Topic 5A: Importance of Energy PDF

# Topic 5A: Importance of Energy ## Importance of energy Energy is essential for life processes. Organisms can be classified according to how they obtain their energy. 1. **Autotrophic:** which obtain energy by synthesising organic compounds from CO₂. (Most of them do this by photosyn...

# Topic 5A: Importance of Energy ## Importance of energy Energy is essential for life processes. Organisms can be classified according to how they obtain their energy. 1. **Autotrophic:** which obtain energy by synthesising organic compounds from CO₂. (Most of them do this by photosynthesis, eg. green plants, algae etc which are photosynthetic organisms. However some of them are chemosynthetic, which obtain energy from metabolic reactions, not sunlight) 2. **Heterotrophic:** organisms which obtain their energy by eating plants or other animals that eat plants. To indirectly use the products of photosynthesis. **Sunlight is therefore the ultimate source of energy for living processes.** ## ATP (Energy Source and Store) **Adenine triphosphate** *Adenine* *P* *Ribose* ATP <=> ADP + Pi + energy **Why is light energy converted to chemical energy in ATP (by photosynthesis)/Importance of ATP** * Light energy cannot be used directly * ATP is universal energy source, so can be used by all organisms * ATP is needed in conversion of GP to GALP **ATP is synthesised in 2 ways:** 1. Using energy released from catabolic reactions 2. **By chemiosmosis (main way/way used in photosynthesis)** * Electrons are excited out of chlorophyll and accepted by electron carrier proteins * Electrons move from carrier to carrier in an ETC of decreasing energy level (in a series of oxidation and reduction reactions) * Energy released by the ETC electron transport chain, is used to pump H⁺ ions (into the thylakoid space) * The high conc. of H⁺ accumulated in thylakoid space, allows H⁺ to diffuse down conc. gradient (through ATP synthase channel proteins) * This movement of H⁺ releases energy to combine ADP and Pi to form ATP. ## Diagram of Chemiosmosis **Stroma** * Light PSII * Hydrogen pump protein as well as an electron carrier * ADP * H⁺ * ↑ ATP * H⁺ * Light PSI * ATP SYNTHASE * P680 (P100) * H⁺ * H⁺ * H⁺ * H⁺ * ↑ * H⁺ * H⁺ * H⁺ * H⁺ **Thylakoid space** * H⁺ ## Basics of Photosynthesis AND Importance of Chlorophyll Photosynthesis is a process used to convert carbon dioxide and water into glucose, using energy trapped by chlorophyll from sunlight, by green plants mainly. **Photosynthesis is an endothermic process, because energy is trapped and taken in** carbon dioxide + water light energy → glucose + oxygen 6CO₂ + 6H₂O light energy → C₆H₁₂O₆ + 6O₂ chlorophyll chlorophyll ΔH ~ +2880 kJ ΔH ~ +2880 kJ ## Chloroplast Structure and Function **Thylakoid membranes** - a system of interconnected flattened fluid-filled sacs. Proteins, including the photosynthetic pigment chlorophyll and electron carriers, are embedded in the membranes and are involved in the light-dependent reactions. **DNA loop** - chloroplasts contain genes for some of their proteins. **Stroma** - the fluid surrounding the thylakoid membranes. Contains all the enzymes needed to carry out the light-independent reactions of photosynthesis. * **Thylakoid space** - fluid within the thylakoid membrane sacs contains enzymes for photolysis. * **Starch grain** stores the product of photosynthesis. * **Granum** a stack of thylakoids joined to one another. Grana (plural) resemble stacks of coins. **Common labelling MCQ** * Granum * Starch grain **Outer membrane** - which is freely permeable to molecules such as CO₂ and H₂O. **Inner membrane** - which contains many transporter molecules. These are membrane proteins which regulate the passage of substances in and out of the chloroplast. These substances include sugars and proteins synthesised in the cytoplasm of the cell but used within the chloroplast. The inner and outer chloroplast membrane form an envelope. ## Structure of Chloroplast in Relation to Photosynthesis 1. It's a double membrane organelle. Space between outer and inner membrane is known as Chloroplast envelope 2. System of membrane disc known as thylakoid. Stack/layer of thylakoid/disc is grana. 