Photosynthesis

Source of EnergyThe sun is the source of energy for our planet. There are four sources of solar radiation. The most important in biology are infrared (IR) rays, ultraviolet (UV) rays and light rays.

PlantsAutotrophs are organisms that can make their own food. Autotrophs are also called producers. The vast majority of autotrophs are green plants. They make their food through photosynthesis.

PhotosynthesisThis is the process by which glucose and oxygen are made from carbon dioxide and water. Light is needed as a source of energy for this process. The green pigment called chlorophyll is necessary as a catalyst for this process. Photosynthesis takes place in plants, especially in the green parts, i.e. leaves, because they contain chlorophyll. Cells need energy to carry out their reactions. This energy is needed in small, usable amounts and is beside supplied by the breakdown of a molecule called Adenosine TriPhosphate (ATP). Light energy is used to make ATP. Role of Photosynthesis:1. Plant can use glucose to re-form ATP at a later stage to provide themselves with cellular energy. 2. Animals consume glucose made by plants and convert it to ATP for their own use. 3. Oxygen is produced from photosynthesis. Most organisms need oxygen to obtain energy in respiration.

Photosynthesis is represented and defined by this balanced equation.

Main Events in Photosynthesis1. Light is Absorbed - Some of the sunlight that strikes a plant is trapped by chlorophyll. Chlorophyll is found in chloroplasts, a subcellular organelle found in plant cells. The trapped sunlight is used to provide the energy that a plant needs to produce gluocse. 2. Water is Split - Some of the trapped sunlight energy is used in the chloroplast to split water molecules. Every hydrogen atom is made of a hydrogen ion (protein H+) and an electron (e-). Water has the formula, H2O. When two water molecules are split, they form four protons, four electrons and a molecule of Oxygen Gas (O2). 3. The Products of the splitting of water - The 3 products formed when water is split act as follows:(i) The electrons are passed to teh chlorophyll. (ii) The protons are relased into a storage pool of protons in the chloroplast for later use. (iii) The oxygen may pass from the chloroplast out into the cytoplasm and eventually out of the leaf into the atmosphere. Alternatively, Oxygen may also be used within the cells or the leaf, in the process of respiration. 4. Light energises ElectronsSome of the sunlight energy trapped by the chlorophyll is passed on to electrons in chlorophyll to form high-energy electrons. 5. Glucose is formedThe high-energy electrons from chlorophyll, along with protons from the pool of stored protons, are combined with carbon dioxide to form a carbohydrate - Glucose, C6H12O6.

Sources of Light, Carbon Dioxide and Water for PhotosynthesisLightSunlight is the normal source of light for photosynthesis. However, sometimes artificial light can be used. For example, electric bulbs. This is often used in greenhouses to stimulate crop growth. Carbon Dioxide, CO2. Plants have two sources of carbon dioxide: 1. External and 2. Internal. Most of the CO2 a plant uses enters the leaf from the atmosphere. Some of the CO2 used in photosynthesis is produced by the leaf cells in the process of respiration. The amount of CO2 available to a plant is relatively low. The lower the level of CO2, the slower the rate of photosynthesis. WaterWater is absored from the soil by the roots of the plants. This water passes up through the plant stem and into the leaves. It is used in the leaves in photosynthesis.

Photosynthesis occurs in two stages - Light Stage and Dark Stage. The reactions of the light stage are dependent on the energy provided by light. The dark stage does not require light; it is light independent. The dark stage depends n the products of the light stage.

Light StageThe events of the light stage of photosynthesis take place in the chloroplast. The chloroplast is a specialised organelle in plants and algae. It is a type of plastid. It contains a high concentration of chlorophyll. Chlorophyll is the pigment which absorbs energy from sunlight. On the right is a diagram of a plant cell.

