WJEC UNIT 3 ATP & Photosynthesis 1
Adenosine Triphosphate (ATP) Revision from unit 1 1. ATP is a nucleotide. Label the components of the ATP molecule below: In the space below draw a simplified diagram of ATP 2
2. ATP releases energy and requires energy to be formed Write an equation to show the reaction that releases energy from ATP. What is the name of this reaction?. How much energy is released from this reaction?. Write an equation to show the reaction that forms ATP What is the name of this reaction?. What is the source of energy for this reaction?. 3. ATP is a universal energy currency What is a universal energy currency?. What are the features of ATP that make it a good universal energy currency? 3
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The Synthesis of ATP Photosynthesis 1. Chloroplast Structure Label the electron micrograph of the chloroplast 5
2. Chloroplast use light energy to phosphorylate ADP to ATP in the stage of photosynthesis called the light dependant reactions where various energy conversion processes result in the formation of a proton gradient between the stroma and thylakoid space. The light dependant stage of photosynthesis occurs in the thylakoid membrane and requires the following structures: A photosystem (also called a light harvesting complex) of which there are two different types these are called photosystem I (PSI) and photosystem II (PSII). Proton pumps. Electron carriers and electron acceptors. ATP synthetase. Light absorbing pigments of which there are many different types. Light energy. Basic structure of the thylakoid membrane Electron carriers/acceptors Thylakoid membrane PSII Proton Pump PSI Real image of thylakoid membranes ATP Synthetase 6
Basic structure of the photosystem Accessory Pigments Antennae Complex Reaction Centre Reaction Centre This contains the Primary pigment which is chlorophyll a. In PSII the chlorophyll a absorbs light of wavelength 680nm. In PSI the chlorophyll a absorbs light of wavelength 700nm. Antennae Complex This contains a number of different light absorbing pigments. Examples of accessory pigments are: chlorophyll b, xanthophyll, beta carotene and carotenoids. The function of the accessory pigments are: To increase the range of wavelength absorbed by the plant. This increases the rate and efficiency of photosynthesis. Light energy is passed down the antennae complex through the accessory pigments and to the reaction centre see dashed arrows in the photosystem diagram above. 7
The absorption of wavelengths of light by light absorbing pigments can be shown on a graph which is called an absorption spectrum graph. A typical absorption spectrum. Every peak on an absorption spectrum graph shows a specific wavelength of light being absorbed by a specific photosynthetic pigment. Every trough represents light being reflected. 8
When light energy is absorbed photosynthesis will occur this can be shown on a graph called an action spectrum graph. A typical action spectrum. An action spectrum has the same general trend as an absorption spectrum. Every peak shows that photosynthesis is occurring while every trough shows that none or less photosynthesis is occurring. 9
Energy The light dependent reactions produce ATP, by cyclic and non-cyclic photophosphorylation, as well as reduced NADP (NADPH 2 ). Non-cyclic photophosphorylation and NADPH 2 production can be represented on an energy graph called the Z scheme. THE Z-SCHEME 4 1 2e - 2e - 2 H + H + ADP ATP 4 2e - + 2H + = H 2 NADP NADPH 2 PSI 1 3 PSII 2e - H 2 O ½O 2 + 2H + 1 10
Summary of the events of non-cyclic photophosphorylation. Step 1. Light energy (of wavelength 680nm) is absorbed by photosystem II. Light energy is transmitted through the antenna complex via the accessory pigment and enters the retraction centre. Here, light energy is absorbed by chlorophyll a the structure of which is shown below: The light energy is used to excite electron to a higher energy level. These excited electrons are then ejected from chlorophyll a, as high energy elections, thereby causing chlorophyll a to become oxidised. The ejected electrons are caught by an electron acceptor. Because chlorophyll a is now oxidised it is highly reactive and requires the lost electrons to be replaced. This is achieved by the photolysis of water. During the photolysis of water, water is split into protons (H + ), oxygen and electrons (e - ). Some of the oxygen is used by the plant in aerobic respiration with the remaining oxygen being released from the leaf and used in animal aerobic respiration. The electrons are used to replace those lost from PS II. 11
