THE WORKING CELL ENERGY FROM SUNLIGHT
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • The Structure of Chloroplasts • As learned previously, the chloroplast is the organelle where photosynthesis takes place. • The chemical compound, chlorophyll is contained in the chloroplast. • Everything in a plant that is green contains chloroplasts and chlorophyll (gives the plant its green color) and carries out photosynthesis.
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • The most chloroplasts are in the leaves where the major site for photosynthesis is located. • Chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue. • Tiny pores found on the surface of the leaf are called stomata, through which CO₂ enters and O₂ exits. • Veins carry the nutrients and water from the roots to the leaves.
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • Organic molecules produced in the leaves are carried by the veins to other parts of the plant. • The chloroplast is structured like the mitochondria: outer and inner membrane, and a thick fluid in the inner membrane called stroma. • In the stroma are the thylakoids which are disk shaped sacs. • Membranes surround each thylakoid. • Stacks of thylakoids are grana.
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • Photosynthesis takes place on the thylakoid membranes or the stroma. • Overview of Photosynthesis • In cellular respiration, electrons “fall” from the glucose to the oxygen. • In photosynthesis, electrons from the water are “boosted uphill” by the sun’s energy. • These electrons, along with CO₂ and H⁺ ions, produce sugar molecules.
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • There are two stages to photosynthesis:the light reactions and the Calvin cycle. • The Light Reactions • Light reactions convert the sun’s energy to chemical energy. • This happens on the thylakoid membrane. • Chlorophyll captures the sun’s energy. • Then they remove the electrons from the water using this energy from the sun. • The water is then broken down to oxygen and hydrogen molecules.
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • Oxygen is the waste product and leaves for the atmosphere through the stomata. • The electrons from the water are used by the chloroplasts to make an energy rich molecule called NADPH. • This is an electron carrier similar to NADH. • The energy from the sun is also used to generate ATP. • The net result is the conversion of the light energy to chemical energy stored in NADPH and ATP. • The Calvin Cycle • Sugars are made from the atoms in CO₂ in the
PHOTOSYNTHESIS USES LIGHT ENERGY TO MAKE FOOD • Calvin cycle, also using the hydrogen ions and the high energy electrons carried in the NADPH. • The enzymes for this process are located outside the thylakoids and dissolved in the stroma. • The “fuel” or energy required to make sugar comes from the ATP made by the light reactions. • The Calvin cycle is also referred to as the “light-independent reactions. • This is because it does not directly require light to begin. • It can’t, however, complete its cycle in the dark because it requires inputs from the NADPH and ATP, produced in light.
REVIEW: Concept Check 8.1, PAGE 162 • Draw and label a simple diagram of a chloroplast that includes the following structure: outer and inner membranes, stroma, thylakoids. • What are the reactants for photosynthesis? What are the products? What is the stimulus? • Name the two main stages of photosynthesis. How are the two stages related?
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • Light Energy and Pigments • Sunlight is a form of electromagnetic energy that travels in waves, like those on a beach. • A wavelength is the distance between two adjacent waves. • The electromagnetic spectrum shows us the ranges of electromagnetic energy from gamma rays to radio waves. • That light which is visible is only a small fraction of the electromagnetic spectrum. • Visible light is from 400 nanometers (nm), violet, to
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY to about 700 nm, red. • The longer the wavelength the less the energy. • UV radiation can damage proteins and nucleic acids as can all wavelengths shorter than visible light. • Pigments and Color • Color is due to chemical compounds called pigments. • If a material contains a pigment, and light shines on it, three things happen to the different wavelengths: • They can be absorbed • They can be transmitted • They can be reflected
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • Pigments in chloroplasts absorb blue-violet and red-orange light. • Some of this absorbed light energy is converted to chemical energy. • Chloroplasts pigments do not absorb green light well, so it passes through (transmitted) or bounces back (reflected). • Leaves look green because the green light is not absorbed well. • Identifying Chloroplast Pigments • Paper chromatography shows us the different pigments in a green leaf.
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • A leaf stain is placed on filter paper which is then placed in a cylinder containing a solvent. • The pigments dissolve in the solvent and travel up the paper, separating out. • Chlorophyll a, which absorbs blue-violet and red light and reflects green light, plays one of the major roles in photosynthesis. • There are other helper pigments in chloroplasts: chlorophyll b (absorbs blue and orange and reflects yellow-green) and carotenoids (absorb blue-green and reflects yellow orange).
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • Harvesting Light Energy • When light strikes a leaf: • In the thylakoid membrane, chlorophyll, along with other molecules, are arranged in clusters called photosystems. • Contained therein are a few hundred pigment molecules, including chlorophyll a and b, and carotenoids. • These are similar to a solar collecting panel, collecting all the light entering. • When a pigment molecule absorbs light energy, the pigment’s electrons gains energy (from a low energy state to a high energy state, which is unstable).
