Photosynthesis PDF

# Photosynthesis ## 13.1 An overview of photosynthesis - On these pages you will learn to: - Carry out investigations on the effects of light intensity, carbon dioxide and temperature on the rate of photosynthesis using whole plants, e.g. aquatic plants such as Elodea and Cabomba - Humans, along...

# Photosynthesis ## 13.1 An overview of photosynthesis - On these pages you will learn to: - Carry out investigations on the effects of light intensity, carbon dioxide and temperature on the rate of photosynthesis using whole plants, e.g. aquatic plants such as Elodea and Cabomba - Humans, along with almost every other living organism, owe their very existence to photosynthesis. The energy we use, whether from the food we respire or from the wood, coal, oil or gas that we burn in our homes, has been captured from sunlight by photosynthesis. Photosynthesis also produces the oxygen we breathe by releasing it from water molecules. - Autotrophic nutrition - The nutrition of organisms can be divided into two categories: - Autotrophic nutrition involves the build-up of simple inorganic molecules such as carbon dioxide and water into complex organic ones like lipids, carbohydrates and proteins using energy from light or from chemical reactions. Plants, algae and some bacteria are autotrophs. - Heterotrophic nutrition involves the breakdown of complex organic molecules into simple soluble ones. Animals, fungi and some bacteria are heterotrophs. - The word autotroph means 'self-feeding' and refers to those organisms such as plants that have no obvious means of obtaining or digesting food - no mouth, teeth, alimentary canal, etc. Instead of obtaining their food by consuming complex organic molecules, they manufacture their own from simple inorganic substances using energy from two possible sources: - Photoautotrophs use light as their source of energy to drive the process of photosynthesis. Examples of photoautotrophs include green plants, algae and photosynthetic bacteria (e.g. cyanobacteria). - Chemoautotrophs use energy from certain chemical reactions. The process is far less common than photosynthesis, but takes place in the nitrifying and denitrifying bacteria that are important in the nitrogen cycle. - An outline of photosynthesis - The overall equation for photosynthesis is: $6CO_2 + 6H_2O + energy \rightarrow C_6H_{12}O_6 + 6O_2$ - Experiments using radioactive isotopes show that all the oxygen produced ($6O_2$) comes from water molecules and not the carbon dioxide molecules. However, the $6H_2O$ in the equation only provides six oxygen atoms, rather than the 12 produced. What happens in practice is that 12 water molecules are used to produce the oxygen, and the hydrogens from the water are used to reduce the carbon dioxide and produce the six water molecules. The equation for photosynthesis is therefore more accurately represented by the equation: - $6CO_2 + 12H_2O +energy \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$ - Photosynthesis is a process in which the light energy, by a series of steps, is converted into chemical energy. There are two main stages to photosynthesis: - Capturing of light energy (light harvesting) by chloroplast pigments such as chlorophyll, carotene and xanthophyll. - The light dependent stage, is a process in which light energy is captured by chloroplast pigments such as chlorophyll, carotene and xanthophyll. An electron flow is created by the effect of light on chlorophyll, known as photoactivation. Photoactivation also causes water to split (photolysis) into hydrogen ions and oxygen. The useful products of the light dependent stage are ATP from chemiosmosis, and reduced NADP. - The light independent stage, in which carbon dioxide is reduced to produce sugars and other organic molecules using the reduced NADP and ATP from the light dependent stage. - Measuring photosynthesis - The rate of photosynthesis is usually found by measuring the volume of oxygen produced by an aquatic plant such as Canadian pondweed. This does not give an altogether accurate measure because: - dissolved oxygen, nitrogen and other gases are often released from the leaf and surrounding water and become included in the gas volume measured - some oxygen produced in photosynthesis will be used up in respiration. - The following account outlines how the rate of photosynthesis at different light intensities can be measured. - The apparatus, known as an Audus photosynthometer is set up as in Figure 3, taking care not to introduce any air bubbles into it and checking that the apparatus is completely air-tight. - The water bath is used to maintain a constant temperature throughout the experiment and should be adjusted as necessary. Better still, an electronically or thermostatically controlled water bath should be used. - Potassium or sodium hydrogencarbonate solution can be used around the plant to provide a source of carbon dioxide - especially important if the experiment