2.1.2 structure of prokaryotic cells and of viruses

• features of prokaryotic cells and of viruses

  cytoplasm lacking membrane-bound organelles

  so genetic material not enclosed in a nucleus
• general structure of prokaryotic cells

  cell-surface membrane

  cell wall -> contains murein and a glycoprotein

  cytoplasm

  small ribosomes

  circular DNA -> free in cytoplasm and not associated with proteins

  capsule (sometimes present)

  plasmids -> small rings of DNA (sometimes present)

  flagella (sometimes present)
• compare the structure of eukaryotic and prokaryotic (1)

  eukaryotic has membrane-bound organelles whereas prokaryotic has no membrane-bound organelles

  eukaryotic has a nucleus containing DNA whereas prokaryotic has no nucleus DNA is free in cytoplasm

  eukaryotic DNA is long & linear & associated with histone proteins whereas prokaryotic DNA is short & circular & not associated with proteins

  eukaryotic has larger (80S) ribosomes (in cytoplasm) whereas prokaryotic has smaller (70S) ribosomes
• compare the structure of eukaryotic and prokaryotic cells (2)

  eukaryotic has cell wall only in plants, algae and fungi containing cellulose or chitin whereas prokaryotic has cell wall in all prokaryotic cells containing murein and a glycoprotein

  eukaryotic has plasmids / capsule never present (sometimes flagella) whereas prokaryotic has plasmids, flagella and a capsule sometimes present

  eukaryotic has a larger overall size whereas prokaryotic has a 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
• general structure of a virus particle

  nucleic acids surrounded by a capsid (protein coat)

  2 attachment proteins allow attachment to specific host cells

  no cytoplasm, ribosomes, cell wall, cell-surface membrane etc

  some also surrounded by a lipid envelope eg HIV