4.1 Nutrition in Plants PDF

Maintenance of life (Nutrition, Transport and Respiration) Syllabus Section 4 4.1: Nutrition in Plants 4.1 Nutrition in Plants Dr M. Ellul 4.1.1: Autotrophic Nutrition Sylla...

Maintenance of life (Nutrition, Transport and Respiration) Syllabus Section 4 4.1: Nutrition in Plants 4.1 Nutrition in Plants Dr M. Ellul 4.1.1: Autotrophic Nutrition Syllabus: Autotrophic nutrition: synthesis of an organic compound from an inorganic source of carbon. Chemosynthesis: using the oxidation of inorganic molecules as a source of energy; photosynthesis: using light as a source of energy. 2 4.1 Nutrition in Plants Dr M. Ellul Learning outcomes Define autotrophic nutrition Distinguish between chemosynthesis and photosynthesis Autotrophic nutrition is the process of building up nutritive organic molecules from inorganic materials. It could be through: Photosynthesis Chemosynthesis Photosynthesis Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae (Kingdom Protoctista). Plants need only light energy, CO2, and H2O to make sugar. The process of photosynthesis takes place in the chloroplasts, specifically using chlorophyll, the green pigment involved in photosynthesis. Photosynthesis takes place in two sequential stages: Light-dependent reactions Light-independent reactions, or Calvin Cycle In light-dependent reactions, the energy from sunlight is absorbed by chlorophyll and converted into chemical energy in the form of electron carrier molecules like ATP and NADPH. In light-independent reactions (the Calvin cycle), carbohydrate molecules are assembled from carbon dioxide using the chemical energy harvested during the light-dependent reactions. The overall chemical reaction involved in photosynthesis is: 6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2. Chemosynthesis Some bacteria build up organic molecules by another process, which utilises chemical energy instead of light energy. This process is called chemosynthesis. In chemosynthesis, one or more carbon molecules (usually carbon dioxide or methane, CH4) is converted into organic matter, using the oxidation of inorganic molecules (such as hydrogen gas, hydrogen sulfide (H2S) or ammonia (NH3)) or methane as a source of energy, rather than sunlight. 3 4.1 Nutrition in Plants Dr M. Ellul Sulfur bacteria These bacteria reside at great depths below the surface of the sea. Hydrogen sulphide seeps from hydrothermal vents, which is oxidised by the bacteria to derive energy. Due to these bacteria, many ecosystems having a lot species of marine animals flourish near these hydrothermal vents. Thiobacillus is a two common species of sulphur bacteria. 2S + 3 O2 + 2H2O → 2H2SO4 + energy During this oxidation process, they release energy that is used to convert carbon dioxide (CO2) into organic compounds like sugars and amino acids. Nitrifying bacteria Nitrifying bacteria initiate oxidation of ammonium to nitrite, which is later oxidised to nitrates to obtain energy. Nitrifying bacteria are chemoautotrophs because they obtain the energy by oxidising inorganic substances ammonia and nitrites to form their organic compounds from CO2. Nitrosomonas: 2NH3 + 3 O2 → 2HNO2 + 2H2O + energy Nitrobacter: 2HNO2 + O2 → 2HNO3 + energy. Iron bacteria These bacteria live in iron-rich environments. They obtain energy by converting iron (II) to the iron (III) state. It is a part of their metabolism, which enables them to derive energy and make food. Ferrobacillus and Leptothrix are common species of iron bacteria. oxygen Fe2+ Fe3+ + energy 4 4.1 Nutrition in Plants Dr M. Ellul 4.1.2: Details of leaf and chloroplast structure and their roles Syllabus: Description of the internal structure of a dicotyledonous leaf; the location of the palisade tissue. Functions of leaf in relation to structure. Structure of a chloroplast as revealed by electron microscopy. To identify the envelope, stroma, grana and lamellar structure. The location of the chloroplast pigments. The role of chloroplast pigments (chlorophyll a and b and carotenoids in converting light energy into chemical energy; primary and accessory pigments) Distinction between absorption and action spectra. 5 4.1 Nutrition in Plants Dr M. Ellul Learning outcomes Draw and describe the internal structure of a dicotyledonous leaf and relate the structure to the function. Draw and describe the chloroplast. Identify the location of the chloroplast pigments. Explain the role of chloroplast pigments (chlorophyll a and b and carotenoids in converting light energy into chemical energy) Distinguish between primary and accessory pigments Distinguish between absorption and action spectra. Understand the principles underlying chromatography. Structure of the leaf Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomata. 6 4.1 Nutrition in Plants Dr M. Ellul Tissue Structure Function Upper and One cell thick. Colourless flattened cells. Protective. lower External walls covered with waxy cuticle Cutin is waterproof and protects from epidermis (made of cutin). desiccation and infection. Lower epidermis contains numerous Stomata are site of gaseous exchange. The stomata. size of the stomata is regulated by guard Each stoma is surrounded by a pair of cells. The guard cells contain chloroplasts. guard cells. Palisade Column-shaped cells with many Main photosynthetic tissue. Chloroplasts mesophyll chloroplasts in a thin layer of cytoplasm. may move towards light. Spongy Irregularly shaped cells fitting together Photosynthetic but contain fewer mesophyll loosely to leave large air spaces chloroplasts than palisade cells. Gaseous exchange can occur through the large air spaces by stomata. Stores starch. Vascular Extensively