transport across cell membrane

• name the structures of a phospholipid?

  hydrophilic head - polar, attracts water

  hydrophobic tails - non polar, repels water

  
• how would phospholipids behave if added to a beaker of water?

  phospholipids form a double layer (bilayer)

  this protects the hydrophobic tails from the water inside and out
• phospholipid arrangement in water?

  hydrophilic heads face the water = extracellular

  hydrophobic tails point towards each other, away from the water = intracellular (fatty acid chains either saturated or unsaturated)
• phospholipid bilayer around organelles?

  the plasma membrane surrounding organelles also contain a phospholipid bilayer - with cytoplasm on the outside and aqueous solution inside the organelle.
• why are cell and organelle membranes described as a fluid mosaic model?

  fluid - movement of molecules

  mosaic - different components embedded within the phospholipid bilayer e.g. glycoproteins, glycolipids
• explain the role of the phospholipid bilayer?

  • allows small, non polar, LIPID soluble substances to enter and leave the cell
  • prevents large, polar, charged or WATER soluble substances entering and leaving the cell
  • gives the membrane flexibility and fluidity
• explain the role of cholesterol molecule?

  restricts the (lateral) movement of other molecules making up the membrane

  makes membrane less fluid/flexible - more ridged

  prevents leakage of water and dissolved ions from the cell

  it is important that it is less fluid at high temperatures this is because if the membranes become too fluid when your hot, the gaps between the phospholipids will increase (too large) and water, dissolved ions could leak in or out cells which can cause dehydration and cells to burst.
• where are protein molecules found?

  protein molecules float about in the bilayer
• what are intrinsic proteins?

  span across the bilayer from one side to the other and are used for transport e.g. channel + carrier proteins
• what are extrinsic proteins?

  attach either to outside or the inner part of the bilayer, do NOT extend across the entire bilayer

  they are connected to proteins or lipids to make glycolipids + glycoproteins

  function as: receptors, enzymes, antigen and for cell recognition
• why do proteins have a different tertiary structure and shape?


  this allows them to be specific to certain molecules
• what do intrinsic and extrinsic protein molecules have in common?

  they both transport molecules from one side of the membrane all the way across to the other side of the bilayer.
• which substances would be able to move through a channel protein?

  • protein channel has tubes filled with water to enable water - soluble, ions (charged) to dissolve and diffuse across the bilayer
  • protein channel is selective and only opens in the presence of certain ions
• explain how carrier proteins work?

  1. specific polar ion/molecule binds to the binding site which is complementary to the protein
  2. carrier protein binds to large molecules e.g. glucose
  3. causes the carrier protein to change shape and 'flips' the molecule across to the other side of the membrane
• what is a glycoprotein?

  carbohydrate bonded to a protein

  • acts as a receptor or antigen
  • synthesised by Golgi apparatus
• what is a glycolipid?

  carbohydrate bonded to a phospholipid

  • acts as a receptor or antigen
  • synthesised by SER + Golgi apparatus
• which substances can pass through the phospholipid bilayer and what type of transport do they use?


  only simple soluble substances e.g. H2O, O2, CO2

  uses simple diffusion
• which substances cant pass through the phospholipid bilayer (on their own)?

  water - soluble substances e.g. glucose because it is large and polar
• definition of simple diffusion?

  movement of molecules from a higher concentration to a lower concentration down a concentration gradient

  • allows small, non polar, lipid soluble substances directly through the phospholipid bilayer
  • doesn't require ATP
• definition of facilitated diffusion?

  movement of molecules from a high concentration to a lower concentration down a concentration gradient using a channel or carrier protein.

  • used for large, polar water - soluble substances
  • enables ions, polar molecules which CANNOT simply diffuse, be transported across the membrane
  • doesn't require ATP
• name three factors that effect membrane fluidity?

  cholesterol

  temperature

  amount of unsaturated fatty acids
• how can cells be adapted for rapid transport across their membranes?

  • large surface area
  • increase temperature
  • increase the number of channel/carrier proteins
• define 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 down a water potential gradient.
• what are the units + symbol for water potential?

  kilopascals (kPa)

  
• what does it mean if the water potential is extremely negative?

  means more solute is dissolved in it
• what is the water potential for pure water (value)?

  0 kpa
• water potentials

  the more concentrated the solution is (more solute) the lower its water potential is.
• importance of mitosis?

  • allows urine formation in the kidneys
  • allows plants to absorb water from the soil
  • the more concentrated the solution is (more solute + less water) the lower the water potential will be
• cells will absorb or lose water depending on which type of solution they are in....

  isotonic - water potential on both sides of the membrane is the same (equilibrium)

  hypotonic - higher water potential, lower solute concentration

  hypertonic - lower water potential, higher solute concentration
• what happens to animal cells + plant cells placed in hypotonic solutions?

  water will move into cell via osmosis

  causes cell to burst and undergo osmotic lysis because they dont have a cell wall

  plant cells wouldn't burst, cell wall becomes turgid (strengthens)
• what happens to animal and plant cells placed in a hypertonic solution?

  water will leave the cell via osmosis

  causing the cell to shrivel
• describe the factors that affect osmosis?

  1. temperature - kinetic energy of water molecules
  2. water potential gradient - higher WPG on one side compared to the other, quicker the water molecules will diffuse
  3. thickness of the membrane - shorter the diffusion pathway the faster the rate of diffusion
• define active transport?

  movement of molecules against their concentration gradient (low to high concentration) using a CARRIER protein and ATP
• importance of active transport?

  allows for control of absorption of molecules/ions

  maximises absorption of substances e.g.

  absorbing glucose + amino acids in small intestine
• explain how carrier proteins are used in active transport?

  1. specific ion/molecule binds to binding site on carrier protein
  2. protein changes shape and 'flips' molecule to the other side
  3. ATP is hydrolysed into ADP + pi which provides energy to move molecule against its concentration gradient
  4. phosphate molecule released from the protein, protein reverted back to its original shape
• what is co - transport?

  movement of two substances at the same time using a carrier protein

  • e.g. sodium ions + glucose