Step 2 The high energy electrons then enter what is called an electron transport chain (ETC). Basically this is a sequence of electron carriers and proton pumps within the thylakoid membrane. A basic representation of the electron transport chain (without proton pumps) is shown below: This represent an electron carrier (open rectangle). The arrows show the movement of the electron (black circle). As the electron carrier gains the electron it becomes reduced. As the electron carrier releases the electron it becomes oxidised. As the electron is released it is passed on to the next electron carrier and so is passed down the electron transport chain via a series of electron carrier that undergo alternating oxidation and reduction. 12
What is the function of these high energy electron as they flow down the ETC? Well, their energy is used to create a proton gradient by providing the proton pumps with the energy they need to pump protons from the stroma and into the thylakoid space. This is shown in the basic diagram below of one thylakoid: H + H + H + H + H + H + H + H + H + H + H + H + H + STROMA ATP H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + H + ADP + Pi H + Thylakoid space Thylakoid membrane As the proton gradient is formed it has within it stored energy. This energy is released as the protons diffuse through the ATP synthetase back into the stroma. This released energy is then used by the enzymatic part of the ATP synthetase to catalysed the reaction between ADP and inorganic phosphate (Pi) to produce ATP. This is how ATP is made by non-cyclic photophosphorylation. 13
Step 3 This step occurs at the same time as step 1, and the same points about electrons being excited and raised to a higher energy level, etc apply to PSI. However, light energy of 700nm is being absorbed. Chlorophyll a in the reaction centre of PS I becomes oxidised and the electron from PS II enter and reduce the chlorophyll a. Step 4 The high energy electron ejected from chlorophyll a of PS I are then passed down a second ETC. The electrons re-combine with the protons formed form the photolysis of water to form hydrogen which then reduces NADP to NADPH 2. The products of non-cyclic photophosphorylation are: Useful products ATP and NADPH 2 these are needed by the Calvin cycle. Waste products Oxygen Details of cyclic photophosphorylation A diagram showing cyclic photophosphorylation is shown on page 14. There is not a great new to lean here. Basically, high energy electrons from PS I can be shunted from going down the ETC to make NADPH 2 and down the ETC that makes ATP. When this occurs the electrons re-enter PS I, get re-ejected and cycle back down the ETC to make ATP. 14
DIAGRAM SHOWING CYCLIC PHOTOPHOSPHORYLATION 2e - H + ADP H + ATP PSI Why does cyclic photophosphorylation occur? The answer is: Non-cyclic photophosphorylation makes ATP and NADPH 2 in roughly equal amounts, but the Calvin cycle requires more ATP than NADPH 2. Cyclic photophosphorylation makes up the ATP deficit required by the Calvin cycle as it does not produce NADPH 2. Simple comparison of cyclic and non-cyclic photophosphorylation. Non-Cyclic Cyclic First source of electrons Water PS I Last election acceptor NADP PS I Products ATP, NADPH 2, Oxygen ATP Photosystems involved PS I and PS II PS I 15
3. Carbon dioxide is fixed into complex sugars by a cyclic series of reaction called the Calvin cycle using both ATP and NADPH 2 and occurring in the stroma of the chloroplast. The Calvin cycle is where carbohydrates are made by the reduction of carbon dioxide. The Calvin cycle is shown on page 17 and a description of it is below: STEP 1 The first reaction in the Calvin cycle involves the reaction of carbon dioxide with ribulose bisphosphate (RuBP). This reaction is catalysed by an enzyme called RUBISCO. The reaction is often called the fixation of carbon dioxide. The product of this step is an unstable 6 carbon compound that rapidly breaks down into 2 molecules of the 3 carbon compound glycerate-3 phosphate (GP). STEP 2 This step requires both ATP and NADPH 2. GP is reduced to two molecules of the three carbon compound triose phosphate. NADPH 2 is required to provide the hydrogen to reduce GP to TP. NADP is reformed during this reaction and goes back to the thylakoid membrane to be reduced again during non-cyclic photophosphorylation. ATP is required to provide energy for the reduction of GP to TP. The ADP form is re-phosphorylated by cyclic/non-cyclic photophosphorylation. STEP 3 Some of the TP formed in step 2 is used to reform RuBP. This occurs initially by TP being converted to ribulose phosphate (RuP), then RuP being phosphorylated to RuBP by the donation of phosphate from ATP to form ADP. As stated in step 2, ADP will be re-phosphorylated. STEP 4. TP that is not used to re-form RuBP leaves the Calvin cycle and, via other chemical reactions, is converted to all the other biological molecules required by the plant, e.g. glucose, sucrose, amino acids, fats DNA etc. 16
