Biology 12 - Photosynthesis PDF

# Biology 12 ## 3.3 Photosynthesis ### Expectations * Describe the energy transformations that occur in photosynthesis. * Describe the role of enzymes in metabolic reactions in chloroplasts. * Investigate and explain how the structure of molecules can influence metabolic rate. * Compare the stru...

# Biology 12 ## 3.3 Photosynthesis ### Expectations * Describe the energy transformations that occur in photosynthesis. * Describe the role of enzymes in metabolic reactions in chloroplasts. * Investigate and explain how the structure of molecules can influence metabolic rate. * Compare the structure and function of chloroplasts and mitochondria. **Photosynthesis is one of the most important chemical processes on Earth.** Photosynthesis involves the use of energy from light to form carbohydrates. Organisms that manufacture their own food (autotrophs), such as plants, algae, cyanobacteria, and photosynthetic bacteria, do so through photosynthesis. Autotrophs form the base of the food chain for virtually all communities of heterotrophs, which must eat to obtain nutrients. The process of photosynthesis also produces the oxygen found in the atmosphere. Oxygen is used by organisms for many processes, such as aerobic cellular respiration. The overall equation for photosynthesis is as follows: $6CO_2 + 6H_2O + energy → C_6H_{12}O_6 + 6O_2$ The process of photosynthesis is believed to have originated in bacteria. Some of these bacteria were able to produce oxygen. Other bacteria were able to carry out a different form of photosynthesis but did not produce oxygen. In 2000, biochemists led by Dr. Carl Bauer at the University of Indiana found that non-oxygen-producing species (purple and green bacteria) are the most ancient photosynthetic bacteria. The oxygen-producing cyanobacteria that exist today (see Figure 3.19) evolved from a non-oxygen-producing bacteria called heliobacteria. As you may have noticed, the overall equation for photosynthesis is exactly the opposite of the equation for aerobic cellular respiration. This does not mean, however, that the reactions follow the same course in reverse. Photosynthesis requires structures and metabolic processes similar to those used in mitochondria: electron transport chains, dissolved enzymes, and a membrane-enclosed space for chemiosmosis. ## Structure of Chloroplasts In plant cells, photosynthesis occurs within chloroplasts. Chloroplasts have a double membrane and contain membrane pockets called thylakoids (see Figure 3.20). Thylakoids occur in stacked, parcel-like structures called grana (singular granum), which are held together by support structures called lamellae. The stroma, a thick, enzyme-rich liquid, fills the interior of each chloroplast. Mesophyll cells in the leaves of plants are specialized for photosynthesis and contain numerous chloroplasts. These cells provide the chloroplasts with the two important ingredients necessary for photosynthesis carbon dioxide and water. Gas exchange (oxygen and carbon dioxide) occurs through pores on the underside of leaves, and water is delivered via veins that extend to the roots of the plant. Within the grana, solar light energy is captured by the thylakoids. This energy is used to form ATP molecules, which fuel the production of carbohydrates. These carbohydrate molecules are then used to synthesize glucose the molecules used in cellular respiration. The thylakoid membrane in the chloroplast is the site of ATP production, using chemiosmosis and complex structures functionally similar to those found in mitochondria. ## Stages of Photosynthesis As the previous section suggests, there are two main stages of photosynthesis: the photo and synthesis stages. The first stage of photosynthesis converts solar energy into chemical energy. The second stage uses this energy to produce PGAL, which is then used to form glucose (see Figure 3.21). The photo reactions require light and are called light-dependent reactions. The synthesis reactions do not require light directly, and are called light-independent reactions. However, light seems to be important in activating enzymes in both the photo and synthesis reactions. As light strikes the leaf of a plant, the energy is captured by pigments in the chloroplasts. These pigments, known as chlorophylls, absorb various wavelengths of visible light (see Figure 3.22). The two most important types of chlorophyll are chlorophyll a and chlorophyll b. Photosynthesis is most active at light wavelengths of about 400 nm to 450 nm and 650 nm to 700 nm. The colour of chlorophyll, green, is a result of the absorption of mainly blue and red parts of the visible light spectrum. In the following MiniLab, you will extract chlorophyll from leaves and examine the colour and properties of both types of