3. Existing as stack/layer increases S.A. for more light absorption 4. Thylakoid membranes are site of light dependent reactions. As they contain photosytem, electron carrier proteins. 5. Thylakoid space provides space for accumulation of H⁺ ions. 6. Grana are joined together by lamellae - extensions of thylakoid membrane, which maintain proper distance between grana to maximise light absorption 7. Stroma is site of light independent reaction. As it contains enzymes eg. RUBISCO involved in Calvin cycle. ## Chlorophyll Chlorophyll is a light-capturing photosynthetic pigment. Its not a single pigment but a mixture of closely related pigments. These include: * Chlorophyll a (blue-green) * Chorophyll b (yellow-green) * Carotenoids (orange carotene and yellow xanthophyll) * Phaeophytin (grey pigment) it is the breakdown product of others. Carotene and xanthophyll(carotenoids) are known as accessory pigments. Because they absorb and pass on energy from light wavelengths that are not efficiently absorbed by primary pigment chlorophyll a. ## Photosystems Photosystems are a collection of chlorophyll a (primary pigment), accessory pigments and associated proteins all within a membrane protein. Their role is to absorb energy from photons of light and excite electron out of photosystem, in ETC. The reaction centre of all photosystems contain chlorophyll a. There are two types of photosystems involved in photosynthesis: * **Photosystem I (PSI/P700)** has a reaction centre with a light absorption peak of 700 nm and therefore known as P700. * **Photosystem II (PSII/P680)** has a reaction centre with a light absorption peak of 680 nm. ## Absorption and Action Spectra * **Absorption spectrum** shows absorption of different wavelengths light by different photosynthetic pigments * **Action spectrum** shows the rate of photosynthesis at each different wavelength. ## Evidence that pigment(s) are used in photosynthesis The cumulative absorption spectra by different pigments produces a similar graph to action spectra. So there is a correlation btwn cumulative/total absorption and action spectra, as they both have peaks and troughs at similar wavelengths. ## Advantage of having different pigments * Each pigment absorbs a diff. wavelength of light AND absorbs diff. amounts of light at each wavelength. * So a greater range of wavelengths is absorbed and available * So, faster rate of photosynthesis can be maintained * Some pigments have protective role in light harvesting, such as accessory pigments (eg. carotenoids) ## Chromatography It is used for separation and identification of chemicals, such as photosynthetic pigments. 1. Grind leaves in propanone and collect filtrate 2. filtrate loaded onto chromatography paper and place paper in suitable solvent. Eg. ethanol, propanone. 3. Obtain solvent front and determine Rf value. Rf values are specific to pigments at a specific solvent. It is important to use the same solvent; * Different solvents permeate cell membrane of chloroplast differently, due to different permeability. * Chlorophylls have different solubility in different solvents, due to their different structures. ## Biochemistry of Photosynthesis Photosynthesis takes place in two stages 1. **Light dependent stage** 2. **Light independent stage** (NOT dark stage, because LIS takes place both in presence and absence of light as it's independent) ## Light Dependent Reactions It occurs on the thylakoid membrane. It has two main functions 1. Break down water molecules in photolysis to provide H⁺ ions for reduction processes. 2. Produce ATP via electron transport chain, needed for light independent reaction. **Process** 1. Photon of light excites electrons (in chlorophyll) to higher energy levels 2. If excited enough to high enough level, electrons leave chlorophyll and are accepted by electron carrier 3. Electrons are then moved in an electron transport chain (from carrier to carrier) and this results in synthesis of ATP, by: * Cyclic photophosphorylation * Non-cyclic photophosphorylation (both phosphorylations happen at the same time. Both produces ATP by chemiosmosis but only non cyclic produces reduced NADP) ## Cyclic Photophosphorylation **Involves only PSI** 1. Light excites electrons in PSI to higher energy levels 2. Electrons leave PSI and is accepted by electron carriers, to be moved in ETC, to synthesis ATP. 3. Electron returns to PSI and can be excited the same way. ## Non-Cyclic Photophosphorylation **Involves both PSI and PSII** 1. Photons hit PSII and excite electrons. 