Transport Through Cell Membranes Flashcards
What is diffusion?
The net movement of molecules from a area of high concentration to an area of low concentration
What is meant by equilibrium?
When there is no net movement of molecules due to both concentrations being equal
What type of process is diffusion?
- Passive = no need of input of energy from the cell in the form of ATP
- Slow = only useful over small distances
What are the 5 factors which affect the rate of diffusion?
1) Temperature: Increasing= more kinetic energy= rate of random movement increases= rate of diffusion increases
2) Concentration gradient: Greater the difference= steeper the gradient= faster the diffusion rate
3) Surface area: Greater surface area= more rapid diffusion E.g in red blood cells biconcave shape + epithelial cells lining small intestine have microvilli
4) Length of diffusion pathway: Thinner the membrane= quicker diffusion
5) Size of molecule: Smaller molecules/ ions= diffuse faster
Explain how diffusion occurs through a phospholipid bilayer
- Phospholipid bilayer is very thin at 7nm= short diffusion pathway
- Fluid nature of phospholipids= small temporary gaps which small molecules will fit
- Middle section of bilayer made up of the hydrophobic/ non polar hydrocarbon fatty acid tails
- Smaller molecules e.g oxygen and carbon dioxide can diffuse directly through membrane
- Slightly larger molecules can diffuse through if they are lipid/fat soluble E.g glycerol, vitamins A/D and hormones such as oestrogen
- Water molecules can diffuse directly through in small quantities (osmosis)
What is facilitated diffusion?
- Where diffusion of molecules is not possible, and requires the use of intrinsic proteins which span the whole width of the phospholipid bilayer. It provides a permanent hydrophilic route through what would be a hydrophobic area (phospholipid tails). Is driven by the kinetic energy of the particles.
- Carrier proteins are used in the case of larger, water-soluble molecules and small polar ions. These are specific to a particular type of molecule and when the molecule binds to the protein it changes shape allowing it to pass through the membrane.
What type of molecules are unable to pass through the membrane via diffusion, and therefore require facilitated diffusion?
- larger water-soluble molecules e.g glucose
- charged, polar molecules despite being small e.g sodium ions and calcium ions
What is active transport?
An active process whereby molecules travel against the concentration gradient/ low concentration to high concentration using energy from ATP
Explain how carrier proteins are used in active transport
- Molecule binds to the binding site of the carrier protein. Site is complementary in shape to the molecule (specific)
- Carrier protein changes shape due to the release of energy fro, the breakdown of ATP
- As the carrier protein changes shape the molecule is moved across the membrane against its concentration gradient
- As the molecule is released the carrier protein goes back to its original shape and is ready to take up more molecules
What is osmosis?
The passive movement of water molecules from a high water potential to a low water potential, through a partially permeable membrane. It relies upon the kinetic energy of the water molecules.
What is water potential? Refer to what it measures, its unit, and what makes water potential high or low.
- Measures the pressure water molecules exert in solution and their ability to move freely.
- units are Kpa
- pure water has highest water potential of 0 = each water molecule is free to move
- as solute is added water potential is decreased = when the solute dissolved in water some of the water molecules surround it making a shell.
What is meant by the term isotonic?
- describes 2 neighbouring solutions which have the same concentration of solute molecules, and therefore same water potential
- there is no net movement of water between these 2 solutions so osmosis doesn’t occur
What is meant by the term hypotonic?
- hypotonic solutions have fewer solute molecules than a neighbouring solution, therefore water potential is higher
- net movement of water molecules from the hypotonic solution to hypertonic due to osmosis
What is meant by the term hypertonic?
- Hypertonic solutions have more solute molecules then a neighbouring solution, therefore giving it a lower water potential
- net movement of water molecules from hypotonic to hypertonic solutions through osmosis
What is haemolysis?
When cells swell up then burst releasing cell contents, due to a net movement of water moving into the cell
