5.6.2: Chloroplasts and photosynthetic pigments

Question 1

What are chloroplasts and where are they found?

Answer 1

• The organelles in plants where photosynthesis takes place.
• Algae have chloroplasts but photosynthetic bacteria do not.

Question 2

Describe the structure and permeability of chloroplasts.

Answer 2

• Disc shaped
• Around 2-10 um long
• Each surrounded by a double membrane, the envelope with an intermembrane space of width 10-20um between the inner and outer membrane.
• The outer membrane is highly permeable.

Question 3

What are the two distinct regions, visible on electron micrographs, inside a chloroplast?

Answer 3

• The fluid-filled matrix called the stroma

- The grana consisting of stacks of thylakoid membranes.

Question 4

Where does the first stage of photosynthesis, the light-dependent stage take place?

Answer 4

In the grana.

Question 5

What are the three distinct membranes in chloroplasts and what are the three internal compartments that they form?

Answer 5

• Outer
• Inner
• Thylakoid
  giving. ..
• Intermembrane space
• stroma
• Thykaloid space

Question 6

How are thykaloids within a granum connected to thykaloids within another granum?

Answer 6

By intergranal lamellae (or intergranal thykaloids)

Question 7

What is a thykaloid?

Answer 7

Flattened membrane-bound sac found inside chloroplasts; containing photosynthetic pigments/photosystems and is the site of the light-dependent stage of photosynthesis.

Question 8

What are stacks of thykaloids called?

Answer 8

• A granum (may contain 100 thykaloids)

- Plural, grana

Question 9

What is the benefit of having many grana in every chloroplast and many chloroplasts in each photosynthetic cell?

Answer 9

Huge surface area for:

• The distribution of photosystems that contain the photosynthetic pigments that trap sunlight energy.
• Electron carriers and ATP synthase enzymes needed to convert light energy to ATP.

Question 10

What holds the photosystems in place?

Answer 10

Proteins embedded into the thykaloid membrane.

Question 11

What is the benefit of having the grana surrounded by the stroma?

Answer 11

The products of the light-dependent stage can easily pass to the stroma to be used in the light-independent stage.

Question 12

What is the stroma and what does it contain?

Answer 12

-The fluid filled matrix. 
Contains: 
-enzymes needed to catalyse the reactions of the light-independent stage of photosynthesis. 
-Starch grains
-Oil droplets 
-Small ribosomes 
-DNA

Question 13

What does the loop of DNA in chloroplasts contain?

Answer 13

• genes that code for some of the proteins needed for photosynthesis.
• These proteins are assembled at the chloroplast ribosomes.

Question 14

Where are photosytems found and what do they contain?

Answer 14

• Within the thykaloid membrane are funnel shaped structures called photosystems.
• They contain photosynthetic pigments.

Question 15

Why is there a need for a variety of photosynthetic pigments?

Answer 15

-Each pigment absorbs light at of a particular wavelength and reflects other wavelengths of light.

Question 16

How does a photosystem work?

Answer 16

-The energy associated with the wavelengths of light captured is funnelled down to the primary pigment reaction centre, consisting of a type of chloryphyll, at the base of the photosystem.

Question 17

What are chlorophylls?

Answer 17

• A mixture of pigments.
• All have a similar molecular structure consisting of a porphyrin group, in which there is a magnesium atom and a long hydrocarbon chain.

Question 18

There are two forms of chlorophyll a. In which ways are they the same?

Answer 18

• Both appear blue-green
• Both situated at the centre of photosystems
• Both absorb red light

Question 19

What is different about the two forms of chlorophyll a?

Answer 19

They have different absorption peaks.

Question 20

Where is P680 found and which wavelength of light is its peak of absorption?

Answer 20

• Photosystem II

- wavelength 680 nm

Question 21

Where is P700 found and which wavelength of light is its peak of absorption?

Answer 21

• Photosystem I

- wavelength 700nm

Question 22

Which wavelength of light does chlorophyll a also absorb (not 680 nm and 700 nm)?

Answer 22

-Some blue light of wavelength around 440 nm.

Question 23

Which wavelengths of light does chlorophyll b absorb?

Answer 23

400-500 nm and 640 nm.

-It appears yellow green.

Question 24

Which wavelengths of light do carotenoids absorb and reflect?

Answer 24

• Blue light if wavelengths 400-500 nm

- They reflect yellow and orange light.

Question 25

Which wavelengths of light do xanthophylls absorb and reflect?

Answer 25

• Blue and green light if wavelengths 375-550 nm

- They reflect yellow light.