Light Dependent Reactions (HL) (HL IB Biology)

• Photosynthesis takes place in two distinct stages:
  • The light-dependent reaction, which relies on light directly
  • The light-independent reaction, which does not use light directly

Where do the light dependent reactions take place?

• Both stages of photosynthesis take place within the chloroplast
• The light-dependent reaction takes place in the thylakoid intermembrane space and across the thylakoid membrane
  • Thylakoids are disc like structures which make up the grana in stacks of up to 100. They contain the photosynthesis pigment chlorophyll. Some may have tubular extensions (intergranal lamellae) which join up with thylakoids in adjacent grana
  • The thylakoid membrane contains a transfer chain where electrons are passed along a number of electron carriers in a series of oxidation-reduction reactions

What happens in the light-dependent reaction?

Three key processes which occur during the light-dependent reaction in the thylakoid membrane include

• Photolysis: The splitting of a water molecule using light energy
  • This occurs in photosystem II
• Chemiosmosis: The synthesis of ATP using an electrochemical gradient produced by H⁺ protons
  • The proton gradient forms across the thylakoid membrane when protons are pumped from the chloroplast matrix into the thylakoid spaces
• Reduction of NADP: NADP⁺ accepts electrons (from photophosphorylation) and H+ protons to become NADPH
  • This occurs in photosystem I

Products of the light-dependent reaction

• During the light-dependent reaction light energy is converted into chemical energy in the form of ATP and reduced NADP
• Oxygen is given off as a waste product of the light-dependent reaction
• The useful products of the light-dependent reaction are transferred to the light-independent reaction within the chloroplast

Diagram to show the location of the light dependent and light independent stages of photosynthesis

Photolysis and the light-dependent reaction

• Photolysis occurs in Photosystem II during the light-dependent reaction of photosynthesis
  
  • The reaction centre acts as an oxidising agent and causes water molecules (that have been moved into the leaf by transport up the xylem vessels) to split during photolysis
• Water splits into protons, electrons and oxygen
  • The oxygen is released as a waste product, it diffuses out of the leaf through stomata
  • The electrons are passed into the electron transport chain
  • The protons are picked up by the carrier molecules NADP forming reduced NADP
• The reaction can be summarised as 2H_₂O → O_₂ + 4H^⁺ + 4e^⁻
• The photolysis of water generates the electrons needed for:
  • Replacement of the electrons lost from the reaction centre in Photosystem II
  • Subsequent reactions of the light-dependent reaction

The effect of oxygen

• Changes to the Earth’s atmosphere, oceans and rock deposition occur due to photosynthesis, and more specifically photolysis
• The first life forms emerged around 4 billion years ago
  • At the time, there was no oxygen in the atmosphere
• About 3.5 billion years ago photosynthetic prokaryotes became the first organisms to carry out photosynthesis
  • This began the release of oxygen into the atmosphere
• Millions of years later algae and plants evolved and also carried out photosynthesis
• Around 2.2 billion years ago, the oxygen concentration in the atmosphere reached 2%
  • This is known as the Great Oxidation Event
• Other changes to the Earth occurred due to photosynthesis
  • Minerals in the oceans were oxidised
  • Photosynthetic bacteria released oxygen into the ocean
  • When dissolved iron was oxidised it formed iron oxide which is a red precipitate that lies on the sea bed
  • Over time a distinctive rock formation was produced - the banded iron formation. Layers of red iron oxide alternate with other mineral oxides
  • Banded iron formations are the most important source of iron ores (and consequently our supply of steel)
  • Methane and CO₂ levels in the air fell, which resulted in an Ice Age
    • This is because methane and CO₂ are important greenhouse gases
• By 600 million years ago, life had evolved into large multicellular organisms, many of which were photosynthetic (plants)
  • This pushed the oxygen concentration of the air up to 20%, peaking at 35% 300 million years ago
  • This contributed to the large size of the animals that roamed the Earth at that time
• The current atmospheric oxygen level is around 21%, due to increased human activity, e.g. burning of fossil fuels, deforestation which remove oxygen from the atmosphere