finely branching network Conducts water and mineral salts to the tissue through the leaf leaf in the xylem. Removes sucrose (product of photosynthesis) in phloem. Provides support to the leaf lamina (leaf blade) [this is aided by collenchyma of the midrib, turgidity of mesophyll cells and sclerenchyma] 7 4.1 Nutrition in Plants Dr M. Ellul Chloroplasts Chloroplasts are a type of plastid. Plastids are double-membrane organelles which are found in the cells of plants and algae. Plastids are responsible for manufacturing and storing of food. The parts of a chloroplast include the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked in grana. Each chloroplast is surrounded by a double- membrane envelope. Chloroplasts are filled with a fluid known as the stroma. The stroma is the site of the light-independent stage of photosynthesis. A separate system of membranes is found in the stroma. This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids. Thylakoids are the site of the light-dependent stage of photosynthesis. The thylakoid membrane contains the pigments, enzymes and electron carriers required for the light-dependent reactions. Thylakoids stack up to form structures known as grana (singular – granum). Grana are connected by membranous channels called lamellae, which ensure the stacks of sacs are connected but distanced from each other. The membranes of the grana create a large surface area to increase the number of light-dependent reactions that can occur. This membrane system provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed. The stroma also contains small (70S) ribosomes, a loop of DNA and starch grains. Sugars formed during photosynthesis are stored as starch inside starch grains. While the mitochondrion has two membrane systems, the chloroplast Label the diagram has three, forming three compartments - Explain. ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ ______________________________ __ 8 4.1 Nutrition in Plants Dr M. Ellul The nature of light Light is a form of electromagnetic radiation – this is a type of energy that travels in waves. Other kinds of electromagnetic radiation include radio waves, microwaves, and X-rays. Together, all the types of electromagnetic radiation make up the electromagnetic spectrum. Every electromagnetic wave has a particular wavelength (distance from one crest to the next) and different types of radiation have different characteristic ranges of wavelength. The whole range of wavelengths of electromagnetic radiation make up the electromagnetic spectrum. A longer wavelength is associated with lower energy and a shorter wavelength is associated with higher energy. The types of radiation on the spectrum, from longest wavelength to shortest, are: radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma ray. Visible light is composed of different colours, each having a different wavelength and energy level. The colours, from longest wavelength to shortest, are: red, orange, yellow, green, blue, indigo, and violet. The visible spectrum is the only part of the electromagnetic spectrum that can be seen by the human eye. It includes electromagnetic radiation whose wavelength is between about 400 nm and 700 nm. Visible light from the sun appears white, but it’s actually made up of multiple wavelengths (colours) of light. Red light has the longest wavelength and the least energy, while violet light has the shortest wavelength and the most energy. Although light and other forms of electromagnetic radiation act as waves under many conditions, they can behave as particles under others. Each particle of electromagnetic radiation, called a photon, has certain amount of energy. Types of radiation with short wavelengths have high-energy photons, whereas types of radiation with long wavelengths have low-energy photons. 9 4.1 Nutrition in Plants Dr M. Ellul Photosynthetic pigments Photosynthetic pigments can be classified into: chlorophylls carotenoids Photosynthetic pigments are located on the chloroplast membranes and the chloroplasts are arranged within the cells in such a way so that the membranes are at right angles to the light sources for maximum absorption. Absorption and action spectra The photosynthetic pigments absorb only specific wavelengths of visible light, while reflecting others. Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum. Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum. The set of wavelengths absorbed by a pigment is its absorption spectrum. An action spectrum is a graph that shows the rate of photosynthesis at different wavelengths of light.. The rate of photosynthesis is at its maximum at the blue-violet and red regions of the light spectrum, since these are the wavelengths of light that chlorophylls and carotenoids can absorb. 10 4.1 Nutrition in Plants Dr M. Ellul The cumulative absorption spectra of all pigments and the action spectrum are strongly correlated: Both graphs have two main peaks – at the blue- violet region and the red region of the light spectrum. Both graphs have a trough in the green-yellow region of the light spectrum. If chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum, then why are most plants green? ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ ___________________________________________________ Chlorophyll A chlorophyll molecule is made up of a ‘head’ group and a tail. The head is a nitrogen-containing porphyrin ring with a magnesium atom at its centre. The head is hydrophylic and so it is found at the surface of the membrane near the stroma. The head is normally placed parallel to the membrane for absorption of light. The tail is a chain of hydrocarbons. The tail is hydrophobic and anchors the molecule to a membrane. 