Metabolism - Photosynthesis Lecture Note

PHOTOSYNTHESIS PHOTOSYNTHESIS Photosynthesis is the process by which cells synthesise organic compounds from inorganic molecules in the presence of sunlight. This process requires a photosynthe.c pigment (chlorophyll) and can only occur in certain organisms (plants, algae, cyanobacteria). In plants, photosynthesis occurs within the chloroplast. 6 CO2 Carbon dioxide + 12 H2O Water Light Chlorophyll C6H12O6 Glucose + 6 O2 Oxygen + 6 H2O Water PIGMENTS In photosynthe.c organisms, the absorp.on of light is mediated by specific pigment molecules. Each of these pigment molecules contains electrons at discrete and specific energy levels. These electrons absorb light at specific wavelengths and become energised. The energy from these excited electrons is harnessed by the cell to make chemical energy (ATP). In plants, the main photosynthe.c pigment is chlorophyll, but other pigments are also used. These pigments are clustered into photosystems to op.mise light absorp.on. CHROMATOGRAPHY Chromatography is an experimental technique that allows for the separa.on of pigments. The pigments are dissolved in a fluid (mobile phase) and passed through a sta.c material (sta.onary phase). The different pigments will travel at different speeds, causing them to separate. A retarda.on factor is calculated and the value is used to iden.fy the pigment. • Rf value = distance the pigment travels (mm) ÷ distance the solvent travels (mm) The Rf value represents a ra.o and hence can never exceed a value of 1. Chromatography experiments will oWen use paper as the sta.c material and acetone as an organic solvent. LIGHT ABSORPTION The electromagne.c spectrum comprises the full range of all types of radia.on energy. All photosynthe.c organisms will absorb wavelengths within the visible region of this spectrum (white light: 400 – 700 nm). Chlorophyll absorbs red and blue light most effec.vely, while green light is reflected. This is because green light is too energe.c to be used efficiently by the plant, while blue light is best able to penetrate the depths of water (reflec.ng the likely aqua.c origins of living organisms). The amount of light absorbed by pigments at any given wavelength is represented by the absorp.on spectra, while the rate of photosynthesis at each wavelength is shown by the ac.on spectra. For both spectra, the overall trend is generally comparable. ACTION SPECTRUM Rate of Photosynthesis Amount of light absorbed ABSORPTION SPECTRUM carotenoid chlorophyll a chlorophyll b 400 500 600 700 400 Wavelength of light (nm) 500 600 700 Wavelength of light (nm) STAGES OF PHOTOSYNTHESIS The process of photosynthesis involves two stages: the light dependent reac.ons and the light independent reac.ons (Calvin cycle). The light dependent reac.ons func.on to convert light energy into chemical energy (ATP) and the light independent reac.ons then use this chemical energy to synthesise organic compounds. In plants, these reac.ons occur within a specialised organelle called the chloroplast (found in leaf .ssues). 1. LIGHT DEPENDENT REACTIONS The light dependent reac.ons require light and occur within the thylakoids (or grana) of the chloroplast. Light is absorbed by photosynthe.c pigments (such as chlorophyll), resul.ng in the excita.on of electrons. The energised electrons then transfer this energy to ATP. Simultaneously, light energy breaks down water molecules (photolysis) into oxygen and hydrogen. The oxygen gas is released as a waste product, while the hydrogen atoms will be used (along with ATP) as an input for the light independent reac.ons (Calvin cycle). The oxygen gas is released from .ny pores on the underside of the leaf called stomata (singular = stoma). Light energy Water Chlorophyll Chemical energy e– Waste Oxygen Hydrogen Hydrogen ATP 2. LIGHT INDEPENDENT REACTIONS The second stage of photosynthesis involve the light independent reac.ons (also called the Calvin cycle). This stage uses the chemical energy (ATP) that was produced in the light dependent stage to synthesise organic compounds (carbohydrates). Molecules of carbon dioxide are combined with hydrogen (from the spli?ng of water) to form organic molecules like glucose. This anabolic process is catalysed by the enzyme Rubisco and uses the chemical energy from ATP to form the covalent bonds required for carbon fixa.on. The inputs of the Calvin cycle are carbon dioxide, ATP and hydrogen; while the product formed is glucose. The light independent reac.ons are localised to the fluid-filled region of the chloroplast (called the stroma). Diffuses into chloroplast Carbon dioxide Organic compounds (e.g. glucose) Light-dependent stage ATP + Hydrogen MEASURING PHOTOSYNTHESIS Metabolic processes can be measured by the rate of input consump.on or the rate of output produc.on. In the case of photosynthesis, the process can therefore be measured by a variety of factors, including: • Oxygen producJon: Oxygen gas is a by-product of photosynthesis and can be measured either via the rate of bubble forma.on (for plants in solu.on) or via a pressure change in an enclosed container • Carbon dioxide uptake: Carbon dioxide interacts with water to form an acidic solu.on (carbonic acid) that lowers pH. Hence, the amount of CO2 produc.on can be measured in solu.on via a pH indicator. • Biomass: Organic compounds contribute to the total dry weight of an organism. Hence, the produc.on of glucose can be indirectly measured according to the change in plant mass (grams per dry weight). LIMITING FACTORS Photosynthesis is a complex metabolic process that involves several dis.nct reac.ons with specific inputs. The law of limi.ng factors states that when a chemical process depends on mulJple essen.al condi.ons being favourable, the rate of reac.on will be limited by the factor that is nearest to its minimum value. Photosynthe.c rate is influenced by numerous condi.ons; including temperature, light and carbon dioxide. Limi.ng Factor In a convoy, the slowest car (limi=ng factor) sets the speed at which the cars will travel (reac=on rate) CARBON ENRICHMENT EXPERIMENTS Human ac.vi.es (such as deforesta.on and combus.on of fossil fuels) are increasing carbon dioxide levels (~400 ppm). Higher levels of carbon dioxide typically increase plant growth and matura.on, however excessive concentra.ons can cause plant damage. Experiments involving enclosed greenhouses and free air carbon dioxide enrichment (FACE) are being undertaken as a way of predic.ng rates of photosynthesis and plant growth in the future. Enclosed greenhouses allow for greater control of extraneous variables (such as sunlight and water exposure), while FACE experiments can incorporate natural condi.ons like heat changes.