Transport Across the Membrane

• Cell Membrane


  Fluid mosaic
• Cell Membrane


  • The fluidity of the cell membrane is needed to control the movement of substances into and out of the cell
• Structure of the cell membrane

  • Phospholipid bilayer
  • Proteins
  • Carbohydrates
• Phospholipid bilayer


  Responsible for the semi-permeability of the cell membrane
• Phospholipid bilayer


  • Hydrophilic head - "water-loving", faces the watery surroundings (inside and outside) of the cell
  • Hydrophobic tails - "water-hating", automatically avoid watery surroundings
• Proteins


  Help in cellular communication and transport across membranes
• Carbohydrates


  Help in cell recognition, the ability to distinguish one cell from another
• Types of transport across the membrane

  • Passive Transport
  • Active Transport
• Passive Transport


  Does not require the use of energy or ATP, driven by the difference in concentration gradient in the cell, occurs when substances move from an area of high concentration to an area of low concentration
• Passive Transport


  • Diffusion
  • Osmosis
  • Facilitated diffusion
• Diffusion


  Movement of molecules from high to low concentration, non-polar molecules like oxygen and carbon dioxide pass through the hydrophobic tails of the double layer, small polar molecules like water can cross the cell membrane directly
• Osmosis


  Focuses on the movement of water molecules across the membrane, the membrane acts like a net, allowing water to pass through but keep solutes trapped in the cell
• Osmotic conditions


  • Isotonic
  • Hypertonic
  • Hypotonic
• Isotonic


  A cell is in its natural environment, concentration of solute of one solution to another solution is the same
• Hypertonic


  The solution outside the cell has higher solute concentration, water will move out of the cell to dissolve the extra solute
• Hypotonic


  The solution outside the cell has lower solute concentration than that of the cell, water (from the surrounding) will move into the cell, causing the cell to swell and burst
• Facilitated diffusion


  Molecules that cannot pass freely across the phospholipid bilayer rely on special gates called transport proteins, movement happens due to concentration gradient
• Active transport


  Energy is needed to transport materials across the membrane because the movement is against the concentration gradient, substances will move from an area of low to high concentration
• Endocytosis


  When the membrane folds inward to enclose material in a vesicle, used by white blood cells to engulf bacteria
• Exocytosis


  A vesicle fuses its membrane with the cell membrane and dumps its contents outside the cell, many endocrine cells use exocytosis to release hormones into the bloodstream