Chapter 11 Photosynthesis 2.2 Cell Metabolism Learning Objectives 2.2.4 Photosynthesis 1. Definition, balanced equation and role of "photosynthesis". 2. An explanation of the process of photosynthesis. 3. Give the location of chlorophyll within cells. 4. Identify the source of light, carbon dioxide and water for photosynthesis. 5. Human intervention: use of artificial light and carbon dioxide enrichment to promote crop growth in greenhouses. 6. Investigate the influence of light intensity or carbon dioxide on the rate of photosynthesis. 7. **Photosynthesis as a two-stage process, The Light Stage and The Dark Stage. 8. **Outline the 2 pathways of the light stage and their products. 9. **Outline the stages of the dark stage and the products. The sun is the primary source of energy on earth. It radiates solar energy which is used by autotrophs to make their own food. Autotrophs - Organisms that make their own food. Photosynthesis is the process by which green plants make their own food. - It is an anabolic process. - It involves the conversion of solar energy to chemical energy. - Carbon Dioxide and Water are used with sunlight to form Glucose and Oxygen. - Photosynthesis occurs in the green parts of plants called chloroplasts which contain a pigment called chlorophyll. -Cells need energy to carry out this process, it is required in the form of ATP, which is initially made from the solar energy, and later from the newly formed Glucose. Equation for photosynthesis: Importance of photosynthesis: Plants use photosynthesis to make food Animals get their food from plants It produces oxygen required by living things for respiration It was responsible for forming fossil fuels (dead plants)
Photosynthesis Requirements and their Sources: Carbon dioxide - supplies the carbon (C) and oxygen (O) from which glucose is made. o Most is absorbed externally from the atmosphere by tiny openings in leaves called stomata. Internally some Carbon Dioxide is formed from the breakdown of Glucose during Respiration. Water - supplies the H which combines with CO2 to form glucose. o Absorbed from the soil by the roots, passes up stem & into leaves. Light - supplies the energy necessary to convert H2O and CO2 into glucose. o Usually sunlight, but artificial lights may be used indoors and in greenhouses to stimulate growth. Chlorophyll - green pigment - formed in chloroplasts. Suitable temperature - photosynthesis is an enzyme controlled reaction and temperature affects the rate of enzyme action. Photosynthesis Products Glucose Stored and used for energy production (Respiration). Oxygen Small Amount used during respiration, most is released as waste back into the atmosphere. Main events in photosynthesis: 1. Light is absorbed light is trapped by chlorophyll found in chloroplasts. 2. Sunlight energy is used to split water 2H 2 O 4H + + 4e - + O 2 (H + = Protons, e - = Electrons) 3. Products of splitting of water Electrons (e - ) are passed to chlorophyll Protons are released into a storage pool in the chloroplast O 2 may pass out of the leaf into the atmosphere or may be used for respiration 4. The electrons that were passed to the chlorophyll become energised by some of the trapped light energy this changes them into high energy electrons 5. The high energy electrons, protons and carbon dioxide join to form glucose (C 6 H 12 O 6 )
Human Intervention If the rate of photosynthesis is increased, the rate of plant growth increases, this can be done in two ways: o o Increasing light can increase growth up to a certain saturation point where no more light can be absorbed and photosynthesis will level. Sometimes artificial sources of carbon dioxide are used to stimulate growth e.g. burning gas in a green house, adding sodium carbonate to the water. Detailed Study HL only Photosynthesis takes place in two stages: Light stage requires light, is controlled by electrons so is temperature independent Dark stage (Calvin Cycle) does not require light, is controlled by enzymes so is temperature dependent Light Stage: Events of the light stage take place in the grana of the chloroplast 1. Light is absorbed White light can be split into 7 components. Chloroplasts contain clusters of pigments, each of the pigments absorb a different colour of light. Plants absorb all colours except green. 2. Light energy is transferred to electrons. Pigments are arranged in clusters, each cluster consists of; - a variety of pigments, - a strategically placed chlorophyll molecule - an electron acceptor. In each cluster the different pigments absorb light of different colours (wavelengths). Each pigment passes energy from one to the other, until it reaches the chlorophyll molecule and passes into the electron acceptor. There the energy passes to the stored electrons and energises them, ready to move along one of 2 pathways. Electron flow: Pathway 1 High energy electrons pass from chlorophyll to the electron acceptor
They then pass to a series of other electron acceptors and back to the same chlorophyll. When electrons move in this way they lose energy. This energy is trapped by ADP and phosphate to form ATP. This pathway is cyclic and is called cyclic phosphorylation. Electron flow: Pathway 2 2 high energy electrons are passed from chlorophyll to the electron acceptor and along a series of electron acceptors These electrons do not return to chlorophyll They lose energy as they pass from acceptor to acceptor. This energy is used to form ATP Eventually the 2 electrons join with NADP + to form NADP - NADP + + 2e - NADP - Chlorophyll is now short of electrons. It gains electron from the splitting of water. Water is split using sunlight energy 2H 2 O 4H + + 4e - + O 2 Protons (H + ) join with NADP - to form NADPH This pathway is non-cyclic photophosphorylation Products of the light stage: - ATP, NADPH used in the dark stage - O 2 released into the atmosphere or used in respiration Dark Stage (Calvin Cycle): Takes place in the stroma of the chloroplast These events are controlled by enzymes, therefore reactions are influenced by temperature Carbon dioxide from the air enters the chloroplast Carbon dioxide combines with H + (protons) and e - to form glucose. H + and e - come from the conversion of NADPH to NADP + NADPH NADP + + 2e - + H + The energy required to form glucose comes from converting ATP to ADP + P NADP + and ADP are reused in the light stage. Carbon dioxide NADPH NADP + ATP ADP + P Glucose