4.3 Intracytoplasmic Membrane Atlas of Bacterial and Archaeal Cell Structure Home

ICM variety

In some species, like this Methyloprofundus sedimenti, the intracytoplasmic membrane is very extensive and appears to be fully separated from the cell membrane. As you can see, it occupies much of the cytoplasmic space.

Source: Ma et al. (2020) Structure: PDB 6KGX

Photocomplexes

Light-harvesting photocomplexes can comprise hundreds of individual proteins and reach tens of nanometers in size, as you can see in this complex from a red alga (the machinery is closely related to that found in cyanobacteria) [37]. Physically tethering enzymes that function in the same pathway increases efficiency by allowing enzymes that catalyze subsequent steps to hand off substrates. You will see more examples of this theme in the following pages; metabolic enzymes in the cell often cluster together into relay teams.

Intracytoplasmic Membrane

Photosynthetic bacteria harvest energy from sunlight using protein complexes (⇩) embedded in their membrane. An easy way to increase the light-gathering capability of such a cell would be to expand the membrane, but then the cell’s volume would also increase. To get around this problem, why not stack the extra membrane inside the cell? Many (but not all) photosynthetic bacteria contain intracytoplasmic membrane, or ICM. In cells like this Rhodopseudomonas palustris, the ICM is continuous with the inner membrane, forming stacks at the middle of the cell. These stacks grow and shrink as needed, depending on light conditions and the cell’s metabolic state. Different species have different, and more extensive, arrangements (⇩). ICM may remind you of the thylakoid membranes of eukaryotic chloroplasts, and for good reason; chloroplasts evolved from photosynthetic bacteria that likely lived symbiotically inside a eukaryotic precursor of modern plants.

Other photosynthetic bacteria have taken the specialization of their light-harvesting membranes a step further, expanding not the entire membrane, but simply the hydrophobic interior. The result is a sort of factory called a chlorosome with hundreds of thousands of photocomplexes packed tightly inside a lipid monolayer. Chlorosomes, and other compartments you will see soon, challenge the idea that organelles exist only in eukaryotes.

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