11 4.1 Nutrition in Plants Dr M. Ellul There are different forms of chlorophyll. Chlorophyll a is the main pigment of photosynthesis and is classified as the primary pigment. Most chloroplasts also contain accessory pigments, pigments that broaden the range of wavelengths at which light can be absorbed and that pass the energy obtained to chlorophyll a or protect against damage. Chlorophyll b, carotenoids and xanthophylls are examples of these pigments. The pigment molecules are arranged in light-harvesting clusters known as photosystems. In a photosystem, the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane. Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre. This is the site where this energy which comes from light, is used to drive a chemical reaction. Therefore, it is here that light energy is converted to chemical energy. 12 4.1 Nutrition in Plants Dr M. Ellul Practical aspect Chromatography of Chloroplast Pigments Chromatography is an experimental technique that is used to separate mixtures. The mixture is dissolved in a solvent called the mobile phase and the dissolved mixture then passes through a static material called the stationary phase. Different components within the mixture travel through the material at different rates depending on the size of the molecules and their solubility in the solvent. This causes the different components to separate. A retardation factor (Rf) can be calculated for each component of the mixture. 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑏𝑦 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡 𝑅𝑓 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑏𝑦 𝑠𝑜𝑙𝑣𝑒𝑛𝑡 There are two common techniques used for separating photosynthetic pigments: Paper chromatography – the mixture of pigments is passed through paper (cellulose). Thin-layer chromatography – the mixture of pigments is passed through a thin layer of adsorbent (eg. silica gel), through which the mixture travels faster and separates more distinctly. Chromatography can be used to separate and identify chloroplast pigments that have been extracted from a leaf as each pigment will have a unique R f value. The Rf value demonstrates how far a dissolved pigment travels through the stationary phase. A smaller Rf value indicates the pigment is less soluble and larger in size. Although specific Rf values depend on the solvent that is being used, in general: Carotenoids have the highest Rf values (usually close to 1) Chlorophyll B has a much lower Rf value Chlorophyll A has an Rf value somewhere between those of carotenoids and chlorophyll B 13 4.1 Nutrition in Plants Dr M. Ellul 4.1.3: Details of Photosynthesis Syllabus: Light-dependent reaction to include cyclic and non-cyclic photophosphorylation in the production of reduced NADP+ (NADPH + H+) and ATP; the evolution of oxygen. Role of the electron transport chain in ATP generation by chemiosmosis (names of carriers are not required). Light-independent reaction to include the fixation of carbon dioxide onto a 5C compound (ribulose bisphosphate – RuBP) to give 3-phosphoglycerate (3PG). The use of reduced NADP+ and ATP from the light-dependent reaction in the synthesis of carbohydrate (glyceraldehyde 3-phosphate – G3P) from 3PG. The regeneration of RuBP. 14 4.1 Nutrition in Plants Dr M. Ellul Learning outcomes Describe the light-dependent reaction. Compare the function of the two photosystems in green plants. Explain how the light reactions generate ATP and NADPH. Describe carbon fixation. Demonstrate how six CO2 molecules can be used to make one glucose. Differentiate between the light-dependent and light-independent reactions. Photosynthesis takes place in three stages: 1. capturing energy from sunlight; 2. using the energy to make ATP and to reduce the compound NADP+, an electron carrier, to NADPH; and 3. using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air. The first two stages require light and are commonly called the light-dependent reactions. The third stage, the formation of organic molecules from CO2, is called carbon fixation. This process takes place via a cyclic series of reactions. As long as ATP and NADPH are available, the carbon fixation reactions can occur either in the presence or in the absence of light, and so these reactions are also called the light- independent reactions. Light dependent reactions occur in the grana and the light independent reactions take place in the stroma of the chloroplasts. Light dependent reactions The light dependent reactions occur in the thylakoid membranes and two photosystems are involved. These are named photosystem I (PSI) and photosystem II (PSII). [the photosystems are named for the order in which they were discovered and not for the order in which they occur in the thylakoid membrane]. Each photosystem is composed of a protein complex called a reaction-centre complex surrounded by several light-harvesting complexes. Each light-harvesting complex consists of various pigment molecules like chlorophyll a, chlorophyll b, and carotenoids bound to proteins. The pigment molecules are closely packed together and serve as an “antenna” for gathering solar energy. When a pigment molecule absorbs a photon, solar energy is passed from one pigment molecule to another until it is passed into the reaction 15 4.1 Nutrition in Plants Dr M. Ellul centre complex. Two chlorophyll a molecules are found the reaction centre. Electrons in the reaction- centre chlorophyll a molecules become excited and escape and move to a nearby electron-acceptor molecule. The electrons can follow two pathways: cyclic electron pathway, called cyclic photophosphorylation - generates ATP non-cyclic pathway, called non-cyclic photophosphorylation - generates both ATP and NADPH In both pathways, ATP is produced through chemiosmotic ATP synthesis. ATP production during photosynthesis is called photophosphorylation because light is involved. Non-cyclic photophosphorylation During this pathway, electrons move from water through PS II to PS I and then on to NADP +. The diagram is called the Z-scheme. The higher up the diagram, the higher the energy level. 