Photosynthetic pigments

CIE A-Level Biology· 45 min read

1. Types and Roles of Pigments★★☆☆☆⏱ 15 min

📘 Definition

Primary vs Accessory Pigments

Primary pigment = chlorophyll a

Primary pigments directly participate in converting light energy to chemical energy by donating excited electrons to the electron transport chain. Accessory pigments absorb light energy and transfer it to the primary pigment in the reaction centre, and do not donate electrons directly.

Example:

Chlorophyll a is the only primary pigment in the reaction centre of photosystems I and II.

All photosynthetic pigments are embedded in the thylakoid membranes of chloroplasts, arranged in photosystems. Different pigments absorb different wavelengths of visible light, extending the range of light that can be used for photosynthesis.

Chlorophyll a: blue-green pigment, absorbs red (~660 nm) and blue-violet (~430 nm) light
Chlorophyll b: yellow-green accessory pigment, extends absorption range of chlorophyll a
Carotenoids (carotene, xanthophyll): orange/yellow accessory pigments, absorb blue-green light, protect chlorophyll from photo-oxidation

📐 Worked Example

Explain why most land plants appear green to the human eye.

1
All photosynthetic chlorophyll pigments absorb most strongly red and blue-violet wavelengths of visible light.
2
Green wavelengths (between 500-600 nm) are barely absorbed by any chlorophyll pigment.
3
Unabsorbed green light is reflected off leaf tissue and transmitted through leaves, reaching the human eye and making plants appear green.

Exam tip:

Always link the colour of a pigment to which wavelengths are reflected, not just which wavelengths are absorbed.

2. Separation by Chromatography★★★☆☆⏱ 20 min

Paper or thin-layer chromatography (TLC) is used to separate mixed photosynthetic pigments extracted from leaf tissue. Separation occurs because pigments differ in their solubility in the mobile solvent and their adsorption to the stationary phase (paper/TCL).

📘 Definition

Rf Value

A dimensionless ratio used to identify unknown separated pigments, which is constant for a given pigment in the same solvent and stationary phase.

Example:

Pigments that are more soluble in the solvent have higher Rf values.

📐 Worked Example

In a paper chromatography experiment, the solvent front travels 10 cm from the origin line. Pigment 1 travels 8.4 cm, Pigment 2 travels 5.9 cm, Pigment 3 travels 3.8 cm, Pigment 4 travels 1.7 cm. Calculate Rf for each pigment and order them by solubility.

1
Recall the formula for Rf:
2
$R_f = \frac{\text{distance moved by pigment}}{\text{distance moved by solvent front}}$
3
Substitute the given values to get:
4
Pigment 1: $R_{f} = 8.4/10 = 0.84$
5
Pigment 2: $R_{f} = 5.9/10 = 0.59$
6
Pigment 3: $R_{f} = 3.8/10 = 0.38$
7
Pigment 4: $R_{f} = 1.7/10 = 0.17$
8
Higher Rf = higher solubility, so solubility order: Carotene (P1) > Xanthophyll (P2) > Chlorophyll a (P3) > Chlorophyll b (P4)

3. Absorption and Action Spectra★★★☆☆⏱ 18 min

Two key graphs describe the relationship between light wavelength and photosynthetic activity. An absorption spectrum shows how much light each pigment absorbs at different wavelengths, while an action spectrum shows the overall rate of photosynthesis at different wavelengths.

Feature	Absorption Spectrum	Action Spectrum
What it measures	Light absorption by pigments	Rate of photosynthesis
Y-axis	% light absorption	O2 production / CO2 uptake
X		a	x	i	s
W	a	v	e	l	e	n	g	t	h	o	f	l	i	g	h	t	(	n	m	)
W	a	v	e	l	e	n	g	t	h	o	f	l	i	g	h	t	(	n	m	)
Purpose	Shows which wavelengths each pigment absorbs	Shows which wavelengths drive photosynthesis

📐 Worked Example

Explain why the shape of the action spectrum closely matches the shape of the absorption spectrum for photosynthetic pigments.

1
Photosynthesis can only occur when light energy is absorbed by photosynthetic pigments in chloroplasts.
2
The rate of photosynthesis, measured by the action spectrum, is directly dependent on how much light is absorbed, measured by the absorption spectrum, at each wavelength.
3
Wavelengths that are highly absorbed (red and blue-violet) produce the highest photosynthetic rate, while poorly absorbed green wavelengths produce the lowest rate. This results in matching peaks and troughs for both graphs.

Exam tip:

If asked to draw either spectrum, always label peaks at ~430 nm (blue) and ~660 nm (red), with a trough at ~550 nm (green).

4. Common Pitfalls

Wrong move:

Claiming all chlorophyll a is primary pigment and never acts as accessory.

Why:

Only chlorophyll a in the reaction centre is primary; other chlorophyll a molecules in the light-harvesting complex are accessory.

Correct move:

State that chlorophyll a acts as the primary pigment in reaction centres, and also functions as an accessory pigment elsewhere in the photosystem.

Wrong move:

Calculating Rf as solvent distance divided by pigment distance.

Why:

This inverts the ratio, giving an incorrect value greater than 1 for most pigments.

Correct move:

Remember: $R_{f} = \frac{d _{p i g m e n t}}{d _{so l v e n t}}$ (pigment over solvent front).

Wrong move:

Stating green light is never used for photosynthesis.

Why:

While green is poorly absorbed, accessory pigments absorb some green light, so photosynthesis still occurs at a low rate.

Correct move:

State that green light is absorbed much less than red/blue, leading to a low rate of photosynthesis, not zero.

Wrong move:

Confusing absorption spectrum with action spectrum in exam answers.

Why:

The two graphs measure different processes, so mixing them up loses marks.

Correct move:

Remember: Absorption = what pigments absorb; Action = what actually drives photosynthesis.

Wrong move:

Drawing the origin line below the solvent level in chromatography.

Why:

Pigments dissolve into the solvent instead of moving up the stationary phase, so no separation occurs.

Correct move:

Always draw the origin line above the initial solvent level in the chromatography chamber.

5. Quick Reference Cheatsheet

Pigment	Colour	Typical Rf (paper)	Role
Carotene	Orange	0.8-0.9	Accessory, photoprotection
Xanthophyll	Yellow	0.5-0.6	Accessory pigment
Chlorophyll a	Blue-green	0.3-0.4	Primary reaction centre, accessory
Chlorophyll b	Yellow-green	0.1-0.2	Accessory, extends absorption range

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Separation of photosynthetic pigments
2023 · 1
Absorption vs action spectrum comparison
2021 · 4
Role of accessory pigments in photosynthesis

What's Next

Understanding photosynthetic pigments is the foundation for learning the light-dependent and light-independent reactions of photosynthesis, which are core heavily tested topics for CIE A-Level Biology. Pigments drive the initial conversion of light energy to chemical energy, so their properties directly explain how photosystem structure supports electron flow and ATP synthesis. You will next explore how the light-dependent reaction uses energy absorbed by pigments to produce reduced NADP and ATP, which feed into the Calvin cycle to synthesize carbohydrates. Mastering this sub-topic also helps you explain how factors like light wavelength and intensity affect overall photosynthetic rate.