Transport across membrane

• Cell membrane


  • Acts as a protective barrier of the cell from its environment
  • Determines which materials can move in and out of the cell
• How the cell membrane works

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• Cell membrane components

  • Phospholipid bilayer
  • Proteins
  • Carbohydrates
• Phospholipid bilayer

  Hydrophilic heads face watery surroundings, hydrophobic tails avoid water
• Cell membrane

  Fluid mosaic of phospholipids, proteins and carbohydrates
• Passive transport

  Transport that does not require energy or ATP, driven by concentration gradient
• Types of passive transport

  • Diffusion
  • Osmosis
  • Facilitated diffusion
• Diffusion

  Movement of molecules from high to low concentration
• Osmosis

  Diffusion of water molecules across a membrane
• Isotonic, hypotonic, hypertonic solutions

  Isotonic - equal solute concentration

  Hypotonic - lower solute concentration

  Hypertonic - higher solute concentration
• Hypertonic solution

  Water moves out of cell, cell shrinks and dies
• Hypotonic solution

  Water moves into cell, cell swells and bursts
• Facilitated diffusion

  Transport of large/polar molecules through membrane proteins
• Active transport

  Transport that requires energy to move substances against concentration gradient
• Active transport

  • Sodium-potassium pump
  • Endocytosis
  • Exocytosis
• Endocytosis is "cellular eating" or "cellular drinking"
• Exocytosis is the opposite of endocytosis, releasing contents outside the cell
• Many endocrine cells use exocytosis to release hormones into the bloodstream