2.8.1 Photosynthesis

• 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

• 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)