chlorophyll. ## Photosystems Light energy is absorbed by a network of chlorophyll molecules known as a photosystem (see Figure 3.23). These chlorophyll molecules are known as antenna pigments because they collect and channel energy. This energy causes electrons in the chlorophyll molecules to become energized. Energy from these electrons is passed from one chlorophyll molecule to another in the photosystem. Eventually the energy reaches the reaction centre, a specific chlorophyll a molecule. Only one in 250 chlorophyll molecules forms a reaction centre. A unit of several hundred antenna pigment molecules together with a reaction centre is called a photosynthetic unit. The large number of antenna pigment molecules in each photosynthetic unit allows the reaction centre to be supplied with the greatest possible amount of energy. Once the energy has reached the reaction centre, an electron acceptor receives the energized electron. Energy from these electrons is used to move H+ ions into the thylakoid interior for ATP production. There are two types of photosystems. Photosystem 700, which absorbs light 700 nm in wavelength, is used by some photosynthetic bacteria. This photosystem contains molecules of chlorophyll a, which is found in cyanobacteria and all photosynthetic eukaryotes (such as green plants). Figure 3.24 shows how electrons pass through photosystem 700. After the electron acceptors receive the energized electrons from the reaction centre, the electrons flow through an electron transport system. Here the electrons are passed from one electron carrier to another. Some of these carriers are cytochrome molecules. As the electrons pass through the system, they release energy that is used to phosphorylate ADP molecules to produce molecules of ATP. This process is called cyclic photophosphorylation, because after the ATP molecules are produced the electrons are cycled back into the photosystem. Only ATP molecules are produced by photosystem 700. ## Non-cyclic Electron Pathway Photosystem 680 absorbs light 680 nm in wavelength. The shorter the wavelength of light, the higher its energy. Therefore, photosystem 680 is more powerful than photosystem 700 because photosystem 680 can capture higher-energy light. In addition to chlorophyll a, photosystem 680 contains molecules of chlorophyll b. It also contains molecules of chlorophyll c, chlorophyll d, and accessory pigments such as carotenes, xanthophylls, and anthocyanins. The pigments other than chlorophyll a aid in absorbing wavelengths of light not absorbed by chlorophyll a. Green plants, algae, and cyanobacteria (unlike other bacteria species) use both photosystems 680 and 700 to carry out photosynthesis. In this case, electrons from photosystem 680 are shunted to photosystem 700, as shown in Figure 3.25. The energy from electrons in photosystem 680 is used to produce ATP molecules. These electrons then move to photosystem 700 where, after becoming energized, they are taken up by NADP+ (nicotinamide adenine dinucleotide). After NADP+ accepts two electrons and a hydrogen ion (H+), it becomes the coenzyme NADPH. The production of NADPH and ATP are endothermic reactions, which require an input of energy. The ATP and NADPH molecules are then used in the synthetic steps to produce glucose. After ATP molecules are produced by photosystem 680, electrons that have passed through the electron transport system are not cycled back into photosystem 680. This type of ATP production is called non-cyclic photophosphorylation. However, photosystem 680 requires electrons to keep the photosystem operating. After photosystem 680 transfers an electron to the electron acceptor, photosystem 680 captures an electron from a Z enzyme. This enzyme is responsible for splitting water molecules into hydrogen ions and oxygen molecules and channeling electrons to the electron acceptor (see Figure 3.25). This process is called photolysis because light energy is required to split bonds within the water molecule. All of the oxygen that we breathe, and all the oxygen in Earth's atmosphere, has been generated through the photolysis stage of photosynthesis. In addition to passing electrons from water to chlorophyll molecules, the Z enzyme that performs photolysis also donates a hydrogen ion from the same water molecule to the reaction-centre of photosystem 680. This hydrogen ion joins the electron in its journey along the electron transport chain. The electron-hydrogen ion combination supplies energy to an electron transport chain comprised of cytochrome enzymes. This chain of enzymes in turn drives a proton pump, similar to the one you learned about in chemiosmosis in the mitochondrion. The photosynthetic proton pump, like proton pumps in the electron transport chain of the mitochondrion, moves H+ ions out of the stroma, into a membrane-enclosed