Transport across cell membranes

• Phospholipids form a bilayer
• The hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane, attracted by the water on both sides.
• The hydrophobic tails of both phospholipid layers point to the centre of the cell membrane, repelled by the water on both sides
• The functions of phospholipids in the membrane include:

  • allow lipid soluble substances to enter and leave the cell
  • Prevent water soluble substances entering and leaving the cell
  • Makes the membrane flexible and self-sealing
• Some proteins occur in the surface of the bilayer and never extend completely across. They give mechanical support to the membrane. OR in conjunction with glycolipids as cell receptors.
• Other proteins, such as protein channels & carrier proteins completely span the phospholipid bilayer from one side to the other.
• Protein channels form water-filled hydrophilic channels to allow water-soluble ions to diffuse across the membrane
• Carrier proteins that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane
• The functions of the proteins in the membrane are to:

  • structural support
  • act as channels transporting water-soluble substances across the membrane
  • allow active transport across the membrane through carrier proteins
  • form cell-surface receptors for identifying cells
  • helps cells adhere to one another
  • act as receptors, e.g. for hormones
• Cholesterol molecules within the phospholipid bilayer add strength to the membranes
• Cholesterol molecules are very hydrophobic and therefore help in preventing loss of water and dissolved ions from the cell
• The functions of cholesterol in the membrane are to:

  • reduce lateral movement of other molecules, including phospholipids
  • make membrane less fluid at high temperatures
  • prevent leakage of water and dissolved ions from the cell
• Glycolipids are made of a carbohydrate covalently bonded with a lipid.
• Carbohydrates from glycolipids extend from the bilayer to the watery environment outside, where it acts as a cell-surface receptor for specific chemicals
• The functions of Glycolipids in the membrane are to:

  • act as recognition sites
  • help maintain the stability of the membrane
  • help cells to attach to one another and form tissues
• Glycoproteins are made of carbohydrate chains that are attached to foreign proteins on the outer surface of the cell membrane
• The functions of glycoproteins in a membrane are to:

  • act as recognition sites
  • helps cells attach to each other and therefore form tissues
  • allows cells to recognize one another
• The cell-surface membrane controls movement of substances in and out of the cell
• Most molecules do not diffuse across the cell-surface membrane because many are:

  • not soluble in lipids meaning they cannot pass through the phospholipid bilayer
  • too large to pass through the channels in the membrane
  • the same charge as the protein channel so are repelled
  • electrically charged (polar) have difficulty passing through the non-polar hydrophobic tails
• Fluid-mosaic model of the cell-surface membrane

  
• The fluid-mosaic model was named this way because:

  • Fluid- the individual phospholipid molecules can move relative to one another, giving the membrane a flexible structure that can change shape
  • mosaic- the proteins embedded in the phospholipid bilayer vary in shape and size, just like a mosaic
• In diffusion:

  • All particles are constantly in motion due to the kinetic energy they possess
  • All motion is random with no set pattern for the way the particles move around
  • Particles are constantly bouncing off of one another as well as surrounding objects
• Diffusion is the net movement of ions or molecules from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed
• Amongst the few molecules that can diffuse across membranes are small and non-polar
• Charged ions and polar molecules do not easily diffuse across the membrane because of the hydrophobic nature of the fatty acid tails of the phospholipids in the membrane. This is made easier (facilitated) by transmembrane channels and carriers
• Facilitated diffusion is a passive process and relies on kinetic energy of the diffusing molecules
• Protein channels and carrier proteins each have different mechanisms that help with facilitated diffusion
• Protein channels are selective, each opening in the presence of a specific ion. If the particular ion isn't present it will remain closed. The specific ion binds with the protein causing it to change shape in a way that it closes on one side and opens on the other
• Carrier proteins span the plasma membrane and when a molecule such as glucose that is specific to the protein is present, it binds with the protein causing the protein to change shape in such a way that the molecule is released to the inside of the membrane. No external energy is required
• Osmosis is the passage of water from a region of high water potential to a region where it has a lower water potential through a selective permeable membrane
• Cell-surface membranes and other plasma membranes such as those around organelles are selectively permeable, meaning they are permeable to water molecules and a few other small molecules, but not to larger molecules
• A solute is any substance that is dissolved in a solvent
• Water potential is the pressure created by water molecules
• The addition of a solute to pure water will lower its water potential
• One way to find the water potential of cells or tissues is to place them in a series of solutions with different water potentials. Where there is no gain or loss of water, the water potential inside the cells must be the same as the solution
• A selectively permeable plasma membrane separates two solutions.

  • the solution on the left has a low concentration of solute molecules. the solution on the right is the opposite
  • Both the solute and water molecules are in random motion due to kinetic energy
  • The membrane only allows water across not solute
  • water diffuses from left to right - down a water potential gradient
  • when the water molecules are even on both sides - dynamic equilibrium is established
• The highest value of water potential is zero, all other values are negative, the more negative the value, the lower the water potential
• Cell surface membranes are very thin and although they are flexible they cannot stretch to a great extent. If placed in a solution of higher water potential they will burst (haemolysis). and if placed in a solution of lower water potential they will shrink and shrivel. This is why animal cells normally live in a liquid which has the same water potential as the cells
• Active transport is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
• In active transport ATP is used to:

  • directly move molecules
  • individually move molecules using a concentration gradient which has already been set up by active transport. This is co-transport