Overview of Photosynthesis (College Board AP® Biology)

The Energetic Basis of Photosynthesis

The Importance of Photosynthesis

• Photosynthesis feeds the whole biosphere

• Photosynthesis is essential to all life, directly or indirectly

  • Light energy is harnessed to convert light energy into chemical energy in ATP and NADPH

  • The chemical energy in ATP and NADPH is in turn converted into chemical energy in food molecules eg. sugar

• 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 Photosynthesis Equation Diagram

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

Chemical Equation for Photosynthesis

The chemical equation for photosynthesis

The Evolution of Photosynthesis and the Oxygenated Atmosphere

• Changes to the Earth’s atmosphere, oceans and rock deposition occur due to photosynthesis

  • Prokaryotic photosynthesis initiated this change and formed the biochemical basis of all eukaryotic photosynthesis seen today

• Evidence indicates that the earth's atmosphere changed through the following process:

  • The first life forms emerged around 4 billion years ago when photosynthesis was first carried out by prokaryotes 

    • 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 oxidized

      • Photosynthetic bacteria released oxygen into the ocean

      • When dissolved iron was oxidized 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 e.g. burning of fossil fuels, deforestation which remove oxygen from the atmosphere

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