osmosis and diffusion

• The cell membrane is a phospholipid

  bilayer

  It controls what enters and exits the cell

  Pores on the surface of the cell act as

  entrances and exits
• Cell membranes are selectively

  permeable – they allow some but not

  all substances to pass through
• Examples of biological

  membranes include:

  Cell membranes

  Nuclear membranes
• Membranes can be:

  Permeable – let everything in and out

  Selectively Permeable – allows some

  substances to pass through

  Impermeable – let nothing in and out
• Diffusion is the movement of

  molecules from a region of high

  concentration to a region of low

  concentration
• diffusion a passive process i.e. no energy is

  required
• Examples of diffusion include:Absorption of food in the intestine
• Osmosis is the diffusion of water.
• Osmosis is the movement of water

  molecules from a region of high water

  concentration to a region of low water

  concentration across a semi permeable

  membrane
• Osmosis is a passive process
• Examples of osmosis include:

  Absorption of water by plant root cells
• Active Transport is the opposite of diffusion

  Substances are pumped through the cell membrane

  against the concentration gradient i.e. from an area

  where they are in low concentration to an area where

  they are in high concentration

  This is an active process and requires energy
• Example of active transport: reabsorption of amino acids and glucose

  in the nephron
• Plant cell walls are fully

  permeable

  Cell membranes are selectively

  permeable
• If a plant cell is surrounded by a less concentrated solution

  then water from outside will move into the cytoplasm and

  vacuole of plant cells

  When the outside water enters the plant cell the vacuole

  becomes bigger and the cytoplasm swells

  This causes the membrane to be pushed out towards the

  cell wall

  When cells are fully “swelled” like this with the

  membranes pushing against the cell wall they are

  described as Turgid
• Turgor is the pressure of the cytoplasm and vacuole

  against the cell wall of a plant
• Turgor pressure gives a plant cell strength
• If the cells are not fully turgid, they become flaccid and

  the plant will bend over an wilt
• Animal Cells in a Solution that

  is the Same Concentration

  If a cell’s contents are at the same concentration as its

  environment, the cell is said to be isotonic

  In this case there is no net movement of water into

  or out of the cell

  Water enters and leaves the cell at the same rate
• Plant Cells in a More Concentrated Solution

  If plant cells are surrounded by a more concentrated solution (for example if plant

  cells were surrounded by salt water) the water inside the cell would move out to the

  more concentrated solution outside

  When this happens the cell wall stays intact but the membrane shrivels up away

  from it

  This is called Plasmolysis

  Cells in this condition are called plasmolysed
• Animal Cells in a Less

  Concentrated Solution

  Animal cells that are in a less concentrated solution (

  hypotonic) have water entering the cell constantly

  The water from the outside will move into the cell

  and may eventually cause it to burst or die

  Animal cells do not have a cell wall to prevent the

  cell from bursting like a plant cell
• Animal Cells in a More

  Concentrated Solution

  If animal cells are in a solution that is

  more concentrated than their cytoplasm

  (hypertonic) then the water may leave the

  animal cell and the cells shrivel up and

  may die

  This condition is called crenation
• Bacteria and Fungi are the most common

  causes of food spoilage
• If a food is placed in a sugary or salty

  solution then any bacteria or fungi present

  will lose the water in their cells to the

  more concentrated solution outside
• If the bacteria and fungal cells loose water the cells will shrivel

  and die and the food will not become contaminated with loads

  of bacteria and fungi
• This prevents the food from spoiling

  Examples include:

  Fish and Meat (bacon) may be stored in a salty solution

  Jams, marmalades and tinned fruits are stored in a sugary

  solution
• To demonstrate osmosis, soften the lengths of visking tubing by soaking them in water.
• Knot one end of each piece of tubing.
• Half fill one piece of visking tubing with sucrose solution.
• Control: half fill the other piece of visking tubing with distilled water.
• Eliminate as much air as possible from the open end of the visking tubing.
• Knot the open end of the visking tubing.
• Wash and pat dry the visking tubing.
• Tie the two ends of the visking tubing together.
• Observe the turgidity and record the mass of each piece of tubing.
• Hang each tube from a glass rod into a beaker of distilled water.
• Leave the tubing hanging for 15 minutes.
• Remove the tubing, dry and re-weigh it.
• Observe the turgidity of the tubing.