2.1.5 Biological Membranes

• What is the main role of the plasma membrane?

  Controls what enters and leaves the cell
• Roles of the plasma membrane:

  1. Controls what enters and leaves the cell
  2. Separates cell contents from environment
  3. Site of chemical reactions
  4. Cell signalling
• Membranes are partially permeable barriers that control the movement of substances in and out of the cell
• What is the main role of internal membranes?

  Compartmentalization -> separates specific processes
• What model describes membrane structure?

  Fluid mosaic model
• Why the fluid mosaic model?

  Fluid -> as the phospholipids move around

  Mosaic -> because there are many different components
• Components of the fluid mosaic model

  • Bilayer -> phospholipids
  • Proteins
  • Cholesterol
  • Glycolipids
  • Glycoproteins
• What is the role of cholesterol in membranes?

  Regulates membrane fluidity
• Give the two types of proteins that can be found in a membrane?

  Intrinsic

  Extrinsic
• What are intrinsic proteins?

  Proteins that are embedded in the bilayer and span the whole bilayer
• What type of protein spans the whole bilayer?

  Intrinsic
• What do intrinsic proteins act as?

  Channel or carrier proteins
• What are extrinsic proteins?

  Proteins that are found on either surface of the bilayer
• What do extrinsic proteins act as?


  Enzymes or receptors
• Glycoproteins are often receptors
• Glycolipids are often antigens
• How does cholesterol give membranes mechanical strength?

  Holds the phospholipid tails together
• How does cholesterol reduce membrane fluidity?

  Inhibits movement of the phospholipids
• Cholesterol makes the membrane less permeable to water and ions
• Channel proteins provide a hydrophilic channel, allowing the movement of polar molecules across a membrane
• What effect does alcohol have on fatty substances?


  Acts as a solvent
• Solvents dissolve lipids
• The shape of a glycoprotein / glycolipid may be complementary to the shape of a signalling molecule in the body, which can be used as binding sites
• Pores allow the movement of molecules that cannot dissolve in the phospholipid bilayer
• Channel proteins provide a hydrophilic channel allowing the movement of polar molecules
• Carrier molecules allow facilitated diffusion
• Name the pore that carries water

  Aquaporin
• Factors that affect the rate of diffusion:

  1. Temperature
  2. Concentration gradient
  3. Size of molecule
  4. Diffusion distance
  5. Surface area
• How does an increase in temperature affect the structure and permeability of a membrane?

  Molecules gain kinetic energy. This increases the membrane's permeability, and any molecule will diffuse quickly. This is because as the phospholipids move about, they leave temporary gaps between them
• What happens to the membranes structure if there is a drastic increase in temperature?

  Bilayer may lose its mechanical stability and the membrane becomes even more permeable. The proteins will denature, and eventually the membrane will become completely permeable
• Solvents dissolve lipids
• Solvents such as alcohol dissolve fatty substances, so increase membrane permeability
• As the concentration of alcohol increases, the membrane is more likely to dissolve
• What is passive transport?


  Movement of molecules across a cell membrane without the use of metabolic energy.
• 3 examples of passive transport


  Diffusion

  Facilitated Diffusion

  Osmosis
• 3 examples of transport requiring ATP


  Active transport

  Endocytosis

  Exocytosis
• What molecules can diffuse across the membrane?

  Small, non-polar molecules.
• What is facilitated diffusion?

  Facilitated diffusion is the passive movement of molecules across a cell membrane with the help of transport proteins.
• What is osmosis?

  Osmosis is the movement of water molecules from an area of lower solute concentration to an area of higher solute concentration across a partially-permeable membrane.
• What is active transport?

  Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient, requiring energy in the form of ATP.