Cell Membranes and Transport

• What does the cell membrane appear as under an electron microscope?

  A double line
• What is the width of the cell membrane?

  7-8 nm
• What is the primary function of the cell membrane?

  Separates the living cell from non-living surroundings
• How does the cell membrane control substance movement?

  It regulates what enters and exits the cell

  
• What role does the cell membrane play in nutrient uptake?

  It controls the uptake of nutrients
• What does the cell membrane allow to pass out of the cell?

  Waste products
• What substances does the cell membrane secrete?

  Enzymes and glycoproteins
• What is one function of the cell membrane related to cell recognition?

  It facilitates cell recognition
• How do phospholipids arrange themselves in the membrane?

  They form bilayers with sheets over one another
• What is the nature of the phosphate head of a phospholipid?

  It is polar and hydrophilic

  
• What is the nature of the fatty acid tails of a phospholipid?

  They are non-polar and hydrophobic

  
• What does the phospholipid bilayer allow to enter and leave the cell?

  Lipid-soluble (non-polar) molecules
• What does the phospholipid bilayer prevent from entering the cell?

  Water-soluble (polar) molecules
• How are membrane proteins arranged in the cell membrane?

  Randomly, unlike phospholipids
• What are extrinsic proteins in the membrane?

  Proteins on the surface or partly embedded
• What is one function of extrinsic proteins?

  • Provide structural support
  • They also form recognition sites by identifying cells
• What do intrinsic proteins do in the cell membrane?

  They span the phospholipid bilayer

  
• What is one function of intrinsic proteins?

  • Act as channels or carriers to facilitate the diffusion of polar (water soluble) molecules, such as ions, across the cell membrane.
  • Other intrinsic proteins form pumps and carry out active transport against a concentration gradient.
• What is the fluid mosaic model?

  A model describing membrane structure
• What does the term "fluid" refer to in the fluid mosaic model?

  Phospholipids can move relative to each other
• What does the term "mosaic" refer to in the fluid mosaic model?

  Proteins form a mosaic pattern in the bilayer
• What are the parts of the cell membrane and their functions?

  • Phospholipid bilayer: Basis of membrane, allows transport of small non-polar molecules.
  • Extrinsic proteins: Provide structural support, form recognition sites.
  • Intrinsic proteins: Span membrane, facilitate transport of polar molecules.
  • Cholesterol: Increases rigidity and stability.
  • Glycolipids: Involved in cell-to-cell recognition.
  • Glycoproteins: Stick out of membranes, involved in recognition.

  
• What does selectively permeable mean in relation to the cell membrane?

  It allows certain substances to pass through
• What types of substances can easily pass through the cell membrane?

  Lipid-soluble (non-polar) substances
• How do small uncharged molecules pass through the membrane?

  • By simple diffusion, e.g. oxygen and carbon dioxide, freely pass through the membrane by simple diffusion.

  (small uncharged molecules are non-polar molecules)

  • Lipid soluble molecules such as glycerol can also pass through the membrane, through the phospholipid bilayer.

  
• What impedes the transport of ions and polar molecules across the membrane?

  • The hydrophobic core of the membrane
  • Charged particles, such as ions, and relatively large charged molecules such as glucose cannot diffuse across the non-polar (hydrophobic) centre of the phospholipid bilayer as they are insoluble in lipid.
• How do intrinsic proteins assist in transport?

  They allow particles to cross the membrane
• What is the role of channels and carriers in the cell membrane?

  Facilitate diffusion of polar molecules

  
• What happens to the cell membrane at temperatures above 40°C?

  It becomes increasingly unstable
• How does increased temperature affect membrane permeability?

  • The cell membrane and tonoplast are stable up to a temperature of 40°C.
  • At temperatures above 40°C the cell membrane and tonoplast become increasingly unstable.
  • Increased heat energy leads to increases kinetic energy.
  • The phospholipids vibrate more and more and move further apart.
  • This increases the permeability of the membrane.
  • The proteins within the membrane denature at high temperatures.
  • As the temperature increases cell membrane and tonoplast permeability increases due to increased disruption of the membranes.
• What effect does increasing ethanol concentration have on membranes?

  • Organic solvents such as ethanol dissolve phospholipids.
  • The greater the concentration of ethanol the more permeable the membranes become.
• How does increasing sodium chloride concentration affect membrane permeability?

  • Sodium ions (Na+ ) attach to the oxygen atoms on the hydrophilic (phosphate) heads of the phospholipid bilayer.
  • This reduces mobility of the phospholipid molecules so less betalain is released.
  • As sodium chloride concentration increases the permeability will decrease.
• What is the effect of increasing detergent concentration on membranes?

  • Detergents reduce surface tension of phospholipids and disperse the membrane.
  • As the concentration of detergent increases the permeability of the membranes increase.
• What is diffusion?

  Net movement of molecules or ions from high to low concentration down a concentration gradient until equilibrium is reached.

  (passive process does not require ATP)
• What is a concentration gradient?

  The difference in concentration between areas
• What happens at equilibrium in diffusion?

  Molecules continue to move with no net movement
• What factors affect the rate of diffusion?

  1. Concentration gradient
  2. Distance
  3. Surface area
  4. Thickness of membrane
  5. Temperature
  6. Particle size
• What type of diffusion occurs for non-polar molecules?

  Simple diffusion across the bilayer
• What is facilitated diffusion?

  • Transport of polar molecules using intrinsic proteins
  • Facilitated diffusion can be limited by the number of available intrinsic proteins.

  
• What are channel proteins?

  • Consist of pores lined with polar groups (hydrophilic).
  • This allows charged ions to pass through (such as Na+ ).
  • Each channel protein is specific for one type of ion.
  • They can also open and close depending on the needs of the cell