space, as illustrated by Figure 3.26. Just as the inner membrane of the mitochondrion contains an ATP synthase complex that opens to the matrix, the thylakoid membrane of the chloroplast contains an ATP synthase complex where H+ ions flow through to the stroma and energize the phosphorylation of ADP. This process is called photophosphorylation. ## The Calvin Cycle In photosynthesis, both the NADPH and the ATP produced by the photo reactions in the thylakoid membrane are used during the synthesis reactions to produce organic molecules from carbon dioxide. ATP and NADPH molecules are formed on the thylakoid membrane by means of the ATP synthase complex and the NADP reductase complex, respectively (see Figure 3.26). The ATP and NADPH molecules formed then leave the thylakoid membrane and enter the stroma, where a series of enzymes perform synthesis reactions in the Calvin cycle. The Calvin cycle is named after biochemist Melvin Calvin. In the late 1940s, Calvin led a team of researchers to determine the steps of this synthesis reaction. Every photosynthetic plant uses the Calvin cycle to form PGAL. PGAL is then used to synthesize many different molecules. Using PGAL as the building block, plants can synthesize amino acids and fatty acids. Other molecules that can be formed from PGAL include fructose phosphate, glucose, sucrose, starch, and cellulose. Although plants synthesize these molecules, not every plant uses the same metabolic pathway. The Calvin cycle has three distinct stages, as shown in Figure 3.27, on the previous page: 1. Stage 1: carbon fixation 2. Stage 2: reduction 3. Stage 3: re-formation of RuBP (ribulose 1,5 bisphosphate) ### Stage 1: Carbon Fixation Carbon fixation is the initial incorporation of carbon into organic molecules. To eventually build complex molecules, such as glucose, plants must first attach carbon to smaller carbon-containing molecules. They do this by taking carbon dioxide from the atmosphere and attaching it to RuBP, ribulose bisphosphate, as shown in Figure 3.28. A six-carbon molecule is the product of this reaction, but this molecule is extremely unstable and immediately splits into two molecules of three-carbon PGA (phosphoglycerate). The enzyme RuBP carboxylase catalyzes this reaction, as shown in Figure 3.28. This reaction is called C3 fixation because it produces two three-carbon molecules of PGA. This molecule then passes into the next stage of the Calvin cycle. C3 fixation is used by plants, such as rice, wheat, and oats, which occur mainly in temperate regions. To form a molecule of glucose (C6H12O6), six carbon atoms must be fixed. Figure 3.27 shows that nine molecules of ATP are required to fix the three carbon atoms in the PGAL that is available to be used for glucose production. Therefore, 18 molecules of ATP are needed to fix the six carbon atoms required to form a glucose molecule. In addition to carbon fixation, RuBP carboxylase oxidizes RuBP with O2 to form CO2 by a process called photorespiration. Photorespiration creates an inefficiency in the carbon fixation process, since both the oxidation of RuBP and carbon fixation are catalyzed by the same enzyme RuBP carboxylase. Both oxygen and carbon dioxide compete to bind with RuBP. The Calvin cycle is an ancient process that developed in an atmosphere with little free oxygen. The rate of reactions in the Calvin cycle increases with temperature to about 25°C. Reaction rate levels out and declines when temperatures approach or exceed 37°C. At warmer temperatures, RuBP carboxylase is mainly involved in oxidizing RuBP, and very little carbon fixation occurs. Thus, plants that live in warmer climates have developed a different approach to fixing carbon. For example, C4 fixation is used by plants, such as sugarcane and corn. In these plants, the Calvin cycle takes place in bundle-sheath cells, as shown in Figure 3.29. Plants that use C4 fixation form the four-carbon oxaloacetate and malate in parenchyma cells. The malate moves into the bundle-sheath cells and a carbon is removed as CO2. Inside the bundle-sheath cells, there is a greater concentration of CO2 and a lower concentration of oxygen than in parenchyma cells at the surface of the leaf. This difference in concentration allows CO₂ to have a greater opportunity to bind with RuBP carboxylase. As a result, the plant can fix sufficient amounts of carbon to produce glucose in the Calvin cycle. In tropical climates, where the temperature often exceeds 28°C, food crops such as corn and sugarcane are commonly grown. Crops that use C3 fixation, however, do not survive well in tropical climates because they fix relatively less carbon and form fewer glucose molecules. Thus, the types of crops that can be grown in warmer climates are limited mainly to plants that use C4 carbon fixation. ### Stage 