Cell bio

• Plasma membrane: Structure. Functions I: Transmembrane transport. Endocytosis. Exocytosis.
• Eukaryotic cell


  Animals, plants, fungi, protists, algae
• Plasma membrane


  • Cytoplasm
  • Membrane-enclosed organelles
  • Larger and numerous ribosomes
  • Cytoskeleton (MF, MT, IF)
• Nucleus


  • Membrane bounded
  • Several, rod-shaped chromosomes: double stranded linear DNA + proteins
  • Nucleolus
• Transcription


  In nucleus
• Translation


  In cytoplasm
• It seems highly likely that eukaryotes originate from prokaryotes, the eukaryotic cell organelles may have evolved through a combination of two processes corresponding to the endosymbiotic theory.
• When the Earth's atmosphere first became rich in oxygen


  ~1.5 billion years ago
• Eukaryote


  With membrane bound nucleus and organelles
• Membranes in the eukaryotic cell

  • Plasma membrane (plasmalemma), defines the boundary of the cell
  • Internal membranes - that enclose individual organelles (cellular compartments) to house specific functions
• Every cell on Earth (prokaryotic or eukaryotic) uses a membrane (plasma membrane) to separate and protect its chemical components from the outside environment.
• Eukaryotic cell plasma membrane and internal cell membranes are constructed on the same principles.
• All cell membranes act as selective barriers.
• The nucleus and mitochondria are each enclosed by two membranes.
• Plasma membrane


  Membrane Structure
• The plasma membrane is so thin that it cannot be seen in the light microscope.
• When properly fixed, sectioned, and stained, with TEM all membranes have a trilaminar appearance: two electron-dense layers separated by an intermediate, electron-lucent layer.
• The total thickness of the plasma membrane is about 8 to 10 nm.
• Internal cell membranes look similar as the plasma membrane.
• Plasma membrane


  • Lipid bilayer containing embedded integral and peripheral membrane proteins
  • Carbohydrates attached to lipids (glycolipids) and to proteins (glycoproteins) extend from the outward-facing surface of the membrane
• Lipid bilayer


  Serves as a permeability barrier to most water-soluble molecules
• Proteins


  Carry out the other functions of the membrane and give different membranes their individual characteristics
• Fluid-mosaic model


  • Fluid – the phospholipid bilayer is viscous and individual phospholipids (and proteins) can move position
  • Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
• Components of the plasma membrane

  • Lipids (phospholipids, cholesterol, glycolipids)
  • Proteins (peripheral and integral proteins)
  • Carbohydrates (glycoproteins, glycolipids)
• Most plasma membranes: 40-60% lipids and 40-50% proteins by weight, with 2-3% of carbohydrates.
• The proportions of the components in the membrane vary with cell type/ organelle type.
• Membrane lipids


  • Phospholipids ~ 50-60% (e.g. glycerophospholipids, sphingolipids)
  • Cholesterol ~ 10-20%
  • Glycolipids (e.g., cerebrosides, gangliosides)
• Cholesterol is absent from the membrane of prokaryotic cells and the mitochondrial inner membrane.
• Phospholipids


  Each phospholipid molecule has a hydrophilic ("water-loving") head and a hydrophobic ("water-fearing") tail
• All lipids in cell membranes are amphipathic!
• Phospholipid bilayers


  Spontaneously close in on themselves to form sealed compartments
• The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable.
• Phospholipid molecules spontaneously form lipid bilayers in an aqueous environment.
• Phospholipid bilayer


  • The fatty-acid chains of the lipid molecules face each other, making the inner portion of the membrane hydrophobic
  • The surfaces of the membrane are formed by the polar head groups of the lipid molecules, thereby making the surfaces hydrophilic
• Membrane phospholipids


  Are motile - the bilayer behaves as a two-dimensional fluid, in which the individual lipid molecules are able to move in their own monolayer
• Lipid molecules do not move spontaneously from one monolayer to the other.
• Flip-flop movements (transvers diffusion)

  Are catalyzed by enzymes such as scramblases and flippases (=lipid transporter proteins)
• These enzymes are essential for initiating and maintaining the asymmetric arrangement of phospholipids that is characteristic of the membranes of animal cells.
• Factors that influence membrane fluidity

  • Temperature
  • The nature of the phospholipids
  • Cholesterol
• Saturated fatty acids


  Have no double bonds - straight tails