Cell Membranes and Transport

• The basic structure of all cell membranes, including cell-surface

  membranes and the membranes around the cell organelles of

  eukaryotes, is the same. The arrangement and any movement includes phospholipids, proteins, glycoproteins and glycolipids in the fluid-mosaic model of membrane structure. Cholesterol may also be present in cell membranes where it restricts the movement of other molecules making up the membrane.
• Cells may be adapted for rapid transport across their internal or

  external membranes by an increase in surface area of, or by an

  increase in the number of protein channels and carrier molecules in,

  their membranes. Membranes are based on a phospholipid bilayer in which proteins are embedded.
• All cells have a cell-surface membrane and, in addition, eukaryotic cells have internal membranes. The basic structure of these membranes is the same and enables control of the passage of substances across exchange surfaces by passive (doesn't require input of energy) or active transport (needs an energy supply such as ATP). These membranes are selectively permeable.
• The cell-surface membrane is a phospholipid bilayer with embedded proteins that are selectively permeable.


  When phospholipids are mixed with water, they arrange themselves in

  a double layer with their hydrophobic (repelled by water) tails pointing inwards and their hydrophilic heads (attracted to water) pointing outwards. This double layer is called a phospholipid bilayer. Lipid soluble substances can pass through the membrane but large polar molecules cannot.
• A cell surface membrane is only about 7 nm thick, so we cannot see all

  the details of its structure. Biologists have produced a model to explain

  its properties. This is called the fluid mosaic model.


  mosaic = different phospholipids and proteins are held together in a mosaic

  fluid = the molecules are moving
• function of the phospholipid


  ~ allow lipid soluble substances enter and leave the cell

  ~ prevent water soluble substances entering and leaving the cell

  ~ make the membrane flexible and self healing
• function of cholesterol in the cell-surface 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
• function of glycolipids in the cell-surface membrane

  ~ act as recognition sites

  ~ help maintain the stability of the membrane

  ~ help cells to attach to one another and so form tissues
• function of glycoproteins in the cell-surface membrane


  ~ acts as recognition sites

  ~ help cells attach to one another and so form tissues

  ~ allows cells to recognise one another, for example lymphocytes can recognise an organdie's own cells
• channel proteins


  form water filled tubes to allow water-soluble ions to diffuse across the membranes
• carrier proteins


  bind to ions or molecules like glucose or amino acids, then change shape in order to move these molecules across membrane
• function of the proteins (channel and carrier) in the cell-surface membrane

  ~provide structural support

  ~channel proteins = act as channels transporting water-soluble substances across the membrane

  ~carrier proteins = allow active transport across the membrane through carrier proteins

  ~from cell-surface receptors for identifying cells

  ~help cells adhere together

  ~act as receptors, e.g. for hormones
• factors that affect membrane permeability (how easy it is for something to pass through) - temperature

  freeze = membrane damaged, so no permeability

  decrease temp. = less permeable so fluidity reduces and particles move less due to less kinetic energy

  increase temp. = more permeable so fluidity increases and particles move more due to more kinetic energy

  boil/too hot = tertiary structure denatures (breaks down), normally only let specific substances to pass through but due to denature, everything is let through
• factors that affect membrane permeability (how easy it is for something to pass through) - detergents


  increase concentration of detergents = breaks down phospholipids (contains fatty acids) and removes the fat/grease, breaks down layer over time and the membrane eventually becomes fully permeable as all phospholipids would be broken down with a high concentration of detergent
• reasons why most molecules don't freely diffuse across the cell-surface membrane:

  ~they aren't soluble in lipids and therefore cant pass through the phospholipid layer

  ~they are too large to pass through the channels in the membrane

  ~molecules are of the same charge as the charge on the protein channels and so, even if they are small enough to pass through, they are repelled

  ~polar (electrically charged) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer
• transport across membranes - simple diffusion


  passive net movement (overall movement of molecules/particles) of a substance from high to low concentration until dynamic equilibrium is reached
• factors that affect the rate of simple diffusion : concentration gradient

  large concentration gradient = increase in the rate of diffusion

  smaller concentration gradient = decrease in the rate of diffusion
• factors that affect the rate of simple diffusion : the area of the exchange surface

  larger surface area = faster rate of diffusion

  smaller surface area = slower rate of diffusion
• factors that affect the rate of simple diffusion : the thickness of the exchange surface

  thicker the exchange surface = slower rate of diffusion (longer distance to travel)

  thinner the exchange surface = faster rate of diffusion (shorter distance to travel)
• what is fick's law?

  rate of diffusion across an exchange surface (e.g. membrane, epithelium) =

  (surface area x difference in concentration gradient)

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  thickness of surface (length of diffusion pathway)