Plants need light energy to make the chemical energy needed to create carbohydrates. Increasing the light intensity will boost the speed of photosynthesis. However, at high light intensities the rate becomes constant.
Experiment
- Place a pond weed Elodea upside in a test tube containing water.
- Place the tube in a beaker of fresh water at 25°C. This helps to maintain a constant temperature around the pond weed.
- Place excess sodium bicarbonate (NaHCO3) in the water to give a constant saturated solution of CO2.
- Place the lamp (the only light source) at distance from the plant.
- Count the number of oxygen bubbles given off by the plant in 1 minute period. This is the rate of photosynthesis at that particular light intensity.
- The gas should be checked to prove that it is indeed oxygen - relights a glowing splint.
- Repeat at different light intensities by moving the lamp to different distances.
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- Graph the results placing light
intensity on the x-axis.
Explanation
- Light energy absorbed by chlorophyll is converted to ATP and H+
- At very low light levels the plant will be respiring only not photosynthesising.
- As the light intensity increases, the rate of photosynthesis increases. However, the rate will not increase beyond a certain level of light intensity.
- At high light intensities the rate becomes constant, even with further increases in light intensity, there are no increases in the rate.
- The plant is unable to harvest the light at these high intensities and the chlorophyll system can be damaged by very intense light levels.
References: skoool.ie
passmyexams.co.uk
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What happens during photosynthesis?
Plants make their own food using photosynthesis. The food is important for the plants and for organisms that feed on the plants. Optimum rates of photosynthesis produce maximum plant yields.
There are several ways of measuring the rate of photosynthesis in the laboratory. These include: These are not perfect methods as the plant will also be respiring, which will use up some oxygen and carbohydrate and increase carbon dioxide output. Several factors can affect the rate of photosynthesis: The amount of chlorophyll also affects the rate of photosynthesis: Without enough light, a plant cannot photosynthesise very quickly - even if there is plenty of water and carbon dioxide and a suitable temperature. Increasing the light intensity increases the rate of photosynthesis, until some other factor - a
limiting factor - becomes in short supply. At very high light intensities, photosynthesis
is slowed and then inhibited, but these light intensities do not occur in nature. Carbon dioxide – with water – is one of the reactants in
photosynthesis.
If the concentration of carbon dioxide is increased, the rate of photosynthesis will therefore increase.Factors affecting photosynthesis
Light intensity
Carbon dioxide concentration
Again, at some point, a different factor may become limiting. Beyond this concentration, further increases in the concentration of carbon dioxide will not result in a faster rate of photosynthesis, and would appear on a graph as a horizontal line.
Temperature
The chemical reactions that combine carbon dioxide and water to produce glucose are controlled by enzymes. As with any other enzyme-controlled reaction, the rate of photosynthesis is affected by temperature.
At low temperatures, the rate of photosynthesis is limited by the number of molecular collisions between enzymes and substrates. At high temperatures, enzymes are denatured.
Chlorophyll
Chlorophyll absorbs the light required to convert carbon dioxide and water into glucose.
Chlorophyll is green - so absorbs the red and blue parts of the electromagnetic spectrum and reflects the green part of the spectrum.
Leaves with more chlorophyll are better able to absorb the light required for photosynthesis.
The compensation point
These graphs have been plotted with rate of photosynthesis against the factor under investigation.
If oxygen production or carbon dioxide uptake is used as a measure of photosynthetic rate, the graphs are slightly different. The line does not go through the origin. This is because oxygen production and carbon dioxide uptake are affected by respiration as well as photosynthesis. For instance, if a graph is plotted of carbon dioxide uptake against light intensity:
The compensation point is the light intensity at which the rate of photosynthesis is equal to the rate of respiration.
A similar graph will be obtained if oxygen production is plotted against light intensity.
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