• 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

• 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

• 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

• 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)