is to extend over a long period. - A source of light that can have its voltage adjusted to change its intensity, is arranged close to the apparatus, which is kept in a dark room to prevent other light (which may vary in intensity) falling on the plant. - The apparatus is kept in the dark for two hours to prevent photosynthesis and allow oxygen already produced by the plant to disperse. - The light source is switched on and a stop clock started. - Oxygen produced by the plant during photosynthesis collects in the funnel end of the capillary tube above the plant. - After 30 minutes this oxygen is drawn up the capillary tube by gently withdrawing the syringe until its volume can be measured on the scale. This can be done directly (if the scale is calibrated in mm³) or, if the scale is not calibrated, calculated using the formula πr²h (where r is the internal radius of the tube and h is the length of the column of oxygen collected). - The gas is drawn up into the syringe, which is then pushed in again before the process is repeated at the same light intensity four or five times and the mean volume of oxygen produced per hour is calculated. - The apparatus is left in the dark for two hours before the procedure is repeated with the light source set at a different light intensity. The actual light intensity can be measured by a light meter placed in the same position relative to the light source as the plant was during the experiment. - An alternative method of varying the light intensity is to change the distance of the light source relative to the plant. The light intensity is inversely proportional to the square of the distance from the plant to the light source, i.e. doubling the distance apart reduces the light intensity by a quarter. - The experiment can be modified to measure the effect of other factors on the rate of photosynthesis as follows: - Wavelength of light - the experiment is repeated using different light sources that emit specific wavelengths or using filters of different colours between the light source and the photosynthometer - Temperature - the temperature of the water bath can be varied and the rates of photosynthesis compared. - Carbon dioxide concentration - different concentrations of potassium or sodium hydrogencarbonate can be used to compare the rate of photosynthesis at different carbon dioxide concentrations. ## 13.2 Adaptations to photosynthesis - leaf and chloroplast structure - The leaf is the main photosynthetic organ of the plant. Within the leaf, photosynthesis occurs in the cells of the palisade and spongy mesophyll tissues. In these cells, the organelles where photosynthesis takes place are the chloroplasts. - Structure of the leaf - Photosynthesis takes place largely in the leaf, the structure of which is shown in Figure 1(a) and (b). Leaves are adapted to bring together the three raw materials of photosynthesis (water, carbon dioxide and light) and remove its products (oxygen and glucose). These adaptations include: - a large surface area that collects as much sunlight as possible - a thin lamina (leaf blade), to keep the diffusion distance short - a transparent cuticle and epidermis that let light through to the photosynthetic palisade cells beneath - numerous stomata for gaseous exchange that open and close in response to changes in light intensity - many air spaces, especially in the spongy mesophyll, to allow diffusion of carbon dioxide and water vapour - a network of vascular tissue made up of xylem that brings water to the leaf cells and phloem that carries away the sugars produced in photosynthesis - Adaptations of palisade mesophyll cells for photosynthesis - Palisade mesophyll cells are adapted to carry out photosynthesis because they: - are closely packed and thin-walled to absorb maximum light - are arranged vertically so there are fewer cross walls that could filter out the light - are packed with numerous chloroplasts that move within the cells and are arranged in the best positions to collect the maximum quantity of light - have a large vacuole that pushes the cytoplasm and chloroplasts to the edge of the cell allowing them to absorb maximum light and leave a short diffusion pathway for carbon dioxide - have a large surface area and moist, thin walls for rapid diffusion of gases. - Structure and role of chloroplasts in photosynthesis - Photosynthesis takes place within cell organelles called chloroplasts, the structure of which is shown in Figure 1d. These vary in shape and size but are typically disc-shaped, 3-10 µm long and 1 µm in diameter. They are surrounded by a double membrane called the chloroplast envelope. The inner membrane is highly selective in what it allows to enter and leave the chloroplast. Inside the chloroplast envelope are two distinct regions: - The stroma is a fluid-filled matrix where the light independent stage of photosynthesis takes place. Within the stroma are a number of other structures such as starch grains and