Living Cells Photosynthesis

Sunlight as a source of energy Green plant use sunlight energy to synthesis their own food, in a process called photosynthesis The process involves the capture of light energy by the plant = Energy fixation The light is trapped by the pigment chlorophyll, which is present in the chloroplasts of the plant cells Once trapped the light energy is converted to chemical energy, in the form of ATP

Summary of photosynthesis The raw materials used in photosynthesis are carbon dioxide (absorbed from the atmosphere, through stomata on the leaf surface) and water (from the soil, which travels up through the plant in vessels called xylem) The plant converts these raw materials with the help of sunlight energy (trapped by chlorophyll), into a plant food source (carbohydrate) glucose (stored as starch) and a by product oxygen (released through the stomata) A carbohydrate is a compound containing the chemical elements carbon, hydrogen and oxygen (See sheets pg 33) Carbon dioxide + Water + light energy Carbohydrate + Oxygen

Sunlight as a source of energy Activity 1.13 – Comparing photosynthesis in light and dark conditions pg 34 Remember that the sunlight energy is absorbed in the first place by chlorophyll in the chloroplasts Activity 1.14 – Comparing photosynthesis in the absence / presence of CO2 pg 35 (top) Photosynthesis occurs as a set of reactions; Photolysis Carbon fixation

Photolysis ‘Photo’ = light , ‘lysis’ = split (See sheets pg 35) Photolysis involves the splitting of water molecules into the elements oxygen and hydrogen, using light energy Photolysis provides the hydrogen and chemical energy in ATP, needed for stage 2 of photosynthesis The oxygen is a by-product

Carbon fixation This involves the hydrogen (produced by photolysis) combining with carbon dioxide (absorbed from the atmosphere) to form the carbohydrate glucose (C6H12O6) The carbon becomes ‘fixed’ into the carbohydrate with the hydrogen (See sheets top pg36)

Conversion of glucose to other carbohydrates During photosynthesis the plant will continue to make glucose, however, not all of this glucose is used by the plant at any one time The spare glucose is synthesised into the larger more complex starch and cellulose carbohydrates The starch is insoluble but can be reversibly converted to glucose when required The cellulose is combined together to form ribbon-like fibres which is a large component of plant cell walls These (reactions) conversions involve enzyme controlled reactions (See sheets bottom pg 36)

Factors affecting the rate of photosynthesis Carbon + Water + Light Carbohydrate + Oxygen dioxide intensity The 3 things required for photosynthesis are carbon dioxide, water and light energy The correct temperature is also required (enzyme-controlled) Water is usually in plentiful supply and rarely in shortage However, concentrations of carbon dioxide, light intensity and temperature can become limited, and are said to be limiting factors. (See sheets top pg 37)

The Elodea Bubbler experiment The elodea bubbler experiment can be used to measure the effect of altering light intensity and carbon dioxide concentration, on the rate of photosynthesis The apparatus can be set up as shown on pg 37 of your sheets, and the number of oxygen bubbles given off over a fixed period of time (usually per minute), is measured, and this will show the rate at which photosynthesis is occurring (See sheets pg 37/38/39) Familiarize yourself with the reasons for using each piece of apparatus.

Early crops in horticulture It is clear from our knowledge of photosynthesis that plants require a good supply of water, light energy and carbon dioxide to grow and develop properly, as well as a suitable temperature If any of the limiting factors are in short supply for a crop of plants, then the rate of photosynthesis will be limited, and therefore the maximum quantity and quality of crop will not be obtained Farmers / Horticulturists can take steps to ensure the best possible conditions are maintained for the optimum quality and quantity of crop production (See sheets bottom pg39) Suitable temperature – Greenhouse / thermostatically controlled heating system Light intensity – Electric lighting Carbon dioxide concentration – Added supply pumped into greenhouse