3.2.3 Transport across cell membranes

• Cell surface membranes


  surround cells 
• Cell Surface Membranes are a barrier


  between the cell and its environment

  — controlling which substances enter and leave the cell
• Cell surface membranes are

  partially permeable — let some molecules through but not others
• Substances can move across the cell surface membrane by

  ● diffusion

  ● osmosis

  ● active transport
• cell - surface membrane

  the plasma membrane
• The membranes around organelles


  divide the cell into different compartments — they act as a barrier between the organelle and the cytoplasm
• Example of membranes around organelles

  ● RNA leaves the nucleus via the nuclear membrane — also called the nuclear envelope 

  ● DNA is too large to pass through the partially permeable membrane 

  — so remains in the nucleus
• Basic structure of all cell membranes

  ● lipids — mainly phospholipids 

  ● proteins

  ● carbohydrates — attached to proteins or lipids 
• In 1972 to describe the arrangement of molecules in the membrane


  the fluid mosaic model was suggested

  
• In the fluid mosaic model the

  ● phospholipid molecules form a continuous , double layer — called a bilayer 

  ● bilayer is ' fluid ' — because the phospholipids are constantly moving
• bilayer is ' fluid '


  — because the phospholipids are constantly moving
• Proteins are scattered through the bilayer like tiles in a mosaic


  include channel proteins and carrier proteins — which allow large molecules and ions to pass through the membrane
• the receptor protiens on the cell surface membrane allow


  cell to detect chemicals released from other cells — the chemicals signal to the cell to respond in some way 

  — e.g the hormone insulin binds to receptor proteins on liver cells which tells the cells to absorb glucose
• glycoproteins


  have a carbohydrate attached
• glycolipids


  have carbohydrate attached
• Transport proteins create hydrophilic channels to


  allow ions and polar molecules to travel through the membrane
• The rate of diffusion across both external and internal cell membranes can


  vary Some specialised cells are adapted for rapid transport across their membranes
• Each carrier protein is specific to


  a particular ion or molecule
• The rate of diffusion depends on :

  • The concentration gradient
  • The thickness of the exchange surface
  • The surface area
• Transport proteins allow the cell to control

  control which substances enter or leave
• fluid mosaic model also helps to explain

  Passive and active movement between cells and their surroundings Cell 
• The rate of diffusion depends on The concentration gradient


  As diffusion takes place , the difference in concentration between the two sides of the membrane decreases until reaches an equilibrium.

  This means that diffusion slows down over time.
• Membranes become less fluid when there is 

  An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
• Phospholipids structurally contain two distinct regions

  a polar head and two nonpolar tails
• The rate of diffusion depends on : The thickness of the exchange surface surface

  the thinner the exchange surface i.e. the shorter the distance the particles have to travel the faster the rate of diffusion .
• Membranes become less fluid when there is

  A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed
• The phosphate head of phospholipid is polar


  hydrophilic therefore soluble in water
• The rate of diffusion depends on : The surface area


  the larger the surface area e.g. of the cell surface membrane , the faster the rate of diffusion
• The lipid tail is non - polar


  non - polar hydrophobic and insoluble in water
• Example Some specialised cells -- epithelial cells in the small intestine

  microvilli - projections formed by the cell - surface membrane folding up on itself increasing the cells surface area.

  A larger surface area means that more particles can be exchanged the same amount of time increasing the rate of diffusion .
• If phospholipids are spread over the surface of water they form

  a single layer with the hydrophilic phosphate heads the water and the hydrophobic fatty acid tails sticking up away from the water
• Diffusion


  The net movement of particles from an area of high concentration to an area of low concentration
• When does diffusion end?


  When equilibrium is reached
• Does diffusion require energy?

  Is a passive process, does not require metabolic energy (only uses kinetic energy)
• Types of molecules that can diffuse.

  Only small, non-polar molecules are able to cross a cell membrane by simple diffusion
• Temperatures below 0 ° C


  phospholipids don't have much energy , so they can't move very much - packed closely together and the membrane is rigid

  channel proteins and carrier proteins in the membrane denature lose structure and function - increasing the permeability of the membrane
• micelle


  If phospholipids are mixed / shaken with water they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other
• Temperatures between 0 and 45 ° 

  The phospholipids can move around and aren't packed as tightly together membrane is partially permeable

  the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane
• Steepness of the concentration gradient and how this factor affects the rate of diffusion


  If there are more molecules on one side of a membrane than on the other at any one moment more molecules will randomly move across the membrane from that side than from the other

  A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion
• Temperatures above 45 C


  phospholipid bilayer starts to melt break down and the membrane becomes more permeable

  Water inside the cell expands , putting pressure on the membrane

  Channel proteins and carrier proteins the membrane denature which increases the permeability of the membrane