Photosynthesis- Photosystems and their Structures + Absorption and Action Spectrum

Question 1

What is a photosystem and where is it found?

Answer 1

A photosystem is a complex containing protein and pigments found in the thylakoid membranes of chloroplasts.

Question 2

What are the types of photosystems and what do they consist of?

Answer 2

There are two types of photosystems,
photosystem I or PSI and
photosystem II or PSII.
Each photosystem consists of an
antenna complex or light harvesting complex containing accessory pigments (which can capture light energy of different wavelengths) and
a reaction centre in each photosystem (where there is a particular type of chlorophyll a present).

Question 3

What is the chlorophyll a of PSI and PSII called respectively and why?

Answer 3

In photosystem I the chlorophyll a is called P700 because it can absorb light of wavelength 700 nm and in photosystem II chlorophyll a is called P680 because it can absorb light of wavelength 680 nm.

Question 4

Light energy trapped by the antenna complex is passed on to what part of the complex and it is converted to what form of energy?

Answer 4

The antenna complex traps light energy of different wavelengths and passes on the energy to the reaction centre of the photosystem where that energy is converted into chemical energy.

Question 5

Describe the structure of chlorophyll.

Answer 5

Chlorophyll is a photoreceptor consisting of a flat head containing Mg. The flattened head is attached to a long hydrocarbon tail which anchors it into the thylakoid membrane.

Question 6

What is the difference between chlorophyll a and B and how does this difference affect their ability?

Answer 6

Chlorophyll a and B differ from each other because of differences in the composition of R groups in their structure.

Because of these differences in structure each type of chlorophyll can absorb light energy of different wavelengths and this enables the plant to absorb light of more wavelengths than if only one of these pigments is present.

Question 7

What can also be present in photosystems?

Answer 7

Accessory pigments are also present in photosystems. These include carotenoids which are made up carotenes and xanthophylls, and anthocyanins

Question 8

What are the peak absorbances for chlorophyll a, chlorophyll B, and carotenoids?

Answer 8

Chlorophyll a- 425 nm (blue) and 675 nm (red)
Chlorophyll B- 470 nm (blue) and 650nm (red)
Carotenoids- 450 nm (blue) and 490 nm (blue/violet)
the least absorbances for all pigments occurs at 500-600 nm (green)