Photosynthesis: Energy from the Sun

Photosynthesis: Energy from the Sun Identifying Photosynthetic Reactants and Products The Two Pathways of Photosynthesis: An Overview The Interactions of Light and Pigments The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Making Carbohydrate from CO2: The Calvin–Benson Cycle Photorespiration and Its Consequences Metabolic Pathways in Plants

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. Light also behaves as if it were a wave. Two things are required for photons to be active in a biological process: Photons must be absorbed by receptive molecules. Photons must have sufficient energy to perform the chemical work required.

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 than the other colors found in sunlight.

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. Other accessory pigments absorb photons between the red and blue wavelengths and then transfer a portion of that energy to chlorophylls. Examples of accessory pigments are the carotenoids, such as b-carotene.

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. The excited state is unstable, and the molecule may return to the ground state. When this happens, some of the absorbed energy is given off as heat and the rest is given off as light energy, or fluorescence. If fluorescence does not occur, 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 antenna 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 antenna complex. In plants, the pigment molecule in the center is always a molecule of chlorophyll a.

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. The electrons of an excited molecule are less tightly held by the nucleus, and more likely to be passed on in a redox reaction to an oxidizing agent. Chl* can react with an oxidizing agent in a reaction such as: Chl* + A ® Chl+ + A–. Chlorophyll becomes a reducing agent and participates in a redox reaction.

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, a process termed electron transport. Two energy rich products of the light reactions, NADPH + H+ and ATP, are the result. Chemiosmotic synthesis of ATP in the thylakoid membrane is called photophosphorylation.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation There are two different systems for transport of electrons in photosynthesis. Noncyclic electron transport produces NADPH + H+ and ATP and O2. Cyclic electron transport produces only ATP.

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation In noncyclic electron transport, two photosystems are required. 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. To keep noncyclic electron transport going, both photosystems must constantly be absorbing light.

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

Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation Cyclic electron transport produces only ATP. The process is called cyclic because the electron passed from an excited P700 molecule cycles back to the same P700 molecule. Water does not enter into the cyclic electron flow reactions, and no O2 is released. In cyclic electron flow, photosystem I acts on its own.

Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP

The Light Reactions: Electron Transport, Reductions, and Photophosphorylation ATP is produced by a chemiosmotic mechanism similar to that of mitochondria, called photophosphorylation. 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 specific protein channels, ATP synthases. The ATP synthases couple the formation of ATP to proton diffusion back across the thylakoid membrane.

Figure 8.11 Chloroplasts Form ATP Chemiosmotically

Making Carbohydrate from CO2: The Calvin–Benson Cycle The 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, and these can not be “stockpiled”. Thus the Calvin-Benson reactions require light indirectly and take place only in the presence of light.

Making Carbohydrate from CO2: The Calvin–Benson Cycle Experiments that revealed that the steps of the Calvin–Benson cycle required radioactively labeled carbon in CO2. Exposure of Chlorella cells 14CO2 for 3 seconds resulted in one compound that was labeled with 14C. The compound was a 3-carbon sugar called 3- phosphoglycerate (3PG). Other products of the cycle were found by increasing the length of time of exposure in a stepwise manner until the whole pathway was revealed.

Figure 8.12 Tracing the Pathway of CO2 (Part 1)

Figure 8.12 Tracing the Pathway of CO2 (Part 2)

Making Carbohydrate from CO2: The Calvin–Benson Cycle The initial reaction of the Calvin–Benson cycle fixes one CO2 into a 5-carbon compound, ribulose 1,5-bisphosphate (RuBP). An intermediate 6-carbon compound forms, which is unstable and breaks down to form two 3-carbon molecules of 3PG. The enzyme that catalyzes the fixation of CO2 is ribulose bisphosphate carboxylase/oxygenase, 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.

Photorespiration and Its Consequences Rubisco is a carboxylase, adding CO2 to RuBP. It can also be an oxygenase, adding O2 to RuBP. These two reactions compete with each other. When RuBP reacts with O2, it cannot react with CO2, which reduces the rate of CO2 fixation.

Photorespiration and Its Consequences RuBP + O2 ® phosphoglycolate + 3PG. The glycolate diffuses into organelles called peroxisomes. In the peroxisomes, a series of reactions converts glycolate to glycine. The glycine diffuses into the mitochondria and is converted to glycerate and CO2.

Figure 8.15 Organelles of Photorespiration

Photorespiration and Its Consequences Photorespiration uses the ATP and NADPH produced in the light reaction. CO2 is released instead of being fixed into a carbohydrate. Rubisco acts as an oxygenase if the CO2 levels are very low and the O2 levels are very high. O2 levels become very high when stomata are closed to prevent water loss (when the weather is hot and dry).

Photorespiration and Its Consequences C3 plants have a layer of mesophyll cells below the leaf surface. Mesophyll cells are full of chloroplasts and rubisco. On hot days the stomata close, O2 builds up, and photorespiration occurs.

Figure 8.16 Leaf Anatomy of C3 and C4 Plants

Photorespiration and Its Consequences C4 plants have two enzymes for CO2 fixation in different chloroplasts, in different locations in the leaf. PEP carboxylase is present in the mesophyll cells. It fixes CO2 to 3-C phosphoenolpyruvate (PEP) to form 4-C oxaloacetate. PEP carboxylase does not have oxygenase activity. It fixes CO2 even when the level of CO2 is extremely low.

Photorespiration and Its Consequences The oxaloacetate diffuses into the bundle sheath cells in the interior of the leaf which contain abundant rubisco. The oxaloacetate loses one C, forming CO2 and regenerating the PEP. The process pumps up the concentration around rubisco to start the Calvin-Benson cycle.

Figure 8.17 (a) The Anatomy and Biochemistry of C4 Carbon Fixation

Figure 8.17 (b) The Anatomy and Biochemistry of C4 Carbon Fixation

Photorespiration and Its Consequences CAM plants use PEP carboxylase to fix and accumulate CO2 while their stomata are closed. These plants conserve water by keeping stomata closed during the daylight hours and opening them at night. In CAM plants, CO2 is fixed in the mesophyll cells to form oxaloacetate, which is then converted to malic acid. The fixation occurs during the night, when less water is lost through the open stomata. During the day, the malic acid moves to the chloroplast, where decarboxylation supplies CO2 for the Calvin–Benson cycle.

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 Photosynthesis and respiration are closely linked by the Calvin–Benson cycle. Some G3P from the Calvin–Benson cycle can be converted into pyruvate, the end product of glycolysis. Some G3P can be converted into hexose phosphates, which can enter glycolysis. Once carbon skeletons from the Calvin–Benson cycle enter glycolysis and the citric acid cycle, they can be used to make lipids, proteins, and other carbohydrates.

Figure 8.18 Metabolic Interactions in a Plant Cell (Part 1)

Figure 8.18 Metabolic Interactions in a Plant Cell (Part 2)

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.