cell membrane

• Function of membranes

  acts as a barrier between cell and environment.

  Cell recognition.

  Cell communication
• How does plasma membrane act as a barrier between the cell and environment?

  Controls the movement in and out of the cell.

  Partially permeable
• Plasma membrane cell recognition

  glycoproteins act as antigens telling the immune system cell is not a foreign pathogen
• Plasma membrane cell communication


  Receptors on plasma membrane bind to hormones or other chemicals released by other cells
• Functions of intracellular membrane

  - Compartmentalises the cell.

  - Forms vesicles.

  - Controls movement in and out of organelle.

  - Site of chemical reactions
• Compartmentalises the cell: intracellular membrane

  internal membrane axis barrier between cell and cytoplasm, - allowing efficient function by keeping enzymes and molecules in one place
• What is the structure of plasma membrane made up of?

  bilayer of phospholipids, proteins and cholesterol
• fluid mosaic model


  - 'fluid' as the phospholipids are constantly moving around.

  - 'Mosaic' as protein molecules are scattered through the phospholipids
• Phospholipids


  - hydrophilic head face extracellular / intracellular fluid

  - 2 hydrophobic tails, point towards each other.

  Main component of plasma membrane.

  Barrier to anything non-lipid soluble, e.g. ions and glucose
• Glycoproteins


  - proteins with carbohydrates, attached

  - Recognition sites

  - antigens
• Glycolipids


  - Phospholipids with carbohydrates attached.

  - Recognition sites.

  - Antigens.

  - Increases membrane stability by forming hydrogen bonds with water molecules
• Cholesterol


  - lipid

  - Between phospholipid tails.

  - Regulates stability and fluidity of plasma membrane by pushing phospholipids together.
• Intrinsic proteins


  - span both bilayers

  - Acts as channel and carrier proteins to transport, water, soluble molecules
• Extrinsic proteins


  Found on surface of plasma membrane.

  Function is enzymes.

  Catalyse chemical reactions inside cells
• How to cell communicate with each other

  Through hormones and neurotransmitters.
• How are molecules released from cells?

  - by exocytosis and travel in the bloodstream to target cells

  - Target cells detect molecule as having complementary receptors.
• example of cell communication

  liver cells insulin receptors on plasma membrane binds to insulin triggering a change inside the cell lowering blood glucose level
• Example of how drugs work

  - binds to cell membrane receptors

  - triggering response in cell or blocking receptor to prevent from working.

  E.g. heroin binds to opioid receptor in brain triggering processes which reduces pain and promote feelings of well-being
• Three factors that affect permeability of cell membrane

  temperature

  pH.

  Ethanol
• Experiment, testing the permeability of cell membrane

  - beetroot in different temperatures for 15 minutes.

  - Colour meter to measure how much light is absorbed by each liquid

  - darker the solution more light absorbed

  - Absorption against temperature graph
• Permeability before freezing


  - increases as proteins and membrane unfold and become deformed.

  - Molecules and membranes have low amount of energy and cannot move around as much.

  - Phospholipids closely packed together, increasing membrane rigidity.

  - ice crystals to form puncturing membrane

  - increasing permeability damaging cell
• between 0-45 Celsius


  - membrane partially permeable.

  - Temp increases components in membrane gain and kinetic energy to move around.

  - Move fluid the membrane most substances allowed through.
• Temperature over 45 Celsius


  - permeability increases rapidly

  - Proteins in membrane become denature and unravel.

  - Water inside cell cytoplasm, expand putting pressure on cell membrane, creating gaps within bilayer
• Ethanol and membrane permeability

  - ethanol is non polar solvent.

  - Dissolves nonpolar substances, e.g. lipids

  - allows substances to leak into and out the cell.

  ⬆️Ethanol concentration = ⬆️ membrane, permeability
• What happens if ethanol concentration is too high

  disintegrate plasma membrane completely killing the cell
• Forms of transport in cell membrane

  osmosis (transport of water, molecules)

  Diffusion (move down, concentration gradient,)

  Active transport (move against concentration, gradient)

  Endocytosis

  Exocytosis
• simple diffusion


  Down concentration gradient.

  Pass through phospholipid bilayer.

  No energy passive process.

  Oxygen and carbon dioxide from alveoli into bloodstream during gas exchange
• 3 things simple diffusion depends on

  - Steepness of concentration gradient

  - Thickness of exchange surface and diffusion distance

  - Surface area
• Investigating diffusion


  PH, indicator phenolphthalein (pink in alkaline, colourless in acid)

  Cubes of agra jelly containing phenolphthalein and diluted sodium hydroxide (alkaline)

  Beaker containing diluted hydrochloric acid.

  Turn pink to colourless as acid diffuses into the cubes neutralising, sodium hydroxide
• Uses of investigating diffusion

  - temperature : high temperature, turn, colourless quicker

  - Surface area : Small cubes with large SA:V turn, colourless, quicker

  - Concentration : highest concentration of hydrochloric acid, turns colourless quicker
• facilitated diffusion


  Down concentration gradient.

  Carrier proteins and channel proteins, HELP.

  Passive process.
• example of facilitated diffusion

  glucose molecules into liver cells through glucose transport protein channels
• 2 factors, facilitated diffusion, depends on

  concentration gradient : steep = faster

  Number of channel or carrier proteins : more = faster
• Osmosis


  movement of water down concentration gradient.

  Across partially permeable membrane.

  Passive
• Example of osmosis


  Plants absorb water from the soil into their roots > stem > leaves where it is used in photosynthesis
• What has the highest water potential?

  pure water
• Isotonic


  when the concentration of two solutions is the same
• Hypotonic


  Having a lower concentration of solute than another solution
• Hypertonic


  Having a higher concentration of solute than another solution.
• 3 factors the rate of osmosis depends on

  Water potential gradient : steeper = faster

  Surface area : larger = faster

  Thickness of exchange surface : thinner = short diffusion distance = faster