2.1 Essential ideas
Figure 2.1.9a – Overview of
Click on Figure 2.1.9a to find the key understandings for this topic:
- Photosynthesis is the production of carbon compounds using light energy.
- Visible light has a range of wavelengths, with violet the shortest wavelength and red the longest.
- Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.
- Oxygen is produced in photosynthesis from the photolysis of water.
- Energy is needed to produce carbohydrates and other compounds from carbon dioxide.
- Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis.
Skill: Drawing absorption and action spectra
- The absorption spectrum shows the relative amounts of light absorbed by photosynthetic pigments at different wavelengths.
- Chlorophyll a and b absorb light effectively in the red and blue areas of the spectrum, but do not absorb light in the green area of the spectrum. This is why chloroplasts (and plants) appear green.
- Other pigments, such as carotenoids, absorb shorter wavelengths (violet, blue) and reflect longer wavelengths (yellow, red), and therefore appear yellow or red.
- Different types of plants have different types and proportions of pigments, so their absorption/action spectra are unique.
- A general example is shown in Figure 2.1.9b.
Figure 2.1.9b – Absorption spectrum of photosynthetic pigments.
Click on the image for tips on how to draw the diagram.
Figure 2.1.9c – Action spectrum for
Click on the image to see how the action and absorption spectra are related.
- The action spectrum shows the rate of photosynthesis (relative to the maximum efficiency) at different wavelengths.
- The action spectrum and absorption spectrum are related. You will notice that there are peaks in the action spectrum at the wavelengths that chlorophyll a absorbs the most light. Chlorophyll a is the primary photosynthetic pigment.
- Smaller peaks are visible where the accessory pigments – chlorophyll b and the carotenoids – absorb the most light. Accessory pigments aid in photosynthesis by transferring energy to chlorophyll a.
Factors that limit the rate of
- Photosynthesis is a collection of metabolic pathways that require different enzymes at each step.
- The rate of photosynthesis is most affected by environmental variation in light intensity, temperature and carbon dioxide. These are known as limiting factors.
- Figure 2.1.9d shows how the rate of photosynthesis changes when one limiting factor is investigated.
- When the environmental conditions for all limiting factors are optimal, photosynthesis proceeds at maximum efficiency.
- Natural conditions change rapidly, so the rate of photosynthesis rarely proceeds at maximum efficiency.
- At any time, only one environmental condition limits the rate of photosynthesis. For example, high temperature is likely to be the limiting factor at midday. At dusk, light intensity is most likely to be the limiting factor, even though the temperature may be lower than the optimum.
Skill: Design of experiments to investigate the effect of limiting factors on
Figure 2.1.9e – A simple design to investigate the effect of limiting factors on the rate of photosynthesis in Elodea, a pondweed
As you plan your experiment, consider ways to modify the original set-up in order to:
- investigate one limiting factor at a time (independent variable)
- determine the number of trials and treatments
- control other factors
- collect quantitative results (dependent variable).
Click on the image for hints!
Refresh your memory on the basics of photosynthesis.
- Depending on the question, accessory pigments may not need to be included in your drawing.
In the lab
- There are many accessory pigments not mentioned on this page. Use paper chromatography to separate photosynthetic pigments. See 2.2.9.
Nature of Science
Think about it
Food for thought
Many students want to know why water is not a limiting factor in photosynthesis. Only a small amount of water is necessary for photosynthesis, but in extremely arid conditions, stomata close to prevent water loss. When stomata are closed, carbon dioxide cannot enter the leaf and carbon dioxide becomes the factor limiting photosynthesis.
This simple graph shows the results of an experiment performed at a constant carbon dioxide concentration. What will the resulting graphs look like if this experiment is repeated at different concentrations of carbon dioxide?
- Get help with lab planning and IA skills in 2.2.5.