Photosynthetic pigments

In a nutshell

Photosynthesis occurs in chloroplasts. Chlorophylls and carotenoids are photosynthetic pigments present in the chloroplasts that absorb the light energy that drives photosynthesis.

Photosynthetic pigments

Chlorophylls are an example of a photosynthetic pigment that absorbs light energy and converts it to chemical energy which is used directly in the first stage of photosynthesis. Carotenoids are another type of photosynthetic pigment however, these are considered an accessory pigment as they transfer the light energy to the chlorophyll. The table below lists some common photosynthetic pigments.

Photosynthetic pigment	Examples	Colour
Chlorophyll	Chlorophyll a	Green
Chlorophyll	Chlorophyll b	Green
Carotenes	$\beta$ -carotene	Orange
Carotenes	Xanthophyll	Yellow
Other	Phycoerithrin	Red
Other	Fucoxanthin	Brown

Plants will have a variety of photosynthetic pigments as they absorb light of different wavelengths. By having a variety, the plant can absorb light more efficiently and at a greater variety of wavelengths.

Example

Chlorophyll a and chlorophyll b both absorb light in the red and blue part of the visible light spectrum but chlorophyll a absorbs more strongly at a wavelength of $430\ nm$ and chlorophyll b absorbs more strongly at a wavelength of $470\ nm$ .

Chlorophyll structure

Chlorophyll have a complex structure with two main parts; the head and the tail. The head is a conjugate protein with a hydrophilic ring and an atom of magnesium in the centre of the ring. The tail is a long hydrophobic chain of hydrocarbons. The tail structure attaches the chlorophyll molecule to the chloroplast and the head structure lies in the membrane of the chloroplast in order to maximise light absorption.

Biology; Energy for biological processes; KS5 Year 12; Photosynthetic pigments — 1. Methyl group in chlorophyll A. This is replaced with a -CHO group in chlorophyll B, 2. Conjugate protein, 3. Hydrocarbon tail

Spectra

There are two main spectra that you need to know about; absorption and action spectra.

Absorption spectra

Absorption spectra show the amount of light that is absorbed by photosynthetic pigments at different wavelengths.

Biology; Energy for biological processes; KS5 Year 12; Photosynthetic pigments — 1. Carotenoid, 2. Chlorophyll b, 3. Chlorophyll a

Action spectra

Action spectra shows the rate that photosynthesis occurs at different wavelengths of light.

Biology; Energy for biological processes; KS5 Year 12; Photosynthetic pigments