4 - Transport across cell membranes

• Cell-surface membrane


  The plasma membrane that surrounds cells and forms the boundary between the cell cytoplasm and the environment
• Cell-surface membrane


  • Allows different conditions to be established inside and outside a cell
  • Controls the movement of substances in and out of a cell
• Phospholipids


  The main component of the cell-surface membrane
• Phospholipids


  • The hydrophilic heads point to the outside of the cell surface membrane
  • The hydrophobic tails point into the centre of the cell membrane
• Proteins


  Embedded in the phospholipid bilayer
• Proteins


  • Some occur in the surface of the bilayer and never extend completely across it
  • Others completely span the phospholipid bilayer from one side to the other
• Cholesterol


  Occur within the phospholipid bilayer of the cell surface membrane
• Glycolipids


  Made up of a carbohydrate covalently bonded with a lipid
• Glycoproteins


  Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane
• Cell-surface membrane permeability


  • Most molecules do not freely diffuse across it because they are not soluble in lipids, too large to pass through the channels, have the same charge as the protein channels, or are electrically charged (polar)
• Fluid-mosaic model


  The way in which all the various molecules are combined into the structure of the cell-surface membrane
• Fluid-mosaic model


  • Fluid - the individual phospholipid molecules can move relative to one another
  • Mosaic - the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern
• Osmosis


  The passage of water from a region where it has a lower water potential to a region where it has a lower water potential through a partially permeable membrane
• Cell surface membranes and other plasma membranes

  • Partially permeable, permeable to water molecules and a few other small molecules, but not to larger molecules
• Solute


  Any substance that is dissolved in a solvent. The solute and the solvent together form a solution
• Water potential


  The pressure created by water molecules
• Water potential is represented by the Greek letter psi
• Water potential is measured in units of pressure, usually Kilopascals
• Under standard conditions of temperature and pressure (25 degrees and 100kPa), pure water is said to have a water potential of zero
• The addition of a solute to pure water

  Lowers its water potential
• The water potential of a solution (water + solute) must always be less than zero, that is, a negative value
• The more solute that is added


  The lower its water potential
• Water will move by osmosis

  From a region of higher water potential to one of lower water potential
• Finding the water potential of cells or tissues


  1. Place them in a series of solutions of different water potentials

  2. Where there is no net gain or loss of water from the cells or tissues, the water potential inside the cells or tissues must be the same as that of the external solution
• The highest value of water potential, that of pure water, is zero, and so all other values are negative
• The more negative the value, the lower the water potential
• If a red blood cell is placed in pure water

  It will absorb water by osmosis because it has a lower water potential
• Cell surface membranes are very thin and, although they are flexible, they cannot stretch to any great extent
• The cell surface membrane will therefore break, bursting the cell and releasing its contents
• To prevent this happening, animal cells normally live in a liquid which has the same water potential as the cells
• Active transport


  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
• Active transport


  1. Directly move molecules

  2. Individually move molecules using a concentration gradient which has already been set up by (direct) active transport (co-transport)
• Co-transport


  The mechanism by which glucose and amino acids are absorbed from the small intestine, where either glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium-potassium pump
• Absorption of glucose in the ileum


  Co-transport of glucose and sodium ions into epithelial cells
• Ileum epithelial cells


  • Possess finger-like projections called microvilli about 0.6 micrometres in length
  • Collectively termed a 'brush border' as they look like bristles on a brush under a microscope
  • Provide more surface area for insertion of carrier proteins for diffusion, facilitated diffusion, and active transport
• Increasing the rate of movement across membranes

  Increase the number of protein channels and carrier proteins in the membrane
• Concentration gradient of glucose and amino acids in the ileum


  Greater concentration within the ileum than within the blood
• Glucose absorption by facilitated diffusion

  1. Glucose moves down the concentration gradient from inside the ileum into the blood

  2. Blood circulation removes glucose, maintaining the concentration gradient
• Glucose and amino acid absorption by co-transport

  1. Sodium ions actively transported out of epithelial cells by sodium-potassium pump

  2. Sodium ions diffuse into epithelial cells down concentration gradient, carrying glucose or amino acids with them

  3. Glucose/amino acids then pass into blood by facilitated diffusion
• Active transport


  • Metabolic energy in the form of ATP is needed
  • Substances are moved against a concentration gradient, that is from a lower to a higher concentration
  • Carrier protein molecules which act as 'pumps' are involved
  • The process is very selective, with specific substances being transported