TRANSPORT ACROSS CELL MEMBRANES

• FUNCTION OF CELL SURFACE MEMBRANES

  • surrounds cells
  • barrier between cell and environment
  • regulates movements of substances into / out of the cell
  • partially permeable
• FUNCTION OF MEMBRANES WITHIN CELLS

  • divides cell into compartments
  • surrounds organelles
  • mitochondria - membrane permeable to respiration substances
  • nucleus - RNA leaves by pores in membrane
  • partially permeable
• STRUCTURE / COMPONENTS IN MEMBRANE

  • phospholipids
  • cholesterol
  • glycolipids
  • glycoproteins
  • proteins

  
• PHOSPHOLIPID ARRANGEMENT

  PHOSPHOLIPID BILAYER

  • polar, charged heads are hydrophilic - attracts / interacts with water
  • non-polar, uncharged fatty acid tails are hydrophobic - repel water
  • naturally arranged with heads facing aqueous environment, shielding tails

  
• HOW DOES THE BILAYER STRUCTURE AID FUNCTION?

  PARTIALLY PERMEABLE AND BARRIER

  • hydrophobic / non-polar molecules can pass through as they do not repel the fatty acid tails
  • small molecules can fit between the phospholipids eg. O2, CO2, H2O
  • polar / charged molecules / ions are water soluble so cannot pass through on their own as they repel the fatty acid tails
  • electrical insulator as charged molecules / ions cannot pass through
• CHOLESTEROL IN MEMBRANE: STRUCTURE

  • type of lipid
  • in all cell membranes
  • fit between the phospholipids
  • bind to hydrophobic tails as mainly non-polar
  • causes the tails to pack more closely together becoming more rigid, less fluid

  
• CHOLESTEROL IN THE MEMBRANE: FUNCTIONS

  • maintains fluidity, stability and strength of the membranes
  • restricts movement of the phospholipids
  • reduce lateral movement of molecules and phospholipids
  • makes membrane less fluid at high temperatures to prevent damage
  • prevents loss of water and dissolved ions as tails are hydrophobic and closer together

  
• GLYCOLIPIDS IN MEMBRANE: STRUCTURE

  • polysaccharide (carbohydrate) chain
  • covalently bonded with a lipid
  • extends from the bilayer into aqueous environment

  
• GLYCOLIPIDS IN MEMBRANE: FUNCTIONS

  • helps cells attach together to form tissues
  • recognition sites
  • maintains stability of membranes
  • cell-surface receptor sites

  
• GLYCOPROTEINS IN MEMBRANE (STRUCTURE)

  • polysaccharide (carbohydrate) chain
  • bonded to an extrinsic protein
  • extends from the bilayer into aqueous environment

  
• GLYCOPROTEINS IN MEMBRANE: FUNCTIONS

  • allows cells to recognise each other eg. Lymphocytes as own cells
  • helps cells attach together to form tissues
  • recognition sites

  
• PROTEINS IN THE MEMBRANE

  TYPES OF PROTEINS

  1. Intrinsic / integral

  • channel proteins
  • carrier proteins

  2. Extrinsic / peripheral

  
• INTRINSIC PROTEINS IN MEMBRANE

  • hydrophobic amino acids on the outsides
  • interact with the tails inside
  • transmembrane
  • transport function

  
• EXTRINSIC PROTEINS

  • on surface
  • structural role
  • enzymes, receptors
  • detect chemicals

  
• PROTEIN CHANNELS

  • form pores in the membrane which charged / polar / hydrophilic particles diffuse through
  • protein channel has a central pore, lined with hydrophilic amino acids and is filled with water
  • protein channels are selective and different protein channels facilitate diffusion of different charged particles so only some chemicals can pass through.
  • EXAMPLE: some only open in response to certain triggers like a chemical or certain ions binding to it like a neurotransmitter or change in voltage across the membrane.

  
• PROTEIN CARRIERS

  • move large molecules across the membrane
  • specific large molecule Attaches to a specific binding site on carrier protein in the membrane. Then the protein changes shape (conformational change) due to its tertiary structure being altered to released the molecule on the opposite side of the membrane.

  
• FLUID MOSAIC MODEL 


  FLUID

  • individual phospholipids can move past each other
  • Flexible structure that is constantly moving and changing shape
  • moves but never exposes fatty acid tails and is stable due to cholesterol

  MOSAIC

  • proteins, glycoproteins and glycolipids in bilayer vary in shape and size
  • creates a mosaic-like pattern from above

  MODEL

  • best representation of membrane structure based on evidence at this time

  
• 1, TEMPERATURE AND PERMEABILITY

  <0°C

  • phospholipids don’t have much energy so can’t move very much
  • packed close together, rigid
  • channel proteins and carrier proteins denature
  • so permeability is high
  • ice crystals may pierce the membrane when they thaw

  
• 2. TEMPERATURE AND PERMEABILITY

  0°C - 45°C

  • lowest permeability at 0°C where proteins aren’t denatured but rigid structure
  • phospholipids gain kinetic energy as the temperature increases
  • they move more and aren’t packed as tightly
  • as temperature increases permeability increases

  
• 3. TEMPERATURE AND PERMEABILITY

  45°C

  • damage to cell surface membranes
  • proteins denature and can’t control what enters or leaves
  • water inside cell expands and puts pressure on membrane and damages it
  • increases fluidity and damage to phospholipid bilayer

  
• SIMPLE DIFFUSION: the net movement of particles from a region of high concentration to a region of low concentration, down the concentration gradient through a partially permeable membrane. Without the use of metabolic energy (ATP from respiration) as it is a passive process.


