1. Unit 2: Chapter 5
Biological Membranes

2. Structure of the Cell Membrane
6 main components
Phospholipids
Proteins
Cholesterol
Carbohydrates
Glycoproteins
glycolipids

3. Phospholipids (review)
Membrane is a bilayer
One layer of polar heads – they are hydrophilic because they love water
One layer of nonpolar tails – they are hydrophobic because they hate water

5. Membranes have proteins!
2 main types
Integral proteins
Inserted into the membrane
May be unilateral – reach only partway across the membrane
Or they may be transmembrane – completely span the membrane
Removal disrupts cell membrane
Peripheral proteins
Not embedded in membrane, but attached to the membrane surface by either integral proteins or filaments from the cytoskeleton
Removal has little effect on cell membrane

7. 7 functions of membrane proteins
Anchoring cell
Passive transport
Active transport
Enzyme activity
Signal transduction – transmitting info into cell
Cell recognition – like ID tags
Junction between cells – cell adhesion

8. Cholesterol
Controls the fluidity of the cell membrane

9. History of the Cell Membrane
2 models
Sandwich Model
Fluid Mosaic Model

10. Sandwich Model Fluid Mosaic Model Davson and Danielli in 1935
Perceived the plasma membrane as a sandwich where the proteins were the bread and the phospholipid bilayer was the meat
Singer and Nicholson in 1972
Said the membrane is fluid and must be
If it solidifies, its permeability will change and enzymes will denature
Said the membrane is mosaic – there are proteins embedded in it

11. Which model was correct?

13. Nature of protein and lipid mobility
Lateral movement of lipids/proteins is quick
Lipids and proteins rarely flip across the lipid bilayer

14. Basic Terms to Understand
Selectively permeable – prevents passage of most materials through the membrane
Solute – what is dissolving (salt, sugar, etc.)
Solvent – what it is dissolving in (water etc.)
Solution – mixture of solvent and solute

15. 7 ways substances can get into a cell
Diffusion
Bulk flow
Osmosis
Facilitated diffusion
Active transport
Vesicle mediated transport
Cell to cell junction

16. Diffusion
Moves materials from a high concentration to a low concentration
Requires no energy – type of passive transport
Easy passage through – oxygen, carbon dioxide, nitrogen, and small polar molecules
Slow passage through – large polar molecules like glucose and charged ions
Proteins allow movement of charged/polar molecules
Particles move until equilibrium

17. Bulk Flow
Molecules move all together in same direction due to hydrostatic pressure

18. Osmosis ALL things undergo diffusion.
Water also diffuses, however, the water diffusion is not evident unless it crosses a membrane.
Osmosis is the diffusion of water across a membrane from high to a low concentration
No energy is required – type of passive transport
Since cells have membranes, osmosis is important to cells

19. Membranes and Osmosis
Tank w/ semipermeable membrane: water may pass, solute can’t
At first the concentration of solute is very high on the left.
But over time, the water moves across the semipermeable membrane, and dilutes the solute.

20. Water Moves Because It is Polar
Because water is polar, it binds to the solute by hydrogen bonds
The concentration of water is higher on the right
Water will then flow across the membrane, down its concentration gradient, to the left

21. Three osmotic environments
Hypertonic
Hypotonic
Isotonic
Same solute concentration inside and outside the cell
Water flows in and out of the cell equally in both directions
Most cells in our body are isotonic

22. Hypertonic High concentration of solute outside the cell
Therefore there is more water inside the cell
Water will move out of the cell
If this process continues, the cell collapses and dies – this is called plasmolysis

23. Hypotonic Low concentration of solute outside the cell
Therefore there is more water outside the cell
Water will move inside the cell
This causes the cell to expand, causing turgor pressure in plant cells Animal cells could burst – this is called cytolysis

25. Plant Cells and Osmotic Pressure
In plants, hypotonic solutions produce osmotic pressure that produces turgor pressure
Turgor means “tight or stiff owing to being very full”
Keeps plant upright; in hypertonic conditions plants wilt
Vacuole fills
Vacuole
shrinks
Hypotonic solution
Hypertonic
Hypertonic

