4.3.1 Photosynthesis Relevant Past Paper Questions Paper
Question
Specification point(s) tested
2013 January
3
a e g i j
2012 June
4
i k p q
2012 January
3
e i j
2011 June
2
b q
2011 January
3 (a mixture of photosynthesis and respiration questions).
k
2010 June
3
k m n
2010 January
5
g o q
Condensed Notes By Specification Point a) Define the terms autotroph and heterotroph . An autotroph is an organism that can use light energy or chemical energy to make organic molecules such as glucose from inorganic molecules such as carbon dioxide. A heterotroph is an organism that relies on organic molecules that it obtains from other organisms. b) State that light energy is used during photosynthesis to produce complex organic molecules. Light energy is used during photosynthesis to produce complex organic molecules. c) Explain how respiration in plants and animals depends upon the products of photosynthesis. The products of photosynthesis are oxygen and organic molecules such as sugars, fatty acids and amino acids that contain stored chemical energy. Respiration relies on these organic molecules. They are the respiratory substrates that are broken down in respiration to release their chemical energy. Aerobic respiration also relies on the oxygen produced in photosynthesis. The oxygen acts as the final acceptor of electrons and protons in respiration. Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
d) State that in plants photosynthesis is a twostage process taking place in chloroplasts. In plants photosynthesis is a twostage process taking place in chloroplasts. e) Explain, with the aid of diagrams and electron micrographs, how the structure of chloroplasts enables them to carry out their functions. Structures within the chloroplast include: Grana (singular granum ), stroma, chloroplast envelope, thylakoids, lamella, DNA, ribosomes, lipid droplets, starch grains. The DNA codes for proteins used in photosynthesis and the ribosomes are the site of synthesis of these proteins. These proteins include: enzymes that make chlorophyll; proteins that make up photosystems; electron carriers; ATP synthase; enzymes involved in the photolysis of water; enzymes involved in the Calvin cycle. The inner chloroplast membrane has transport proteins allowing to control the movement of substances between the cytoplasm and the stroma. Grana provide a large surface area for pigments, electron carriers and ATP synthase, all of which are needed for the lightdependent stage of photosynthesis. Photosynthetic pigments are arranged into photosystems to maximise the absorption of light energy. The stroma contains enzymes enzymes needed for the lightindependent stage of photosynthesis. f) Define the term photosynthetic pigment . A photosynthetic pigment is a molecule that absorbs light energy for use in photosynthesis. Each pigment has a range of wavelengths that it absorbs and a peak absorption. It reflects other wavelengths. g) Explain the importance of photosynthetic pigments in photosynthesis. Chloroplasts contain molecules of primary pigment (chlorophyll a) and accessory pigment (e.g. chlorophyll b and carotenoids). The primary and accessory pigments are arranged in the thylakoid membranes in funnelshaped protein complexes called photosystems, where they harvest light energy by absorbing photons There are two types of photosystem, photosystem I and photosystem II. They both have a reaction center which contains chlorophyll a. Chlorophyll absorbs red and blue light, but reflects green light.The different types of chlorophyll have slightly different peak absorptions. The carotenoids absorb the wavelengths of light that are not well absorbed by the chlorophylls. Having a range of different pigments in the photosystem allows a range of wavelengths of light to be absorbed. Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
The reaction center chlorophyll a molecules in photosystem I and photosystem II have slightly different peak absorptions. This is because they are found in different protein environments. The reaction center chlorophyll in photosystem I has a peak absorption of 700nm and is therefore known as P700. The reaction center chlorophyll in photosystem II has a peak absorption of 680nm and is therefore known as P680. h) State that the lightdependent stage takes place in thylakoid membranes and that the lightindependent stage takes place in the stroma. The lightdependent stage takes place in thylakoid membranes and the lightindependent stage (Calvin cycle) occurs in the stroma of the chloroplast. i) Outline how light energy is converted to chemical energy (ATP and reduced NADP) in the lightdependent stage (reference should be made to cyclic and noncyclic photophosphorylation, but no biochemical detail is required). In the lightdependent stage of photosynthesis light energy is captured and used to produce ATP (from ADP and P ) and reduced NADP (from NADP, protons and electrons). This is known as photophosphorylation. i Oxygen (O ) is also produced in the lightdependent stage, as a byproduct. 