Main events of the Light Stage1. Light is absorbed - White light is composed of different wavelengths of light. Chloroplasts contain a range of pigments, including chlorophyll. Each of the chlorophyll pigments absorbs a different colour of light. Plants absorb all the colours of white light except green. This is why most plants are green. 2. Light energy is transferred to electrons - Chloroplast pigments are arranged in clusters in the chloroplast. Each cluster consists of a variety of pigments - these include a stragetically placed chlorophyll molecule and an electron acceptor. In each cluster, the different pigments absorb light energy of different wavelengths. The function of the cluster is to absorb as much light as possible. The different pigments transfer the absorbed energy from one to another until it reaches the chlorophyll associated with the electron accpeotr. The energy is then transferred to electrons, causing it to become energised. These electrons are known as 'High-Energy' electrons. these are passed from the chlorophyll to the electron accpeot. They flow the here alone one of two different pathways. 3. Electron Flow: Pathway 1 - 'Cyclic PhotoPhosphorylation'The high energy electrons pass from chlorophyll to the electron acceptor and along a series of electron acceptors and back to the chlorophyll molecule. All these molecules are located in the chloroplast. The electrons are recycled to the chlorophyll. When they return to the chlorophyll, they lose energy. The energy they release is trapped by ADP and a phosphate group. ADP stands for Adenosine DiPhosphate. ADP and Phosphate form ATP and Water. The addition of the phosphate to a molecule is known as phosphorylation. In this pathway, the source of energy is light. The production of ATP is known as photophosphorylation. This is a cyclic pathway because the electrons flow from the chlorophyll to the electron acceptor and back to the chlorophyll. Therefore this pathway is referred to as 'Cyclic PhotoPhosphorylation'.  4. Electron Flow: Pathway 2 - 'Non-Cyclic PhotoPhosphorylation'Two high-energy electrons at a time are passed from chlorophyll to the electron acceptor and along a series of electron acceptors. This occurs within the chloroplast. The energised electrons do not return to the chlorophyll. Instead, they lose some energy as they pass from acceptor to acceptor. The energy they release is used to make ATP - ATP + energy + P ---> ATP + Water. Eventually the two electrons combine with NADP+ to temporarily form NADP-:NADP+ + 2 electrons ----> NADP-The chlorophyll molecule is now short of electrons. It gains new electrons from the splitting of water. This reaction is called photolysis. When two water molecules are split by light, they form 4 protons, 4 electrons and oxygen (O2). Two of these electrons replace the electrons lost by the chlorophyll. The protons formed by photolysis are stored in a pool of protons in the chloroplast. These are attracted to the NADP- and combine with it to reduce it to NADPH:NADP- + proton ---> NADPH. As a result of this pathway, two electrons pass from a water molecules to chlorophyll. They are energised by light and flow to the electron acceptor. From here, they move along the series of electron acccpeots and are used to form NADPH. As they flow, some of their energy is used to convery ADP to ATP.  In this pathway the electrons start with water and end up in NADPH. They do not recycle. This pathway is therefore referred to as 'Non-Cyclic PhotoPhosphorylation'.

End products of the Light Stage1. ATP is made - supply energy to the dark stage. 2. NADPH is made - supply protons and energised electrons to the dark stage. 3. Oxygen is made when water is split - used for respiration or release into the atmostphere.

Calvin CycleThe dark stage takes place in a different part of the chloroplast. These events were discovered by Melvin Calvin. It is sometimes referred to as the 'Calvin Cycle'. This is a light-independent stage and occurs in the stroma of chloroplasts.

Main Events of the Dark StageFormation of Glucose - CO2 from the air enters the chloroplast. These CO2 molecules combine with hydrogen ions (H+) and electrons to form glucose. CO2 is reduced to Glucose. This is becuase the hydrogen ions and electrons come from the conversion of NADPH to NADP+; the electrons are reduced. The energy to form glucose comes from ATP, made in the light stage, to ADP and Phosphate:ATP + Water ---> ADP + P + energy. All the NADP+ and ADP moelcules produced in the dark stage are reused in the light stage.

Introduction to Photosynthesis

Light Stage

Dark Stage