2.1.2 Structure of prokaryotic cells and of viruses

• What are the distinguishing features of prokaryotic cells?

  ● Cytoplasm lacking membrane-bound organelles

  ● So genetic material not enclosed in a nucleus
• Describe the structure of eukaryotic cells

  • Has membrane-bound organelles eg. mitochondria, endoplasmic reticulum
  • Has a nucleus containing DNA
  • DNA is long & linear & associated with histone proteins
  • Larger (80S) ribosomes (in cytoplasm)
  • Cell wall only in plants, algae and fungi containing cellulose or chitin
  • Plasmids / capsule never present (sometimes flagella)
  • Larger overall size
• Describe the structure of prokaryotic cells

  • No membrane-bound organelles eg. no mitochondria, endoplasmic reticulum
  • No nucleus - DNA is is free in cytoplasm
  • DNA is short & circular & not associated with proteins
  • Smaller (70S) ribosomes
  • Cell wall in all prokaryotic cells containing murein, a glycoprotein
  • Plasmids, flagella and a capsule sometimes present
  • Much smaller overall size
• Explain why viruses are described as acellular and non-living

  • ● Acellular - not made of cells, no cell membrane / cytoplasm / organelles
  • ● Non-living - have no metabolism, cannot independently move / respire / replicate / excrete
• Describe the general structure of a virus particle

  1. Nucleic acids surrounded by a capsid (protein coat)
  2. Attachment proteins allow attachment to specific host cells
  3. No cytoplasm, ribosomes, cell wall, cell-surface membrane etc.
  4. Some also surrounded by a lipid envelope eg. HIV