2.5

• 3 functions of membranes at cell surface(plasma membranes)

  1)recognition from other cells

  2)cell communication/signalling

  3)barrier between the cell and it's environment, controlling which substance enter and leave the cell- they're partially permeable
• What are the 3 functions of membranes within a cell?

  • form vesicles to transport substance between different areas of the cell
  • Membranes within cells, site of chemical reaction
  • Membranes around the organelle divide the cell into different compartments-act as a barrier between the organelle and cytoplasm
• Basic components of all membranes

  • lipids(mainly phospholipids)
  • proteins
  • carbohydrates(usually attached to proteins or lipids)
• Fluid mosaic model

  Arrangement of the molecules in the membrane
• Components in the fluid mosaic model

  • Phospholipid molecules form a continuous, double layer called a bilayer
  • Bilayer is 'fluid' because phospholipids are constantly moving
  • Cholesterol molecules are present within the bilayer
  • Protein molecules are scattered through the bilayer(like mosaic tiles)
  • Some proteins have a polysaccharide chain attached - called glycoproteins
  • Some lipids have a polysaccharide chain attached - called glycolipids
• Why do phospholipids form a barrier to dissolved substances?

  • The head is hydrophilic - it attracts water - face outwards
  • The tail is hydrophobic - it repels water - face inwards
  • Molecules automatically arrange themselves into a bilayer
  • Centre of bilayer is hydrophobic so the membrane doesn't allow water-soluble substances eg ions through it-it acts as a barrier to these dissolved substances
  • But fat-soluble substances, can dissolve in the bilayer and pass directly through the membrane
• Cholesterol give the membrane stability

  • Cholesterol is a type of lipid
  • All cell membranes except bacterial cell membranes
  • Cholesterol fit between the phospholipids and bind to the hydrophobic tails, causing them to pact more closely together. This makes the membrane less fluid and more rigid
  • At lower temperatures, it prevents phospholipids from packing too close together and so increases membrane fluidity
• What do glycolipids and glycoproteins act as ?

  • Act as receptors for message molecules
• How do glycoproteins/lipids act as receptors?

  • stabilise the membrane by forming hydrogen bonds with surrounding water molecules
  • sites where drugs, hormones and antibodies bind too
  • act as receptors for cell signalling
  • antigens
• How does temperature affect the membrane permeability?

  Increasing the temperature increases the membrane permeability

  (directly proportional)
• Membrane permeability affected by temperatures below 0

  • The phospholipids don't have much energy, so they can't move very much. They're packed closely together and the membrane is rigid
  • But channel and carrier proteins in the membrane deform, increasing the permeability of the membrane
  • Ice crystals may form and pierce the membrane making it highly permeable when it thaws
• How is the membrane permeability affected by temperature between 0 and 45 ?

  1)The phospholipids can move around and aren't packed as tightly together-the membrane is partially permeable.

  2)As temperature increase the phospholipids move more because they have more energy-this increases the permeability of the membrane
• How is the membrane permeability affected by temperature above 45?

  1)The phospholipids bilayer starts to melt-membrane becomes more permeable

  2)Water inside the cell expands, putting pressure on the membrane

  3)Channel and carrier proteins deform so they can't control what enters or leaves the cell -this increases the permeability of the membrane
• Diffusion?

  1)Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration

  2)Particles diffuse down a concentration gradient

  3)Diffusion is a passive process
• What type of molecules can diffuse through the cell membrane and why?

  1)Small, non-polar molecules (such as carbon dioxide and oxygen) are able to diffuse easily through spaces between phospholipids

  2)Small uncharged polar molecules(such as water)are small enough to fit between phospholipids, so is able to diffuse across plasma membranes
• What are the 4 factors that affect the rate of diffusion and why?

  1)Concentration gradient-the higher it is, the faster the rate of diffusion

  2)The thickness of the exchange surface-the thinner the exchange surface (shorter the distance the particles have to travel), the faster the rate of diffusion

  3)The surface area-the larger surface area, the faster the rate of diffusion

  4)The temperature-the warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster
• What is facilitated diffusion and why is it needed?

  1)Some larger molecules, ions and polar molecules don't diffuse directly through the phospholipid bilayer of the cell membrane

  2)Instead they diffuse through carrier or channel proteins in the cell membrane this called facilitated diffusion

  3)Particles move down a concentration gradient, from a higher to a lower concentration

  4)It's a passive process
• How do carrier proteins move molecules into or out of the cell down a concentration gradient ?

  Different carrier proteins facilitate the diffusion of different molecules

  1)First, a large molecule attaches to a carrier protein in the membrane

  2)Then, the protein changes shape

  3)This releases the molecules on the opposite side of the membrane
• How does endocytosis work?

  1)Some molecules are way too large to be taken into cells by carrier proteins

  2)Instead a cell surrounds a substance with a section of its plasma membrane

  3)The membrane then pinches off to form a vesicle inside the cell containing the ingested substance

  4)Like active transport, this process also uses ATP for energy
• How does exocytosis work?

  1)Vesicles containing substances produced by the cell pinch off from the sacs of the Golgi apparatus and moves towards the plasma membrane

  2)The vesicles fuse with the plasma membrane and release their contains outside the cell

  3)Exocytosis uses ATP as an energy
• How do channel proteins move molecules into or out of the cell?

  1)Channel proteins form pores in the membrane for charged particles to diffuse through (down their concentration gradient)

  2)Different channel proteins facilitate the diffusion of different charged particles
• What is active transport?

  1)Active transport uses energy to move molecules and ions across the plasma membranes, against a concentration gradient (from a low concentration to a higher concentration)

  2)This process involves carrier proteins
• How does active transport work?

  
  

  • molecule attaches to the carrier protein
  • ATP is hydrolysed in the cell to release energy.

  This energy is used to make the carrier protein change shape. This moves the molecule across the membrane, releasing it on the other side.
• What process do cells use to take in substances?


  Endocytosis
• What process do cells use to secrete substances ?


  Exocytosis
• Osmosis:

  • Diffusion of water molecules across a partially membrane down a water potential gradient.

  This means water molecules move from an area of higher water potential (higher concentration) to an area of lower water potential

  (lower concentration)

  
  

  Pure water has the highest water potential of 0, all solutions have a lower water potential than pure water
• What is a hypotonic solution?

  • Solution with a higher water protentional than the cell
  • Net movement of water is into the cell
• What is an isotonic solution?

  Solution with the same water potential as cell
• What is a hypertonic solution?

  • Solution with a lower water potential than the cell
  • Net movement of water is out of the cell
• How are animal cells affected by water potential of a surrounding solution?

  • Hypotonic solution: cell bursts
  • Isotonic solution-water molecules pass into and out of the cell in equal amounts, no net movement
  • Hypertonic solution: cell shrinks
• How are plant cells affected by water potential of a surrounding solution?


  • Hypotonic solution: the vacuole swells. The vacuole and cytoplasm push against the cell wall and the cell becomes turgid(swollen)
  • Isotonic solution-water molecules move into and out of the cell in equal amounts, no net movement
  • Hypertonic solution: the cell becomes flaccid (limp). The cytoplasm and the membrane pull away from the cell wall. This is called plasmolysis