2.9 Photosynthesis

Lesson one: Absorption and action spectra for photosynthesis


Understand what is meant by absorption and action spectra for photosynthetic pigments. Understand how to draw absorption and action spectra.


  • Photosynthesis: the production of carbon compounds in cells using light energy
  • Photosynthetic pigment: a compound in cells that absorbs light energy in specific wavelengths eg. chlorophyll a
  • Wavelength: a property of light that causes the colour to change (eg. wavelength of 400nm or 400 nanometers is blue)
  • Visible light: the range of light which can be detected by the human eye (400nm – 700nm)
  • Absorption spectra: a graph showing the % of light absorbed by a pigment at each wavelength (eg. chlorophyll absorbs mostly red and blue light)
  • Action spectra: a graph showing the rate of photosynthesis at each wavelength of light (photosynthesis tends to be faster in red and blue light)

Entry task:

Using this virtual online lab (click below), find out if plants grow faster in red, blue, or green light. Be ready to discuss your findings with the class, and also decide if the seven statements below are true or false.


Statements (decide if they are true or false)

  1. These plants grew faster in green light
  2. These plants grew slowest in green light
  3. Growth rate is related to rate of photosynthesis
  4. Plants look green because they absorb and use green light
  5. Plants look green because they reject and do not use green light
  6. All plants are green
  7. All plants have the same photosynthetic pigments (eg. chlorophyll)

Nature of photosynthesis

Photosynthesis is the production of carbon compounds in cells using light energy. This energy is normally provided by sunlight, although photosynthesis will occur under artificial light e.g. in greenhouses
The equation for photosynthesis is quite similar to the equation for respiration – observe: how is it different?



Photosynthesis equation0911photosynthesisequation

The equations are almost the reverse of each other. This is because photosynthesis is designed to fix light energy in sugars, and respiration is designed to release energy from sugars.

Task -Consider the experiment below:





Surprising fact: 50-80% of the photosynthesis that goes on is not carried out by higher plants. It is the algae in the oceans that perform most of the photosynthesis. These are specifically a group of algae called phytoplankton

image credit earthsky.org

 Absorption spectra and action spectra

The absorption spectra and the action spectra are both investigated in photosynthesis research

memory trigger:  Plants absorb light according to the absorption spectra, and use light according to the action spectra.

Absorption spectra

image credit: wikipedia.org

image credit: wikipedia.org

Task describe Key features of absorption spectrum: 

  • Chlorophylls have peaks of absorbance in _________ and __________ light
  • Chlorophylls have a trough of absorbance in _________light
  • Chlorophyll a and chlorophyll b are ____________ to each other but not ___________ the same

 The action spectrum

image credit: philpot education

image credit: philpot education

Task: Key features of action spectrum

  • Photosynthesis rate peaks at _______________nm, and ______________nm which correspond to the colours ______________and______________.
  • Photosynthesis rate has a trough at ________________nm, which corresponds to _________light

Comparing the absorption and action spectra

image credit: philpot education

image credit: philpot education

Task comparison: The comparison shows a close ______________ in the rate of photosynthesis (action spectrum) and the absorption of different wavelengths( _________ spectrum). This is because the energy needed for ______________is absorbed at specific ________by photosynthetic_______________ 

Investigating the rate photosynthesis

Investigating the rate of photosynthesis involved the following main methods for estimating rate:

  1. growth rates (although these do not correlate well during the seedling stage)
  2. oxygen production (for land plants the oxygen must be contained, for aquatic displacement of water may be used
  3. carbon fixation, or the rate of CO2 —) Glucose (radioactive carbon dioxide can be used here)


Limiting factors:


A limiting factor is something that is in short supply and therefore determines the rate of a process. A good example would be sunlight during the night.

Limiting factors for photosynthesis may be:

  1. Temperature
  2. Light
  3. Carbon dioxide concentration
  4. Water availability via xylem

Q) How can you tell if a factor is limiting? 

A) If a factor is limiting, then increasing it will cause an increase in the rate of photosynthesis because the limiting factor is the one controlling the rate of reaction.

Task: Question 4 in Oxford companion page 139

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