Light Reactions:

– require chlorophyll and take place in thykaloid membranes

– absorb light energy and transfer it to ADP and NADP+

– energy is transferred to carbohydrate molecules in the Calvin cycle

Carbon Fixation:

– incorporate molecules of CO2(g) into organic molecules, such as glucose

– require energy from ATP and NADPH

– takes place in the stoma by means of the Calvin cycle

Light (Electromagnetic (EM) radiation:

– is a form of energy that travels as photons

– photons are wave packets characterized by their wavelength and energy

– photons with short wavelengths have high energy and photons with long wavelengths have low energy

Electromagnetic Spectrum:

– Sun light is a mixture of photons of different energies

– photons of different energies can be separated when passed through a transparent prism in a spectroscope forming the electromagnetic spectrum

Plant Growth Requirements

– humans can see light between 380nm (violet)-750nm(red), but the rest is invisible

– thylakoid membranes have clusters of photosynthetic pigments, called photosystems, that absorb photons


Chlorophyll a and b:

– absorb photons in the blue-violet and red regions of the spectrum

– reflect photons in the green region of the spectrum

chlorophyll a is the only pigment that can transfer light energy to the carbon fixation reactions

cholorphyl b acts as an accessory pigment, absorbing photons that chlorophyll a absorbs poorly or not at all

Accessory Pigments

– transfer their energy to chlorophyll a

– chlorophyll a and b combined with accessory pigments, absorb light within the entire visible spectrum


– able to absorb light energy in the range from 400nm to 500nm

Plant carotenoids:

The Calvin Cycle

-some carotenoids absorb light which would otherwise damage chlorophyll, and then they lose light as heat

Human carotenoids

– similar compounds to plant cartenoids are thought to protect the eye from excessive photon damage

b-carotene is converted to rhodopsin, a photopigment in the eye that helps vertebrates see in low-light conditions

– carrots are rich in b-carotene help maintain good vision

Leaf Colour:

– leaves appear green because of the high concentration of chlorophyll in the chloroplasts of leaf cells

– leaves appear different colours in autumn as plants stop producing chlorophyll and disassemble those already in the leaves; causes the yellow (xanthophylls), red (carotenoids) and brown pigments to become visible


  1. Just to note that the wavelength numbers on the spectrum diagram are the wrong way round – blue should be 400 nm and red 700 nm

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