National 5 Biology Course Notes Unit 1 : Cell Biology Part 7 : Photosynthesis

Word equation for photosynthesis In photosynthesis, a plant uses light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose. Oxygen is also produced as a by-product and is released from the plant. Photosynthesis can be summarised by this word equation: Light Absorbed by chlorophyll Carbon Dioxide + Water Glucose + Oxygen However photosynthesis is not a single chemical reaction, it is a series of reactions, each controlled by its own enzyme. Two stages of photosynthesis The photosynthesis reactions can be divided into two main stages: 1.Light reactions – This stage uses light energy trapped by chlorophyll 2. Carbon fixation – In this stage, which does not require light, carbon dioxide is converted to glucose using substances produced in the light reactions

Light reactions These reactions use light energy from the sun which is trapped by chlorophyll in the chloroplasts. There are two light reactions: Photolysis Light energy absorbed by chlorophyll is used to split water into oxygen and hydrogen atoms Water Hydrogen + Oxygen ADP + Pi ATP [Pi = phosphate] Production of ATP Light energy absorbed by chlorophyll is used to change ADP + phosphate to ATP This reaction changes light energy into chemical energy (stored in ATP) Oxygen produced by photolysis is released from the plant as a by-product. The other products of the light reactions – hydrogen atoms and ATP are used in the second stage of photosynthesis, carbon fixation

Carbon fixation This is the second stage of photosynthesis. It involves a series of enzyme controlled reactions joining together carbon dioxide and hydrogen to form glucose. This stage requires ATP and hydrogen, passed on from the light reactions Carbon dioxide Glucose ATP ADP + Pi The hydrogen needed to change carbon dioxide to glucose comes from splitting water (photolysis). The energy needed to make glucose from CO 2 comes from changing ATP (from the light reactions) back to ADP and Pi How the plant uses glucose The glucose produced during photosynthesis can be used in various ways by the plant cell 1.It can be broken down during respiration to release energy in respiration 2.It can be converted to starch and stored. 3.It can be converted to cellulose and used to create cell walls

Limiting Factors A limiting factor is something that slows the rate of a process when it is in short supply. Limiting factors for photosynthesis are 1.Light Intensity 2.Temperature 3.Carbon dioxide concentration Rate of photosynthesis Light intensity X When the graph is like this the variable on the X axis is not the limiting factor Limiting factor graphs The graph shows the effect of light intensity on the rate of photosynthesis When the graph is like this the variable on the X axis is the limiting factor Up to point X, increasing light intensity will increase the rate of photosynthesis, so light intensity is the limiting factor After X, increasing light intensity does not increase the rate of photosynthesis, so light intensity is no longer the limiting factor (the limiting factor is now either temperature or carbon dioxide concentration)