26. Dialysis Dialysis is the diffusion of solutes across a membrane
The selectively permeable membrane allows small sugar molecules to move across the membrane, but large proteins cannot

27. Facilitated Diffusion
Transport proteins move materials through membrane
3 kinds of transport proteins
Uniport – carries a single molecule across the membrane
Symport – moves 2 different molecules at the same time in the same direction
Antiport – exhanges 2 molecules in opposite directions
This is passive transport

29. Vesicle Mediated Transport
When vesicles or vacuoles fuse with the membrane to move substances in or out of the cell
2 main types
Exocytosis – when vesicle expels contents outside the cell
Endocytosis – when vesicles bring substances into the cell
3 types
This is Active Transport

30. 3 types of Endocytosis Phagocytosis – solid being taken into the cell
Pinocytosis – liquid being taken into the cell
Receptor Mediated Endocytosis – substance binds to a specific receptor on the cell before it is brought in

31. Exocytosis

32. phagocytosis

33. water
Pinocytosis

34. Receptor Mediated Endocytosis

35. Active Transport Energy is needed
Moves materials against the concentration gradient
Main example: sodium potassium pump

36. Sodium Potassium Pump Occurs in animal cells
Required ATP (active transport)
Exchanges 3 Na+ ions on inside for 2 K+ ions on outside
This exchange is uneven so an electric potential is generated and so the membrane is now considered to be polarized
Let’s see this in action

38. How do cells stick together and let materials?
Plasmodesmata
Gap junctions
Desmosomes
Tight junctions

39. Plasmodesmata They are channels that allow movement
of certain molecules
and ions between plant cells

40. Gap Junctions Cytoplasmic channels between neighboring animal cells
Let adjacent cells communicate
Small dissolved molecules and electrical signals may pass from one cell to the other
Very similar to plasmodesmata

42. Desmosomes Function as rivets and join animal cells together
They are reinforced by intermediate filaments made of keratin
Still permits materials to move around them in the intercellular space

43. Tight Junctions
Continuous belt around animal cells that fuse membranes of neighboring cells
It is leak proof
Contains no intercellular space

44. Water Potential Chemical potential of water
Measure of the energy available for reaction or movement
Measures the ability of water to move and water always moves from areas of higher potential to areas of low potential
Has the symbol Ψ (psi)

45. Water Potential Continued…
Is measured in the unit bars
The formula for calculated water potential is osmotic potential (solute) + pressure potential
So, the equation is
Ψ = Ψs + Ψp

46. Equation Components Ψp = pressure on the system
= 0, if the system or container is open
** all of our problems will be open so
Ψp will always equal 0.
Ψs = change in water potential due to solute molecules
The more solute, the lower the water potential

47. Ψs = -iCRT
Where
i = ionization constant
= # of ions in the solute
= 1 when there are no ions present
C = Molar Concentration
usually given in problem
equal to Molarity (M), or moles/volume
R = pressure constant
= Liters X Bars/moles x Kelvin
this number never changes
T = temperature
needs to be in Kelvin
Conversion of Celsius to Kelvin is
K = Celsius

48. Ψ= Ψs + Ψp Ψ = -iCRT + 0 Ψ = -(1)( 0) (0.0831)(23 + 273) + 0 Ψ = 0 + 0
Suppose we have a beaker of distilled water at room temperature. (0M) What is the water potential?
Ψ= Ψs + Ψp
Ψ = -iCRT + 0
Ψ = -(1)( 0) (0.0831)( ) + 0
Ψ = 0 + 0
Ψ = 0

49. Ψ = Ψ s + Ψ p Ψ = -iCRT + 0 Ψ = -(1)(2.5)(0.0831)(25 + 273)
Suppose we have an open beaker A that contains Sucrose (2.5M) at 25 C. What is the water potential?
Ψ = Ψ s + Ψ p
Ψ = -iCRT + 0
Ψ = -(1)(2.5)(0.0831)( )
Ψ = bars