Structure of Prokaryotic Cells (AQA A Level Biology)

• Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of prokaryote
• Prokaryotic cells are much smaller than eukaryotic cells (between 100 - 1000 times smaller)
• They also differ from eukaryotic cells in having:
  • A cytoplasm that lacks membrane-bound organelles
  • Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S)
  • No nucleus (instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins)
  • A cell wall that contains murein (a glycoprotein)

• In addition, many prokaryotic cells have a few other structures that differentiate the species from others and act as a selective advantage, examples of these are:
  • Plasmids
  • Capsules
  • Flagellum

• Plasmids are small loops of DNA that are separate from the main circular DNA molecule
  • Plasmids contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance)

• Some prokaryotes (e.g. bacteria) are surrounded by a final outer layer known as a capsule. This is sometimes called the slime capsule
  • It helps to protect bacteria from drying out and from attack by cells of the immune system of the host organism

• Flagellum (plural = flagella) are long, tail-like structure that rotate, enabling the prokaryote to move (a bit like a propeller)
  • Some prokaryotes have more than one

Additional structures unique to prokaryotic cells

Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are believed to have emerged as the first living organisms on Earth