B4 transport across cell membranes

• all cells and organelles are surrounded by a partially permeable layer called a plasma membrane
• the cell-surface membrane is a plasma membrane which surrounds cells, forming a boundary between the cytoplasm and the environment
• the functions of the cell-surface membrane are to allow different conditions to be established inside and outside a cell, and to control the movement of substances in and out of the cell
• phospholipids form a bilayer in the cell-surface membrane, because the hydrophilic heads are attracted to water so point to the outside of the membrane, and the hydrophobic tails are repelled by water so point to the inside of the membrane
• the functions of phospholipids in the bilayer are to:

  • allow lipid-soluble substances to enter and exit the cell
  • prevent water-soluble substances from entering and exiting the cell
  • make the membrane flexible and self-sealing
• proteins are embedded throughout the phospholipid bilayer, either extrinsic proteins on the surface or intrinsic proteins spanning the whole bilayer, these are carrier or channel proteins
• the functions of proteins in the phospholipid bilayer are to:

  • provide structural support
  • act as channels transporting water-soluble substances across the membrane
  • allow active transport across the membrane
  • form cell-surface receptors with glycolipids
  • help with cell adhesion
• cholesterol molecules occur within the phospholipid bilayer, they are very hydrophobic and pull together the fatty acid tails of phospholipids, restricting their movement
• the functions of cholesterol molecules in the phospholipid bilayer are to:

  • reduce lateral movement of phospholipids
  • decrease membrane fluidity
  • prevent leakage of water and dissolved ions out of the cell
• glycolipids are a carbohydrate covalently bonded with a lipid, they extend from the phospholipid bilayer outside of the cell
• the functions of glycolipids in the bilayer are to:

  • act as receptors
  • maintain membrane stability
  • help with cell adhesion
• glycoproteins are a carbohydrate attached to an extrinsic protein, they extend from the phospholipid bilayer outside of the cell
• the functions of glycoproteins in the bilayer are to:

  • act as recognition sites
  • help with cell adhesion
  • allow cell recognition
• the cell-surface membrane prevents molecules from freely diffusing across it if they are: not lipid-soluble, too large, the same charge as protein channels, or polar
• molecules which are not lipid-soluble cannot pass through cell-surface membrane as they cannot pass through the phospholipid bilayer
• molecules which are too large cannot pass through the cell-surface membrans as they are too large to fit through the channel proteins
• molecules which are of the same charge as the protein channels cannot pass through the cell-surface membrane as they would be repelled
• molecules which are polar cannot pass through the cell-surface membrane as they cannot pass the non-polar hydrophobic tails of the phospholipid bilayer
• the fluid-mosaic model is used to represent the way the various components are combined into the structure of the cell-surface membrane
• the fluid-mosaic model is fluid as individual phospholipid molecules can move relative to one another, and mosaic as the proteins embedded in the phospholipid bilayer vary in shape, size and pattern
• simple diffusion across a cell-surface membrane is the passive movement of small, non-polar, lipid-soluble molecules from an area of high concentration to an area of low concentration, across the phospholipid bilayer
• facilitated diffusion across a cell-surface membrane is the use of carrier proteins and channel proteins to transport substances, which would otherwise not be able to cross the phospholipid bilayer, from an area of high concentration to an area of low concentration
• channel proteins are water-filled hydrophilic channels which allow specific water-soluble ions to pass through them, they are selective so open when the specific ion is present, and otherwise remain closed
• channel proteins work as the ions bind to the protein, causing it to undergo a conformational change and change shape in a way that closes it to one side of the membrane and opens it to the other side
• carrier proteins are intrinsic proteins which allow specific molecules to pass through the membrane
• carrier proteins work as the molecules bind to the protein, causing it to undergo a conformational change and change shape in a way that releases the molecule to the inside of the membrane
• osmosis is the movement of water from an area of high water potential to an area of low water potential, across a partially permeable membrane
• a cell-surface membrane is partially permeable as it allows the passage of water and a few other small molecules, but not of large molecules
• water potential is the pressure created by water molecules, represented by $\Psi$
• under standard conditions of 298K and 100kPa, pure water has a water potential of zero
• the addition of solute to pure water always lowers its water potential, the more that is added, the lower the water potential will become
• the water potential of a solution must always be negative
• active transport is the movement of molecules or ions into or out of a cell from an area of low concentration to an area of high concentration, using ATP and carrier proteins
• ATP is used in active transport to directly move molecules against their concentration gradient, making it an active process instead of a passive process
• active transport process:

  • the specific ion or molecule binds to a receptor site on an intrinsic carrier proteins
  • ATP binds to the protein, it is hydrolysed into ADP and Pi
  • the protein undergoes a conformational change, changing shape and opening to the opposite side of the membrane
  • the ion or molecule is released to the opposite side of the membrane
  • the phosphate molecule is released from the protein so it reverts to its original shape and the process can be repeated
• co-transport uses ions to move substances into and out of cells, by creating a concentration gradient using active transport
• the process of co-transport of glucose/amino acids in the ileum is:

  • Na+ ions are actively transported out of epithelial cells by the sodium-potassium pump and into the blood
  • the concentration of Na+ ions in the lumen is greater than in epithelial cells, establishing a concentration gradient
  • Na+ ions diffuse into epithelial cells down the concentration gradient through a co-transport carrier protein, carrying glucose/amino acids with them
  • the glucose/amino acids pass into the blood by facilitated diffusion using a carrier protein