Transport across cell membranes

• FLUID MOSAIC MODEL:

  Describes the structure of the plasma membrane (plasma membranes= all membranes around and within cells) as a mosaic of phospholipids, cholesterol, proteins and carbohydrates (which gives the membrane a fluid character).
• FUNCTION OF THE PLASMA MEMBRANE:

  The plasma membrane defines the border of cells & most organelles. Its selectively permeable (which means it allows some materials to freely enter or leave while other materials cannot move freely)
• STRUCTURE OF PHOSPHOLIPIDS:

  A phospholipid= a molecule consisting of glycerol, 2 fatty acid chains, and a phosphate -containing group. The molecules arrange themselves in a bilayer ranging from 5-10 nm in thickness. The hydrophilic phosphate head faces outwards attracted by water on both sides & the hydrophobic fatty acids faces inwards repelled by the water on both sides. Forms a barrier to water-soluble molecules
• PROTEINS:

  Scattered throughout the bilayer. Includes channel & carrier proteins. Allows large molecules and ions to pass through. Receptor proteins are on the membrane’s surface. Some proteins are in a fixed position, some can move sideways. Include glycoproteins and glycolipids. They provide structural support and act as channels transporting water soluble ions across. Carrier proteins (to allow active transport), cell surface receptors (identifying cells)
• STRUCTURE OF CHOLESTEROL:

  Is a lipid that sits with phospholipids in the core of the membrane. Cholesterol is not found in bacterial cell membranes, and makes the membranes they are in more rigid/ add strength to the membranes as it reduces lateral movement of molecules in the phospholipids. Pulls together fatty acid tails of the phospholipids -> limited movement -> makes it rigid. This means they help maintain the shape of animal cells. Hydrophobic so prevents the loss of dissolved ions & water in the cell.
• GLYCOPROTEINS:

  Carbohydrate chains are attached to proteins on the outer surface of the cell membrane. They act as recognition sites, help cells to attach to each others and form tissues, allows cells to recognise one another eg lymphocytes can recognise an organism's own cells
• PERMEABILITY OF CELL MEMBRANES (is influenced by factors):

  • TEMPERATURE: Higher temps -> increase fluidity of the membrane -> increasing its permeability. Using a water bath can help keep the temp constant.
  • pH: Affects the protein structure in the cell membrane. Buffer solutions can control the pH.
  • SOLVENT CONCENTRATION: More easily the phospholipid bilayer dissolves, the more permeable the membrane is. Solvent concentration can be controlled using the same solvent at same concentration for each trial.
• INVESTIGATING CELL MEMBRANE PERMEABILITY:

  1. COLLECT SAMPLES: Cut discs of same thickness eg of beetroot & rinse (removes excess pigment )
  2. ADD ETHANOL: Prepare ~5 concentrations of ethanol (eg 0%, 10%, 20%..) in beakers. Submerge discs into each beaker for 10 mins and mix frequently.
  3. REMOVE THE DISCS: from their solutions
  4. CALIBRATE THE COLORIMETER: using a cuvette of distilled water.
  5. MEASURE ABSORBANCE: then plot the results in a graph (concentration x-axis, absorbance y-axis). Darker the solution= more pigment released (higher reading of absorbance)
• SIMPLE DIFFUSION:

  • DEFINITION: Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration. Describes the passive movement of particles in fluids (liquids and gases.
  • MOLECULE/ NET MOVEMENT: Molecules move randomly but tend to move in any space available until evenly distributed. The net movement would be towards an area of lower concentration/ down a concentration gradient.
  • EG: Substances such as carbon dioxide, oxygen, and urea move in and out of cells across cell membranes via diffusion
• FACILITATED DIFFUSION:

  • LARGE/ POLAR MOLECULES: Some membranes are too large to move through a membrane. Others are polar molecules that are repelled by the hydrophobic part of the membrane. These materials move across the membrane by facilitated diffusion.
• FACILITATED DIFFUSION:

  DEFINITION: Describes the net movement of particles down a concentration gradient (ie a from a region of high concentration to a region of low concentration). Molecules diffuse through carrier proteins or channel proteins spanning across the membrane in facilitated diffusion. It's a passive process so does not  require energy.
• FACILITATED DIFFUSION:

  • CHANNEL PROTEINS: allow polar compounds to avoid the non-polar central layer of the plasma membrane (non-polar layer would prevent/slow entry into the cell). Aquaporins =channel proteins that allow water soluble substances to pass through. Some are water-filled tubes. Rate of fd depends on the size of the conc gradient, & as molecules moves into the epithelial cells the conc gradient falls, so fd cannot absorb all the molecules (so co transport also used). Some channel proteins= always open, some open to a trigger eg chemical binding/ voltage
• FACILITATED DIFFUSION CARRIER PROTEINS: 

  1)Allows  a substance (e.g. ions, glucose, amino acids) to bind to them

  2) Causes a change in the shape of the carrier protein. 

