Photosynthesis

1.         Photosynthesis converts light energy to the chemical energy of food.

            a.         Organisms obtain organic compounds by one of two major modes: autotrophic nutrition or heterotrophic nutrition. Autotrophs produce their organic molecules from CO₂ and other inorganic raw materials obtained from the environment. Heterotrophs live on organic compounds produced by other organisms. (photosynthesis produces about 160 B tons of carbohydrate per year)

            b.         All green parts of a plant have chloroplasts (500,000/mm² of leaf and 30-40/mesophyll cell) but the leaves are the major site of photosynthesis for most plants. There are about half a million chloroplasts per square millimeter of leaf surface. The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.

            c.         Different pigments absorb photons of different wavelengths, and the wavelengths that are absorbed disappear. A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light. Other pigments with different structures absorb light of different wavelengths.

            d.         The general equation is 6CO₂ + 6H₂O + light energy ➝ C₆H₁₂O₆ + 6O₂. Note that the reaction is the reverse of cellular respiration.

            e.         Light energy is captured by low energy CO₂ and converted into high energy C₆H₁₂O₆

            f.         Photosynthesis is a redox reaction. It reverses the direction of electron flow in respiration. In other words, water is split and electrons transferred with H⁺ from water to CO₂, reducing it to sugar. In cellular respiration electrons move from high energy glucose to O₂ making low energy water; in photosynthesis, low energy water is split and electrons move to CO₂ making high energy glucose. The energy required for the reduction is provided by the sun.

2.         Leaf structure

            a.         Each chloroplast has two membranes around a central aqueous space, the stroma.

            b.         In the stroma is an elaborate system of interconnected membrane-bound sacs, the thylakoids. Chlorophyll is located in the thylakoids.

            c.         Thylakoids are arranged into stacks called grana.

            d.         O₂ exits and CO₂ enters the leaf through microscopic pores called stomata in the leaf.

            e.         Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. A typical mesophyll cell has 30–40 chloroplasts.

            f.         Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant.

            g.         Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are many disks called thylakoids. Chlorophyll is located in the membranes of the thylakoids.

3.         Photosynthesis occurs in two phases: the light reactions occur on the thylakoid membranes, while the Calvin cycle occurs in the stroma.

            a.         The light reactions (the photo part) convert solar energy to chemical energy.

                        i.         In the light reactions, light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP⁺, forming NADPH(an electron carrier like NADH) carries those electrons to the Calvin cycle.

                        ii.        When a molecule absorbs a photon, one of that molecule’s electrons is excited to a higher energy level. Excited electrons are unstable and usually release the energy as heat in a billionth of a second.

                        iii.       In the thylakoid membrane, chlorophyll is organized along with special proteins into photosystems. A photosystem is composed of a reaction center surrounded by a light-harvesting complex of a few hundred pigment molecules.

                        iv.       When any pigment molecule absorbs a photon, the energy is transmitted from molecule to molecule until it reaches a particular chlorophyll molecule called the reaction center.

                        v.         At the reaction center is a primary electron acceptor, which accepts an excited electron from the reaction center chlorophyll.

                        vi.       There are two types of photosystems in the thylakoid membrane: Photosystem I (PS I) and Photosystem II (PS II). These two photosystems work together to use light energy to generate ATP and NADPH.

                        vii.      During the light reactions, there are two possible routes for electron flow:

                                    (1)       Noncyclic electron flow, the predominant route, produces both ATP and NADPH.

                                                (a)       Photosystem II absorbs a photon of light and an electron is excited to a higher energy state.

                                                (b)       This electron is captured by the primary electron acceptor, leaving the reaction center oxidized.

                                                (c)       Electrons from water are used replace those lost by the reaction center. This reaction creates O₂ as a by-product.

                                                (d)       Excited electrons pass along an electron transport chain before ending up at a the reaction center of photosystem I. As they move along the chain, their energy is used to produce ATP. This ATP is used by the Calvin cycle.

                                                (e)       Meanwhile, light energy has excited an electron of the reaction center of PS I. The electron was captured by PS I’s primary electron acceptor, but is replaced by an electron that reaches the bottom of the electron transport chain from PS II.