16 4.1 Nutrition in Plants Dr M. Ellul Photosystem II has a primary pigment (chlorophyll a) that absorbs light at a wavelength of 680nm and is therefore called P680. Light is absorbed by photosystem II and passed to the photosystem II primary pigment (P680). An electron in chlorophyll a molecule found in the reaction-centre is excited to a higher energy level and is emitted from the chlorophyll molecule in a process known as photoactivation. This excited electron is passed down a chain of electron carriers (electron transport chain). During this process to ATP is synthesised from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis. The ATP then passes to the light-independent reactions. Photosystem II contains a water-splitting enzyme called the oxygen-evolving complex which catalyses the breakdown (photolysis) of water by light: H2O → 2H+ + 2e- + ½O2 As the excited electrons leave the primary pigment of photosystem II and are passed on to photosystem I, they are replaced by electrons from the photolysis of water. At the same time as photoactivation of electrons in photosystem II, electrons in photosystem I also undergo photoactivation. Photosystem I has a primary pigment that absorbs light at a wavelength of 700nm and is therefore called P700. The excited electrons from photosystem I also pass along an electron transport chain. These electrons combine with hydrogen ions (produced by the photolysis of water) and the carrier molecule NADP+ to give reduced NADP: H+ + 2e- + NADP+ → NADPH The NADPH then passes to the light-independent reactions to be used in the synthesis of carbohydrate. When these hydrogen ions flow down their electrochemical gradient through an ATP synthase complex, ATP synthesis is produced by chemiosmosis. The electron from Photosystem II enters PSI replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. This energy is used in Carbon Fixation. 17 4.1 Nutrition in Plants Dr M. Ellul Putting it all together What are the inputs and outputs of non-cyclic photophosphorylation? Input Output Using the image below, label the following: thylakoid lumen, stroma, photosystem I (PSI), photosystem II (PSII), electron transport chain, ATP synthase, light entering (at both photosystems), the splitting of H2O. Then, draw the passage of the electrons from PSII through the ETC to PSI and the H+ gradient. Cyclic photophosphorylation This system uses only photosystem I and produces ATP alone. Light is absorbed by photosystem I and passed to the photosystem I primary pigment (P700). As electrons pass through the electron transport chain they provide energy to transport protons (H +) from the stroma to the thylakoid lumen via a proton pump. A build-up of protons in the thylakoid lumen can then be used to drive the synthesis of ATP from ADP and an inorganic phosphate group (Pi) by the process of chemiosmosis. Excited electrons are then passed back to PSI again, completing the cycle. 18 4.1 Nutrition in Plants Dr M. Ellul Light independent reactions The series of reactions that make up the light-independent stage forms a cycle, which is known as the Calvin cycle. The reactions occur in the stroma and are controlled by enzymes. This stage produces complex organic molecules, including (but not limited to) carbohydrates, such as: Starch (for storage) Sucrose (for translocation around the plant) Cellulose (for making cell walls) The light-independent stage does not require energy from light and can therefore take place in light or darkness. However, as it requires inputs of ATP and NADPH from the light-dependent stage, it cannot continue indefinitely in darkness, as these inputs will run out. The Calvin cycle reactions can be divided into three main stages: carbon dioxide fixation reduction regeneration of RuBP Carbon dioxide fixation A CO2 molecule combines with ribulose-1,5-bisphosphate (RuBP) (5C). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglycerate (3PG). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco. Reduction In the second stage, ATP and NADPH are used to convert the 3PG molecules into molecules of a three- carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make G3P. Regeneration of RuBP Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP. Regeneration requires ATP and involves a complex network of reactions. For one G3P to exit the cycle and go towards glucose synthesis, three CO2 molecules must enter the cycle, providing three new atoms of fixed carbon. When three CO2 molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP. 19 4.1 Nutrition in Plants Dr M. Ellul Further reactions The molecules of glyceraldehyde-3-phosphate that leave the cycle are removed from the chloroplast by a specific transport protein in the inner chloroplast membrane. This protein exchanges it for phosphate ions that are needed to make ATP. The G3P is used to synthesise other carbohydrates such as glucose, sucrose, starch and cellulose. It is also used in the synthesis of other compounds such as lipids and amino acids. What are the inputs and outputs of the Calvin cycle? Input Output 20 4.1 Nutrition in Plants Dr M. Ellul Label the diagram of the Calvin cycle. It has been started with a total of 3 CO2 molecules entering the cycle (remember, one CO2 enters at a time) Reflect on what you learnt. Jot down the main points of the light dependent and light independent reactions. 