lipid droplets. - The grana are stacks of up to 100 disc-like structures called thylakoids, where the light dependent stage of photosynthesis takes place. - Chloroplasts are adapted to their function of harvesting sunlight and carrying out the light dependent and light independent stages of photosynthesis in the following ways: - The granal membranes provide a large surface area for the attachment of the photosynthetic pigments, electron carriers and enzymes that carry out the light dependent reaction. - A network of proteins in the grana hold the photosynthetic pigments in a very precise manner that forms special units called photosystems. - The granal membranes have many ATP synthase complexes attached to them, which manufacture ATP by chemiosmosis. - The fluid of the stroma has all the enzymes needed to carry out the light independent stage (Calvin cycle). - The stroma fluid surrounds the grana and so the products of the light dependent stage in the grana can readily pass into the stroma. - Chloroplasts contain both circular DNA and 70S ribosomes so they can quickly and easily manufacture some of the proteins needed for photosynthesis. ## 13.3 Chloroplast pigments and light harvesting - There are a number of pigments found in chloroplasts that act together to capture the light necessary for photosynthesis. The three most important groups of these pigments are the chlorophylls, carotene and xanthophyll. Apart from carbon, hydrogen and oxygen, they also contain the elements magnesium and nitrogen, which are obtained from minerals taken up from the soil by the roots. - Chlorophyll - Chlorophyll is not a single substance, but rather a group of similar green pigments of which chlorophyll a, the most important photosynthetic pigment, and chlorophyll b are the most common. These pigments strongly absorb light in the blue and red wavelengths of the spectrum. Photoactivation of chlorophyll involves chlorophyll a. Chlorophylls are made up of a complex ring called a porphyrin ring, which has the same basic structure as the 'haem' group of the blood pigment haemoglobin but at its centre there is a magnesium atom. - Carotene and xanthophyll - Carotene and xanthophyll have a basic structure comprising two small rings linked by a long hydrocarbon chain. They range in colour from pale yellow, through orange to red. The greater the number of double bonds in the hydrocarbon chain, the deeper the colour. Carotene and xanthophyll are known as accessory pigments because they are not directly involved in the light-dependent reaction of photosynthesis. Instead they absorb light wavelengths that are not efficiently absorbed by chlorophyll a and pass the energy they capture to chlorophyll a for use in the light dependent stage. - Chromatography - The various photosynthetic pigments can be separated from one another by means of chromatography. It involves moving the mixture of molecules over a stationary phase. The separation of the molecules depends on their solubility and molecular mass. To separate photosynthetic pigments, the mixture of pigments is concentrated in a spot at one end of a paper strip and then dipped in a solvent which moves up the paper by capillarity, carrying the molecules with it. The different pigments separate out at different distances from the original spot. Each pigment can be identified by its Rf value, calculated by dividing the distance travelled by the pigment by the distance travelled by the solvent front. For any particular solvent used, each pigment has a characteristic Rf value. - Absorption and action spectra - Radiant energy comes in discrete packages called quanta. A single quantum of light is called a photon. Light also has a wave nature and so forms part of the electromagnetic spectrum. Visible light is made up of different wavelengths. The shorter the wavelength the greater the quantity of energy it possesses. A pigment, such as one of the chlorophylls, will absorb some wavelengths of light more than others. If the amount of light it absorbs at each wavelength is plotted on a graph, we obtain what is called the absorption spectrum. - The remaining pigment molecules (accessory pigments) of the photosystem absorb light energy (photons). These molecules are called the antenna complex. They are held tightly together by proteins that act as a framework holding the pigment molecules in the best positions to allow energy be transferred between them. - The photon absorbed by an accessory pigment creates an excitation energy that is passed along a chain of pigment molecules to the reaction centre. - Energy from many pigment molecules in the antenna complex is funnelled in this way to the reaction centre. Energy from one photon excites an electron in each of the primary pigment molecules (special pair) of the reaction centre. These electrons play an important part in the light dependent stage. - Light harvesting - In 1932, plant physiologists Emerson and Arnold discovered that it