Biological Membranes

• State the roles of biological membranes:

  • to keep organelles separate from the cytoplasm and the rest of the cell (external environment)
  • partially permeable barrier
  • controls the entry and exists of materials
  • holds the components of metabolic pathways in place
  • involved in cell recognition and signalling
• Cell surface membranes are also called plasma membranes
• State the roles of plasma membranes:

  • to keep each organelle separate from the cytoplasm and the rest of the cell (external environment)
  • controls the entry and exits of materials
  • contain enzymes
  • contains antigens
  • release chemicals that signal to other cells
  • contains receptors
  • the site of chemical reactions
• Why do cell surface membranes contain antigens?

  Immune recognition : so that the immune system can recognise the cell and not attack it
• Why does plasma membranes contain receptors?

  to be the site for cell communication or signalling for chemical signals
• Hormones and drugs may bind to membrane bound receptors and cause a change in the cell's activity
• Define the Fluid Mosaic model:

  the structure of cell membranes with proteins embedded in a sea of phospholipids
• What does the Fluid Mosaic Model explain?

  how cell membranes could be more dynamic and interact more with the cells' environment
• List the features of a plasma membrane according to the fluid mosaic model:

  • cholesterol
  • phospholipids
  • glycolipid
  • glycoprotein
  • carbohydrate molecules / glycocalyx
  • peripheral proteins/ extrinsic
  • integral proteins/ intrinsic
  • carrier proteins
  • channel proteins
  • phospholipid bilayer
  • cytoskeleton filaments
• What kind of proteins are channel and carrier proteins?

  integral
• What are peripheral proteins?

  Proteins that are loosely bound to the surface of the cell membrane.
• What are integral proteins?

  proteins that are permanently bound through the cell membrane
• Define Glycolipids:

  lipids / phospholipids with a chain of carbohydrate molecules attached
• Define Glycoproteins:

  proteins with a chain of carbohydrate molecules attached
• All cells and organelles have a phospholipid bilayer (cell membrane)
• What is the function of peripheral proteins?

  • mechanical support
  • form glycoproteins and glycolipids
  • cell recognition as receptors
• What is the function of integral proteins?

  Transport molecules across the cell membrane
• Define Carrier Proteins:

  proteins that bind to ions and larger molecules and change shape to transport the molecules across the cell membrane
• Define Channel Proteins:

  water-filled tubes lined with hydrophilic amino acids to transport water-soluble molecules across the cell membrane
• Why are cell membranes less fluid in hot temperatures?

  to prevent water and dissolved molecules or ions from leaking out the cell membrane
• The total thickness of a cell membrane is 5 - 10 nm
• The membrane forming the myelin sheath in neurons are about 20 % protein and 76 % lipid
• Explain the factor of increasing temperature on the rate of diffusion and permeability in cell membranes.

  • high temperature causes a high kinetic energy of phospholipids
  • which increases fluidity
  • making the cell membrane more permeable
  • Position and functions of embedded proteins may change
  • this may affect endocytosis and can change the ability of cells to signal by increasing exocytosis
  • if the temperature is too high, channel and carrier proteins and components of the cytoskeleton denatures
  • the structure of the cell membrane can break
  • Cholesterol reduces the increase in membrane fluidity
• State the modes of transport in cell membranes.

  • simple diffusion
  • facilitated diffusion
  • active transport
  • osmosis
  • endocytosis
  • exocytosis
• State the two types of bulk transport.

  endocytosis and exocytosis
• Define Bulk Transport.

  The process by which large substances made from smaller substances enter the cell without crossing the membrane.

  E.g. endocytosis and exocytosis
• Define Passive Process:

  the process depends on kinetic energy
• Define Active Process:

  the process depends on ATP
• What is the roles of glycolipids?

  Cell signalling and cell recognition
• Channel and carrier proteins are both involved in what kind of transport?

  Facilitated diffusion
• Carrier proteins are involved in which kinds of transport?

  facilitated diffusion and active transport
• State the factors affecting membrane permeability:

  • temperature
  • dissolved solvents
  • pH
• Define Omosis:

  A passive process of the net movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane, down the concentration gradient until a dynamic equilibrium is reached
• What does the symbol ψ represent?

  water potential
• Name the states an animal cell can become due to the affect of osmosis:

  H2O IN: lysed / cytolysed

  H2O OUT: shrivelled / crenated
• Define Cytolysis:

  The bursting of an animal cell due to the uptake of too much water through osmosis.
• Define Crenation:

  The shrinking of an animal cell when placed in a hypertonic solution due to large amounts of water moving out through osmosis
• Name the states a plant cell can become due to the affect of osmosis:

  H2O IN: turgid

  H2O OUT: plasmolysed / flaccid
• Define Plasmolysis:

  The shrinking of a plant cell when placed in a hypertonic solution due to large amounts of water moving out through osmosis resulting in the plasma membrane detaching from the rigid cell wall.
• Unit for water potential:

  kPa