21 4.1 Nutrition in Plants Dr M. Ellul The light-dependent stage Location: the thylakoid membranes in the chloroplast Substances in: H2O, NADP+, ADP, Pi Useful products: NADPH, ATP Waste products: O2 Energy transferred from: light Energy transferred to: ATP, NADPH 1. Light energy excites electrons in chlorophyll molecules. 2. The excited electrons are emitted from the chlorophyll molecules.  The energy carried by these electrons is used: to reduce NADP + to NADPH;  to move hydrogen ions across the thylakoid membrane to create a potential energy store;  to split water into hydrogen ions, electrons and oxygen. 3. The electrons from the water are used to replace the electrons emitted by the chlorophyll molecules. 4. The potential energy store is used to make ATP from ADP and P i. The light-independent stage Location: the stroma of the chloroplast Substances in: CO2, ATP, NADPH Useful products: carbohydrate (3-carbon sugar) Waste products: NADP+, ADP, Pi Energy transferred from: ATP, NADPH Energy transferred to: carbohydrate 1. The enzyme ribulose 1,3-bisphosphate carboxylase (Rubisco) fixes carbon dioxide by converting one carbon dioxide molecule and one 5C molecule (RuBP) into two 3C molecules. 2. The 3C molecules (3PG) are then phosphorylated using ATP. This increases the energy of the molecules. 3. The 3C molecules (BPG) are then reduced using NADPH to give 3C sugar molecules. 4. One out of every six 3C sugar molecules (G3P) produced is used to make other carbohydrates including glucose. 5. Five out of every six 3C sugar molecules (G3P) produced are recycled to make more 5C molecules (RuBP) to fix carbon dioxide. 22 4.1 Nutrition in Plants Dr M. Ellul 4.1.4: Factors affecting Photosynthesis Syllabus: The effect of light intensity and wavelength, carbon dioxide concentration and temperature on the rate of photosynthesis. The concept of limiting factors; compensation point 23 4.1 Nutrition in Plants Dr M. Ellul Learning outcomes Explain the concept of limiting factors. Describe and explain the effect of light intensity on the rate of photosynthesis. Describe and explain the effect of different wavelengths of light on the rate of photosynthesis. Describe and explain the effect of carbon dioxide concentration on the rate of photosynthesis. Describe and explain the effect of temperature on the rate of photosynthesis. Identify compensation points. The concept of limiting factors The principle of limiting factors can be stated as: When a chemical process is affected by more than one factor its rate is limited by that factor which is nearest its minimum value: it is that factor which directly affects the process if its quantity is changed. Interpreting graphs of limiting factors In the section of the graph where the rate is increasing (the line is going up), the limiting factor is whatever the label on the x-axis (the bottom axis) of the graph is. In the section of the graph where the rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on the x-axis – choose from temperature, light intensity or carbon dioxide concentration. 24 4.1 Nutrition in Plants Dr M. Ellul Light The photosynthetic rate increases in proportion to light intensity. At some point, if light intensity continues increasing, the relationship above will no longer apply and the rate of photosynthesis will reach a plateau. At this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting the rate of photosynthesis. The factors which could be limiting the rate when the line on the graph is horizontal include temperature being too low or too high, or not enough carbon dioxide. Except for shaded plants, light intensity is not normally a limiting factor for photosynthesis. Very high light intensities may bleach chlorophyll and so retard photosynthesis, but plants that naturally grow in these conditions have protection such as thick cuticles and hairy leaves. Wavelength of light In the light-dependent reaction, light of a specific wavelength was absorbed by the photosynthetic pigments in the photosystems. Chlorophyll a and chlorophyll b absorb best in the red and blue wavelengths respectively, and carotene absorbs towards the blue wavelengths as well. Plants appear green because the photosynthetic pigments do not absorb green light (so it is reflected). Therefore, the light not only needs to be of the right intensity, but also the right wavelength. Plot a graph showing how the rate of photosynthesis varies with wavelength. Carbon dioxide concentration Carbon dioxide is needed during the light independent reactions where it is fixed into organic compounds. The normal atmospheric concentration of CO2 is 0.03-0.04% but increasing the concentration increases the photosynthetic rate. The short-term optimum is about 0.5% but this can cause damages over long 25  4.1 Nutrition in Plants Dr M. Ellul periods, when 0.1% is better. The group of C4 plants are more efficient at removing carbon dioxide from the atmosphere. Temperature Many reactions within photosynthesis are controlled by enzymes and therefore temperature sensitive. For temperate plants, the optimum temperature for growth is 25°C. Discuss why temperature does not have a significant effect on the light- dependent reactions. However, the Calvin cycle is affected by temperature. ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________ 26 4.1 Nutrition in Plants Dr M. Ellul Combining the limiting factors Enzyme controlled reactions in the light independent reactions are sensitive to temperature. Therefore, an increase in temperature is going to increase the rate of photosynthesis. Carbon dioxide concentration can also be a limiting factor in the light independent reactions. The following diagram summarizes the effect of different limiting factors on the Calvin cycle Compensation points Photosynthesis uses carbon dioxide and evolves oxygen whereas respiration uses oxygen and gives out carbon dioxide. At night, a plant respires only and gives out carbon dioxide. Photosynthesis only commences when light becomes available at dawn. At one point photosynthesis and respiration exactly balance each other and there is no net exchange of oxygen and carbon dioxide. This is the compensation point. (More precisely, this is the light compensation point, i.e., the light intensity at which net gas exchange is zero). Beyond this compensation point the plant may increasingly photosynthesise as conditions of temperature and light improve. The plant at this stage still respires producing carbon dioxide in its cells and all of this carbon dioxide is utilised. However, much more carbon dioxide is needed which diffuses in from the air. In the evening when dusk arrives a point is reached when the rate of photosynthesis falls due to the decrease in 27 4.1 Nutrition in Plants Dr M. Ellul light and the onset of darkness. The amount of carbon dioxide produced at one point is totally utilised in photosynthesis. This is another compensation point. The carbon dioxide compensation point is the carbon dioxide concentration at which net gas exchange is zero for a given light intensity. Practical Aspects (notes taken from www.savemyexams.co.uk) Investigating the Rate of Photosynthesis: Redox Indicators The light-dependent reactions of photosynthesis take place in the thylakoid membrane and involve the release of high-energy electrons from chlorophyll a molecules These electrons are picked up by electron acceptors and then passed down the electron transport chain However, if a redox indicator (such as DCPIP or methylene blue) is present, the indicator takes up the electrons instead This causes the indicator to change colour o DCPIP: oxidised (blue) → accepts electrons → reduced (colourless) o Methylene blue: oxidised (blue) → accepts electrons → reduced (colourless) o The colour of the reduced solution may appear green because the chlorophyll have a green colour The rate at which the redox indicator changes colour from its oxidised state to its reduced state can be used as a measure of the rate of photosynthesis o When light is at a higher intensity, or at more preferable light wavelengths, the rate of photoactivation of electrons is faster, therefore the rate of reduction of the indicator is faster 28 4.1 Nutrition in Plants Dr M. Ellul Light activates electrons from chlorophyll molecules during the light-dependent reaction. Redox indicators accept the excited electrons from the photosystem, becoming reduced and therefore changing colour Method Step 1: Leaves are crushed in a liquid known as an isolation medium o This produces a concentrated leaf extract that contains a suspension of intact and functional chloroplasts o The medium must have the same water potential as the leaf cells (so the chloroplasts don’t shrivel or burst) and contain a buffer (to keep the pH constant). It should also be ice-cold (to avoid damaging the chloroplasts and to maintain membrane structure) Step 2: Small tubes are set up with different intensities, or different colours (wavelengths) of light shining of them o If different intensities of light are used, they must all be of the same wavelength (same colour of light) o If different wavelengths of light are used, they must all be of the same light intensity Step 3: DCPIP of methylene blue indicator is added to each tube, as well as a small volume of the leaf extract Step 4: The time taken for the redox indicator to go colourless is recorded o This is a measure of the rate of photosynthesis 29 4.1 Nutrition in Plants Dr M. Ellul Investigating the Rate of Photosynthesis: Aquatic Plants Investigations to determine the effects of light intensity, carbon dioxide concentration and temperature on the rate of photosynthesis can be carried out using aquatic plants, such as Elodea or Cabomba (types of pondweed) The effect of these limiting factors on the rate of photosynthesis can be investigated in the following ways: o Light intensity – change the distance (d) of a light source from the plant (light intensity is proportional to 1/d2) o Carbon dioxide concentration – add different quantities of sodium hydrogencarbonate (NaHCO3) to the water surrounding the plant, this dissolves to produce CO2 o Temperature (of the solution surrounding the plant) – place the boiling tube containing the submerged plant in water baths of different temperatures Whilst changing one of these factors during the investigation (as described below), ensure the other two remain constant o For example, when investigating the effect of light intensity on the rate of photosynthesis, a glass tank should be placed in between the lamp and the boiling tube containing the pondweed to absorb heat from the lamp – this prevents the solution surrounding the plant from changing temperature Method Step 1: Ensure the water is well aerated before use by bubbling air through it o This will ensure oxygen gas given off by the plant during the investigation form bubbles and do not dissolve in the water Step 2: Ensure the plant has been well illuminated before use o This will ensure that the plant contains all the enzymes required for photosynthesis and that any changes of rate are due to the independent variable Step 3: Set up the apparatus in a darkened room o Ensure the pondweed is