took hundreds of chlorophyll molecules to produce a single molecule of oxygen. This led them to conclude that light for photosynthesis, rather than being absorbed by independent pigment molecules, is captured by groups of chlorophyll molecules along with their accessory pigments. These groups are now known as photosystems and are located in the photosynthetic membranes (thylakoids and intergranal lamellae). They operate as follows: - Each photosystem is a collection of chlorophyll a molecules, accessory pigments and associated proteins all fixed within a protein matrix. - One particular pair (primary pigments) of chlorophyll a molecules, often termed the special pair, acts as a reaction centre for each photosystem. - Photosystem I (PSI) has a reaction centre with a light absorption peak of 700nm and is therefore known as P700. Photosystem I occurs mostly on inter-granal lamellae of the chloroplast. - Photosystem II (PSII) has a reaction centre with a light absorption peak of 680 nm and is therefore known as P680. Photosystem II occurs mostly on the granal lamellae of the chloroplast. ## 13.4 Photosynthesis - the light dependent stage - The light dependent stage of photosynthesis takes place in the thylakoids of the chloroplasts. It involves the capture of light whose energy is used for two purposes: - To add an inorganic phosphate molecule to ADP, to make ATP. As this process of phosphorylation is brought about by light it is known as photophosphorylation. - To split water into protons, electrons and oxygen. As the splitting is caused by light, it is known as photolysis. - Photoactivation of chlorophyll - When light is passed to the reaction centre chlorophyll molecules, a pair of electrons is raised to a higher energy level. This is known as the photoactivation of chlorophyll. These electrons are said to be in an excited state and are taken up by a molecule called an electron carrier or electron acceptor. Each chlorophyll molecule has been oxidised while the carrier, which has gained electrons, has been reduced. The electrons are now passed along a number of electron carriers in a series of redox reactions. Each new carrier is at a slightly lower energy level than the previous one, and so the electrons lose energy at each stage. This energy is used to transfer hydrogen ions (protons) across the thylakoid membrane into the thylakoid space (lumen). ATP is produced as a result of chemiosmosis, as described in Topic 12.2. Protons that build up in the thylakoid space flow through the ATP synthase complex of the thylakoid membrane, thereby providing the energy to combine inorganic phosphate with ADP to form ATP. This process is called photophosphorylation, as it requires light (not oxygen as in oxidative phosphorylation). The events are shown in Figure 2. The question now is, what happens to the electrons? There are two alternative processes they can enter: either cyclic photophosphorylation or non-cyclic photophosphorylation. - Cyclic photophosphorylation - Cyclic photophosphorylation uses only photosystem I. When light raises an electron in a reaction centre chlorophyll molecule to an excited state, the electron is taken up by an electron acceptor and simply passed back to the same chlorophyll molecule via a sequence of electron carriers, i.e. it is recycled. While this does not produce any reduced NADP, it does generate sufficient energy to combine inorganic phosphate with ADP. The ATP so produced is then used in the light independent stage or is used directly, as in guard cells where it is used to pump potassium ions into the guard cells, thereby reducing water potential and leading to water entering them by osmosis and increasing their turgidity, with the result that the stoma opens. - Non-cyclic photophosphorylation - Non-cyclic photophosphorylation uses both photosystem I and photosystem II. Electrons raised to an excited state in photosystem II are taken up by an electron acceptor and passed along a sequence of electron carriers to then replace the electrons lost in photosystem I. The electrons raised to an excited state from photosystem I are taken up by an electron acceptor and are then taken up by NADP+ (nicotinamide adenine dinucleotide phosphate) and passed into the light independent stage of photosynthesis. This leaves the reaction centre chlorophyll molecules of photosystem II short of electrons and therefore positivey charged. Before the photosystem can operate again these electrons must be replaced. The replacement electrons are provided from water molecules that are split using light energy. This photolysis of water also yields hydrogen ions (protons) into the thylakoid lumen, where they contribute to the build-up of the proton gradient. Hydrogen ions in the stroma can be used for the reduction of NADP. - Photolysis - Photolysis is the splitting of water as a direct consequence of the photoactivation of chlorophyll. It occurs only in photosystem II, which is associated with