2.4 Cell Membranes & Transport

• Function of the cell surface membrane

  Create an enclosed space which separates the internal cell environment from the external environment. They also control the exchange of materials across them
• Describe the structure of phospholipids

  Contain two distinct regions: a polar head and two non polar tails

  • The phosphate head is polar (hydrophilic) and is soluble in water
  • The lipid tail is non-polar (hydrophobic) and is insoluble in water
• Define what a phospholipid bilayer is

  Two layered phospholipids forming sheets
• What type of proteins can be found in the phospholipid bilayer

  Intrinsic (integral) or extrinsic (peripheral)
• Where can intrinsic proteins be found

  Embedded in the membrane
• Where can extrinsic proteins be found

  on the outer or surface of the membrane
• Why does the fluid mosaic model describe cell membranes as ‘fluid’

  1 The phospholipids and proteins can move around via diffusion

  2 Phospholipids move sideways within their own layers

  3 Proteins move around in the bilayer
• Why does the fluid mosaic model describe cell membranes as ‘mosaics’

  The scattered pattern produced by the proteins within the phospholipid bilayer look like a mosaic when viewed from above
• Structure of the of the cell surface membrane

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• Name the three types of lipids found in the cell membrane

  1 Phospholipids

  2 Cholesterol

  3 Glycolipids
• Name the two types of proteins found in the cell membrane

  1 Glycoproteins

  2 Other proteins such as transport proteins
• Describe the phospholipid component of the cell membrane

  Forms a bilayer

  • Hydrophobic tails (fatty acid chains) point in towards the membrane interior
  • Hydrophilic heads (phosphate groups) point out towards the membrane surface
• Describe the cholesterol component found in the cell membrane

  Also have hydrophobic tails and hydrophilic head

  • Fit between phospholipid molecules
  • Are absent in prokaryotes membranes
• Describe the glycolipids found in the cell membrane

  Lipids with carbohydrate chains attached

  • The carbohydrate chains project out into whatever fluid is surrounding the cell
  • Found on the outer phospholipid monolayer
• Describe the glycoproteins found in the cell membrane

  Proteins with carbohydrate chains attached

  • Also project out into whatever fluid is surrounding the cell
  • Found on the outer phospholipid monolayer
• Function of the phospholipids in the cell surface membrane

  1 Act as a barrier to most water-soluble substances this is because of the non-polar fatty acid tails which prevent polar molecules or ions from passing across the membrane

  2 This ensures water-soluble molecules such as sugars, amino acids and proteins cannot escape the cell and unwanted water-soluble molecules cannot get in
• What is one function of cholesterol in the cell membrane?

  Regulates the fluidity of the membrane
• How does cholesterol prevent phospholipids from packing too closely together?

  It is found between phospholipids
• What effect does cholesterol have on the membrane at low temperatures?

  It prevents the membrane from freezing and fracturing
• How does cholesterol stabilize the cell membrane at higher temperatures?

  By stopping phospholipids from becoming too fluid
• What is the mechanism by which cholesterol stabilizes phospholipid tails?

  By binding to the hydrophobic tails of phospholipids
• What is one contribution of cholesterol to the cell membrane's properties?

  Increases impermeability to ions
• What would happen to the cell membrane without cholesterol?

  The membrane would break down and the cell would burst
• Function of glycolipids and glycoproteins in the cell membrane

  Act as receptor molecules

  • Allows them to bind with certain substances on the cell’s surface
• Function of proteins in the cell surface membrane

  Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
• Name the two types of proteins found in the cell membrane

  1 Channel (pore) protein

  2 Carrier protein
• Define Diffusion

  The net movement of particles (molecules or ions) from a region of its higher concentration to a region of its lower concentration down a concentration gradient
• Factors that affect the rate of diffusion

  1 The concentration gradient

  2 Temperature

  3 Surface Area

  4 Properties of the molecules or ions
• Explain how the concentration gradient will affect the rate of diffusion

  A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion
• Explain how temperature affects the rate of diffusion

  As temperature increases, molecules and ions have more kinetic energy at higher temperatures. They move faster which results in a higher rate of diffusion
• Explain how surface area affects the rate of diffusion

  The greater the surface area across which diffusion is taking place, the greater the number of molecules or ions that can cross it.

  • Surface area can be increased by the folding of the cell membrane such as in the cristae in mitochondria
• What substances cannot diffuse through the phospholipid bilayer of the cell membrane

  1 Large polar molecules such as glucose and amino acids

  2 Ions such as Na+ and Cl-
• Define Facilitated Diffusion

  transport of substances across the phospholipid bilayer from an area of higher concentration to an area of lower concentration with the help of a transport molecule. 
• Name the two proteins that enable facilitated diffusion

  1 Channel (pore) proteins

  2 Carrier proteins

  Both HIGHLY specific
• Describe channel proteins

  Water-filled pores

  • They allow charged substances (ions) to diffuse through the cell membrane
  • The pore can move in order to close or open which allows them to control the exchange of ions

  
• Describe Carrier proteins

  Can switch between two shapes

  • The binding site open to one side of the membrane first and then open to the other side of the membrane when the carrier protein switches shape

  
• Define Osmosis

  The net movement of water molecules from a region of high water potential to a region of lower water potential, through a partially permeable membrane, moving down a concentration gradient
• What does it mean if the membrane is partially membrane

  It allows small molecules through such as water but not larger molecules
• Define water potential

  The tendency of water to move out of a solution
• Water potential of pure water

  0 kPa