Introduction to photosynthesis

Nature of light


Light reactions

Calvin cycle and C4

The Nature of Light:

"Light absorbs packets of energy."

Another way of saying this:

All light (electromagnetic radiation) travels at a constant speed. However, all light travels in waves. Different kinds of light are defined by the the wavelength of their light.


All this is fine and good, but why does light travel in a wave in the first place? Light contains energy.

The energy gives a specific kind of light its frequency. Light with a lot of energy has a high frequency, i.e. the wavelength of light oscillates back and forth rapidly 'because' it has energy. Light with little energy oscillates slowly. This means that different kinds of light (or light of a specific wavelength) have specific amounts of energy that they carry in their waves. The amount of energy contained in a specific kind of light over a distance is different from the amount of energy contained in every other kind of light over that same distance, and is called a quantum.

An measurement or way of defining light is called a photon. A quantum is defined as the amount of energy each specific wavelength of light contains in a photon.

The sun gives off a full range of light of varying wavelengths called the electromagnetic spectrum.

The electromagnetic spectrum includes:

.001 - 1 nm

1-10 nm

10-1000 nm

(380nm -750 )







Visible light




Fortunately for us, the upper atmosphere of our wonderful planet filters out most of the really high energy light before it reaches us and the plants.

What is leftover consists of about

  • 4% UV,
  • 52% IR or weaker light, and
  • 44% visible light.

All of this light is available to plants to use, but they only use visible light in the process of photosynthesis.


IR light does not contain enough energy for photosynthesis .

UV on the other hand has too much energy, and in a sense can't be controlled by plants. UV light intercepted by plants (and us) can create free radicals, which can break chemical bonds in an organism. This is detrimental to the plant. Plants in fact have pigments to protect them from UV light.

Visible light, however, seem to be just right for plants to use to move electrons around.