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • The electron then falls back to the ground state, transferring energy to another molecule which, in turn, excites an electron in the receiving molecule, and so on. • This happens to the energy until it reaches its destination, the reaction center of the photosystem. • This consists of chlorophyll a next to another molecule called a primary electron acceptor. • The primary electron acceptor traps the excited electron form the chlorophyll a molecule.
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • Other molecules in the thylakoid membrane use the energy to make ATP and NADPH. • Chemical Products of the Light Reactions • There are two systems involved in light reactions. • The first traps the light energy and then transfers the excited electrons to an electron transport chain. • This is the “water-splitting photosystem” because the electrons are replaced by splitting a molecule of water releasing oxygen and hydrogen ions.
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • The two photosystems are connected by an electron transport chain which releases energy, which is used by the chloroplast to make ATP, similar to ATP production in cellular respiration. • As in cellular respiration, the electron transport chain pumps hydrogen ions across a membrane (inner mitochondrial membrane in respiration and the thylakoid membrane in photosynthesis). • In respiration food is the source for electrons and in photosynthesis light excited electrons from chlorophyll travel down the chain.
THE LIGHT REACTIONS CONVERT LIGHT ENERGY TO CHEMICAL ENERGY • The second photosystem is the “NADPH-producing photosystem.” • In this, NADPH is produced from transferring the excited electrons and hydrogen ions to NADP⁺. • Light reactions convert light energy to the chemical energy of ATP and NADPH. • Sugar has yet to be produced and this is done in the Calvin cycle, utilizing ATP and NADPH produced by the light reactions.
REVIEW: CONCEPT CHECK 8.2, page 167 • Explain why a leaf appears green. • Describe what happens when a molecule of chlorophyll a absorbs light. • Besides oxygen, what two molecules are produced by the light reactions? • Where in the chloroplast do the light reactions take place?
THE CALVIN CYCLE MAKES SUGAR FROM CARBON DIOXIDE • A Trip Around the Calvin Cycle • Similar to the Krebs cycle, this is a regenerative process with the starting material being a compound called RuBP, a sugar with five carbons. • The chemical inputs are carbon dioxide and ATP and NADPH from the light reactions. • In the Calvin cycle, the carbon comes from the CO₂, the energy from the ATP, and the high-energy electrons and hydrogen ions from the NADPH. • The result is an energy laden sugar molecule.
THE CALVIN CYCLE MAKES SUGAR FROM CARBON DIOXIDE • This molecule is called G3P, not yet glucose, which the plant uses to make glucose and other organic molecules. • Summary of Photosynthesis 6 CO₂ + 6 H₂O →→→ C₆H₁₂O₆ + 6 O₂ • Light reactions in the thylakoid membranes • Light energy to chemical energy = ATP + NADPH • Light reactions use water from the equation and release oxygen • Calvin cycle, in the stroma, uses ATP and NADPH and converts CO₂ to sugar.
REVIEW: CONCEPT CHECK 8.3, page 170 • What are the inputs and outputs of the Calvin cycle? • Which stage of photosynthesis uses each reactant from the overall photosynthesis equation? Which stage generates each product from the overall photosynthesis equation? • Why is the Calvin cycle called a cycle? • What molecule is the direct product of photosynthesis? How is that molecule then used by plant cells?
PHOTOSYNTHESIS HAS A GLOBAL IMPACT • The Carbon Cycle • The carbon cycle is a process in which carbon moves from inorganic to organic compounds and back. • Using photosynthesis, the producers such as grass convert the inorganic carbon dioxide to organic compounds. • The consumers, including Cape buffalo, eat the producers obtaining their organic compounds. • These consumers are in turn eaten by lions, so eventually the CO₂ is returned to the atmosphere
PHOTOSYNTHESIS HAS A GLOBAL IMPACT by the producers and consumers • In the previous pictures, CO₂ in the atmosphere, from the consumers, leads to photosynthesis by producers building organic compounds which leads to cellular respiration by the producers and consumers releasing CO₂.. • It is estimated that photosynthesis by plants produces 160 billion metric tons of organic material per year, or 25 stacks of biology books reaching from Earth to the sun.
PHOTOSYNTHESIS HAS A GLOBAL IMPACT • Photosynthesis and Global Climate • Carbon dioxide is the key element in the carbon cycle • It is a reactant in photosynthesis by plants and a product in cellular respiration. • Figuring all the CO₂ produced by organisms of Earth, there is a very large amount produced in the atmosphere. • CO₂ in the atmosphere aids in the greenhouse effect, keeping Earth’s temperature 10⁰C warmer than it otherwise would be.