Transport across cell membranes

• Phospholipid molecules have a water soluble head

  (hydrophilic head ) and a water insoluble tail

  (hydrophobic tail)
• In the cell membrane the phospholipids

  arrange themselves as a bilayer around

  the cell.
• The membrane is made of two main parts

  phospholipids and proteins
• Proteins are dotted about

  the membrane in a

  mosaic like pattern,

  some on the surface,

  some buried while others

  go all the way through.
• The proteins in the cell membrane have a variety of functions.


  • structural support

  • provide channels allowing transport of small molecules,

  • provide antigen markers that identify the cells own tissue type
• The proteins in the cell membrane can act as 


  •carriers to pump molecules across the membrane

  (membrane proteins for active transport)

  •enzymes that catalyse reactions

  •receptor for hormones
• The cell membrane provides a barrier between the interior

  of the cell and its environment
• The cell membrane controls

  what substances can enter and

  exit a cell.
• The cell membrane is

  selectively permeable.
• very small molecules,

  such as water, oxygen

  and carbon dioxide can

  pass directly through

  the phospholipids in the

  cell membrane
• Larger molecules, such

  as glucose require a

  specific transport

  protein to facilitate their

  movement across the cell

  membrane.
• Very large molecules

  such as proteins or

  starch are too big to

  move through the cell

  membrane which is

  said to be

  impermeable to them.
• Selectively permeable means that only small soluble

  molecules can pass across the membrane whereas

  large, insoluble molecules cannot
• Substances that can pass through the cell membrane


  Water

  Glucose

  Oxygen

  Carbon dioxide

  Amino acids
• Substances that cannot pass through the cell membrane


  Protein

  Starch

  Fats
• The cell membrane creates different conditions inside and

  outside of the cell

  This means there can be more or less molecules of a

  substance inside or outside the cell
• A concentration gradient is the difference in substance concentration between two areas, driving diffusion from high to low concentration.
• The cell membrane creates different conditions inside and

  outside of the cell

  This means there can be more or less molecules of a

  substance inside or outside the cell
• diffusion only takes place if

  the concentration of the

  substance is different on

  either side of the

  membrane ( a

  concentration gradient)
• the higher the concentration difference, the faster

  the rate of diffusion
• Passive transport is when a substance moves down a

  concentration gradient
• Passive transport does not require energy
• Diffusion is the movement of molecules from an area of

  high(er) concentration down a concentration gradient to a

  low(er) concentration.
• Diffusion is a passive process, it requires no energy

  Diffusion will continue until all the molecules are evenly

  distributed.
• Molecules move from a high concentration to a low

  concentration down a concentration gradient
• •Diffusion is important to

  cells because

  • They can gain the

  raw materials for

  respiration

  • They can remove

  harmful waste

  products
• Osmosis is the movement of water from a higher

  water concentration to a lower water concentration

  through a selectively permeable membrane
• Osmosis is a passive process, it does not require

  energy.
• Potato cylinders or model cells made with Visking tubing

  are often used to demonstrate osmosis


  They must be blotted dry before reweighing to

  remove excess water which will affect the weight
• If a cell is placed in a solution which is more dilute than

  its self (i.e. it hs a higher water concentration i.e.pure

  water) it will gain water.
• If a cell is placed in a solution which is more

  concentrated than itself ( less water), it will lose water.
• An increase in mass


  • Cell placed in a solution with a higher water concentration

  than inside the cell

  • Water moves into the cell by osmosis
• A decrease in mass


  • Cell placed in a solution with a lower water concentration

  than inside the cell

  •Water moves out of the cell by osmosis
• The greater the change in mass, the greater the

  difference in water concentration between the solution

  and the cells
• The smaller the change in mass, the closer the

  concentration of the solution to the cell
• If a cell is placed in a solution which is the same

  concentration as the cell contents water will move

  equally in both directions, so there will be no net

  gain or loss of water.
• If there is no change in mass, the concentration of the

  solution is the same as the cell
• Cell placed in more

  dilute solution

  Cell gains water

  Cell swells (becomes

  turgid) but does not

  burst due to cell wall
• Cell placed in

  equal solution

  no change
• Cell placed in more

  concentrated solution

  Cell loses water

  Cell membrane

  pulled away from

  wall, cytoplasm and

  vacuole shrinks. (cell

  is plasmolysed)