2.1 Essential ideas

2.1.9 Photosynthesis

Figure 2.1.9a – Overview of photosynthesis

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 photosynthesis

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

  • 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.

Figure 2.1.9d – Factors affecting photosynthesis. The dotted line shows the optimum condition for each of the limiting factors. Figure 2.1.9d – Factors affecting photosynthesis. The dotted line shows the optimum condition for each of the limiting factors. 

  • 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 photosynthesis

Figure 2.1.9e – A simple design to investigate the effect of limiting factors on the rate of photosynthesis in Elodea, a pondweedFigure 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:

Click on the image for hints!

Refresh your memory on the basics of photosynthesis.

Figure 2.1.9eFigure 2.1.9f – Chloroplasts
Photosynthesis takes place in the chloroplasts, specialised organelles containing protein pigments that absorb visible light.

Exam tip

  • 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

Experimental design: Controlling relevant variables in photosynthesis experiments is essential.

Think about it

  • On Figure 2.1.9b, you can see that there is very little absorption of light at 600nm by chlorophyll a, but Figure 2.1.9c shows a relatively high rate of photosynthesis at that wavelength. Suggest a reason.
  • Explain the effect of temperature on the rate of photosynthesis.

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.

Figure 2.1.9gFigure 2.1.9g
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?

Course link

  • Get help with lab planning and IA skills in 2.2.5.