OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(c) (i) the importance of photosynthetic pigments in photosynthesis
Full specification wording
Photosynthesis: (c) (i) the importance of photosynthetic pigments in photosynthesis
Photosynthetic pigments absorb light energy and pass it to the reaction centre, so they are essential for the light-dependent stage.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 3 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Photosynthetic pigments absorb light energy and pass it to the reaction centre, so they are essential for the light-dependent stage. Chlorophyll a is the primary pigment at the reaction centre; accessory pigments such as chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a. This widens the range of wavelengths usable, increasing the efficiency of light absorption. Carotenoids also protect chlorophyll from damage by excess light. Because each pigment absorbs a characteristic range of wavelengths, an absorption spectrum shows which colours are captured, and an action spectrum shows the rate of photosynthesis at each wavelength; the two broadly correspond. A question may ask why several pigments are advantageous rather than one, or which pigment is at the reaction centre.
What earns the marks
- Pigments absorb light energy and transfer it to the reaction centre for the light-dependent stage.
- Chlorophyll a is the primary pigment at the reaction centre.
- Accessory pigments (chlorophyll b, carotenoids) absorb wavelengths chlorophyll a absorbs poorly and pass energy to chlorophyll a.
- Having several pigments widens the range of wavelengths absorbed, increasing efficiency.
- Carotenoids help protect chlorophyll from damage by excess light.
Common mistakes and how to put them right
Claiming accessory pigments carry out the light-dependent reactions themselves.
Put it right by saying they transfer energy to chlorophyll a at the reaction centre.
Stating that all pigments absorb the same wavelengths.
Put it right by noting each pigment has a characteristic absorption range.
Confusing absorption spectrum with action spectrum.
Put it right: State that absorption shows light taken up by pigment while action shows rate of photosynthesis at each wavelength.
Exam tips
- Ask what the stem wants: the role of pigments, the identity of the primary pigment, or the advantage of several pigments.
- Reject options implying accessory pigments replace chlorophyll a at the reaction centre.
- Link pigment diversity to widened wavelength range when judging 'why' options.
Try the lesson
The start of the Photosynthesis: (c) (i) the importance of photosynthetic pigments in photosynthesis lesson, made from checked teaching notes for this specification point.
Have a go first
What is the main role of photosynthetic pigments such as chlorophyll in photosynthesis?
Listen
Photosynthetic pigments absorb light energy and pass it to the reaction centre, so they are essential for the light-dependent stage. Chlorophyll a is the primary pigment at the reaction centre; accessory pigments such as chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a. This widens the range of wavelengths usable, increasing the efficiency of light absorption. Carotenoids also protect chlorophyll from damage by excess light. Because each pigment absorbs a characteristic range of wavelengths, an absorption spectrum shows which colours are captured, and an action spectrum shows the rate of photosynthesis at each wavelength; the two broadly correspond. A question may ask why several pigments are advantageous rather than one, or which pigment is at the reaction centre. A strong answer earns marks for these points. Pigments absorb light energy and transfer it to the reaction centre for the light-dependent stage. Chlorophyll a is the primary pigment at the reaction centre. Accessory pigments (chlorophyll b, carotenoids) absorb wavelengths chlorophyll a absorbs poorly and pass energy to chlorophyll a. Having several pigments widens the range of wavelengths absorbed, increasing efficiency. A common slip: Claiming accessory pigments carry out the light-dependent reactions themselves. Instead, saying they transfer energy to chlorophyll a at the reaction centre. Exam tip: Ask what the stem wants: the role of pigments, the identity of the primary pigment, or the advantage of several pigments.
Key words
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Primary pigment
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Accessory pigments
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Widened wavelength range and efficiency
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Protective role of carotenoids
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See every definition
- Primary pigment
- Chlorophyll a is the primary pigment at the reaction centre, where light energy is passed for the light-dependent stage.
- Accessory pigments
- Chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly and transfer the energy to chlorophyll a.
- Widened wavelength range and efficiency
- Having several pigments widens the range of wavelengths usable, increasing the efficiency of light absorption.
- Protective role of carotenoids
- Carotenoids also protect chlorophyll from damage by excess light.
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