transport across membranes

• the membrane is fluid, it moves giving the membrane flexibility
• it’s a mosaic because the different shapes proteins (glycoproteins and cholesterol) are embedded in the bilateral
• phospholipid bilayer made up of

  • hydrophilic head facing out and hydrophobic tails facing in
  • function; allows small UNCHARGED NON POLAR LIPID SOLUBLE molecules through
  • provides flexibility as constantly moving
• cholesterol

  • is the in bilayer
  • increases strength and stability of the membrane
• glycoprotein

  • glycogen attached to a membrane protein
  • act as receptors for hormones
  • antigens for cell recognition
  • points for cell adhesion
• glycolipid is glycogen attached to a membrane by phosholipid

  and acts as a receptor for hormones

  antigens for cell recognition

  points for cell adhesion
• factors affecting membrane permeability:

  • temperature (can denature the proteins )
  • pH
  • solvents (can dissolve phospholipids
• a solvent is a substance that dissolves other substances, called solutes, to form a solution
• on a graph to show the effect of temperature on membrane permeability

  at the start the phospholipids don’t have much energy so can move very much, they are closely packed together and the membrane is rigid. but channel proteins and carrier proteins in the membrane denature (losing structure and function) which increases permeability of the membrane, ice crystals may form and pierce the membrane making it highly permeable when it thaws
• on a graph to show the effect of temperature on membrane permeability, at around 20° the phospholipids can move around and stents packed as tightly together

  • the membrane is partially permeable
  • temp increases so the phospholipids move more due to increased energy
  • which increases the permeability of the membrane
• on a graph to show the effect of temperature on membrane permeability

  at around 40°, the phospholipid bilayer starts to melt and break down and the membrane becomes more permeable,

  • water inside the cell expands
  • putting pressure on the membrane
  • channel and carrier proteins in the membrane denature so canr control what enters or leaves
  • increasing the permeability of the cell
• a mineral ion is a charged molecule
• a drug wanting to rapidly enter a cell should be:

  • highly permeable
  • lipid soluble
• Cl- ions would need to pass through the cell surface membrane through proteins in the membrane with facilitated diffusion
• the protein content of a energy releasing organelle is higher because mitochondria take in and release large molecules (glucose, oxygen co2) ,during respiration, which only move through protein channels as they are big
• a function of the membrane surrounding a chloroplast is to keep enzymes needed for photosynthesis all in one place making the process more efficient
• Part 1 required practical: investigating how temperature affects membrane permeability

  1. use a scalpel to carefully cut five equal sized pieces of beetroot, rinse the pieces to remove any pigment
  2. add the five pieces to different test tubes each containing 5cm3 of water
  3. place each test tube in water bath at a different temperature eg, 10,20,30,40,50 for the same length of time
• solvents include alcohols like ethanol and methanol, these can dissolve the lipids in the phospholipid bilayer, increasing the membranes permeability
• factors affecting diffusion

  • concentration gradient
  • surface area of exchange surface
  • thickness of exchange surface (diffusion distance)
• concentration gradient (diffusion)

  • the steeper the concentration gradient the faster the rate of diffusion,
  • ventilation and blood circulation maintains a steep o2 conc between the alveoli and the blood
• surface area of the exchange surface (diffusion)

  • larger the surface area of the cell membrane, the faster the rate of diffusion
  • lots of alveoli in the lung
  • lots of microvilli in the small intestine
• thickness of exchange surface (diffusion)

  • the thinner the exchange surface the shorter the distance the molecules have to travel so faster rate of diffusion
• diffusion across the phospholipid membrane

  • number of channel/ carrier proteins in the membrane
  • size and charge of the diffusing molecules
  • temperature
• diffusion across the membrane is affected by

  size and charge of the diffusing molecules

  • eg, lipid soluble/ non polar molecules diffuse more quickly through the phospholipid bilayer
  • but water soluble/ polar diffuse slower through protein channels/ carrier
• diffusion across the phospholipid is affected by

  • temperature :
  • kinetic energy of particles diffusing
• Diffusion


  The net movement of molecules from a high concentration to a low concentration. It is passive and random

  Small non-polar molecules move through the phospholipids such as oxygen and carbon dioxide
• Factors affecting diffusion


  The concentration gradient - the bigger the gradient the faster the rate of diffusion

  Temperature - a higher temperature increases kinetic energy therefore increases the rate of diffusion

  Surface Area - a greater surface area increases the rate of diffusion
• Facilitated Diffusion


  The passive net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient through a channel or carrier transport protein
• Factors affecting facilitates diffusion

  Surface area

  Diffusion distance

  Concentration difference

  Number of protein channels
• Active Transport


  The energy requiring movement of molecules from a low concentration to a high concentration across a partially permeable membrane using carrier proteins and ATP

  Energy is provided by ATP which is produced during respiration
• The process of active transport

  ATP binds to the carrier protein and is hydrolysed to ADP + P. The phosphate molecule then binds to the protein causing it to change shape and release the molecule on the other side of the membrane.
• Osmosis


  The movement of water molecules from an area of high water potential to an area of low water potential across a partially permeable membrane
• Water Potential


  The measure of water molecules to move freely in a solution

  Some water molecules collide with the membrane as they move around at random. This exerts a pressure on the membrane - this is water potential

  It is measured in kPa, the unit of pressure
• Water potential of pure water

  0 kPa - the molecules can move very easily
• Water potential of a dilute solution

  -100 kPa - the water molecules cannot move very easily
• Water potential of a concentrated solution

  -200 kPa - it is very difficult for the water molecules to move
• Hypotonic Solution


  Has a higher water potential than the cell
• Hypertonic Solution


  Has a lower water potential than the cell
• Isotonic Solution


  Has the same water potential as the cell
• Incipient Plasmolysis


  This means 'about to plasmolyse'

  This is when the cytoplasm of plant cells just begins to shrink away from the cell wall

  50% of cells will be plasmolysed when the mean solute potential equals the water potential of the external solution