Transport across membrane

• why is bulk transport different to other transport systems?

  involves membranes pinching off
• what is the difference between passive and active transport?

  passive allows molecules to pass across membrane through a concentration with no ATP required, whereas active transport forces molecules against concentration gradient with ATP required
• where does energy for passive transport come from?

  no cellular energy - energy comes from kinetic energy from the particles
• what is diffusion?

  The net movement of particles down a concentration gradient, from a region of high concentration to a region of low concentration
• what is simple diffusion?

  Small non-polar substances (O, CO2, lipid soluble molecules, steroid hormones, small polar molecules - water) can diffuse straight across the phospholipid bilayer without need for specialised channels or carriers
• what will increase rate of diffusion into the cell?

  • increase temp
  • increase concentration gradient
  • increase surface area
  • decrease amount of substance inside cell
  • increase amount of substance outside cell
• what will decrease rate of diffusion into a cell?

  • decrease temp
  • decrease concentration gradient
  • decrease surface area
  • increase diffusion path
  • increase amount of substance inside cell
  • adding more water outside cell
• what is Fick’s Law?

  rate of diffusion is proportional to : (Surface Area * Concentration)/Distance
• how will small polar molecules diffuse across a membrane?

  slowly, won’t be able to move through non-polar tails (water, urea) - can be transported through aquaporins
• how do small non polar molecules diffuse across a membrane?

  quickly, no need for carrier proteins (oxygen, carbon dioxide, caffeine, alcohol)
• how do large polar molecules diffuse across a membrane?

  not at all - glucose, amino acids (will need facilitated diffusion)
• how do large non polar molecules diffuse across a membrane?

  quickly - steroid hormones
• how do small/large ionic molecules diffuse across a membrane?

  not at all, channels are very selective and may also be gated (nitrogen ions, any ions)
• what molecules use facilitated diffusion?

  highly charged molecules, large molecules, hydrophilic particles (glucose, amino acids) - use channel or carrier proteins
• what are ion channels?

  • channel-mediated diffusion - provide a route for charged molecules such as sodium ions down the concentration gradient
  • are very selective, only permitting the passage of a single type of ion
  • may also be gated - diffusion only happens when gate is open
• what are carrier proteins?

  • carrier-mediated diffusion - provides a route for large molecules like glucose, down the concentration gradient
  • solute molecule binds to carrier protein, it undergoes a conformational change depositing solute molecule on opposite side of membrane
• how is facilitated diffusion affected?

  • number of channel proteins
  • temperature
  • concentration gradient steepness
  • thickness of membrane
  • surface area
• what is active transport?

  the movement of molecules or ions out of a cell from a region of lower concentration to a region of higher concentration, this process requires ATP and carrier proteins.
• what are some specific uses of active transport?

  • uptake of glucose and amino acids in the small intestine
  • absorption of mineral ions by plant roots
  • excretion of hydrogen ions and urea by kidneys
  • exchange of sodium and potassium ions in neurons and muscle cells
• what are the 5 steps of how active transport in brought about?

  • transport is through carrier proteins spanning the cell membrane
  • molecule binds to a receptor complementary in shape on the protein
  • ATP binds to the carrier protein from the inside of the cell and it is hydrolysed into ADP + Pi
  • This causes the carrier protein to change shape and release the molecule to the other side
  • The phosphate ion is then released and the protein returns to its original shape
• what is endocytosis?

  The bulk transport of material into the cell, split into phagocytosis, pinocytosis and receptor-mediated endocytosis
• what is exocytosis?

  The bulk transport of material out of the cell
• what is the difference between phagocytosis and pinocytosis?

  Phagocytosis is movement of solid materials whereas pinocytosis is the movement of liquids
• what is the difference between pinocytosis and reverse pinocytosis?

  pinocytosis is liquid taken into the cell whereas reverse pinocytosis is liquid taken out of a cell
• why is ATP needed in bulk transport?

  • changing membrane shape
  • vesicles forming and fusing
• what are the steps of endocytosis?

  • engulf extracellular material
  • plasma membrane changes shape (using ATP)
  • vesicles form and is taken into cytoplasm where is may join with a lysosome forming a phagolysosome
• what are the steps of receptor-mediated endocytosis?

  • pits with receptors on membrane surface - coats the pits
  • LDL binds to receptors
  • clathrin link up - creates a sturdy shell
  • vesicles pinches off - clathrin detaches
  • endosomes allow LDL to be separated from LDL receptor
• what is osmosis?

  The diffusion (net movement) of water molecules from a region of high water potential (concentration of free water molecules) to a region of lower water potential, through a partially-permeable membrane down a water potential gradient
• what is water potential?

  the tendency of water molecules in a system to move, it is measured in KiloPascals (kPa) - units of pressure

  • pure water has the highest water potential, value of 0kPa
  • solutions have a lower water potential than pure water, have a negative water potential
• what is the solute potential?

  the contribution that solutes make to the water potential of a solution, is a negative value

  • greater the amount of solute, the lower the water potential - because of water molecules binding to the solute molecules, reducing the number of water molecules that are free to diffuse
• what is pressure potential?

  • affects water potential - greater the pressure, the higher the water potential
  • always a positive value
• how do you calculate water potential?

  water potential = solute potential + pressure potential
• what is tonicity?

  the ability of an extracellular solution to make water move into or out of a cell by osmosis

  • HYPER = higher solution
  • HYPO = lower solution
  • ISO = same solution
• what does LDL stand for?

  low density lipoproteins