Module 2.1.5- Biological membranes

• Plasma membrane

  A selectively-permeable phospholipid bilayer forming the boundary of the cells
• Phospholipid bilayer


  a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends facing outward.

  
• Fluid mosaic model


  model that describes the arrangement and movement of the molecules that make up a cell membrane

  
• Components of a cell surface membrane

  phospholipids, proteins, carbohydrates, cholesterol, glycoproteins, glycolipids, pore
• Function of glycoproteins


  Intrinsic proteins. A protein with one or more covalently attached carbohydrates.

  act as recognition site for chemicals e.g. hormones.

  role in cell adhesion
• Function of glycolipids


  Membrane carbohydrates that are covalently bonded to lipids.

  act as recognition site for chemicals. act as antigens or cell markers and can be recognised by the immune system as self or non-self
• Function of cholesterol


  maintains fluidity and stability of the membrane
• Function of intrinsic proteins


  Specific protein molecule that spans the bilayer

  Carrier and channel proteins (water-filled pores) involved in facilitated diffusion (or active transport if carrier).

  Can be glycoproteins .
• Function of extrinsic proteins


  -provide mechanical support

  -act as receptors for molecules
• Effect of temperature on plasma membrane


  Higher temperature causes an increase in KE and phospholipids move more becoming more fluid (less tightly packed) and begin to lose its structure. Eventually it will break down completely and become very permeable.
• Effect of solvents on phospholipids


  Organic solvents such as ethanol will damage cell membranes by disrupting the structure as they dissolve lipids (more fluid and more permeable). Many organic solvents are less polar than water.
• Diffusion (passive)


  the net ,or , overall movement of particles from a region of higher concentration to a region of lower concentration
• What particles can freely pass down the membrane

  Small non-polar and polar molecules
• Factors affecting rate of diffusion

  ↑ concentration gradient ↑ diffusion

  ↑ temperature ↑ diffusion

  ↑ SA:V ↑ diffusion

  ↓ diffusion pathway ↑ diffusion
• Facilitated diffusion


  Movement of specific molecules across cell membranes through protein channels e.g. carrier proteins
• Active transport


  Energy-requiring process that moves material across a cell membrane against a concentration difference using carrier proteins
• Active transport with carrier protein


  A membrane protein is used to help molecules across the cell membrane.

  Molecule binds to receptors in the channel of the carrier protein

  On the inside ATP binds to protein and is hydrolysed into ADP and Phosphate

  Binding causes protein to change shape

  Molecule is released and ATP is reformed.

  
• Bulk transport


  The process by which large particles and macromolecules are transported through plasma membranes. e.g. exocytosis and endocytosis
• Endocytosis


  A process in which a cell engulfs extracellular material through an inward folding of its plasma membrane. Phagocytosis and pinocytosis. It is an active process
• Exocytosis


  release of substances out a cell by the fusion of a vesicle with the membrane. It is an active process
• Osmosis


  Diffusion of water through a selectively permeable membrane from an area of high water potential to an area of low water potential
• Water potential


  the pressure exerted by water molecules as they collide with a membrane or container
• Hydrostatic pressure


  Pressure exerted by a volume of fluid against a wall, membrane, or some other structure that encloses the fluid.
• Cytolysis


  the bursting of a cell due to water moving in
• Crenation


  This happens when a cell shrinks and shrivels; can result in cell death if severe due to water moving out
• Osmosis in a red blood cell diagram

  
• Isotonic


  when the concentration of two solutions is the same
• Hypertonic


  when comparing two solutions, the solution with the greater concentration of solutes
• Hypotonic solution


  A solution in which the concentration of solutes is less than that of the cell that resides in the solution
• Turgor pressure


  Pressure that a fluid exerts against a structure that contains it. Mainly in a plant.
• Plasmolysis


  This happens when a cell shrinks inside its cell wall while the cell wall remains intact since water moves out
• When there is a higher water potential inside a plant cell, the cell...

  ... swells and becomes turgid