CC3

• Name 6 functions of membranes

  • Allows cell to change shape
  • Controls Entry + Exit of substances
  • Communication with the external environment
  • Hold the cells organelles
  • Site of chemical reactions
  • Transport and packagimg within a cell
• Membranes are made of phospholipids
• Phospholipid structure

  • Phospahte head is hydrophillic which attracts water
  • Fatty acid tail is hydrophobic so repels water
  • Glycerol backbone
• What is the name of the membrane model?

  Fluid mosaic model
• Explain the fluid mosaic model

  It is fluid because the phospholipids can move by rotating on their axis and swapping with one opposite or next to them.

  It is a 'mosaic' because proteins on the surface are placed randomly.
• The cell membrane is impenetrable to water soluble substances due to the phospholipid membrane. There are intrinsic and extrinsic proteins which allows water soluble substances to pass through the membrane.
• Water is polar and lipids are non-polar. This means hydrogen bonds cannot form between them
• Osmosis is the net passive diffusion of water molecules from an area of high water potential to an area of low water potential through a selectively permeable membrane.
• Net movement is the movement of water molecules in both directions across the membrane at the same time
• Net movement equation


  Movement in one direction - Movement in the opposite direction
• Water potential is the measure of free energy of water molecules and its tendency to move from one solution to another from a high to low water potential.
• What is water potential measured in?

  Kilopascals (kPA)
• Pure water has the highest water potential value of 0kPA
• All solutions have a more negative water potential as water molecules weakly bond to the solvent meaning they cant move as freely.
• Turgor pressure is the pressure exerted by the cell contents on the cell wall. This causes the cell wall to push against the expanding cytoplasm which creates a pressure potential by resistance
• A hypertonic solution means there is more solute and less water outside of the cell. This means water move OUT of the cell by osmosis down a water potential gradient. This means the cells shrinks = plasmolysis
• A hypotonic solution means there is less solute and more water outside of the cell. This means water moves INTO the cell by osmosis down a water potential gradient. This means the cells expands = turgid
• An isotonic solution means there is an equal volume of solute and water inside and outside of the cell. There is no net movement as water moves in and out at the same rate. The cell is said to be flaccid.
• A passive process does not require ATP and moves molecules from an area of high concentration to an area of lower concentration
• An active process requires ATP and moves molecules against their concentration gradient.
• Diffusion is a passive process that doesnt require ATP and moves molecues from an area of high concentration to an area of lower concentration
• Simple diffusion is the movement of molecules from an area of high concentration to an area of low concentration directly throgh the phospholipid bilayer. Moleculs are limited to small, nonpolar and lipid soluble substances like oxygen and carbon dioxide.
• Facilitated diffusion is the movement of molecules from an area of high concentration to an area of lower concentration across a membrane using channel and carrier proteins. They transport large, polar and water soluble sustances.
• Name 6 factors that affect the rate of diffusion

  • Temperature
  • Size + Solubility
  • Membrane surface area
  • Length of diffusion pathway
  • Steepness of concentration gradient
  • Membrane permeability
• Co-transport is a type of facilitated diffusion and is a passive process that uses carrier proteins to transport 2 molecules simutaneously across the membrane.
• Cotransport of Sodium + Glucose

  1. Sodium is moved out of the cell inot the blood by a sodium potassium pump using active transport
  2. This reduces the concentration of sodium ions in the cell
  3. This allows glucose and sodium to be brought into the cell by facilitated diffusion as sodium can move from high to low concentration.
  4. Glucose binds to a carrier protein and moves out the cell into the blood by facilitated diffusion
• Active transport requires ATP and moves molecules against their concentration gradient. A molecule binds to a specific carrier protein which has a complementary shape. ATP changes the chape of the protein to allow the molecule to move across the membrane.
• Bulk transport is the process of moving large amounts of substances across the cell membrane.
• Name 4 types of bulk transport

  • Exocytosis
  • Endocytosis
  • Phagocytosis
  • Pinocytosis
• Exocytosis is an active process that requires ATP and is when molecules are secreted out of the cell through vesicles which fuse with the plasma membrane.
• Endocytosis is an active process that reuires ATP and moves substnaces into the cell through infoldings of the plasma membrane
• Phagocytosis is a type of endocytosis which moves solid particles or whole cells into the cell. The contents is digested by hydrolytic enzymes which are secreted by lysosomes
• Pinocytosis is a type of endocytosis which moves liquids into the cell. The contents is digested by hydrolytic enzymes that are secreted by lysosomes.