an enzyme known as the oxygen evolving complex. Having lost an electron, the chlorophyll molecule needs to replace it. In the case of non-cyclic photophosphorylation, it does this using electrons from water molecules that are split by the oxygen evolving complex into protons, electrons and oxygen according to the following equation: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ - The electrons replace those lost by the chlorophyll molecules. The protons reduce NADP to NADPH + H+, which then enters the light independent stage where it reduces carbon dioxide. - The oxygen by-product is either used in respiration or diffuses out of the leaf as a waste product of photosynthesis. - The Z-scheme - All the processes of the light independent stage are closely linked. These events are summarised in Figure 3, which illustrates the zig-zag energy levels of the electrons. As the diagram resembles a Z on its side, the complete process is called the Z-scheme. ## 13.5 Photosynthesis - the light independent stage - The products of the light dependent stage of photosynthesis, namely ATP and reduced NADP are used to reduce carbon dioxide in the second part of photosynthesis. Unlike the first stage, this stage does not require light directly and is therefore called the light independent stage. In practice, it requires the products of the light dependent stage and so rapidly stops when light is absent. The light independent reaction stage takes place in the stroma of the chloroplasts. The details of this stage were worked out by Melvin Calvin and his co-workers. The process is therefore often known as the Calvin cycle. - The Calvin cycle - In the following account of the Calvin cycle, the numbered stages are shown in Figure 1. Each step in the Calvin cycle is enzyme controlled. - Carbon dioxide from the atmosphere diffuses into the leaf through stomata and dissolves in water around the walls of the palisade cells. It then diffuses through the cell surface membrane, cytoplasm and chloroplast envelope into the stroma of the chloroplast. - In the stroma, the carbon dioxide combines with the five-carbon compound ribulose bisphosphate using the enzyme ribulose bisphosphate carboxylase, to form an unstable six-carbon compound. - The unstable six-carbon compound immediately breaks down into two molecules of the three-carbon glycerate 3-phosphate (GP). - Using one of the ATP molecules from the light dependent reaction, the GP (glycerate 3-phosphate) is converted into a 3 carbon sugar triose phosphate (TP). - Reduced NADP from the light dependent stage provides hydrogen for the reduction of GP to TP (glycerate-3-phosphate to triose phosphate). - Triose phosphate molecules combine in pairs to form six-carbon (hexose) sugars. - The six-carbon sugars can be polymerised into starch. - Five out of every six triose phosphate molecules produced are used to regenerate ribulose bisphosphate using the remainder of the ATP from the light dependent stage as the source of energy. - Formation of other substances for use by the plant - Plants, like other organisms, are made up of a range of complex organic molecules. The bulk of these are carbohydrates, lipids and proteins. Unlike animals and other heterotrophic organisms, plants cannot obtain these substances by taking them in from the outside. They must synthesise them from the various compounds of the Calvin cycle. - Carbohydrates, e.g. sucrose (the carbohydrate which is transported in the phloem) are made by combining the two hexose sugars, glucose and fructose. Glucose is used as a respiratory substrate. - Starch (the storage carbohydrate) and cellulose (the essential component of cell walls) are made by polymerising glucose in different ways. - Lipids are made up of glycerol and fatty acids. Plants make glycerol from triose phosphate and fatty acids from glycerate 3-phosphate (GP). Lipids are used in plant cells for storage and to form phospholipids in their cell membranes. - Proteins are made up of amino acids that in turn can be produced from glycerate 3-phosphate (GP) via acetyl coenzyme A and the intermediates of Krebs cycle. Proteins are important components of cell membranes and all enzymes are proteins. ## 13.6 Limiting factors affecting photosynthesis - In any complex process such as photosynthesis, the factors that affect its rate all operate simultaneously. However, the rate of the process at any given moment is not affected by a combination of all the factors, but rather by just one the one whose level is at the least favourable value. This factor is called the limiting factor because it alone limits the rate at which the process can take place. However much the levels of the other factors change, they do not alter the rate of the process. - To take the example of light intensity limiting the rate of photosynthesis: - In complete darkness, it is the absence of light alone that prevents photosynthesis occurring. - No matter how much we raise or lower the