IB Biology SL

Revision Notes

2.8.1 Photosynthesis

Photosynthesis Defined

  • Simple, inorganic compounds are converted into complex organic ones by photosynthesis
    • The energy required is provided by light
  • Photosynthesis occurs in autotrophic organisms such as plants, algae and cyanobacteria
    • H2O and CO2 are the raw materials
  • Photosynthesis is a form of energy conversion, from light energy to chemical energy, stored in biomass
  • Energy is stored within the bonds of these organic compounds
  • Photosynthesis can be thought of as the exact reverse of respiration
    • Respiration is the process by which energy is released from organic molecules in living cells
  • The overall chemical equation for photosynthesis is as follows:

Photosynthesis, downloadable IGCSE & GCSE Chemistry revision notes

The basic equation of photosynthesis as it takes place in a leaf

Balanced chemical equation for photosynthesis, IGCSE & GCSE Biology revision notes

The chemical equation for photosynthesis

Exam Tip

Remember, energy is never created or destroyed; it is only ever converted from one form to another!

Visible Light Wavelengths

  • Chloroplasts contain pigments in order to absorb light
  • Pigments are coloured, which means they absorb some wavelengths (or colours) of the white light that the Sun radiates
    • The remaining light is reflected, giving the pigment its colour
  • Chloroplasts contain several different photosynthetic pigments, so that they can absorb multiple different wavelengths of light
    • The main photosynthetic pigment is chlorophyll
  • Violet light has the shortest wavelength of light in the visible spectrum (around 400nm)
  • Red light has the longest wavelength of light in the visible spectrum (around 700nm)
  • Green light has a wavelength in the middle of this range (around 550nm)
  • The absorption of light varies with wavelength, as does the rate of photosynthesis that a plant can carry out
  • When plants are exposed to light of a specific wavelength, the rate of photosynthesis can be measured as well as the absorbance (the % of the light that is absorbed by the plants)
  • There are peaks in both plots at the blue and red ends of the spectrum, where photosynthesis can occur
  • There are troughs in both plots for green light, which is not absorbed and so cannot provide energy for photosynthesis

Chlorophyll and Visible Light Wavelengths, downloadable IB Biology revision notes

The effect of visible light wavelength on the % absorbance of chlorophyll b and the rate of photosynthesis

Exam Tip

You don’t have to memorise the wavelengths of different colours of light, but you need to know that visible light has a wavelength of between 400 and 700 nanometres (nm).

Chlorophyll

  • Plant cells contain chloroplasts which are the site of photosynthesis
  • The main photosynthetic pigment is chlorophyll
  • Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours
    • Chlorophyll appears green because it absorbs red and blue light
    • The green light is reflected away and so leaves appear green to the eye
    • This explains why the majority of plants are green (with variations in the shades of green that we can see)
  • Red and blue light provides the energy needed for photosynthesis
  • Chlorophyll exists in two main forms, a and b
  • There are two groups of pigments: primary pigments known as chlorophylls and accessory pigments known as carotenoids
  • Chlorophylls absorb wavelengths in the blue-violet and red regions of the light spectrum
  • Carotenoids absorb wavelengths of light mainly in the blue-violet region of the spectrum
  • The combination of pigments maximises the amount of white light energy that can be captured

Exam Tip

Remember – chlorophyll is not the only photosynthetic pigment, others exist to maximise light energy absorption.

Photolysis of Water

  • Oxygen is produced in photosynthesis from the photolysis of water
    • Photo – means ‘with light’
    • Lysis – means ‘breaking apart’
  • Water is broken apart using light energy; this is called photolysis
  • This releases electrons (e), protons (H+) and the waste product, oxygen gas

2H2O → 4e + 4H+ + O2

  • Whilst oxygen is a waste product, the electrons and protons play a crucial role in the further reactions of photosynthesis
    • Though oxygen is a waste product, in practice, a plant will use some of the oxygen it produces in photosynthesis for its own respiration (during the day)

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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