Topic 5.6 Photosynthetic pigments

Question 1

What is an absorption spectrum?

Answer 1

A graph that shows the percentage of each wavelength of light that a pigment absorbs

Question 2

What is an action spectrum?

Answer 2

A graph that shows the overall rate of photosynthesis at each wavelength of light. Strongly corresponding to absorbtion spectrum of chlorophyll a (the most abundent pigment)

Question 3

Name the two main groups of photosynthetic pigment

Answer 3

• Chlorophyll (made of chlolophyll a and b)
• Carotonoids (carotene and xanthophylls)

Question 4

Where are photosyntehtic pigments found?

Answer 4

Embedded in the thylakoid membrane within chloroplasts

Question 5

Explain the role of chlorophyll a

Answer 5

Primary photosynthetic pigment.

Mainly absorbs wavelengths in violet-blue (430 nm) and orange- red (662 nm) parts of spectrum.

Emits electrons after absorbing photons of light.

Question 6

Explain the role of chlorophyll b

Answer 6

Accessory pigment with absorption peaks 453 nm and 642 nm

Question 7

Explain the role of carotenoids

Answer 7

Act as an antioxident to prevent damage to other pigments via non-photochemical quenching of excess photons.

Mainly absorb wavelengths in the blue-green part of the spectrum.

Question 8

Why do so many plants have a variety of photosynthetic pigments?

Answer 8

To widen the range of wavelengths of light they can absorb to ensure maximum rate of photosynthesis. Particularly important for plants in shaded conditions.

Question 9

State the purpose and principle of paper chromatography

Answer 9

Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper).

Question 10

Outline a method for extracting photosynthetic pigments

Answer 10

Use a pestle and mortar to grind leaf with an extraction solvent e.g. propanone

Question 11

Outline how paper chromatography can be used to separate photosynthetic pigments

Answer 11

1. Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin) 1cm above bottom of paper
2. Place chromatography paper in solvent. (origin should be above solvent level)
3. Allow solvent to run until it almost touches the other end of the paper. Pigments move different distances

Question 12

What are R_f values? How can they be calculated?

Answer 12

Ratios that allow comparison of how far molecules have moved in chromatograms.

R_f value = distance moved between origin and centre of pigment spot/ distance between origin and solvent front.