temperature or change the concentration of carbon dioxide, there will be no photosynthesis. Light or rather the absence of it, is the factor determining the rate of photosynthesis at that moment. - If we provide light, however, the rate of photosynthesis will increase. - As we add more light, the more the rate increases. This does not continue indefinitely, however, because there comes a point at which further increases in light intensity have no effect on the rate of photosynthesis. - At this point some other factor, such as the concentration of carbon dioxide, is in short supply and so limits the process. - Carbon dioxide is now the limiting factor and only an increase in its level will increase the rate of photosynthesis. - In the same way as happened with light, providing more carbon dioxide will lead to more photosynthesis. - Further increases in carbon dioxide levels will fail to have any effect. - At this point a different factor, e.g., temperature, is the limiting factor and only an alteration in its level will affect the rate of photosynthesis. - The law of limiting factors can therefore be expressed as: At any given moment, the rate of a physiological process is limited by the one factor which is at its least favourable value, and by that factor alone. - Effect of light intensity on the rate of photosynthesis - When light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity. The rate of photosynthesis is usually measured in two ways: - the volume of oxygen produced by a plant - the volume of carbon dioxide taken up by a plant. - These measurements do not, however, provide an absolute measure of photosynthesis because: - some of the oxygen produced in photosynthesis is used in cellular respiration and so never leaves the plant and therefore cannot be measured - some carbon dioxide from cellular respiration is used up in photosynthesis and therefore the volume taken up from the atmosphere is less than that actually used in photosynthesis - As light intensity is increased, the volume of oxygen produced and carbon dioxide absorbed due to photosynthesis will increase to a point at which it is exactly balanced by the oxygen absorbed and carbon dioxide produced by respiration. At this point there will be no net exchange of gases into or out of the plant. This is known as the light compensation point. Further increases in light intensity will cause a proportional increase in the rate of photosynthesis and increasing volumes of oxygen will be given off and carbon dioxide taken up. A point will be reached at which further increases in light intensity will have no effect on photosynthesis. At this point some other factor such as carbon dioxide concentration or temperature is limiting the reaction. - Effect of carbon dioxide concentration on the rate of photosynthesis - Carbon dioxide is present in the atmosphere at a concentration of around 0.04%. This level continues to increase as the result of human activities such as burning fossil fuels and the clearing of rain forests. It is still one of the rarest gases present and is often the factor that limits the rate of photosynthesis under normal conditions. The optimum concentration of carbon dioxide for a consistently high rate of photosynthesis is 0.1% and growers of some glasshouse crops like tomatoes enrich the air in the glasshouses with more carbon dioxide to provide higher yields. - Effect of temperature on the rate of photosynthesis - Provided that other factors are not limiting, the rate of photosynthesis increases in direct proportion to the temperature. Between the temperatures of 0°C and 25°C the rate of photosynthesis is approximately doubled for each 10°C rise in temperature. Above the optimum temperature of 25°C the rate levels off and then declines - largely as a result of enzyme denaturation. Purely photochemical reactions are not usually affected by temperature, and so the fact that photosynthesis is temperature sensitive suggested to early researchers that there was also a totally chemical process involved as well as a photochemical one. We now know that this chemical process is the light independent stage and that there are enzymes involved in the light dependent stage. - Increasing crop yields in glasshouses - Food production depends on photosynthesis. As the rate of photosynthesis is determined by the factor that is in shortest supply (limiting factor) it follows that there is commercial value in determining which factor is limiting photosynthesis at any one time. By supplying more of this factor, photosynthesis, and hence food production, can be increased. It is not practical to control the environment of crops in natural conditions. Plants grown in glasshouses are a different matter. In the enclosed environment of a glasshouse it is possible to regulate temperature, humidity, light intensity and carbon dioxide concentration. Scientists are able to predict the effects of changing these factors on the rate of photosynthesis. They