submerged in sodium hydrogencarbonate solution (1%) – this ensures the pondweed has a controlled supply of carbon dioxide (a reactant in photosynthesis) Step 4: Cut the stem of the pondweed cleanly just before placing into the boiling tube Step 5: Measure the volume of gas collected in the gas-syringe in a set period of time (eg. 5 minutes) Step 6: Change the independent variable (ie. change the light intensity, carbon dioxide concentration or temperature depending on which limiting factor you are investigating) and repeat step 5 Step 7: Record the results in a table and plot a graph of volume of oxygen produced per minute against the distance from the lamp (if investigating light intensity), carbon dioxide concentration, or temperature 30 4.1 Nutrition in Plants Dr M. Ellul The set up of the experiment to measure the rate of photosynthesis of an aquatic plant (pond weed) by measuring the rate of oxygen gas produced. All three limiting factors can be assessed this way Exam Tip Learn the 3 limiting factors and how each one can be altered in an laboratory environment: Light intensity – the distance of the light source from the plant (intensity ∝ 1/d2) Temperature - changing the temperature of the water bath the test tube sits in Carbon dioxide - the amount of NaHCO3 dissolved in the water the pondweed is in Also remember that the variables not being tested (the control variables) must be kept constant. 31 4.1 Nutrition in Plants Dr M. Ellul 4.1.5: C3 and C4 plants Syllabus: C3 and C4 Plants: Photorespiration (details of full biochemical pathways not required). C3 and C4 pathways as examples of ecological adaptation. CAM plants. Comparison of the internal leaf structure of C4 as compared to that of the C3 leaf. The two types of chloroplasts in a C4 leaf. 32 4.1 Nutrition in Plants Dr M. Ellul Learning outcomes Distinguish between C3 and C4 plants. Describe photorespiration. Describe how C4 plants avoid photorespiration. Describe how CAM plants avoid photorespiration. Modes of photosynthesis Most plants put CO2 directly into the Calvin cycle. Therefore, the first stable organic compound formed is the glyceraldehyde 3-phosphate. Since that molecule contains three carbon atoms, these plants are called C3 plants. For all plants, hot summer weather increases the amount of water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomata closed during hot, dry weather. This also restricts the amount of carbon dioxide that can enter the plant to drive photosynthesis. Therefore, when stomata are closed, C3 plants produce less sugar. This drives up the concentration of oxygen in the cells, and these conditions initiate an alternative process called photorespiration. Photorespiration describes the process whereby rubisco binds O2 in the place of CO2. The resulting product splits, and the 2-carbon compound, glycolate, that forms leaves the chloroplast. Glycolate diffuses into a peroxisome and it is converted to glycine. Mitochondria convert glycine to serine, releasing CO2. However, this alternative route does not produce any ATP or sugar. Photorespiration is generally considered a wasteful process. Why is photorespiration wasteful? 1. Energy expenditure: Photorespiration consumes ATP. This energy expenditure is wasteful because it reduces the overall energy available for the plant to carry out essential processes. 2. Carbon loss: During photorespiration, carbon is lost in the form of CO2, which is released instead of being fixed into organic molecules through photosynthesis. 3. Reduced photosynthetic efficiency: Photorespiration competes with photosynthesis for the same substrates, such as ribulose-1,5-bisphosphate (RuBP). When photorespiration is active, it reduces the availability of RuBP for the Calvin cycle, which is the primary pathway for CO 2 fixation during 33 4.1 Nutrition in Plants Dr M. Ellul photosynthesis. This reduces the plant's overall photosynthetic efficiency and can lead to lower rates of carbon dioxide assimilation. 4. Reduced plant growth: The energy and resources diverted to photorespiration could have been used for productive processes like biomass production and yield. Many plants have evolved various mechanisms to minimize photorespiration and increase their photosynthetic efficiency, such as C4 and CAM photosynthesis. These adaptations allow certain plant species to thrive in conditions where photorespiration would otherwise be highly wasteful. C4 plants The C4 plants include maize, sugarcane and tropical grasses. C4 plants are mainly found in tropical and warm-temperate regions. C4 plants show a characteristic anatomy in their leaves, often called Kranz anatomy. The vascular bundles are surrounded by two rings of cells, the inner layer is of bundle sheath cells while the outer layer is of mesophyll cells. The inner bundle sheath cells have many starch-rich chloroplast which lack grana (agranal chloroplast) and is thus different from those in mesophyll cells. The outer mesophyll cells have chloroplasts with grana (granal chloroplast). They lack starch grains. In Kranz anatomy, the bundle sheath cells are large and no mesophyll cell is more than two or three cells away from the nearest bundle sheath cell. The bundle sheath cells and mesophyll cells are connected with each other by plasmodesmata which allow the movement of metabolites between the cells. For the operation of the C4 pathway, contribution of both the cells is required. The chloroplasts found in the mesophyll and bundle sheath cells are dimorphic, that is they have different forms. Mesophyll chloroplasts Bundle sheath chloroplasts Large grana - light-dependent reactions favoured, No grana - light-dependent reactions occur so