2.1.5 - biological membranes

• Fluid mosaic model of membranes is fluid because of the phospholipid bilayer which has individual phospholipids that can move giving it a flexible shape
• Fluid mosaic model of membranes is mosaic because of the intrinsic and extrinsic proteins of different shapes and sizes embedded in the bilayer
• In membranes, cholesterol connects phospholipids and reduces fluidity to maintain mechanical Stability of the bilayer and resist effects of temperature change.
• In membranes, glycolipids are used for cell signalling and cell recognition
• In membranes, intrinsic proteins are electron carriers; channel proteins; carrier proteins
• Membranes within cells provide an internal transport system. They are selectively permeable to regulate passage of molecules; provide a reaction surface; isolate organelles from the cytoplasm for specific metabolic reactions.
• The cell surface membrane isolates cytoplasm from external environment; selectively permeable to regulate transport of substances; involved in cell signalling/recognition
• Temperature affects membrane permeability

  high temperature denatures the membrane proteins; phospholipid molecules have more kinetic energy and move further apart so membrane is more permeable
• Ph affects membrane permeability as it changes the tertiary structure of membrane proteins so they are denatured
• Solvent effects membrane permeability as some organic solvents may dissolve the membrane
• Colorimetry can be used to investigate membrane permeability with temperature.
• Results of beetroot investigation:

  the higher the temperature, the higher the colorimeter value is.

  More pigment leaked from the beetroot so the membrane become more permeable as temperature increased.
• Simple diffusion is the net movement of small, lipid soluble molecules from an area of high concentration to an area of lower concentration, down a concentration gradient.
• Simple diffusion is a passive process so requires no energy from ATP hydrolysis.
• Facilitated diffusion the movement of larger or polar molecules from an area of high concentration to an area of low concentration via specific channel or carrier proteins.
• facilitated diffusion is a passive process
• Channel proteins are pores allowing transport of ions
• Osmosis is the net movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane.
• Water potential is the pressure created by water molecules.
• The water potential of pure water is 0kPa. The more solute is added the more negative the value of water potential.
• When animal cells are placed in solution with a lower water potential:

  water moves out by osmosis

  Cell shrinks or shrivels up - known as crenation
• When animal cells are placed in solution with the same water potential:

  No net movement of water

  cell stays the same shape
• When animal cells are placed in solution with a higher water potential:

  water moves in by osmosis

  cell expands and may burst - known as cytolysis
• When plant cells are placed in solution with a higher water potential:

  water moves in by osmosis

  cell begins to swell and push against cell wall - known as turgid

  gives plants strength and shape
• compartmentalisation within cells; keeps organelle contents separate from the cytoplasm
• Cell signalling is communication between cells; where cells work together to trigger a response
• Active transport is the movement of substances against their concentration gradient across a cell membrane using ATP and protein carriers
• Exocytosis is the bulk transport of molecules, too large to pass through a cell membrane, out of a cell
• For the colorimeter experiment, cuvettes are used to find a Colorimeter value in the colorimeter
• In exocytosis:

  the cytoskeleton contracts; to move the vesicle to the plasma membrane; the membrane of the vesicle and plasma membrane fuse; releasing the contents of the vesicle
• In the osmosis practical:

  For sugar solution there is a higher water potential inside the potato; water moves out by osmosis; mass of potato decreases

  for distilled water there is a higher water potential in the solution, water moves in by osmosis; mass of potato increases