Photosynthesis
Plants ‘eat’ by photosynthesising. Without it, we wouldn’t be able to survive. It’s the basis of food chains and generates the oxygen we breathe.
Photosynthetic reaction
Photosynthesis is the process by which plants use light energy from the sun to make food in the form of glucose. Plants use glucose to produce energy through respiration. The energy is used for important biological processes, such as cell division or the active transport of mineral ions.
Photosynthesis takes place inside organelles called chloroplasts. It’s represented by the equation:
The reaction uses energy so it’s an endothermic reaction. (Remember that endothermic reactions absorb energy while exothermic reactions release energy.)
Energy is transferred from the environment to the chloroplasts by light.
Rate of photosynthesis
The rate of photosynthesis depends on temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
Temperature – in general, the higher the temperature the faster the rate of photosynthesis because the enzymes and reacting molecules have more kinetic energy so they collide into each other more frequently. If the temperature becomes too high, the rate of photosynthesis decreases because the enzymes involved in photosynthesis are denatured.
Light intensity – the higher the light intensity, the faster the rate of photosynthesis. Light energy is a requirement for photosynthesis and without it the reactions of photosynthesis cannot take place. Eventually the rate of photosynthesis will stop increasing because something else (e.g. temperature or carbon dioxide concentration) is a limiting factor.
Carbon dioxide concentration – as the concentration of carbon dioxide increases, the rate of photosynthesis increases because carbon dioxide is a reactant in the reaction. Eventually the rate of photosynthesis will stop increasing because something else (such as light intensity) has become a limiting factor.
Amount of chlorophyll – the more chlorophyll present in the leaf, the higher the rate of photosynthesis. Chlorophyll is the green pigment found in chloroplasts which is responsible for absorbing light energy. Plants which live in darker conditions will synthesise more chlorophyll to absorb more light. Some plant diseases will inhibit the production of chlorophyll and will therefore prevent a plant from photosynthesising as efficiently.
We can calculate the rate of photosynthesis by determining the volume of oxygen produced by a plant in a given time. To do this, we read off the y-axis to find the volume of gas produced and divide this by the value on the x-axis, which tells us how long it took for that volume of gas to be formed.
Knowledge about limiting factors can be used to increase the rate of photosynthesis of plants growing in greenhouses to make more profit. Here are some measures that farmers can put in place to increase rates of photosynthesis:
Use artificial lighting with an increased light intensity
Use artificial lighting which provide optimum wavelengths for photosynthesis
Use paraffin heaters or liquid carbon dioxide to increase carbon dioxide levels
Place bags of fungi around the greenhouse which will release carbon dioxide through respiration
Heat the greenhouse to an optimum temperature
Uses of glucose from photosynthesis
The glucose produced in photosynthesis may be:
Used in respiration to release energy
Converted into starch and stored for later use
Converted into fat and stored for later use
Used to make cellulose for cell walls
Converted into amino acids to build proteins