chloroplasts and photosynthetic pigments

Define photosynthesis

The production of carbon compounds in cells using light energy

Describe the structure of a chloroplast

• thylakoids

• Thylakoid space/lumen

• Granum

• Stroma

• Starch grains

• Outer and inner membrane

• 70S ribosome

• Circular DNA

How is the thylakoid adapted for its function

• large surface area for light absorbing photosystems

• Membrane provides site for electron transport chain and chemiosmosis

How is the thylakoid space/lumen adapted for its function

Very small volume so a steep proton gradient builds very fast (for accumulation of protons)

How is the grana adapted for its function

Stack of thylakoid membranes to maximise light absorption

How is the stroma adapted for its function

Contains enzymes used in Calvin cycle/light independent reaction

What is the function of the double membrane of a chloroplast

Allows separation from the rest of the cell

How is the presence of DNA and ribosomes useful in the chloroplasts

For protein synthesis for production of enzymes (eg RUBISCO)

What is the function of the starch grains in chloroplasts

To store carbohydrates produces in photosynthesis

What is a pigment

A substance that absorbs light

What is chlorophyll and how does it aid photosynthesis

• pigment responsible for absorbing light

• Releases electrons used to produce ATP

What photosynthetic pigments are found in the chloroplast (4)

• Chlorophyll a (main pigment)

• Chlorophyll b (accessory pigment)

• Carotenoids (accessory pigment)

• Xanthophylls (accessory pigment)

Where are the photosynthetic pigments found in the chloroplast

In photosystems

Where in a photosystem are accessory pigments found in the chloroplasts

In photosystems

What is the role of pigments in the light harvesting system

• absorb light energy from a range of different wavelengths

• Transfer energy to reaction centre (containing chlorophyll a)

What is the role of the reaction centre

• releases high energy electrons

• Used in light dependant stage of photosynthesis

Outline the role of pigments in the process of photosynthesis

• pigments in photosystems absorb light energy

• Light energy excites electrons

• Energy passed from pigment to pigments

• Energy reaches reaction centre where chlorophyll a is found

• Accessory pigments allow for wider range of wavelengths to be absorbed

Compare chlorophyll’s absorption in the different parts of the visible spectrum

• absorbs blue light most strongly, also absorbs red light

• Reflects green light

Describe what an absorptionspectrum shows

Wavelengths of light absorbed by each pigment

Describe the shape of a photosynthesis action spectrum

• graph shows overall rate of photosynthesis at each wavelength of light

• Maximum at blue light, then red light

• Lowest at green light

Define action spectrum

Graph showing overall rate of photosynthesis at each wavelength of light

Distinguish between absorption spectrum and action spectrum

• absorption spectrum - wavelengths of light absorbed by each

• Action spectrum - overall rate of photosynthesis at each wavelength

How can mixtures of photosynthetic pigments be separated

Chromatography

Describe the process of chromatography to separate coloured pigments

1. Place Spot of sample on pencil line on chromatography paper/plate

2. Repeat to concentrate extract

3. Suspend paper in suitable solvent (Solvent below base line)

4. Let solvent move up paper, remove before solvent reaches top

5. Mark solvent front

6. Calculate Rf value

7. Compare with data book value for Rf

Give a suitable solvent for extracting photosynthetic pigments from plant tissues

• propanone

• Alcohol

• Ether