2 There are two types of phosphorylation, cyclic and noncyclic. Noncyclic photophosphorylation produces ATP and reduced NADP, whereas cyclic photophosphorylation produces only ATP. Noncyclic photophosphorylation involves photosystems I and II, whereas cyclic photophosphorylation involves only photosystem I. In both types of photophosphorylation light is absorbed by a pigment molecule in a photosystem and the energy absorbed is then funnelled towards the primary pigment reaction center. When light energy is absorbed by a pigment molecule in a photosystem, an electron from that pigment molecule becomes excited (moves to a higher energy level). This electron then becomes deexcited and releases energy. The energy is absorbed by a neighbouring pigment molecule. In this way, the energy is passed from pigment molecule to pigment molecule until it reaches the reaction center, where there are a special pair of chlorophyll a molecules. Once an electron in the primary pigment (the chlorophyll a in the reaction center) becomes excited, that excited electron physically leaves the photosystem (meaning that it will have to be replaced). The process of noncyclic photophosphorylation occurs as follows. Light energy hits photosystem II, is absorbed by an accessory pigment and is then funnelled to the primary pigment reaction center where it excites an electron. This excited electron leaves photosystem II and passes through a series of electron carriers (protein complexes embedded in the thylakoid membrane). As it passes through it deexcites and the energy released used to pump protons across the thylakoid membrane from the stroma to thylakoid lumen, setting up a concentration gradient. This proton gradient is used to synthesise ATP as explained below. The electron then passes to photosystem I where it is reexcited by light energy hitting photosystem I. The excited electron is then passed to more electron carriers and then into the stroma where it combines with protons and NADP to form reduced NADP. The electrons that are lost from photosystem II are replaced with electrons from the photolysis of water (see section j). The process of cyclic photophosphorylation occurs as follows. Light energy hits photosystem I, is absorbed by an accessory pigment and is then funneled to the primary pigment reaction center where it excites an Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
electron. This excited electron leaves photosystem I and passes through a series of electron carriers. As it passes through it deexcites and the energy released used to pump protons across the thylakoid membrane from the stroma to thylakoid lumen, setting up a concentration gradient. This proton gradient is used to synthesise ATP as explained below. The electron then passes back to photosystem I where it will be excited again. The process carries on in this cycle. Since no electrons are lost (no reduced NADP is formed) there is no need for the photolysis of water. The proton gradient established by noncyclic and cyclic photophosphorylation is used to power the production of ATP. The protons flow down their gradient through ATP synthase (a large protein complex embedded in the thylakoid membrane). The energy of the flowing protons causes ATP synthase to rotate and the energy of this rotation is used to power the production of ATP from ADP and Pi . j) Explain the role of water in the lightdependent stage. Water provides protons and electrons that are used in the lightdependent stage. There is an enzyme within photosystem II which, in the presence of light splits water into oxygen, protons and electrons (H2 O > 1/2O2 + 2e +2H+ ). This is called photolysis . The protons are used in chemiosmosis and then, along with the electrons, they combine with NADP to produce reduced NADP. k) Outline how the products of the lightdependent stage are used in the lightindependent stage (Calvin cycle) to produce triose phosphate (TP) (reference should be made to ribulose bisphosphate (RuBP), ribulose bisphosphate carboxylase (rubisco) and glycerate3phosphate (GP), but no other biochemical detail is required). The ATP and reduced NADP that are produced in the lightdependent stage are used in the lightindependent stage to convert GP to TP. The other input to the lightindependent stage is carbon dioxide which diffuses into the leaf through the open stomata. The lightindependent stage is