Prokaryotic Cell Structure: Inclusion Bodies and Organelles Used for Photosynthesis

II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL STRUCTURE

3f. Inclusion Bodies and Organelles Used for Photosynthesis

The overall purpose of this Learning Object is:
1) to briefly introduce the organelles used for photosynthesis in photosynthetic bacteria; and
2) to briefly introduce the chemical makeup and functions of variouis bacterial inclusion bodies.

LEARNING OBJECTIVES FOR THIS SECTION

In this section on Prokaryotic Cell Structure we are looking at the various organelles or structures that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane;
a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and
often various external structures such as a glycocalyx, flagella, and pili.

Structures located within the cytoplasm of bacteria include the nucleoid, ribosomes, and in some bacteria, plasmids, endospores, inclusion bodies, and organelles used for photosynthesis. We will now look at inclusion bodies and organelles used for photosynthesis.

A. Organelles Used in Bacterial Photosynthesis

There are three major groups of photosynthetic bacteria: cyanobacteria, purple bacteria, and green bacteria.

The cyanobacteria carry out oxygenic photosynthesis, that is, they use water as an electron donor and generate oxygen during photosynthesis.The photosynthetic system is located in an extensive thylakoid membrane system that is lined with particles called phycobilisomes.

Photograph of the cyanobacteria Anabaena.

Photograph of the cyanobacteria Oscillatoria.

The green bacteria carry out anoxygenic photosynthesis. They use reduced molecules such as H₂, H₂S, S, and organic molecules as an electron source and generate NADH and NADPH. The photosynthetic system is located in ellipoidal vesicles called chlorosomes that are independent of the cytoplasmic membrane.

The purple bacteria carry out anoxygenic photosynthesis. They use reduced molecules such as H₂, H₂S, S, and organic molecules as an electron source and generate NADH and NADPH. The photosynthetic system is located in spherical or lamellar membrane systems that are continuous with the cytoplasmic membrane.

B. Other Inclusion Bodies

Cyanobacteria contain large inclusion bodies called cyanophycin granules that store nitrogen for the bacteria.

Cyanobacteria, thiobacilli, and nitrifying bacteria, organisms that reduce CO₂ in order to produce carbohydrates, possess carboxysomes containing an enzyme used for CO₂ fixation.

Purple and green photosynthetic bacteria as well as some other aquatic bacteria contain gas vacuoles. These are aggregates of hollow protein cylinders called gas vesicles that are permeable to atmospheric gas, enabling the organism to regulate buoyancy.

Some bacteria produce inorganic inclusion bodies in their cytoplasm, including volutin granules that store phosphate, and sulfur granules that store sulfur.

Some bacteria produce organic inclusion bodies containing either polyhydroxybutyrate granules or glycogen granules as an energy reserve.

Some motile aquatic bacteria are able to orient themselves by responding to the magnetic fields of the earth because they possess magnetosomes, membrane-bound crystals of magnetite or other iron-containing substances that function as tiny magnets.