chapter 4 - transport across cell membranes

• What is the primary function of the plasma membrane?

  It controls the movement of molecules across the membrane.
• What are the 5 main components of the plasma membrane?

  Phospholipids, proteins, cholesterol, glycoproteins and glycolipids
• What is the structure of phospholipids in the plasma membrane?

  • Hydrophilic heads face outward, attracted by water.
  • Hydrophobic tails point inward, repelled by water.
• What are the functions that phospholipids play in the membrane?

  Allow lipid-soluble substances to enter and leave the cell

  Prevent water-soluble substances entering and leaving the cell

  Make the membrane flexible and self-sealing
• What is one function of proteins in the plasma membrane?

  They provide mechanical support to the cell.
• How do protein channels function in the plasma membrane?

  they form water-filled tubes to allow water-soluble ions to diffuse across the membrane
• What is the role of cholesterol in the plasma membrane?

  it adds strength and is hydrophobic which prevents loss of water & dissolved ions.

  they pull together the tails, limiting movement
• What are the functions of the plasma membrane components?

  • Phospholipids: allow lipid-soluble substances to enter and leave the cell, prevents water-soluble substances entering and leaving and makes the membrane flexible & self-sealing.
  • Proteins: mechanical support, allow active transport across membrane.
  • Cholesterol: maintains fluidity and stability, reduces lateral movement & prevents leakage of water & dissolved ions.
• How do the hydrophobic and hydrophilic properties of phospholipids contribute to membrane structure?

  The hydrophobic tails face inward, creating a barrier, while the hydrophilic heads face outward, interacting with water.
• What is the significance of the plasma membrane's selective permeability?

  It allows the cell to control which substances enter and exit the cell
• What role do cholesterol molecules play in the cell-surface membrane?


  They add strength to the membranes and are very hydrophobic to prevent the loss of water and dissolved ions
• Where do cholesterol molecules occur in the cell membrane?

  Within the phospholipid bilayer of the cell-surface membrane.
• How do cholesterol molecules contribute to the strength of cell membranes?

  They add strength to the membranes.
• Why are cholesterol molecules important for preventing water loss from the cell?

  They are very hydrophobic.
• What effect do cholesterol molecules have on the fatty acid tails of phospholipid molecules?

  They pull together the fatty acid tails, limiting their movement.
• How do cholesterol molecules affect the rigidity of the membrane?

  They limit movement without making the membrane too rigid.
• What are the functions of cholesterol in the membrane?

  • Reduce lateral movement of other molecules including phospholipids
  • Make the membrane less fluid at high temperatures
  • Prevent leakage of water and dissolved ions from the cell
• What is one function of cholesterol in the membrane related to fluidity?

  It makes the membrane less fluid at high temperatures.
• How does cholesterol help in preventing leakage from the cell?

  It prevents leakage of water and dissolved ions from the cell.
• What are glycolipids made up of?

  Carbohydrate covalently bonded to a lipid
• Where does the carbohydrate portion extend from?

  From the phospholipid bilayer into the watery environment outside of the cell
• What are glycoproteins made up of?


  carbohydrate chains attached to many extrinsic proteins on the outer surface of the membrane
• Glycolipids:

  • act as recognition sites
  • maintain the stability of the membrane
  • helps cells to attach to one another and so form tissues

  Glycoproteins:

  • act as recognition sites
  • helps cells to attach to one another and so form tissues
  • allow cells to recognise one another
• why can't most molecules freely diffuse across the cell-surface membrane?

  • too large to pass through the channels
  • are the same charge as the charge on the protein channels and so are repelled
  • not soluble in lipids and so cannot pass through the phospholipid layer
  • they are electrically charged and so have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
• what is the arrangement of the molecules in the cell-surface membrane known as?

  fluid-mosaic model
• why is it called the fluid-mosaic model?

  • fluid - the individual phospholipid molecules can move relative to one another, which gives it a flexible structure that is constantly changing in shape
  • mosaic - proteins that are embedded in the bilayer vary in shape, size and pattern
• Facilitated diffusion occurs when substances move through protein channels or carrier proteins, allowing them to passively cross the plasma membrane without requiring energy.
• what is the definition of diffusion?


  the net movement of particles from an area of high concentration to an area of low concentration until it is evenly distributed
• what is simple diffusion?

  a passive transport in which the energy comes from the natural motion of the particles
• what is facilitated diffusion?

  a passive process in which molecules, such as charged ions and polar molecules, is facilitated by transmembrane channels and carriers that span the membrane
• what does facilitated diffusion rely on?

  • only on the kinetic energy of the diffusing molecules
  • occurs down a concentration gradient
  • occurs on the plasma membrane where there are specific proteins
• what are the two proteins involved?

  1. protein channels
  2. carrier proteins
• protein channels:

  • form water-filled channels across the membrane which allows specific water-soluble ions to pass through
  • the channels are selective, opening in the presence of a specific ion and closes if ion is not present
  • the ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it on the other side

  carrier proteins:

  • a molecule that is specific to the protein that is present, it binds with the protein which causes it to change shape in a way that the molecule is released to the inside of the membrane
  • no external energy is needed
• how do protein channels control the entry & exit of ions?

  the channels are selective, each opening in the presence of a specific ion and remains closed if the specific ion is not present
• what is the definition of osmosis?

  the diffusion of water molecules across a partially permeable membrane from an area of higher water potential to an area of lower water potential
• what are solutions?

  mixtures made up of a solute (e.g. glucose) dissolved in a solvent (e.g. water)
• what is the definition of water potential?

  the pressure exerted by water molecules on the membrane surrounding a solution
• what is water potential measured in?

  kiloPascals(kPa)
• what does pure water have a water potential of?

  0kPa
• • high water potential - higher water concentration, not a lot of solute dissolved
  • low water potential - lower water concentration, a lot of solute dissolved