  3) This change in shape moves the substance from the outside of the cell to the inside, allowing the carrier proteins to carry them across the bilayer via active transport
• FACTORS AFFECTING DIFFUSION:

  • CONC GRADIENT: Bigger the difference in conc between 2 areas, greater the conc gradient, faster the rate of diffusion
  • TEMPERATURE: Higher the temp, faster rate of diffusion
  • MEMBRANE SA: Larger the membrane sa, faster the rate of diffusion
  • THICKNESS OF MEMBRANE: The thicker the exchange surface, the slower the rate of diffusion
  • CHANNEL/ CARRIER PROTEINS: Higher the concentration gradient, faster the rate of diffusion UNTIL an equilibrium is reached,& the rate will level off. The more channel/carrier proteins, faster rate of fd.
• OSMOSIS:

  • DEFINITION: The diffusion of water across a partially permeable membrane from a dilute solution (high water potential) to a concentrated solution (low water potential)
• OSMOSIS:

  • PARTIALLY PERMEABLE MEMBRANE: Allows water through but not larger molecules dissolved in water
• OSMOSIS:

  • WATER MOVEMENT: Water will move to make the concentrations equal at both sides of the membrane. Water potential= likelihood of water molecules diffusing.
  • NET MOVEMENT OF WATER:  Water molecules will move through the membrane in both directions but the net (overall) movement will be from the dilute solution to the concentrated solution.
  • WATER POTENTIAL: Pure water has the greatest water potential, concentrated solutions have the lowest water potential, isotonic solutions have equal water potentials on either side of the membrane.
• OSMOSIS:

  RATE OF OSMOSIS: Lower the water potential gradient, slower the rate of osmosis. Thicker the membrane, slower the rate of osmosis. Smaller the surface area, slower the rate of osmosis.
• WATER POTENTIAL OF PLANT CELLS (eg potato):

  1. CUT SAMPLE: cut potato (same size, shape, mass) Weigh each, record results.
  2. PLACE IN TUBES: Submerge chips in boiling tubes, with each containing diff sucrose concentrations (eg 0.1, 0.2… dm⁻³). Heat in water bath 30°C
  3. REWEIGH: Blot them, weigh and measure them
  4. PLOT GRAPH: Conc= x-axis, mass change= y-axis. Should be neg and pos values on the y-axis
  5. INTERPRET RESULTS FROM GRAPH: Conc where line crosses x -axis= water potential of the potato=isotonic point (water hasn't moved in/out the tissue.The water potential is the same inside as outside).
• ACTIVE TRANSPORT:

  • DEFINITION: Active transport is the process where dissolved molecules move from a less concentrated solution to a more concentrated solution. As net movement of molecules is against the concentration gradient, energy (from respiration) is used.
• ACTIVE TRASNPORT:

  • CARRIER PROTEINS: Important membrane adaptation for active transport= carrier proteins to facilitate movement.
• ACTIVE TRASNPORT:

  • EG SUGAR ABSORPTION IN THE GUT: Active transport allows sugar molecules (needed for cell respiration) to be absorbed into the blood from the gut, even if sugar concentration of the blood is higher
  • EG MINERAL ABSORPTION IN PLANTS: Active transport allows root hairs of plants to absorb mineral ions (necessary for healthy growth) even if concentrations of minerals is usually lower in the soil than the root hair.
• ACTIVE TRASNPORT:

  FACTORS AFFECTING RATE: Surface area of cell membranes, amount of carrier proteins in cell membranes.
• ACTIVE TRANSPORT & CARRIER PROTEINS:

  1. The molecule/ ion attaches to a receptor site on the carrier protein (this takes place at the side with the lower molecule conc)
  2. A molecule of ATP binds to the carrier protein
  3. The ATP hydrolyses producing phosphate + molecule of ADP
  4. The phosphate attaches to the carrier protein-> change shapes
  5. Shape change-> the carrier protein transports the molecule/ ion to the other side of the membrane
  6. The ADP & phosphate will reform ATP during respiration by ATP synthesis
• CO-TRANSPORT:

  • DEFINITION: The mechanism of transport of glucose and amino acids into the epithelial cells in the ileum.
• CO-TRANSPORT:

  • SODIUM-POTASSIUM ATPase: Sodium ions are actively transported into the blood by sodium-potassium ATPase from the inside of epithelial cells (lining the ileum). The concentration of Na is now lower inside the cell than outside
  • SODIUM-GLUCOSE COTRANSPORTER: 2 Na ions from the lumen of the ileum bind to a sodium-glucose cotransporter (a transmembrane protein). At the same time the cotransporter binds with a molecule of glucose from the ileum and moves it into the cell. The sodium ions diffuse into the cell, moving the glucose into the cell at the same time
• ACTIVE TRANSPORT- COTRANSPORT (ileum):

  • Glucose moves down the concentration gradient to the epithelial cells via carrier proteins (facilitated diffusion)
  • Facilitated diffusion depends on the size of the concentration gradient . When it falls not all of the glucose can be transported, so are transported via active transport
  • The sodium-potassium pump uses ATP to actively transport sodium ions into the bloodstream while actively transporting potassium into the epithelial cells
• ACTIVE TRANSPORT- COTRANSPORT (ileum):

  • The active transport of glucose ensures its transported quickly
  • The energy for the glucose transport comes from the sodium concentration gradient