Cellular organisaton

• Eukaryote cells


  Are compartmentalised
• Cellular compartments


  • endoplasmic reticulum (ER)
  • membrane-bound ribosomes
  • free ribosomes
  • microtubules
  • dense core vesicles
  • clathrin-negative vesicles
  • clathrin-positive compartments and vesicles
  • mitochondria
• Compartments are separated by membranes
• Functions of membranes


  • Transport
  • Enzyme activity
  • Cell communication
  • Connecting cells
  • Cell recognition
• Membrane structure


  Membranes are sheet-like structures, 2 molecules thick, form closed boundaries (compartments), most membranes 5 – 10 nm thick
• Membrane composition


  Membranes are composed of lipids and proteins
• Lipid molecules are much smaller than proteins so approximately 50 lipid molecules per protein in a plasma membrane
• Membrane proteins


  Vary in structure, association with the membrane and function
• Enveloped viruses


  Such as HIV and influenza are enclosed by membranes, usually derived from the host cell
• Bacteria possess membranes that have a different composition to animal cell membranes
• Phospholipids


  Are phosphate-containing lipids and the most abundant membrane lipid
• Types of phospholipids


  • Phosphoglycerides
  • Sphingolipids
• Lots of variation in lipids due to combinations of head groups, hydrocarbon chain lengths and saturation states
• The plasma membrane is asymmetrical
• Asymmetrical components of the plasma membrane

  • Phosphatidylcholine
  • Sphingomyelin
  • Glycolipid
  • Phosphatidylethanolamine
  • Phosphatidylserine
• Membranes are fluid


  Movement: Lateral movement occurs ~10^7 times per second; Flip-flop movement occurs <1 per month (uncatalysed)
• Fluidity depends on


  Non-covalent bonds, membrane composition and temperature
• The plasma membrane is able to regulate the cell’s molecular traffic
• Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
• Polar molecules, such as sugars, do not cross the membrane easily
• Types of transport across membranes

  • Passive transport
  • Active transport
  • ATP
  • Diffusion
  • Facilitated diffusion
  • Channel protein
  • Carrier protein
• A typical animal cell devotes almost 1/3 of its energy to running the Na+/K+ pump
• Solutes trapped within the cell by the plasma membrane


  • Proteins
  • Sugars
  • Amino acids
  • Nucleotides
  • Ions
• Water will move into the cell by osmosis
• If unregulated, the cell will burst
• Sodium potassium pump


  Cells control intracellular osmolarity by actively pumping out inorganic ions such as Na+ across the plasma membrane
• This compensates for their excess of organic solutes
• Receptor mediated endocytosis


  Clathrin molecules assemble at the membrane and cause the membrane to bend inwards
• Eventually the budding vesicle is snipped off from the membrane, at which point clathrin is removed, revealing an endosome
• Exocytosis


  Secreted proteins are transported to the plasma membrane in vesicles, which then fuse with the plasma membrane releasing their contents outside the cell
• Secreted proteins may be soluble inside the vesicle or embedded in the vesicle membrane
• Secreted proteins are synthesised in the rough ER (signal peptide) and packaged by the Golgi body
• Cells are compartmentalised and busy
• Signal sequences


  • Glycosylation
  • Phosphorylation
  • Ubiquitination
• Signal sequences


  Instructions for where a protein belongs in a cell
• How proteins move between compartments

  • Gated transport
  • Transmembrane transport
  • Vesicular transport
• Transport mechanisms for proteins

  • Between compartments
  • Within compartments
• Nuclear envelope


  Composed of two concentric membranes which are perforated by nuclear pores
• Proteins are imported into the nucleus and mRNA exported through the nuclear pore complexes
• A typical mammalian cell contains 3000 – 4000 nuclear pore complexes (NPCs)