Rubisco CO 2 Step 1. Carbon dioxide fixation Step 3. Regeneration of CO 2 acceptor ADP Ribulose Bisphosphate Unstabe 6 carbon compound ATP Ribulose phosphate Glycerate Phosphate ATP Step 4. Output Triose Phosphate NADP ADP NADPH 2 Step 2. Reduction 17
4. The concept of limiting factors As you can see from the previous sections, photosynthesis is a complex series of reactions. The rate at which these reactions occur, and hence, the rate of photosynthesis is determined by the rate of the slowest reaction in the series. For example, the Calvin cycle is dependent on the light dependent reactions for ATP and NADPH 2. At low light intensities, the rate at which these products are produced is too slow to allow the Calvin cycle to proceed at maximum rate so light is the limiting factor. The principle of limiting factors can be defined as: When a chemical process is affected by more than one factor, its rate is limited by that factor that is nearest to its minimum value. It is this factor that will directly affect a process if its quantity is changed. The most important factors that can limit the rate of photosynthesis are: 1. Light intensity 2. Carbon dioxide concentration 3. Temperature The effect of the above factors on the rate of photosynthesis can be plotted on a graph called a reaction rate graph. To explain the principle of limiting factors, consider the graph (and questions) on page 19 where light intensity is the independent variable. 18
Rate of Photosynthesis E B C A D Light Intensity Q What is the limiting factor in region A? A Remember that a limiting factor is the factor that is nearest to its minimum value. So at the start of the x axis the light intensity is very low. As the light intensity increases it has the effect of increasing the rate of photosynthesis. So light intensity is the limiting factor in region A. Q What is represented by the curve at B and C? A In region B some other factor is becoming a limiting factor as well as the light intensity. In region C light intensity is no longer the limiting factor, i.e. it is no longer at its minimal value. Further increase in light intensity has no effect on the rate of photosynthesis in region C. Q What does point D represent on the curve? A This is known as the saturation point for the light intensity. It s the point beyond which an increase in light intensity will no longer cause an increase in the rate of photosynthesis. Q What does point E represent on the curve. A The maximum rate of photosynthesis that is attainable under the conditions of the experiment. 19
All the rate limiting factors show the same trend as the graph above. All show an initial linear increase in photosynthetic rate where the factor being investigated (independent variable) limiting, followed by a decrease in the rate of increase and stabilising of the rate as another factor, or factors, becomes limiting. Further explanation of the factors that limit the rate of photosynthesis 1. Light When considering the effect of light on photosynthesis its important to distinguish between: light intensity, wavelength of light and duration of light exposure. 1.1 Light intensity In low light intensities the rate of photosynthesis increase linearly with increasing light intensities. Gradually the rate of increase falls off as the other factors become limiting. Illumination on a clear summers day is about 100 000 lux, whereas light saturation for photosynthesis is reach at about 10 000 lux. Therefore, except for shaded plants, light is not normally a major limiting factor. 1.2 Light duration Photosynthesis only occurs in light, but is otherwise not unaffected by light duration. 1.3 Wavelength of light The effect of light wavelength is revealed by the action spectrum for photosynthesis as shown previously. 20
2. Carbon Dioxide Concentration Carbon dioxide is needed in the Calvin cycle where it is fixed into carbohydrates. Under normal condition in the field carbon dioxide is the major limiting factor in photosynthesis. The concentration of carbon dioxide in the atmosphere is 0.03%, but increases in the rate of photosynthesis can be achieved by increasing the carbon dioxide concentration. The optimum concentration of carbon dioxide is about 0.5%. 3. Temperature The reactions of the Calvin Cycle are enzyme controlled (and to some extent are the light dependent reactions). 21