  • until equilibrium is reached where particles have spread out evenly, diffusion stops.
  • the direction of movement is random.
  • simple diffusion means molecules diffuse directly through the membrane.

  
• WHAT CAN GO THROUGH MEMBRANE BY SIMPLE DIFFUSION AND WHY?

  Particles that move freely through a membrane can diffuse through it. They must be able to flow and have kinetic energy.

  • GASES - oxygen and carbon dioxide diffuse into and out of the cell for respiration.
  • HYDROPHOBIC MOLECULES - don’t repel from the hydrophobic fatty acid tails.
  • NON-POLAR / UNCHARGED - don’t repel from the hydrophobic acid tails
  • WATER - small molecule
  • LIPID-SOLUBLE (capable of dissolving lipids) - dissolve in phospholipid bilayer to move through it
  • SMALL MOLECULES - fit between phospholipids
• FACTORS AFFECTING RATE OF SIMPLE DIFFUSION

  1, CONCENTRATION GRADIENT

  • greater = faster
  • as diffusion takes place, concentration difference reduces until equilibrium so diffusion slows down until it’s reached
• FACTORS AFFECTING RATE OF SIMPLE DIFFUSION

  2. SURFACE AREA

  • greater = faster
  • more area / space exposed for particles to use and diffuse through
• FACTORS AFFECTING RATE OF SIMPLE DIFFUSION

  3. DISTANCE / THICKNESS OF SURFACE

  • shorter / thinner = faster
  • particles have less distance to cover
• FACTORS AFFECTING RATE OF SIMPLE DIFFUSION

  4. TEMPERATURE

  • warmer = faster
  • more kinetic energy, move faster
• FACTORS AFFECTING RATE OF SIMPLE DIFFUSION

  5. PARTICLE SIZE / MASS

  • Smaller / lighter = faster
  • at any given temperature, the diffusion of a smaller particle is faster than bigger as smaller particles can move faster
• FICK’S LAW AND CALCULATING RATE

  RATE OF DIFFUSION a (SURFACE AREA X CONCENTRATION GRADIENT) / THICKNESS OF MEMBRANE

  HIGHEST WHEN:

  • surface area = increases
  • concentration gradient = increases
  • thickness of membrane = decreases

  ON A GRAPH:

  • straight line = gradient
  • curved = tangent

  
• EXAMPLES OF EFFICIENT DIFFUSION

  LUNGS - ALVEOLI:

  • thin exchange surface - 1 cell thick
  • large surface area
  • steep concentration gradient of oxygen and carbon dioxide
  • oxygen moves from alveoli to blood
  • carbon dioxide moves from blood to alveoli

  
• EXAMPLES OF EFFICIENT DIFFUSION

  SMALL INTESTINE - MICROVILLI:

  • Microvilli on epithelial cells provide 600 x surface area
  • thin walls
  • steep concentration

  
• LIMITATIONS IMPOSED BY THE PHOSPHOLIPID BILAYER

  • only certain molecules can passively pass through the phospholipid bilayer
  • this is due to it being made of phospholipids which have a polar, hydrophilic head and non polar, hydrophobic fatty acid tails shielded by the heads.
  • They are arranged closely in the fluid mosaic model too which only allows small molecules to pass through.

  
• FACILITATED DIFFUSION: passive net movement of particles across a cell membrane from an area of high concentration to an area of low concentration, down a concentration gradient through a transport protein in the cell membrane. 


  • use integral, transmembrane proteins to transport molecules through the membrane in protein channels and carrier proteins.
  • a uniporter is a membrane transport protein that transports a single species of substrate (charged or uncharged) across a cell membrane.

  
• WHAT CAN PASS THROUGH BY FACILITATED DIFFUSION?

  • larger molecules like glucose or amino acids
  • charged / polar / hydrophilic molecules
  • water soluble molecules
  • molecules that can’t readily pass through the cell membrane via simple diffusion
• FACTORS AFFECTING RATE OF FACILITATED DIFFUSION

  1. CONCENTRATION GRADIENT

  • Greater = faster
  • Up to a point if all proteins are in use. As equilibrium reaches, rate plateaus.

  2. NUMBER OF PROTEIN CHANNELS / CARRIERS

  • More = faster
  • once all in use, the rate of diffusion cannot increase even if other factors Increase.
• OSMOSIS: the movement of water from an area of higher water potential (less negative) to an area of lower water potential (more negative) over a partially permeable membrane. this is a passive process and doesn’t require metabolic energy (ATP from respiration).


  • only the movement of water molecules
  • AQUAPORINS: special types of protein channels that allow the facilitated diffusion of water through cell membranes. Kidney cells are adapted to have lots of them to help them in reabsorption of water.

  
• WATER POTENTIAL: the likelihood (potential) of water molecules to diffuse out of or into a solution.


  • pure water is 0. Can’t get a water potential higher than .
  • adding a solute lowers water potential, making it more negative. more concentrated solution is more negative with more solute.
  • higher water potential in hypotonic.
  • lower water potential in hypertonic.

  
• WATER POTENTIAL

  Water moves from:

  • hypotonic —> hypertonic
  • high water potential —> low water potential
• HYPOTONIC: water potential is higher (closer to 0) than water potential of cell.
• ISOTONIC: water potential same inside and outside the cell.
• HYPERTONIC: water potential of solution is lower (more -ve) than water potential of cell.