Summary of Section 3

• Prokaryotes do not have a nucleus or other internal membrane-bound organelles and their cytoplasm appears relatively homogeneous under the light microscope; their DNA appears pale, because the rest of the cell is densely packed with ribosomes, which are far more electron-dense and so appear dark.
• Most bacteria have a cell wall that lies outside the cell membrane. The cell wall of Gram-positive bacteria is composed of a thick layer of peptidoglycan, while Gram-negative bacteria have a thinner layer of peptidoglycan within the periplasmic space between the inner and outer cell membranes.
• Some bacteria possess membrane specialisations (relatively common examples are flagella and pili) and a few exhibit complex folding of the cell membrane.