2.8.2 Photosynthesis Continued

• Changes to the Earth’s atmosphere, oceans and rock deposition occur due to photosynthesis
• 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
    • 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 eg. burning of fossil fuels, deforestation which remove oxygen from the atmosphere

• Chemical reactions can be exothermic or endothermic
• Photosynthesis is an example of an endothermic reaction and an anabolic reaction, where the required energy input is in the form of light energy
• Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide
• The energy is not lost - it is stored in chemical form in the carbohydrates that are produced

Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis

• Each of these factors can limit the rate of photosynthesis when they are below the optimal level
  • Temperature
  • Light intensity
  • Carbon dioxide concentration

• These are known as the limiting factors of photosynthesis
  • A limiting factor is a variable that holds back the rate of a chemical reaction
  • If that variable is increased, the reaction rate also increases
• Under any set of conditions, only one of these factors will be limiting the rate of photosynthesis
  • At night, light intensity will be very low, so that is the limiting factor
  • On a cold, sunny day, temperature will be the limiting factor
    • An increase in the light intensity will not increase the rate of photosynthesis because the temperature is, at that point, the limiting factor

Analogy - Limiting Factors

Imagine you're on a group hike and you have to stick together with your two teammates. You are not allowed to finish the hike separately. If your legs are feeling weak, you will be walking slowly. The other two hikers will have to slow down to walk at your speed; the team progresses at the speed of the slowest member. However, after lunch, you may be feeling strengthened by the food and your legs feel good; this time, it's your teammate who is the slowest, so has taken over from you as the limiting factor. Your team only finishes when all three of you finish, and that determines your overall rate of progress.