4 - Transport Across Cell Membranes

• What is meant by the 'fluid-mosaic model'?

  The cell membrane contains multiple components (mosaic) that are free to move in relation to each other
• What are the 5 key components in the cell membrane?

  1. Phospholipids (bilayer)
  2. Proteins
  3. Glycoproteins
  4. Glycolipids
  5. Cholesterol

  
• What is the role of phospholipids in the cell membrane?

  Phospholipids align as a bilayer due to the hydrophilic heads being attracted to water and the hydrophobic tails being repelled by water.
• What is the role of proteins in the cell membrane?

  Proteins are embedded across the cell surface membrane. Integral proteins are protein carriers and channels for transport which allow certain large or otherwise impermeable components to pass through the membrane (i.e. ions, glucose, sucrose)
• What are the role of glycoproteins & glycolipids in the cell membrane?

  Glycoproteins/Glycolipids are carbohydrates attached to peripheral proteins and are involved in cell recognition and as receptors
• What is the role of cholesterol in the cell membrane?

  Cholesterol is present in some membranes too and this will restrict the lateral movement of other molecules in the membrane. This is useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions from leaking out of the cell.
• Cell membranes are partially permeable. Which molecules can pass through the membrane by simple diffusion?

  • Small, non-polar molecules (O2, CO2, N2)
  • Small, uncharged, polar molecules (H2O, glycerol)
• Cell membranes are partially permeable. Which molecules cannot pass through the membrane by simple diffusion?

  • Large, uncharged, polar (glucose, sucrose)
  • Ions (Cl-, K+, Na+)
• What is simple diffusion?

  The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require ATP, and therefore it is a passive process.

  
• What is facilitated diffusion?

  This is a passive process (it does not require ATP) but it differs from simple diffusion as proteins are used to transport molecules. The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins

  
• What are the two types of proteins that are involved in facilitated diffusion and how do they work?

  1. Protein channels form tubes filled with water and this enables water-soluble ions to pass through the membrane. This is still selective, as the channel proteins only open in the presence of certain ions when they bind to the protein.
  2. Carrier proteins will bind with a molecule, such as glucose, which causes a change in the shape of the protein. This shape change enables the molecule to be released to the other side of the membrane
• What is osmosis?

  Osmosis is the movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane

  
• What is the water potential of distilled water?

  0
• What is a hypertonic solution?

  The water potential of a solution is more negative than the cell.
• What is an isotonic solution?

  The water potential is the same in the solution and the cell within the solution
• What is a hypotonic solution?

  The water potential of a solution is more positive (closer to zero) than the cell
• What is an active transport?

  The movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins.
• What's goes on in the process of active transport?

  Certain molecules can bind to the receptor site on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi. This causes the protein to change shape and open towards the inside of the membrane.

  
  

  This causes the molecule to be released on the other side of the membrane. The Pi molecule is then released from the protein, and this results in the protein reverting to its original shape. This is how ATP and carrier proteins are used in active transport.

  
• Which two molecules are absorbed by co-transport in the ileum?

  1. Glucose
  2. Amino Acids
• What are the steps of co-transport?

  1. Sodium ions are actively transported out of the epithelial cell into the blood in the capillary.
  2. This reduces the sodium ion concentration in the epithelial cell.
  3. Sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell.
  4. The protein the sodium ions diffuse through is a co-transporter protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient.
  5. Glucose then moves by facilitated diffusion from the epithelial cell to the blood.

  
• How does surface area affect the rate of movement across a membrane?

  The larger the surface area, the more space there is for transport and therefore this increases the rate of movement.
• How does the number of channel/carrier proteins affect the rate of movement across a membrane?

  If there are more channel/carrier proteins embedded within a membrane, then this will increases the surface area and the rate of movement will increase.
• How does concentration gradient affect the rate of movement across a membrane?

  If the concentration gradient is steeper then the rate of movement will increase because there are more molecules in the concentrated area and therefore more molecules to move across.
• What are examples of cells adapted for rapid transport across the internal or external membranes?

  1. Microvilli
  2. Chloroplasts (thylakoid membrane has a large surface area and many embedded proteins)
• Structure of a microvilli