Transport across cell membranes

• Molecules that form cell-surface membrane structure

  • Phospholipids
  • Proteins
  • Cholesterol
  • Glycolipids
  • Glycoproteins
• Phospholipids

  • Hydrophilic heads of both phospholipid layers point outwards
  • Hydrophobic tails point inwards
• Proteins


  • Some occur on the surface of the bilayer, either give mechanical support to membrane, or act as cell receptors for molecules such as hormones
  • Others span the bilayer from one side to the other
• Cholesterol

  • Hydrophobic, pull together fatty acid tails of phospholipid molecules
• Glycolipids


  • Made up of a carbohydrate covalently bonded with a lipid. Extends from phospholipid bilayer into the watery environment outside of cell - acts as a cell surface receptor for specific chemicals
• Glycoproteins


  • Carbohydrate chains attached to extrinsic proteins. Also acts as cell surface receptors, for hormones and neurotransmitters
• Most molecules do not freely diffuse across the membrane because many of them are: Not soluble in lipids, Too large to pass through the protein channels, Of the same charge as the protein channels, and so are repelled, Electrically charged (polar) - difficult to pass through non-polar hydrophobic tails in phospholipid bilayer
• Fluid-mosaic model

  The arrangement of the various molecules in the cell surface membrane is referred to as the fluid-mosaic model because: Fluid - individual phospholipid molecules can move relative to one another. Membrane is a flexible structure constantly changing shape. Mosaic - due to the way the proteins are embedded in the bilayer and how they vary in size and shape
• Simple diffusion

  The net movement of molecules or ions form a region where they are more highly concentrated to one where their concentration is lower until they are evenly distributed
• Facilitated diffusion

  Charged ions and polar molecules do not diffuse easily - their movement is facilitated by transmembrane channels and carriers. Is a passive process - relies on kinetic energy of diffusing molecules
• Protein channels

  • Form water-filled hydrophilic channels across the membrane - allow specific water-soluble ions to pass through
  • Have selective control over the entry and exit of ions
  • Ions bind with the protein causing it to change shape in a way that opens it to one side of the membrane and closes it to the other side
• Carrier proteins

  • When a molecule that is specific to the protein is present, it binds with the protein
  • Protein changes shape to release the molecule inside the membrane
• Osmosis

  The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
• Solute


  Any substance that is dissolved in a solvent
• Water potential

  • Measured in units of pressure, kiloPascals (kPa)
  • Refers to the pressure created by water molecules
  • Pure water potential has a kPa of 0
  • The addition of a solute to pure water will lower its WP
  • The WP of a solution (water + solute) must always be a negative number
  • The more solute added, the more negative the WP
• Active transport

  The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
• Active transport

  • Metabolic energy in the form of ATP is needed
  • Substances are moved against a concentration gradient
  • Carrier proteins act as 'pumps'
  • Selective process
• Active transport

  1. Carrier proteins bind to molecules or ions that are being transported on one side of the plasma membrane

  2. Molecules or ions bind to receptor sites on the carrier protein

  3. On the inside of the cell - ATP binds to the protein, causing it to split into ADP and Pi = protein molecule changes shape and opens to other side of the plasma membrane

  4. Molecules or ions are released

  5. Pi released from protein, causes the protein to revert to its original shape
• Epithelial cells lining the ileum have microvilli - these provide greater surface area for carrier proteins. Increasing density of protein channels and carrier proteins can increase transport across membranes
• Greater concentration of glucose and amino acids within the ileum (compared to the blood)


  Creates a concentration gradient down which glucose moves from ileum to blood by facilitated diffusion
• Glucose and amino acids that remain after facilitated diffusion are absorbed by active transport - co-transported by the sodium-potassium pump


  1. Sodium ions are actively transported out of the epithelial cells, by the SP pump, into the blood. Takes place in a protein-carrier molecule

  2. Maintains a higher concentration of sodium ions in the lumen of the intestine

  3. Sodium ions diffuse into the epithelial cells down the concentration gradient, as they do so, they carry amino acid or glucose molecules with them

  4. Glucose and amino acids pass into blood by facilitated diffusion