also known as the Calvin cycle and proceeds as follows: ● CO2 reacts with ribulose bisphosphate (RuBP) to produce two molecules of glycerate3phosphate (GP). This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase oxygenase (rubisco). ● Some of the GP is used to make amino acids and fatty acids. ● The rest of the GP is reduced by reduced NADP and phosphorylated by ATP to produce triose phosphate (TP). ● Five sixths of the TP molecules are recycled to RuBP. This allows the cycle to continue. ● The other sixth of the TP molecules are used to make hexose sugars (such as glucose and fructose) and glycerol. l) Explain the role of carbon dioxide in the lightindependent stage (Calvin cycle). Carbon dioxide is the source of carbon for the production of complex organic molecules. It is fixed in the Calvin cycle to produce triose phosphate which can then be used to make many different types of organic molecules including carbohydrates, lipids and proteins. These molecules provide an energy source for all living things as well as having a huge range of structural and functional roles. m) State that TP can be used to make carbohydrates, lipids and amino acids. Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
Triose phosphate can be used to make carbohydrates, lipids and amino acids. n) State that most TP is recycled to RuBP. The majority (five sixths) of the triose phosphate is recycled to ribulose bisphosphate. o) Describe the effect on the rate of photosynthesis, and on levels on GP, RuBP and TP, of changing carbon dioxide concentration, light intensity and temperature. Increasing carbon dioxide concentration increases the rate of photosynthesis, because carbon dioxide is needed in the Calvin cycle for the production of triose phosphate. Decreasing carbon dioxide concentration causes levels of RuBP to increase and levels of GP and TP to fall. This is because GP is still converted into TP, amino acids and fatty acids, and TP is still converted into RuBP, sugars and glycerol, but RuBP is no longer combined with CO2 to produce GP. Increasing the light intensity increases the rate of photosynthesis. Decreasing the light intensity causes levels of GP to increase and levels of TP and RuBP to fall. This is because low light intensity reduces the rate of the lightdependent stage, which reduces the rate at which ATP and reduced NADP are produced. ATP and reduced NADP are required in the Calvin cycle to convert GP to TP. Increasing the temperature increases the rate of photosynthesis. This is because many of the stages in photosynthesis are controlled by enzymes and an increase in temperature causes an increase in enzyme activity. However, above a certain temperature (about 25°C depending on the plant) the rate of photosynthesis levels off and then falls as the enzymes begin to work less efficiently and then denature. If the temperature is too high the enzymes of the Calvin cycle will denature. This could cause the levels of GP, TP and RuBP to all fall if TP and GP are still being used up in the production of sugars, fatty acids and amino acids. However, if the temperature also denatures the enzymes that control these further synthesis reactions, then the levels of GP, TP and RuBP will not change. p) Discuss limiting factors in photosynthesis with reference to carbon dioxide concentration, light intensity and temperature. There is only ever one limiting factor at any given time. This is the factor which is currently limiting the rate of photosynthesis. This factor will usually be carbon dioxide concentration, light intensity or temperature. It is possible to identify the limiting factor because an increase in the limiting factor causes an increase in the rate of photosynthesis. An increase in any other factor will not cause an increase in the rate of photosynthesis. For example, if light intensity is currently the limiting factor then the only way to increase the rate of photosynthesis is to increase light intensity. Increasing temperature or carbon dioxide concentration will have no effect. q) Describe how to investigate experimentally the factors that affect the rate of photosynthesis. There are various things that can be measured in order to estimate the rate of photosynthesis. One is rate of carbon dioxide uptake, one is the rate of oxygen production and another is the rate of change in mass (mass increases as carbon dioxide is fixed). Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