2: Reduction In the second stage of the Calvin cycle, the stroma performs the necessary enzymatic reactions that reduce PGA to form PGAL. This happens in two stages. First, ATP molecules donate phosphate groups to the PGA molecules, converting them to bisphosphoglycerate, or PGAP molecules (see Figure 3.31). Secondly, an NADPH molecule, which was produced during the photo reactions, donates a hydrogen ion and two electrons to PGAP. This reduces PGAP to glyceraldehyde phosphate, or PGAL – the building block for anabolic processes including the synthesis of glucose. The oxidized NADP+ can return to the thylakoid membrane to be reduced again. ### Stage 3: Re-formation of RuBP Recall from Figure 3.27 and Figure 3.28 that RuBP, ribulose bisphosphate, is required in the carbon fixation stage of the Calvin cycle. RuBP is used to produce PGA, needed for the reduction stage of the cycle. Because PGAL is needed to reform RuBP, the majority of PGAL molecules, do not contribute to glucose production. The Calvin cycle reactions must occur twice to create one molecule of glucose. This is because for every three times that the Calvin cycle reactions occur, five PGAL are used to re-form three RuBP, ribulose bisphosphate, as shown in Figure 3.32. Notice from Figure 3.27 that 5 three-carbon PGALs contain the same number of carbon atoms as 3 five-carbon RuBPs. To summarize the synthesis reactions of the Calvin cycle: * Stage 1: Carbon fixation, which takes carbon atoms from atmospheric carbon dioxide molecules and incorporates these atoms into organic molecules. * Stage 2: Reduction, which involves the formation of PGAP and its reduction to PGAL. * Stage 3: Re-formation of RuBP, which uses most of the PGAL molecules formed in the reduction stage to produce RuBP. This is then used to form more PGA in the Calvin cycle. ## Glucose: The Ultimate Food Source After glucose is produced in the synthesis reactions, plant cells can use glucose for glycolysis, followed by aerobic respiration in the mitochondria. The products and intermediary molecules of aerobic respiration provide the carbon-based molecules necessary to build amino acids, as well as the precursors to nucleic acids and lipids. However, there are many other ways that plants use glucose, for example, * the conversion of glucose to starch, * the formation of cellulose from glucose, and * the conversion of glucose to sucrose. These three ways will now be described. Autotrophs, such as green plants, produce a molecule used for energy storage, called starch. The starch is a large, branched polysaccharide composed of hundreds of glucose molecules linked by condensation reactions. Plants convert glucose to starch in the stroma. During peak hours of bright daylight, plants may produce more starch than they can use. This starch is stored in cells and is ready to be broken down into glucose for use in cellular processes. In the Thinking Lab on page 92, you will consider how the structure of starch can influence metabolic processes. In another series of reactions, plants may form another kind of polysaccharide that is the building block of cell walls cellulose. PGAL is first exported from the chloroplast into the cytoplasm where condensation reactions take place to link glucose molecules. The formation of sucrose (the transport sugar in plants) also occurs in the cytoplasm. In order for glycolysis and cellular respiration to take place in the cytosol and mitochondria of plants, glucose is required. Because plants cannot move glucose molecules through the phloem (vascular tissue that transports organic material), they convert PGAL to glucose in the cytoplasm of leaf mesophyll cells. Glucose and fructose are then converted to sucrose. Sucrose is a molecule of fructose covalently bonded to a molecule of glucose. After sucrose is formed it is actively transported to the phloem, and then moved to locations in the plant that metabolize glucose. ## Photosynthesis Versus Aerobic Cellular Respiration Both plant and animal cells have mitochondria and carry out aerobic cellular respiration. However, only plants use photosynthesis. The cellular organelle for photosynthesis is the chloroplast, while the cellular organelle for aerobic cellular respiration is the mitochondrion. Figure 3.33 compares the processes of photosynthesis and respiration. Both processes have an electron transport chain located on membranes in the chloroplast and mitochondrion. ATP is produced on these membranes through the process of chemiosmosis. In photosynthesis, water is oxidized and oxygen is produced. In aerobic cellular respiration, oxygen is reduced to form water. Reactions in the chloroplast and mitochondrion are catalyzed by enzymes. These enzymes help to reduce CO2 to glucose in the chloroplast and oxidize glucose to CO2 in the mitochondrion.