gen bio (structure of cell membrane)

• Cells maintain an appropriate amount of all molecules within them to function effectively
• Plasma membrane


  Acts as a semi-permeable membrane allowing the entry and exit of certain materials
• Cell membrane discovered by Swiss botanist Carl Naegeli Carl Eduard Cramer

• Despite the existence of microscopes from 1600, no-one thought that the cell membrane existed because all they could see was the cell wall
• Naegeli and Cramer noted that the surface of the cell was not continuous and that it was impermeable to pigments added to the solution around the cell
• Naegeli and Cramer found that the photoplasmic surface was more dense and viscous when compared to the cytoplasm
• Plasma membrane


  The surface that Naegeli and Cramer called the plasma membrane
• Ernst Overton obtained the first insight into the chemical nature of the membrane


  1890s
• Overton realised that the substances entering the cell had to be dissolved in the outer boundary of cell due to lipids
• Irvin Langmuir found that the membrane was made of a monolayer of lipids arranged vertically with hydrocarbon chains away from water and carboxyl groups in contact with the surface of water


  1917
• E. Gorter and F. Grendel were the first to find that the membrane was made of two layers of lipids (lipid bilayer) with hydrophilic heads and hydrophobic tails


  1925
• Hugh Davson and James Danielle proposed that the membrane is made of lipid bilayer with a lining of globular proteins on both outer and inner surface


  1935
• Davson and Danielle found that selective permeability was because of the presence of protein lined pores within the lipid bilayer, which allowed the passage of polar solutes and ions into and out of cell


  1950
• S. Jonathan Singer and Garth Nicholson proposed the Fluid Mosaic Model which is considered as the central dogma of membrane biology


  1972
• Fluid Mosaic Model


  Describes the structure of cell membrane as a lipid bilayer with proteins embedded in it and which is free to move laterally within the membrane
• Components of the membrane

  • Membrane lipids
  • Membrane proteins
• Membrane lipids


  • Cholesterol
  • Phospholipids
  • Glycolipid
• Phospholipids


  Lipids that form the bilayer of plasma membrane, with hydrophobic tails facing each other and hydrophilic heads facing the cytoplasm and extracellular fluid
• Cholesterol


  Helps to 'pack' the phospholipids in the membrane, determines the permeability of membrane to water soluble constituents of body fluids
• Glycolipids


  Lipids with a carbohydrate attached, provide energy and act as markers for cellular recognition
• Membrane proteins


  • Integral membrane proteins
  • Peripheral proteins
  • Glycoproteins
• Integral membrane proteins


  Proteins embedded within the lipid bilayer that act as channels through which substances diffuse between extracellular and intracellular fluid
• Peripheral proteins


  Attached to the inner surface of the membrane and do not penetrate the lipid bilayer, function as enzymes and receptors
• Glycoproteins


  Embedded in the cell membrane, formed by attachment of carbohydrate molecules to proteins, help in cell-cell communication and molecule transport across the membrane
• Membrane transport


  The movement of particles across or through the phospholipid bilayer of the plasma membrane to maintain cellular life
• Factors affecting membrane transport

  • Permeability of the membrane
  • Transmembrane solute concentration
  • Size of the solute
  • Charge of the solute
• Chemicals that can pass through the membrane

  • Water
  • Carbon dioxide
  • Oxygen
  • Small polar molecules such as ammonia
  • Lipids such as cholesterol
• Chemicals that cannot pass through the membrane

  • All ions including hydrogen ions
  • Large polar molecules like glucose
  • Amino acids
  • Macromolecules such as proteins, polysaccharides
• Passive transport


  Transport of substances down the gradient across the membrane without the expenditure of cell energy
• Types of passive transport

  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
• Simple diffusion


  The net movement of solutes from an area of higher concentration to an area of lower concentration until the concentration of solute reaches the state of equilibrium
• Small uncharged polar molecules like water, urea, ethanol can also diffuse through the lipid bilayer</b>
• Factors affecting diffusion across the cell membrane

  • Size of solute
  • Solute polarity
  • Temperature
  • Lipid solubility
• Facilitated diffusion


  Movement of molecules across the cell membrane with the help of special transport proteins embedded within the phospholipid bilayer
• Osmosis


  The movement of solvent molecules (usually water) across a selectively permeable membrane from an area of higher concentration to an area of lower concentration
• Isotonic solution


  A solution in which the cell maintains its normal shape and volume, where solute concentration inside and outside the cell is the same
• Hypotonic solution


  A solution with lower solute concentration than inside the cell, causing the cell to swell and burst as it causes movement of water to inside of cell
• 
  

  A solution with higher solute concentration than inside the cell, causing osmosis of water from inside of cell to outside leading to shrinkage of cell
• Active transport


  The transport of molecules across the cell membrane from an area of lower concentration to an area of higher concentration using cellular energy
• Forms of active transport

  • Primary active transport
  • Secondary active transport