Photosynthesis: Energy from the Sun

Identifying Photosynthetic Reactants and Products Photosynthesis, the biochemical process by which plants capture energy from sunlight and store it in carbohydrates, is the very basis of life on Earth.

Identifying Photosynthetic Reactants and Products By the 1800s, scientists had learned: Three ingredients are needed for photosynthesis: water, CO2, and light. There are two products: carbohydrates and O2. The water, which comes primarily from the soil, is transported through the roots to the leaves. The CO2 is taken in from the air through stomata, or pores, in the leaves.

Figure 8.1 The Ingredients for Photosynthesis

Identifying Photosynthetic Reactants and Products By 1804, scientists had summarized the overall chemical reaction of photosynthesis: CO2 + H2O + light energy ® sugar + O2 More recently, using H2O and CO2 labeled with radioactive isotopes, it has been determined that the actual reaction is: 6 CO2 + 12 H2O ® C6H12O6 + 6 O2 + 6 H2O

The Two Pathways of Photosynthesis: An Overview Photosynthesis occurs in the chloroplasts of green plant cells and consists of many reactions. Photosynthesis can be divided into two pathways: The light reaction is driven by light energy captured by chlorophyll. It produces ATP and NADPH + H+. The Calvin–Benson cycle does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.

Figure 8.3 An Overview of Photosynthesis (Part 1)

Figure 8.3 An Overview of Photosynthesis (Part 2)

The Interactions of Light and Pigments Visible light is part of the electromagnetic radiation spectrum. It comes in discrete packets called photons.

Figure 8.5 The Electromagnetic Spectrum

The Interactions of Light and Pigments When a photon and a pigment molecule meet, one of three things happens: The photon may bounce off, pass through,or be absorbed by the molecule. If absorbed, the energy of the photon is acquired by the molecule. The molecule is raised from its ground state to an excited state of higher energy.

Figure 8.4 Exciting a Molecule

The Interactions of Light and Pigments Molecules that absorb wavelengths in the visible range are called pigments. When a beam of white light shines on an object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red. In the case of chlorophyll, plants look green because they absorb green light less effectively.

The Interactions of Light and Pigments A molecule can absorb radiant energy of only certain wavelengths. If we plot the absorption by the compound as a function of wavelength, the result is an absorption spectrum. If absorption results in an activity of some sort, then a plot of the effectiveness of the light as a function of wavelength is called an action spectrum.

Figure 8.6 Absorption and Action Spectra

The Interactions of Light and Pigments Plants have two predominant chlorophylls: chlorophyll a and chlorophyll b. These chlorophylls absorb blue and red wavelengths, which are near the ends of the visible spectrum.

Figure 8.7 The Molecular Structure of Chlorophyll

The Interactions of Light and Pigments A pigment molecule enters an excited state when it absorbs a photon. If the molecule does not return to ground state, the pigment molecule may pass some of the absorbed energy to other pigment molecules.

The Interactions of Light and Pigments Pigments in photosynthetic organisms are arranged into systems. In these systems, pigments are packed together and attached to thylakoid membrane proteins to enable the transfer of energy. The excitation energy is passed to the reaction center of the complex.

Figure 8.8 Energy Transfer and Electron Transport

The Interactions of Light and Pigments Excited chlorophyll (Chl*) in the reaction center acts as a reducing agent. Chl* can react with an oxidizing agent in a reaction such as: Chl* + A ® Chl+ + A–.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation The energized electron that leaves the Chl* in the reaction center immediately participates in a series of redox reactions. The electron flows through a series of carriers in the thylakoid membrane (electron transport). Two energy rich products of the light reactions, NADPH + H+ and ATP, are the result.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation There are two different reactions in photosynthesis: Light Reactions: produces NADPH + H+ and ATP and O2. Dark Reactions: uses CO2 , produces only ATP.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation In the light reactions, two photosystems are used. Photosystems are light-driven molecular units that consist of many chlorophyll molecules and accessory pigments bound to proteins in separate energy-absorbing antenna systems.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Photosystem I uses light energy to reduce NADP+ to NADPH + H+. The reaction center contains a chlorophyll a molecule called P700 because it best absorbs light at a wavelength of 700 nm.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Photosystem II uses light energy to oxidize water molecules, producing electrons, protons, and O2. The reaction center contains a chlorophyll a molecule called P680 because it best absorbs light at a wavelength of 680 nm.

Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP NADP+ → NADPH

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation To produce ATP, high-energy electrons move through a series of redox reactions, releasing energy that is used to transport protons across the membrane. Active proton transport results in the proton- motive force: a difference in pH and electric charge across the membrane.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation The electron carriers in the thylakoid membrane are oriented so as to move protons into the interior of the thylakoid, and the inside becomes acidic with respect to the outside. This difference in pH leads to the diffusion of H+ out of the thylakoid through ATP synthase.

Figure 8.11 Chloroplasts Form ATP Chemiosmotically

Making Carbohydrate from CO2: The Calvin–Benson Cycle The dark reactions of the Calvin-Benson cycle take place in the stroma of the chloroplasts. This cycle does not use sunlight directly; but it requires the ATP and NADPH + H+ produced in the light reactions. Thus the Calvin-Benson reactions require light indirectly and takes place only in the presence of light. The Calvin-Benson cycle uses CO2.

Making Carbohydrate from CO2: The Calvin–Benson Cycle The initial reaction of the Calvin–Benson cycle fixes one CO2 into a 5-carbon compound. The enzyme that catalyzes the fixation of CO2 is called rubisco. Rubisco is the most abundant protein in the world.

Figure 8.14 RuBP Is the Carbon Dioxide Acceptor

Making Carbohydrate from CO2: The Calvin–Benson Cycle The Calvin–Benson cycle consists of three processes: Fixation of CO2, by combination with RuBP (catalyzed by rubisco) Conversion of fixed CO2 into carbohydrate (G3P) (this step uses ATP and NADPH) Regeneration of the CO2 acceptor RuBP by ATP

Making Carbohydrate from CO2: The Calvin–Benson Cycle The end product of the cycle is glyceraldehyde 3- phosphate, G3P. There are two fates for the G3P: One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose. Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.

Figure 8.13 The Calvin-Benson Cycle

Making Carbohydrate from CO2: The Calvin–Benson Cycle The products of the Calvin–Benson cycle are vitally important to the biosphere as they are the total energy yield from sunlight conversion by green plants. Most of the stored energy is released by glycolysis and cellular respiration by the plant itself. Some of the carbon of glucose becomes part of amino acids, lipids, and nucleic acids. Some of the stored energy is consumed by heterotrophs, where glycolysis and respiration release the stored energy.

Metabolic Pathways in Plants Green plants are autotrophs and can synthesize all their molecules from three simple starting materials: CO2, H2O, and NH4. To satisfy their need for ATP, plants, like all other organisms, carry out cellular respiration. Both aerobic respiration and fermentation can occur in plants, although respiration is more common. Cellular respiration takes place both in the dark and in the light.

Metabolic Pathways in Plants Energy flows from sunlight to reduced carbon in photosynthesis to ATP in respiration. Energy can be stored in macromolecules such as polysaccharides, lipids, and proteins. For plants to grow, energy storage must exceed energy released or overall carbon fixation by photosynthesis must exceed respiration. The capture and movement of sun energy becomes the basis for ecological food chains.