photosynthesis: extended study (HL) light 6H2O + 6CO2 C6H12O6 + 6O2 chlorophyll Photosynthesis is a complex reaction that uses the sun’s energy to manufacture sugar from carbon dioxide and water. The reaction has two main stages. The first stage is called the light-dependent stage, as it only takes place in the presence of light. In these reactions, NADPH and ATP are made and water is split, releasing oxygen. This process uses the chlorophyll pigment molecules. The second stage is dependent on the products of the light phase, i.e. NADPH and ATP. Carbon dioxide is reduced to sugar using these products. As light is not directly required in these reactions, it is called the light-independent stage. UNIT 2 THE CELL The light-dependent stage (HL) The energised electrons leave chlorophyll and go to an acceptor molecule. UNIT 2 THE CELL The role of chlorophyll in photosynthesis (HL) There are two possible pathways that these electrons can take. In the first pathway, they will return to a chlorophyll molecule via a series of carrier molecules. UNIT 2 THE CELL The role of chlorophyll in photosynthesis (HL) In the second possible pathway: The electrons are passed from the acceptor to NADP+. This results in chlorophyll being deficient in electrons. The electrons needed to replenish the chlorophyll come from the splitting of water. H2O 2H+ + 2e− + ½O2 (is released as a gas) UNIT 2 THE CELL The role of chlorophyll in photosynthesis (HL) The NADP+ receives two electrons from the chlorophyll to become NADP–. H+ ions (from a pool of such protons in the chloroplast) are attracted to the NADP− forming NADPH. NADP+ + 2e− + H+ NADPH The H+ ions released by the splitting of water replenish the proton pool in the chloroplast. The remaining oxygen is released as a gas. UNIT 2 THE CELL The light-dependent stage (HL) UNIT 2 THE CELL The light-independent stage (HL) There are three products of the light-dependent stage: NADPH, ATP and oxygen gas. The NADPH is used to reduce (convert) carbon dioxide to glucose. The ATP provides energy for this production of glucose. The oxygen is released as a waste gas or used in respiration. Increasing or decreasing the light intensity is the main factor that controls the rate of these reactions. UNIT 2 THE CELL The light-independent stage (HL) UNIT 2 THE CELL The light-independent stage (HL) The light-independent stage needs NADPH and ATP if it is to work. In this reaction: A carbon dioxide molecule is reduced (hydrogen is added) using NADPH. The energy needed to allow the reaction to take place comes from ATP. The carbon dioxide is converted into a carbohydrate Cx(H2O)y. The simplest of these carbohydrates would be glucose (C6H12O6). UNIT 2 THE CELL Products of the light-independent stage [HL] Glucose: Used for respiration or stored (as starch). NADP–, ADP + P: Return to the light- dependent stage. UNIT 2 THE CELL Products of the light-independent stage [HL] The reactions in the light-independent stage are controlled by enzymes. The levels of CO2 and the temperature will affect the rate of these reactions as well as the amount of NADPH and ATP available. The levels of these last two (NADPH and ATP) are determined by the rate of the light-dependent stage. UNIT 2 THE CELL Summary of photosynthesis (HL) UNIT 2 THE CELL Learning outcomes After studying this topic, you should be able to: 1 Define photosynthesis. 2 State the role of photosynthesis. 3 Give a balanced equation for the overall reaction of photosynthesis. 4 Identify the location of chlorophyll. 5 Describe how light energy causes electrons to be released by chlorophyll, which combine with protons and carbon dioxide to form carbohydrate. The protons come from the splitting of water, which also releases oxygen. UNIT 2 THE CELL Learning outcomes After studying this topic, you should be able to: 6 Identify the source of light, water and carbon dioxide for photosynthesis. 7 Carry out an experiment to show the effect of light intensity or carbon dioxide concentration on photosynthesis. 8 Refer to the methods by which humans can artificially modify the rates of photosynthesis in horticulture. 9 Describe the two stages of photosynthesis: the light-dependent stage producing ATP, NADPH and oxygen; and the light- independent stage, which uses CO2, ATP and NADPH and produces carbohydrate. UNIT 2 THE CELL