                                                (f)        Photoexcited electrons are passed from PS I’s primary electron acceptor down a second electron transport chain.

                                                (g)       Electrons from this chain are transferred to NADP+, forming NADPH, which will carry these high-energy electrons to the Calvin cycle.

                                    (2)       Under certain conditions, excited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow.

                                                (a)       Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and return to the oxidized reaction center.

                                                (b)       As electrons flow along the electron transport chain, they generate ATP by but there is no production of NADPH and no release of oxygen.

                                                (c)       Noncyclic electron flow produces ATP and NADPH in roughly equal quantities but the Calvin cycle consumes more ATP than NADPH.

                                                (d)       Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.

                        viii.     Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis. In both organelles, an electron transport chain pumps protons across a membrane as electrons are passed along a series of increasingly electronegative carriers.

            b.         The Calvin cycle (the synthesis part) uses ATP and NADPH from the light reactions to convert CO₂ from the atmosphere to a three-carbon sugar. The purpose is to reduce low energy CO₂ into a high energy sugar. The energy needed comes from ATP while the electrons needed come from NADPH - both produced by the light reactions.

                        i.         The Calvin cycle has three phases.

                                    (1)       Phase 1: Carbon fixation

                                                (a)       In the carbon fixation phase, each CO₂ molecule is attached to a 5C (RuBP) sugar to form a 6C sugar. This is catalyzed by the enzyme rubisco, the most abundant protein in chloroplasts and probably the most abundant protein on Earth.

                                                (b)       The 6C intermediate is unstable and splits in half to form two 3 carbon molecules.

                                    (2)       Phase 2: Reduction

                                                (a)       During reduction, each 3C intermediate is reduced by a pair of electrons from NADPH.

                                                (b)       The product is G3P (PGAL), some of which can exit the cycle and be used by the plant cell to make glucose (for energy), starch, cellulose, and other molecules.

                                    (3)       Phase 3: Regeneration

                                                (a)       The remaining G3P are used to regenerate RuBP. In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged to regenerate three molecules of RuBP.

                        ii.        About 50% of the product from photosynthesis is consumed as fuel for cellular respiration in plant mitochondria. Most of the rest is used to make cellulose.

4.         Photorespiration is a problem in hot, arid climates.

            a.         The stomata are not only the major route for gas exchange (CO₂ in and O₂ out), but also for the evaporative loss of water. On hot, dry days, plants close their stomata to conserve water. This causes problems for photosynthesis.

            b.         In most plants (called C₃ plants), carbon fixation is accomplished by rubisco.

            c.         When these plants partially close their stomata on a hot, dry day, CO₂ levels drop as CO₂ is consumed in the Calvin cycle. At the same time, O₂ levels rise as the light reaction converts light to chemical energy.

            d.         While rubisco normally accepts CO₂, it also can recognize O₂ and when the O₂:CO₂ ratio increases (on a hot, dry day with closed stomata), it accidentally adds O₂ to the 3C molecule.

            e.         When rubisco adds O₂ to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece. The two-carbon fragment is exported from the chloroplast and oxidized to CO₂ by mitochondria in a process called photorespiration.

                        i.         Unlike normal respiration, this process produces no ATP but actually consumes it.

                        ii.        Unlike photosynthesis, photorespiration does not produce organic molecules; rather, it removes carbon from the Calvin cycle, decreasing photosynthetic output.

                        iii.       A hypothesis for the existence of photorespiration is that it is evolutionary baggage.

                                    (1)       When rubisco first evolved, the atmosphere had far less O₂ and more CO₂ than it does today.

                                    (2)       The inability of the active site of rubisco to exclude O₂ would have made little difference. In today’s atmosphere it does make a difference.

                        iv.       Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day.

            f.         Evolution has produced a solution to this problem in hot, arid climates. C₄ plants (like corn and sugar cane) and CAM plants (like cacti) fix carbon by, respectively, using an enzyme other than rubisco and by fixing carbon at night while the stomata are open.