Carbon dioxide concentration, light intensity and temperature can be varied to see the effect on the rate of photosynthesis. Light intensity can be varied by changing the distance between the light source and the leaf. Light intensity is equal to one divided by the square of the distance (light intensity = 1/d2 ). Hydrogen carbonate can be used as a source of CO . The CO concentration can be varied by changing the amount of 2 2 hydrogen carbonate added. Temperature can be varied by using water baths at different temperatures. Photosynthometer experiment: One piece of apparatus that can be used to measure the rate of photosynthesis is a photosynthometer (also known as an Audus microburette). In this experiment the rate of photosynthesis is measured by measuring the rate of oxygen production by an aquatic plant, such as Elodea. A shoot from the aquatic plant is taken a placed upsidedown in a test tube which is inside a water bath. The test tube contains hydrogen carbonate solution, which supplies carbon dioxide to the plant. A light source is provided so that the plant will photosynthesise. As the plant photosynthesises it produces oxygen that leaves the plant and floats up through the water. There is a capillary tube placed over the plant to collect the oxygen given off. Part of the capillary tube has a scale next to it. Gas is allowed to collect in the tube for a certain amount of time and then a syringe that is connected to the tube is used to pull the bubble into the region that has the scale. The volume of gas produced can be calculated by multiplying the length of the bubble by the crosssectional area of the capillary tube, and the crosssectional area can be calculated from the radius of the tube using the formula crosssectional area = π x radius2 . The volume of gas produced per unit time can be used as a measure of the rate of photosynthesis. There are several limitations to this experiment: 1. Not all of the oxygen produced is actually collected. There are several reasons for this: firstly, some of the oxygen is used in respiration; secondly, some oxygen will dissolve in the water; thirdly, some oxygen may escape the collection apparatus; finally, some oxygen may remain trapped inside the leaf as a bubble. 2. The method assumes that all of the gas collected is oxygen, when in fact some of it is nitrogen and some is carbon dioxide. There was nitrogen within the leaf before the experiment was started, and some of this will leave the leaf during the experiment.There will also be some carbon dioxide in the gas collected, due to the fact that there was carbon dioxide dissolved in the water (some that was there to start with and some from the added hydrogen carbonate) and some of this will come out of solution 3. There are disadvantages to using the rate of oxygen production. One is that oxygen is only produced in the lightdependent stage, meaning that this method only measures the rate of the lightdependent stage. Another disadvantage is the fact that some of the oxygen produced in photosynthesis will be used for respiration and therefore not leave the plant. This means that this technique will produce an underestimate of the rate of photosynthesis. Leaf disc experiment: Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net
Cut out leaf discs (e.g. from cress cotyledons). Place them in a syringe and halffill the syringe with dilute sodium hydrogencarbonate solution. Hold the syringe upright with your finger over the end and gently pull on the plunger. This pulls the air out of the air spaces of the spongy mesophyll and replaces it with hydrogen carbonate. This causes them the density of the syringes to increase, so they fall to the bottom. The discs are then transferred to a beaker with a light source where they photosynthesise. As they photosynthesis, the oxygen produced causes the density to decrease until they rise up to the top again. The time taken to rise can be used as a measure of the rate of photosynthesis. Indicator experiment: This experiment measures the rate of photosynthesis by measuring the rate of CO uptake. A shoot of an 2 aquatic plant is placed in a test tube that contains hydrogen carbonate solution and an indicator (such as hydrogen carbonate indicator). Carbon dioxide make water acidic. As carbon dioxide is used up in photosynthesis, the water becomes less acidic (pH increases). Therefore the timing of the colour change of the indicator can be used to measure the rate of photosynthesis. There are disadvantages to using the rate of carbon dioxide uptake. One is the fact that carbon dioxide is only used in the lightindependent stage, meaning that this method only measures the rate of the lightindependent stage. Another is that some of the carbon dioxide produced in respiration will be used for photosynthesis, meaning that this technique will produce an underestimate of the rate of photosynthesis.
Copyright George Weller 2016 For more notes and worksheets, visit www.georgeweller.net