5) Plasma membranes

• Membranes are flexible and able to break and fuse easily.
• Functions:

  • separating cell contents from outside environment
  • separating the contents and activities of organelles from each other
  • cell recognition and signalling
  • holding the components of some metabolic pathways in place (e.g membrane bound enzymes)
  • regulating the transport of materials into and out of cells
• Structure of plasma membranes:

  • double layer of phospholipids
  • phosphate group, phosphoester bond, glycerol, ester bond (2) and fatty acid (2)
  • hydrophilic head which is soluble
  • hydrophobic head which is insoluble
• What are these labels

  A) hydrophobic tail

  B) Hydrophilic head

  
• What is the fluid mosaic model?

  Phospholipids are free to move within the layer relative to each other, giving the membrane flexibility, and because the proteins embedded in the bilayer vary in size, shape and position. This model forms the basic of understanding of membranes today.
• What can pass through the membrane?

  hydrophobic molecules

  non-polar molecules

  small, uncharged, non-polar molecules
• What cannot pass through the membrane?

  large, uncharged, polar molecules

  ions

  hydrophilic molecules
• Why does the membrane require integrity?

  • pathogen cannot enter
  • some substances can be transported through
  • some substances cannot enter
  • withstand hydrostatic pressure
  • to keep organelles in the cell
  • maintain concentration gradient
• Factors affecting cell membrane integrity and fluidity:

  • temperature
  • cholesterol
  • saturated or unsaturated fatty acid
• In low temperatures: Phospholipid cluster together, don't have energy to move, low fluidity. Crystallised

  In high temperatures: phospholipid has more energy and move around, high fluidity
• Cholesterol is a fluidity buffer

  In high temperatures decreases fluidity

  In low temperatures increases fluidity
• Saturated fatty acid - stacks neatly so fluidity decreases

  Unsaturated fatty acid - stacks unneatly, so fluidity increases as there is more distance between phospholipids
• Affects of organic solvents on membrane structure (ethanol) :

  • will dissolve membranes, disrupting cells
  • causes damage to membranes
  • when membrane is more disrupted it become more fluid and more permeable
  • so loss of contents, lose control of entry of substances, impossible to sustain concentration gradient, loss of function and enzymes in membrane no longer work
• membrane fluidity test:

  tested using beetroot

  tonoplast and cell surface membrane is disrupted, increases permeability causing pigment to diffuse into the solution

  put in baths at different temperatures

  use colorimeter to test for absorbance
• diffusion - resulting from the random movement of molecules in which there is a netflow of molecules from a region of low to high concentration
• Factors affecting rate of diffusion:

  • temperature - the higher the temperature, the higher the rate of diffusion
  • concentration difference - the greater the difference between concentration gradients the faster the rate of diffusion
  • surface area - larger the surface areas faster the rate of diffusion
  • thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion
  • diffusion distance
• Ficks law:

  Rate of diffusion (is proportion to) surface area X concentration difference / thickness of membrane
• Simple diffusion:

  • small molecules
  • high to low concentration gradient
  • uncharged because of hydrophobic interior
  • passive process
  • non-polar
• Facilitated diffusion:

  • used protein channels
  • carries charged particles
  • polar molecules
  • hydrophilic
  • passive process
  • down the concentration gradient
• Channel proteins:

  • constantly open
  • provide hydrophilic channel
  • passive movement
  • bind to specific ions
• intrinsic/integral proteins:

  • transmembrane proteins that are embedded through both layers of the membrane. They have amino acids with hydrophobic R groups on their external surfaces which interact with the hydrophobic core of the membrane keeping them in place.
  • e.g carrier and channel proteins
• Extrinsic proteins:

  • present in one side of the bilayer
  • normally have hydrophilic R group on their outer surface and interact with polar heads of the phospholipids or with intrinsic proteins
• Active processes:

  • active transport
  • endocytosis
  • exocytosis
• active processes:

  • movement of molecules from a place of low concentration to high concentration
  • requires energy and carrier proteins
  • energy supplied by hydrolysis of ATP releasing phosphate that binds to carrier protein causing conformational change
• Bulk transport

  • large molecules such as enzymes, hormones and whole cells such as bacteria re too large to move through membrane channels and carrier proteins
  • exocytosis and endocytosis
• Endocytosis:

  • bulk transport of materials into the cell
  • membrane invagulates, membrane fuses and makes a vesicle

  two types

  phagocytosis is for solids

  pinocytosis is for liquids
• Exocytosis:

  • bulk movement of materials out of a cell
  • vesicle formed by golgi apparatus, moves towards and fuses with cell surface membrane
  • the contents are then released outside of the cell
• In Bulk transport ATP is required for:

  • movement of vesicles along the cytoskeleton
  • changing the shape of membrane to engulf materials
  • fusion of the cell membrane as vesicles form
  • fusion of the cell membrane as it meets vesicles or materials from outside the cell
• Osmosis - the movement of water molecules across a selectively permeable membrane from a region of high water concentration to a region of low water concentration
• if animal cell placed in:

  isotonic solution - water constantly enters and leaves, but at equal rates

  hypotonic solution - water moves out and cell shrinks and become crenulated

  hypertonic - water moves in and cell swells and bursts
• cytolysis - cell breaks and bursts
• Cytolysis in animal cells:

  1. water will move into the cell by osmosis
  2. this increases hydrostatic pressure inside the cell
  3. the cell surface membrane which cannot stretch and cannot withstand pressure
  4. cell membrane will break and burst, this is called cytolysis

  occurs when cell is put in a hypotonic solution
• animal cells don't have a cell wall to protect from hydrostatic pressure

  so multi-cellular animals must, therefore have osmo-regulatory mechanisms which keep the water potential of bodily fluids within narrow ranges
• Plant cell in a high water concentration - water enters and cell swells and becomes turgid

  plant cell in low water concentration - water leaves cell, cell becomes flaccid (shrinks) and plasmolyse
• Plant cell wall:

  • made of cellulose
  • completely permeable to water and solutes
  • provides strength and rigidity
  • not continuous (punctured by plasmodesmata so that adjacent cell can share cytoplasm)
• Extrinsic protein roles:

  • binding sites/receptors e.g for hormones
  • antigen
  • bind cell together
  • involved in cell signalling
• Factors that effect membrane permeability:

  temperature - high temperature denatures membrane proteins/phospholipid molecules, have more energy and move further apart

  pH - changes tertiary structure of the membrane proteins

  solvent - dissolves membrane
• Carrier proteins

  • binds to complementary molecule, causing a complementary change which releases a molecule on other side of the membrane
  • facilitated diffusion
  • in active transport it required energy from the hydrolysis of atp
• Active transport process:

  1. molecule binds to receptor carrier protein
  2. ATP binds to carrier protein and is hydrolysed into ADP and phosphate
  3. binding of phosphate causes protein to change shape
  4. molecule is released to the inside of the cell
  5. phosphate molecule is released and recombines with ADP to form ATP
  6. carrier protein returns to original shape
• glycoproteins

  • intrinsic proteins
  • embedded in the cell surface membrane with attached carbohydrate chain
  • used for cell adhesion and receptors for chemical signalling