2. Excited electrons (lost from PSII) are accepted by electron carriers to move in an ETC to PSI 3. Meanwhile, photolysis of water releases electrons, that replace the electrons lost by PSII. 4. Electrons from PSI are also excited and accepted by electron carrier NADP, which also accepts H⁺ from photolysis of water to become reduced NADP. **Photolysis Equation** H₂O ----> 2H⁺ + 2e^- + 1/2O₂ accepted by NADP to become reduced NADP lost from leaf or used in respiration **Only the products of Non cyclic photophosphorylation are used in light independent reactions** ## Light Independent Reactions It takes place in the stroma. It uses ATP and reduced NADP from non-cyclic photophosphorylation of LDS, to convert GP into GALP. It takes place in a cyclical pathway called Calvin cycle, where the first step is **carbon fixation** catalysed by enzyme **RUBISCO.** 1. Carbon dioxide combines with a 5-carbon compound called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase (RuBISCO), the most abundant enzyme in the world. * **1. Carbon fixation** 5 Ten out of every 12 GALPs are involved in the recreation of RuBP. The ten GALP molecules rearrange to form six 5-carbon compounds; then phosphorylation using ATP forms RuBP. 6ADP 6RuBP (5C) 6CO₂ * **2. 6C intermediate splits into two (3C) GP** 12GP (3C) * **12reduced NADP 12NADP** * **12ATP 12ADP + 12P;** * **3. GP converted to GALP** (10GALP) 12GALP (3C) Π 6ATP (2GALP) glucose (6C) (hexose) This 3-carbon compound is reduced to form a 3-carbon sugar phosphate called glyceraldehyde 3-phosphate (GALP). The hydrogen for the reduction comes from the reduced NADP from the light-dependent reactions. ATP from the light-dependent reactions provides the energy required for the reaction. * **4. Two out of every 12 GALPs formed are involved in the creation of a 6-carbon sugar (hexose) which can be converted to other organic compounds, for example amino acids or lipids.** Simple sugars(glucose) has formula C₆H₁₂O₆ * C and O from carbon dioxide * H from water ## Using the Product of Photosynthesis * GALP is the primary end product of photosynthesis * GALP can be used to regenerate RuBP * Carbohydrates 2 mol GALP joined to form 1 mol glucose. The glucose can be used to produce di and polysaccharides in condensation rxns, using enzymes and energy from ATP. (GALP can be directly respired to produce ATPs required for these conversions.) * Lipids glucose produced is used to make glycerol and GP is used to make fatty acids, to synthesis lipids. * Proteins and Nucleic acids the glucose is converted to correct no. of C. Then combined with nitrates or phosphates (from soil) ## Limiting factors of Photosynthesis These are factors that limit the rate of photosynthesis when they are in low levels even if other factors are not limiting. 1. **Light** - both light intensity and wavelength affects rate of photosynthesis. Low light intensity results in less excitation of electrons so less ATP produced. Less photolysis less H⁺ ions, hence less reduced NADP produced. 2. **Carbon dioxide** - less CO₂ means less carbon fixation, so less GP, GALP produced lowering rate of photosynthesis. 3. **Temperature** - increasing temp. increase KE of RUBISCO and substrates, so more collision and ES complexes form. Enzyme activity increases to increase rate of photosynthesis. **25* is optimum temp.** **light dependent reactions are independent of temp. changes** ## Graphs of Limiting Factors ### Changes in Carbon dioxide concentration ### Changes in Light intensity * Reducing light intensity will reduce the rate of the light-dependent stage of photosynthesis. This will reduce the quantity of ATP and reduced NADP produced. ATP and reduced NADP are needed to convert GP to TP. The concentration of GP will therefore increase and the concentration of TP will decrease. As there will be less TP to regenerate RuBP, the concentration of RuBP will also decrease. The reverse will happen when the light intensity is increased.