Biology 12

86 MHR • Unit 1 Metabolic Processes

Photosystems

Light energy is absorbed by a network of

chlorophyll molecules known as a photosystem

(see Figure 3.23). These chlorophyll molecules are

known as antenna pigmentsbecause they collect

and channel energy. This energy causes electrons

in the chlorophyll molecules to become energized.

Energy from these electrons is passed from one

chlorophyll molecule to another in the photosystem.

Eventually the energy reaches the reaction centre,

a specific chlorophyll amolecule. Only one in 250

chlorophyll molecules forms a reaction centre. A

unit of several hundred antenna pigment molecules

together with a reaction centre is called a

photosynthetic unit. The large number of antenna

pigment molecules in each photosynthetic unit

allows the reaction centre to be supplied with the

greatest possible amount of energy. Once the energy

has reached the reaction centre, an electron

acceptorreceives the energized electron. Energy

from these electrons is used to move H+ions into

the thylakoid interior for ATP production.

Figure 3.23A photosystem works by passing light energy

from one molecule of chlorophyll to another.

Cyclic Electron Pathway

There are two types of photosystems. Photosystem

700, which absorbs light 700 nm in wavelength,

is used by some photosynthetic bacteria. This

photosystem contains molecules of chlorophyll a,

which is found in cyanobacteria and all

photosynthetic eukaryotes (such as green plants).

Figure 3.24 shows how electrons pass through

photosystem 700. After the electron acceptors

receive the energized electrons from the reaction

centre, the electrons flow through an electron

transport system. Here the electrons are passed

from one electron carrier to another. Some of these

carriers are cytochrome molecules. As the electrons

pass through the system, they release energy that is

used to phosphorylate ADP molecules to produce

molecules of ATP. This process is called cyclic

photophosphorylation, because after the ATP

molecules are produced the electrons are cycled

back into the photosystem. Only ATP molecules

are produced by photosystem 700.

Figure 3.24The cyclic electron pathway. In photosystem 700,

electrons are recycled after their energy is used to form ATP

molecules.

http://www.mcgrawhill.ca/links/biology12

To find out more about how photosynthetic bacteria perform

photosynthesis, go to the web site above, and click on Web

Links. Note the types of bacteria that use photosynthesis to

produce ATP. Prepare a chart that compares and contrasts the

methods by which bacteria and plants perform photosynthesis.

WEB LINK

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