1. Membrane Structure

2. Membrane Structure
The Dawson – Danielli model proposed in 1935 used a lipid bi-layer model
Model suggest it was covered on both sides by a thin layer of globular protein

3. Membrane Structure
Led by the Dawson – Danielli Model, Singer and Nicolson proposed that proteins are inserted into the phospholipid layer and do not form a layer on the phospholipid bilayer surfaces
Believed that the proteins formed a mosaic floating in a fluid layer of phospholipids
With only slight changes the Singer and Nicolson model was adopted and the Fluid Mosaic Model is used today.

4. Fluid Mosaic Model

5. Phospholipids
Each phospholipid is composed of a three-carbon compound called glycerol
Two of the glycerol carbons have fatty acids
Non-polar and not water soluble - Hydrophobic
The third carbon is attached to a highly polar organic alcohol that includes a bond to a phosphate group
Organic alcohol with phosphate is highly polar and water soluble – Hydrophilic (amphipathic)

6. Phospholipids Have two regions, with different properties:
2 hydrocarbon tails which are Hydrophobic
A phosphate head, that is negatively charged to which Hydrophilic

7. Phospholipids Bilayer
When mixed in water they become arranged in double layers
Heads face outward and tails inward
This is a stable structure because of the bonds that form between phosphate heads and the surrounding water.
Tails form hydrophobic interactions
This is a weak intermolecular interaction
Look at the combinations of bonds collectively as strong

8. Phospholipids Bilayer

9. Membrane Structure Cholesterol Is a component of animal cells
Have a role in determining membrane fluidity, which changes with temperature
Cholesterol restricts the movement of phospholipid molecules – reduces the fluidity
Allow membranes to function effectively at a wide range of temperatures
Also reduces permeability
Plant cells do not have cholesterol molecules
Plants depend on saturated and unsaturated fatty acids

10. Membrane Structure Proteins: Create diversity in membrane function
Proteins of various types are embedded in the fluid matrix of the phospholipid bilayer
This creates the mosaic effect referred to in the fluid mosaic model

11. Proteins Integral Proteins – embedded in phospholipids
Show amphipathic character (both hydrophobic & hydrophilic regions)
Hydrophobic areas are towards the middle
Hydrophilic are on the ends close to water
Peripheral Proteins – loosely attached to the surface of the membrane
Often anchored to an integral proteins

12. Membrane Protein Functions
Hormone Binding Site
Insulin receptor
Immobilized enzymes with the active site on the outside
In the small intestine
Cell Adhesion
Cell to Cell communication
Channels for passive transport
Pumps for active transport
Uses ATP

13. Membrane Protein Functions

14. Membrane Transport There are 2 types of cellular transport:
Passive Transport
Active Transport
Passive transport
Requires no energy
Moves from down the concentration gradient
Some molecules pass through the membrane
Some molecules use channels for facilitated diffusion

15. Diffusion
Diffusion can only occur across the membrane if the phospholipids bi-layer is permeable to the particles
Hydrophobic center does not let ions with + or – charges to pass easily
Polar molecules with partial + / - charges over the surface can diffuse at slow rates
Small particles can pass more easily than large ones
Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration

16. Diffusion
Membranes allow some substances to diffuse through but not others.
Partially Permeable
Some substances move between the phospholipid molecules in the membrane
Simple Diffusion

17. Facilitated Diffusion
Channels in the membrane in which ions and other particles can pass into or out the cell
Channels can be a single or group of protein molecules
The diameter and properties (like charge) ensure that only one type of particle passes
Involves specific carrier proteins that can change shape to accomplish this task but requires no energy.

18. Osmosis
Osmosis is the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration (high water concentration) to a region of higher solute concentration (low water concentration).
Water goes to the direction of the solute

19. Osmosis Equal concentration of solute Lower concentration of solute
Higher concentration of solute
Equal concentration of solute
H2O
Solute molecule
Selectively permeable membrane
Water molecule
Solute molecule with
cluster of water molecules
Net flow of water

20. Estimating Osmolarity
A concentration gradient of water allows the movement to occur as a result of solute concentration.
Hypertonic – a solution with higher concentration
of solute.
Hypotonic – a solution with lower solute
concentration
Water will go from Hypotonic (hypo = below) to
Hypertonic (hyper = above)
Isotonic – solution in which the concentration of
solute are equal.
Iso = equal

21. Estimating Osmolarity
The osmolarity of a solution is the number of moles of solute particles per unit volume of the solution.
Pure water has an osmolarity of zero
The greater the concentration of solutes the high the osmolarity

22. Osmosis in Donor Organs
Osmosis can cause cells in human tissue or organs to swell up and burst.
Shrink too due to net gain of loss of water by osmosis.
To prevent this tissues/organs used in medical procedures such as kidney transplants must be bathed in a solution with the same osmolarity as human cytoplasm.
Solution is called Isotonic Saline

23. Membrane Transport Active Transport
Cells sometimes take in substances even when there is a higher concentration of the substance inside the cell than outside
The substance is absorbed against the concentration gradient
Cells can even pump cells out even though there is a larger concentration outside
Energy is needed for this to occur
ATP is required

24. Pump Proteins
Protein pumps in the membrane are used for active transport.
Each pump only transports particular substances
This way the cell can control what is absorbed and what is expelled
Pumps work in a specific direction

25. Sodium-Potassium Pump
In axons from neurons they contain a pump protein that moves sodium ions and potassium ions across the membrane.
Antiporter – sodium and potassium are pumped in opposite directions
Energy comes from ATP
One ATP provides enough energy to pump 2 potassium ions in and 3 sodium ions out of the cell.
The concentration gradients are needed for the transmission of nerve impulses in axons

26. Sodium-Potassium Pump
A mechanism for moving sodium (Na) and potassium (K) ions
It has 5 stages:
A specific protein binds to three intracellular Na ions
The binding of Na ions causes phosphorylation by ATP. ATP has three attached phosphates. When it carries out phosphorylation, one phosphorylation, one phosphate is lost resulting in a two phosphate compound called ADP.
The phosphorylation causes the protein to change its shape, thus expelling Na ions to the exterior.
Two extra cellular K ions bind to different regions of the protein and this causes the release of the phosphate group
The loss of the phosphate group restores the protein’s original shape, thus causing the release of the K ions into the intracellular space.

27. Sodium-Potassium Pump

28. Endocytosis
The mass movement INTO the cell by the membrane ‘pinching’ into a vacuole

29. Endocytosis Endocytosis can occur in three ways Phagocytosis - solids
Pinocytosis - liquids
Receptor-mediated endocytosis

30. Exocytosis
The mass movement OUT of the cell by the fusion of a vacuole and the membrane
Both are possible because the of the fluid properties of the membrane (able to break and reform easily, phospholipids not attached just attracted)
Example of endocytosis can be found in secretory cells (saliva)