can then advise commercial growers on the optimum conditions that should be created in order to increase the rate of photosynthesis and hence the growth of their crops. It may seem logical to simply increase the level of all factors, with a view to increasing photosynthesis to a high value and ensure maximum yield. In practice different plants have different optimum conditions and too high a level of a particular factor may reduce yield or kill the plant altogether. For example, high temperatures may increase the yield of one species but denature the enzymes of another, and kill the plants of that species. It is also uneconomic and wasteful to use energy to raise temperature or to increase carbon dioxide concentrations or light intensity beyond what is necessary. Precise control of the environment is therefore essential. This can be brought about in ways ranging from totally manual control to the use of advanced computerised systems. To take the example of carbon dioxide concentration, the average concentration in the atmosphere is around 400 parts per million (ppm). It has been shown that by raising this level to 1000 ppm the yields from tomato plants can be increased by 20% or more. ## 13.7 Adaptations to photosynthesis - C4 plants- - In Topic 13.5 we learnt about the action of ribulose bisphosphate carboxylase (rubisco) during photosynthesis. During the Calvin cycle, this important enzyme combines carbon dioxide with a five-carbon ribulose bisphosphate molecule to form a six-carbon compound. This compound is unstable and immediately splits into two three-carbon compounds. Plants which photosynthesise in this way are therefore called C3 plants. - Rubisco also catalyses a second reaction in which ribulose bisphosphate combines with oxygen rather than carbon dioxide. This process is called photorespiration and it releases carbon dioxide. This works against the Calvin cycle in which carbon dioxide is incorporated into molecules rather than released from them. The two reactions take place at the same active site on the rubisco enzyme and therefore compete with one another. At a temperature of 25°C, around 20% of carbon dioxide fixed by photosynthesis is lost to photorespiration. The higher the temperature, the greater this loss, with up to 50% of photosynthetically fixed carbon being lost in this way. - To overcome this wastage, some plants have adapted to warmer environments by evolving a different photosynthetic pathway. Among these plants arc maize and sorghum (Figure 2) and the pathway they use is called the C4 pathway. - The C4 pathway - Plants which use C4 photosynthesis, add a carbon dioxide molecule to a three-carbon molecule called phosphoenol pyruvate (PEP) instead of the five-carbon ribulose bisphosphate. A molecule of oxaloacetate is formed that has four carbon atoms, hence the name, C4 pathway. The enzyme that catalyses this reaction is called PEP carboxylase and, importantly, it does not carry out oxidation and so there is no photorespiration. In addition, PEP carboxylase has a greater affinity for carbon dioxide than rubisco does. It also operates at a higher optimum temperature without being denatured. - The oxaloacetate formed is converted to malate, which passes from the mesophyll cells to special cells called bundle sheath cells. Carbon dioxide is lost from malate and enters the Calvin cycle in the usual way. The three-carbon pyruvate, which forms as a result, passes back into the mesophyll cells and is converted to phosphoenol pyruvate ready to accept another carbon dioxide molecule. A summary of C4 photosynthesis is shown in Figure 1. - Adaptation of the leaf in C4 plants - If you look at the arrangement of the cells in the leaf of a C4 plant as shown in Figure 1, you will see they are different from those of a C3 plant. You will notice that around the vascular bundle is a tight ring of bundle sheath cells which itself is surrounded by a ring of tightly fitting mesophyll cells. This arrangement ensures that the bundle sheath cells are isolated from the air inside the leaf. This prevents photorespiration by preventing oxygen reaching the bundle sheath cells. It also prevents carbon dioxide being lost, which therefore accumulates within them. This store of carbon dioxide can be used when supplies from outside the leaf are in short supply. An adaptation of this kind is very useful for plants that grow in hot climates and where stomata may close at midday to prevent excessive water loss. - The light dependent stage, where oxygen is produced, takes place in mesophyll cells and not bundle sheath cells. This spatial separation of light dependent and the initial carbon fixation of the light independent stages prevents photorespiration taking place by keeping rubisco separated from oxygen. A further adaptation is the presence of numerous plasmodesmata between bundle sheath cells and mesophyll cells. These allow more rapid movement of malate and pyruvate between the two.