plenty of ATP, reduced NADP and O2 are at very low rate, so little ATP, reduced generated. NADP or O2 are generated. Virtually no RuBP carboxylase (rubisco)- so no CO2 High concentration of RuBP carboxylase fixation (rubisco)– efficient CO2 fixation Little starch Abundant starch 34 4.1 Nutrition in Plants Dr M. Ellul The pathway that C4 plants take is known as Hatch-Slack pathway. In C4 plants, the light-dependent reactions and the Calvin cycle are physically separated, with the light-dependent reactions occurring in the mesophyll cells and the Calvin cycle occurring in the bundle-sheath cells. First, atmospheric CO2 is fixed in the mesophyll cells to form a simple, 4-carbon organic acid known as oxaloacetate. This step is carried out by an enzyme, PEP carboxylase, that does not bind with O2. Oxaloacetate is then converted to malate (4C) that is transported into the bundle-sheath cells (via the plasmodesmata). Inside the bundle sheath, malate breaks down and releases a molecule of CO2 The CO2 is fixed by rubisco and made into sugars via the Calvin cycle. Pyruvate is also produced in this step and moves back into the mesophyll cell, where it is converted into PEP, using ATP in the process. So, this process uses energy. However, because the mesophyll cells constantly pump CO2 into neighbouring bundle-sheath cells in the form of malate, there’s always a high concentration of CO2 right around rubisco and so sugars can be produced. C4 plants are common in habitats that are hot, but are less abundant in areas that are cooler. In hot conditions, the benefits of reduced photorespiration likely exceed the ATP cost of moving CO2 from the mesophyll cell to the bundle-sheath cell. CAM plants CAM stands for crassulacean acid metabolism. CAM plants include cacti and pineapple. These live in extremely hot, dry areas like deserts and therefore can only safely open their stomata at night when the weather is cool. Thus, there is no chance for them to get the CO2 needed for the light independent reaction during the daytime. At night when they can open their stomata and take in CO2, these plants incorporate the CO2 into various organic compounds to store it. In the daytime, when the light dependent reaction is occurring and ATP is available (but the stomata must remain closed), they take the CO2 from these organic compounds and put it into the Calvin cycle. CAM plants separate light dependent reactions and the Calvin cycle in time (temporal separation). At night, CAM plants open their stomata, and CO2 diffuses into the leaves. This CO2 is fixed into oxaloacetate by PEP carboxylase (the same step used C4). It is then converted to malate. The malate is stored inside vacuoles until the next day. In the daylight, the CAM plants do not open their stomata, but they can carry 35  4.1 Nutrition in Plants Dr M. Ellul out photosynthesis because malate is transported out of the vacuole and broken down to release CO2, which enters the Calvin cycle. This controlled release maintains a high concentration of CO2 around rubisco. Characteristic C3 Plants C4 Plants CAM Plants Carbon Fixation Calvin Cycle (in mesophyll Calvin Cycle (in bundle Crassulacean Acid cells) sheath cells) Metabolism (in mesophyll cells at night) Initial Fixation Ribulose-1,5-bisphosphate Phosphoenolpyruvate PEP carboxylase (at Enzyme carboxylase/oxygenase carboxylase (PEP night) and RuBisCO (RuBisCO) carboxylase) (during the day) Anatomical None Kranz anatomy - None (but stomata can Adaptations Separation into close during the day) mesophyll and bundle sheath cells Location of CO2 Mesophyll cells Mesophyll cells Mesophyll cells (at Fixation night) CO2 Concentration at High (ambient levels) High (concentrated in High (concentrated in RuBisCO bundle sheath cells) vacuoles at night) Photorespiration Significant, especially Minimal due to high CO2 Minimal due to night- under high temperatures concentration in bundle time CO2 uptake and low CO2 conditions sheath cells Water Use Efficiency Moderate High High Environmental Well-suited for temperate Well-suited for hot and Well-suited for arid Conditions climates; sensitive to high dry conditions; less conditions with limited temperatures and low CO2 sensitive to temperature water availability and CO2 changes Energy Efficiency Lower due to Higher due to reduced Higher due to reduced photorespiration photorespiration photorespiration Examples Wheat, rice, soybeans Corn (maize), sugarcane, Succulents (e.g., cacti), some grasses pineapple 36 4.1 Nutrition in Plants Dr M. Ellul Self-Assessment: Review questions 1. What are autotrophs? 2. What is the primary purpose of photosynthesis? 3. True or false: photosynthesis has two stages. 4. True or false: the light independent reactions are also known as the dark reactions because they can only happen in the absence of light. 5. In which organelle does photosynthesis take place? More specifically, where does each stage of photosynthesis take place? 6. How is photosynthesis a redox process? 7. Do you support this claim: the light dependent reactions do not depend on the Calvin cycle. Why or why not? 8. Do you support this claim: the Calvin cycle does not depend on the light dependent reactions. Why or why not? 9. Distinguish between cyclic and non-cyclic photophosphorylation. 10. Justify this claim: rubisco is the enzyme responsible for producing the world’s food. 11. In the Calvin cycle, how many molecules of CO2 are needed to produce one 12-carbon sugar? 12. What are the three phases of the Calvin cycle? 13. What is the purpose of each phase of the Calvin cycle? 14. Why does photorespiration occur? Reflect on what you learnt: Draw a concept map summarising photosynthesis. 37