IB DP Biology: SL复习笔记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:


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


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


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).


  • 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)