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The plasma membrane and material transport
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  • 1. 04. The plasma membrane & material transport.Source: Campbell et al. (2011) Ian Anderson Saint Ignatius College Geelong
  • 2. Knowledge and skills. Describe the molecular structure of plasma membranes. Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of plasma membranes. List the functions of membrane proteins. Describe the different ways that molecules or substances cross membranes, including • Diffusion • Active transport • Osmosis • Endocytosis • Facilitated diffusion • Exocytosis
  • 3. Plasma membrane. The boundary that separates a cell from its surroundings. Maintains the internal environment of a cell by controlling the movement of substances into and out of cell (i.e. partially permeable).  Enabling the cytosol to have a different composition from the surrounding environment. ~7-9 nm thick.
  • 4. Fluid-mosaic model. Composed of a double layer (bilayer) of lipids (phospholipids), called a phospholipid bilayer.  The phospholipid molecules are in constant motion, sliding past one another and the other molecules embedded in the plasma membrane. Proteins can  Traverse the plasma membrane and stick out from both surfaces; be partially submerged in the inner or outer surface; or be found on the surface.  Main roles are in helping transport molecules across the plasma membrane; acting as attachment points for other cells; and functioning as identity tags for cells.
  • 5. Fluid-mosaic model. Carbohydrates are attached to some proteins that protrude to the outside of the cell (together called glycoproteins).  Play an important role in cell-to-cell interactions Cholesterol molecules can be found in the middle between the two phospholipid bilayers.  Helps to stabilize the plasma membrane and keep it flexible.
  • 6. Fluid-mosaic model. Source: Enger et al. (2011)
  • 7. Phospholipids. Lipids that contain a phospate group at one end of each molecule.  Hydrophilic (‘water loving’) at their phosphate end.  Hydrophobic (‘water fearing’) along their fatty acid tail region. When in an aqueous solution, the phospholipid molecules line up with their hydrophobic tails pointing away from the solution  phospholipid bilayer.
  • 8. Phospholipids. Source: http://bioweb.wku.edu/courses/biol115/wyatt/biochem/lipid/Lipid_2.asp
  • 9. Movement in and out of cells. All cells must be able to exchange (take in and expel) substances with their environment in order to survive, grow and reproduce. Movement of substances across the plasma membrane depends on  Surface area available for exchange  Chemical properties of the substance being exchanges, plus  Concentration gradients, temperature, etc.
  • 10. Surface area-volume ratio. Surface area-volume ratio of a cell influences the rate of entry and exit of substances into and out of it. As a structure increases in size, its SA:V decreases. The SA:V ratio differs according to the shape of the structure.  Ratio is highest in flattened shapes and lowest in spheres.
  • 11. Surface area-volume ratio.
  • 12. Movement across the plasma membrane. Plasma membrane is a partially permeable boundary.  i.e. only some dissolved materials are able to cross it. Various processes used to allow materials to cross plasma membrane  Diffusion  Facilitated diffusion  Osmosis  Active transport  Endocytosis and exocytosis
  • 13. Diffusion.The net movement of molecules along a concentration gradient, from a region of high concentration to a region of low concentration. Does not require energy (i.e. passive).  Occurs due to the random movement of molecules. Always occurs down a concentration gradient.  The larger the concentration gradient, the more rapid the rate of diffusion.
  • 14. Diffusion. Diffusion across the plasma membrane will occur as long as the molecule can pass through.  Lipid-soluble substances (such as vitamins A & D and alcohol) diffuse through the lipid bilayer.  Small molecules such as water, O2 and CO2 can diffuse between the phospholipid molecules.  Large molecules, polar molecules and small ions generally are unable to diffuse across the plasma membrane without assistance.
  • 15. Facilitated diffusion. The movements of a substance across the plasma membrane from a region of higher concentration of a substance to a region of lower concentration, assisted (or facilitated) by specific membrane proteins.  Membrane proteins are either carrier proteins or ion channels.  Carrier proteins attaches to the molecule, with the resultant change in shape allowing the molecule to be shifted across the plasma membrane and then released.  Ion channels act like gates by opening and closing the channel. Does not require energy.
  • 16. Facilitated diffusion. Facilitated diffusion is more rapid than simple diffusion. The protein channels through the plasma membrane are specific for particular molecules. Mainly involves substances that cannot diffuse across the plasma membrane by dissolving in the lipid bilayer of the membrane.  Polar molecules will diffuse through via carrier proteins and small ions via the ion channels.  e.g. the movement of glucose across the membrane of red blood cells.
  • 17. Facilitated diffusion. Source: Campbell et al. (2011)
  • 18. Osmosis. The net movement of free water molecules through a partially permeable membrane, from a dilute to a more concentrated solution. A special case of diffusion (i.e. movement of water molecules not the movement of solute molecules). Osmotic gradient; osmotic pressure. e.g. Absorption of water from food in the gut; reabsorption of water in kidneys.
  • 19. Osmosis. Osmotic influence on cells.  Cells are affected by the amount of dissolved materials in the water that surrounds them.  Hypertonic v hypotonic v isotonic solutions.  Hypertonic soln = more dissolved material & less water.  Hypotonic soln = less dissolved material and more water.  Isotonic soln = both solutions the Source: Enger et al. (2011) same concentration of water.
  • 20. Osmosis.
  • 21. Active transport. The net movement of dissolved substances into or out of a cell against the concentration gradient. Requires energy. Enables cells to maintain stable internal conditions in spite of extreme variation in the external surroundings. Occurs through protein channels which are very selective for specific ions and molecules.  e.g. Sodium ions are pumped out of cells and potassium ions are pumped in, both against the concentration gradient.  Plant root cells take up nitrate ions from very dilute solutions in the soil.
  • 22. Active transport. e.g. active transport of salt occurs in many freshwater organisms.  Freshwater fish lose salt by diffusion across their skin- cell plasma membranes into their surrounding freshwater environment. Energy (in the form of ATP) is used to actively transport salt molecules against the concentration gradient. Some molecules can be transported across plasma membranes either by facilitated diffusion or active transport e.g. glucose (most cells v liver cells).
  • 23. Active transport. Source: Walpole et al. (2011)
  • 24. Endocytosis & exocytosis. Movement of large molecules and particles in bulk across the plasma membrane. Requires energy. Exocytosis = movement of a substance out of a cell.  Transport vesicles migrate to the plasma membrane, fuse with it, and release their contents.  e.g. Digestive enzymes in a transport vesicle. Endocytosis = movement of a substance into a cell.  Molecules enter cells within vesicles that pinch inward from the plasma membrane.  Phagocytosis v pinocytosis  e.g. white blood cells engulf microorganisms by phagocytosis.
  • 25. Endocytosis & exocytosis. Endocytosis and exocytosis in a white blood cell. Source: Enger et al. (2011)