Xem mẫu

Cambridge International A Level Biology 286 Chapter 13: Photosynthesis Learning outcomes You should be able to: ■■ describe the absorption of light energy in the light dependent stage of photosynthesis ■■ explain the transfer of this energy to the light independent stage of photosynthesis and its use in the production of complex organic molecules ■■ describe the role of chloroplast pigments in the absorption of light energy ■■ discuss how the structure of a chloroplast fits it for its functions ■■ explain how environmental factors influence the rate of photosynthesis ■■ describe how C4 plants are adapted for high rates of carbon fixation at high temperatures Chapter 13: Photosynthesis Fuel from algae Despite millions of hours of research, we still have not managed to set up a chemical manufacturing system that can harvest light energy and use it to make complex chemicals, in the way that plants and some protoctists do. So, why not just let the cells do it for us? Figure 13.1 shows a photobioreactor – a series of tubes containing the single-celled photosynthetic organism Chlorella. Provide light, carbon dioxide and minerals, and the cells photosynthesise. Bioreactors like this are being used around the world to produce biomass for animal feed, and chemicals that can be used as food additives or in the manufacture of cosmetics. They can also be used to convert energy from the Sun into ethanol or biodiesel but, so far, the bioreactors cannot produce biomass cheaply enough to compete with the use of fossil fuels. Figure 13.1 A photobioreactor. An energy transfer process As you have seen at the beginning of Chapter 12, the process of photosynthesis transfers light energy into chemical potential energy of organic molecules. This energy can then be released for work in respiration (Figure 12.2). Almost all the energy transferred to all the ATP molecules in all living organisms is derived from light energy used in photosynthesis by autotrophs. Such photoautotrophs include green plants, the photosynthetic prokaryotes and both single-celled and many-celled protoctists (including the green, red and brown algae). A few autotrophs do not depend on light energy, but use chemical energy sources. These chemoautotrophs include the nitrifying bacteria that are so important in the nitrogen cycle. Nitrifying bacteria obtain their energy from oxidising ammonia (NH3) to nitrite (NO2 ), or nitrite to nitrate (NO3 ). An outline of the process Photosynthesis is the trapping (fixation) of carbon dioxide and its subsequent reduction to carbohydrate, a starch grain granum b ribosomes light outer membrane inner membrane light chloroplast envelope 287 lipid droplet stroma lamella thylakoid stroma thylakoid membrane using hydrogen from water. It takes place inside chloroplasts (Figure 13.2) photosystem primary pigment reaction centre thylakoid accessory pigments Figure 13.2 a A diagram of a chloroplast. b A photosystem: a light-harvesting cluster of photosynthetic pigments in a chloroplast thylakoid membrane. Only a few of the pigment molecules are shown. Cambridge International A Level Biology An overall equation for photosynthesis in green plants is: light energy in the presence n CO + n H O of chlorophyll (CH O)n + n O carbon water carbohydrate oxygen dioxide Hexose sugars and starch are commonly formed, so the following equation is often used: light energy in the presence 6CO + 6H O of chlorophyll C H O + 6O carbon water carbohydrate oxygen dioxide Two sets of reactions are involved. These are the light dependent reactions, for which light energy is necessary, and the light independent reactions, for which light energy is not needed. The light dependent reactions only take place in the presence of suitable pigments that absorb certain wavelengths of light (pages 295–296). Light energy is necessary for the splitting (photolysis) of water into hydrogen and oxygen; oxygen is a waste product. Light energy is also needed to provide chemical 288 energy, in the form of ATP, for the reduction of carbon dioxide to carbohydrate in the light independent reactions. The photosynthetic pigments involved fall into two categories: primary pigments and accessory pigments. The pigments are arranged in light-harvesting clusters called photosystems of which there are two types, I and II. In a photosystem, several hundred accessory pigment molecules surround a primary pigment molecule, and the energy of the light absorbed by the different pigments is passed to the primary pigment (Figure 13.2b). The primary pigments are two forms of chlorophyll (pages 295–296). These primary pigments are said to act as reaction centres. The light dependent reactions ofphotosynthesis The light dependent reactions include the splitting of water by photolysis to give hydrogen ions (protons) and the synthesis of ATP in photophosphorylation. The hydrogen ions combine with a carrier molecule NADP (page 275), to make reduced NADP. ATP and reduced NADP are passed from the light dependent to the light independent reactions. Photophosphorylation of ADP to ATP can be cyclic or non-cyclic, depending