Photosynthesis Overview (College Board AP® Biology): Study Guide

Photosynthesis & energy

• Photosynthesis allows organisms to capture and store energy for use in biological processes

• During photosynthesis light energy enables the conversion of simple, inorganic compounds into sugars

  • The energy captured is stored within the bonds of these new organic compounds

• The simple equation for photosynthesis is:

The evolution of photosynthesis

• Photosynthesis first evolved in prokaryotic organisms, such as cyanobacteria

  • Prokaryotic photosynthetic pathways provided the foundation for the evolution of eukaryotic photosynthesis

Oxygenation of the atmosphere

• Evidence indicates that Earth's atmosphere changed due to photosynthesis in early cyanobacteria

  • 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 cyanobacteria became the first organisms to carry out photosynthesis, beginning the release of oxygen into the atmosphere

Chlorophylls & energy

• Chlorophylls are photosynthetic pigments that absorb energy from light

• Pigments are arranged in structures called photosystems which are embedded in the internal thylakoid membranes of chloroplasts

  • Within a photosystem different pigment molecules are positioned in funnel-like structures to absorb as much light energy as possible

• There are two different types of photosystem:

  • photosystem I (PSI), also referred to as P700

    • The chlorophyll a in this system has a maximum absorption of light at 700 nm

  • photosystem II (PSII), also referred to as P680

    • The chlorophyll a in this system has a maximum absorption of light at 680 nm

• As the chlorophylls absorb energy from light their electrons are boosted to a higher energy level within photosystems I and II

• High energy electrons are transferred between photosystems via the electron transport chain in the thylakoid membrane