on the pattern of electron flow in one or both types of photosystem. Cyclic photophosphorylation Cyclic photophosphorylation involves only photosystem I. Light is absorbed by photosystem I and is passed to the primary pigment. An electron in the chlorophyll molecule is excited to a higher energy level and is emitted from the chlorophyll molecule. This is called photoactivation. Instead of falling back into the photosystem and losing its energy as thermal energy or as fluorescence, the excited electron is captured by an electron acceptor and passed back to a chlorophyll molecule via a chain of electron carriers. During this process, enough energy is released to synthesise ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis (page 270). The ATP then passes to the light independent reactions. Non-cyclic photophosphorylation Non-cyclic photophosphorylation involves both photosystems in the so-called ‘Z scheme’ of electron flow (Figure 13.3). Light is absorbed by both photosystems and excited electrons are emitted from the primary pigments of both reaction centres. These electrons are absorbed by electron acceptors and pass along chains of electron carriers, leaving the photosystems positively charged. The primary pigment of photosystem I absorbs electrons from photosystem II. Its primary pigment receives replacement electrons from the splitting (photolysis) of water. As in cyclic photophosphorylation, ATP is synthesised as the electrons lose energy while passing along the carrier chain. Photolysis of water Photosystem II includes a water-splitting enzyme that catalyses the breakdown of water: H2O → 2H++ 2e−+ 2 O2 Oxygen is a waste product of this process. The hydrogen ions combine with electrons from photosystem I and the carrier molecule NADP to give reduced NADP. 2H++ 2e−+ NADP → reduced NADP Reduced NADP passes to the light independent reactions and is used in the synthesis of carbohydrate. The photolysis of water can be demonstrated by the Hill reaction. The Hill reaction Redox reactions are oxidation–reduction reactions and involve the transfer of electrons from an electron donor (reducing agent) to an electron acceptor (oxidising agent). Sometimes hydrogen atoms are transferred, so that dehydrogenation is equivalent to oxidation. Chapter 13: Photosynthesis chains of electron carriers 2e– ADP + Pi 2e– ATP H2O primary pigment 1O2 photosystem I NADP + 2H+ reduced NADP Key flow of electrons in non-cyclic + photophosphorylation primary pigment light flow of electrons in cyclic photosystem II photophosphorylation increasing energy level light Figure 13.3 The ‘Z scheme’ of electron flow in photophosphorylation. In 1939, Robert Hill showed that isolated chloroplasts had ‘reducing power’ and liberated oxygen from water in the presence of an oxidising agent. The ‘reducing (dichlorophenolindophenol), can substitute for the plant’s NADP in this system (Figure 13.4). DCPIP becomes colourless when reduced: power’ was demonstrated by using a redox agent that changed colour on reduction. This technique can be oxidised DCPIP chloroplasts in light reduced DCPIP used to investigate the effect of light intensity or of light wavelength on the rate of photosynthesis of a suspension of chloroplasts. Hill used Fe3+ ions as his acceptor, but various redox agents, such as the blue dye DCPIP (blue) (colourless) 289 H2O 1O2 Figure 13.4 shows classroom results of this reaction. BOX 13.1: Investigating the Hill reaction Chloroplasts can be isolated from a leafy plant, such as lettuce or spinach, by liquidising the leaves in ice-cold buffer and then filtering or centrifuging the resulting suspension to remove unwanted debris. Working quickly and using chilled glassware, small tubes of buffered chloroplast suspension with added DCPIP solution are placed in different light intensities or in different wavelengths of light and the blue colour assessed at intervals. The rate of loss of blue colour (as measured in a colorimeter or by matching the tubes against known concentrations of DCPIP solution) is a measure of the effect of the factor being investigated (light intensity or the wavelength of light) on chloroplast activity. blue 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 placed in light Key chloroplasts in light chloroplasts in dark for five minutes, then in light Figure 13.4 The Hill reaction. Chloroplasts were extracted from lettuce and placed in buffer solution with DCPIP. The colorimeter reading is proportional to the amount of DCPIP remaining unreduced. 0.2 colourless 0 2 4 6 8 10 12 14 16 Time/minutes Cambridge International A Level Biology QUESTIONS 13.1 Examine the two curves shown in Figure 13.4and explain: a the downward trend of the two curves b the differences between the two curves. 13.2 Explain what contribution the discovery of the Hill reaction made to an understanding of the process of CO2 (1C) RuBP ribulose bisphosphate (5C) unstable intermediate (6C) photosynthesis. The light independent reactions of photosynthesis GP Calvin cycle × 2 glycerate reduced 3-phosphate NADP NADP ... - tailieumienphi.vn
nguon tai.lieu . vn