1. Membrane Structure & Function
Ch. 7
Membrane Structure & Function

2. You have a WARM-UP this morning. Use your Ch. 6 notes.
Good Morning
You have a WARM-UP this morning. Use your Ch. 6 notes.
Finish by 1:00

3. POGIL Time
Groups of 4
Choose a READER, MANAGER, RECORDER, REPORTER
Work together!

4. Turn in the POGIL if…
You are the person who’s BIRTHDAY is CLOSEST to today.

5. Amoeba Sisters!
Inside the Cell Membrane

6. The Cell Membrane

7. Aaaah, one of those structure–function
Phospholipids
Phosphate
“attracted to water”
Phosphate head
hydrophilic
Fatty acid tails
hydrophobic
Arranged as a bilayer
Fatty acid
“repelled by water”
Aaaah, one of those structure–function
examples

8. Phospholipids Amphipathic = hydrophilic head, hydrophobic tail
Form a hydrophobic barrier: keeps hydrophilic molecules out

9. Arranged as a Phospholipid bilayer
Serves as a cellular barrier / border
sugar
H2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
lipids

10. Cell membrane defines cell
Cell membrane separates living cell from aqueous environment
thin barrier = 8nm thick
Controls traffic in & out of the cell
allows some substances to cross more easily than others
hydrophobic (nonpolar) vs. hydrophilic (polar)

11. Cell Membrane Fluid Mosaic Model
Fluid: membrane held together by weak interactions
Mosaic: phospholipids, proteins, carbs

12. Membrane fluidity
Low temps: phospholipids w/unsaturated tails (kinks prevent close packing)
Cholesterol resists changes by:
limit fluidity at high temps
hinder close packing at low temps
Adaptations:
bacteria in hot springs (unusual lipids);
winter wheat ( unsaturated phospholipids) –adapted to cold winter

13. Cell membrane is more than lipids…
Transmembrane proteins embedded in phospholipid bilayer
create semi-permeabe channels
lipid bilayer
membrane
protein channels in lipid bilyer membrane

14. Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins
Classes of membrane proteins:
Peripheral Proteins
Integral Proteins

15. Membrane Proteins PERIPHERAL PROTEINS
Loosely bound to surface of membrane (NOT embedded)
ex: cell surface identity marker (antigens)
Extracellular or cytoplasmic sides of membrane
Held in place by the cytoskeleton or ECM
Provides stronger framework

16. Membrane Proteins INTEGRAL PROTEINS
penetrate lipid bilayer, usually across whole membrane
Embedded in membrane
Transmembrane protein
(hydrophilic heads/tails and hydrophobic middles)
ex: transport proteins
channels, permeases (pumps)

17. Transmembrane protein structure
Hydrophobic interior
Hydrophilic ends

18. Integral & Peripheral proteins

19. Permeability to polar molecules?
Membrane becomes semi-permeable via protein channels
specific channels allow specific material across cell membrane
inside cell
H2O aa
sugar
salt
outside cell
NH3

20. Proteins domains anchor molecule
Within membrane
nonpolar amino acids
hydrophobic
anchors protein into membrane
On outer surfaces of membrane in fluid
polar amino acids
hydrophilic
extend into extracellular fluid & into cytosol
Polar areas
of protein
Nonpolar areas of protein

21. H+
NH2 H+
COOH
Cytoplasm
Retinal
chromophore
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
Examples
aquaporin = water channel in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
H2O H+
proton pump channel in photosynthetic bacteria
function through conformational change = protein changes shape
H2O

22. Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Enzyme activity
Cell surface receptor
“Antigen”
Signal transduction - transmitting a signal from outside the cell to the cell nucleus, like receiving a hormone which triggers a receptor on the inside of the cell that then signals to the nucleus that a protein must be made.
Cell surface identity marker
Cell adhesion
Attachment to the cytoskeleton

23. Membrane Carbohydrates
Play a key role in cell-cell recognition
ability of a cell to distinguish one cell from another
Antigens
Glycolipids, glycoproteins
important in organ & tissue development
basis for rejection of foreign cells by immune system
The four human blood groups (A, B, AB, and O) differ in the external carbohydrates on red blood cells.

24. Movement across the Cell Membrane

25. Selective Permeability
Small molecules (polar or nonpolar) cross easily (hydrocarbons, hydrophobic molecules, CO2, O2)
Hydrophobic core prevents passage of ions, large polar molecules
Transport proteins span the membrane to help move these molecules

26. Passive Transport NO ENERGY (ATP) needed!
Diffusion down concentration gradient (high  low concentration)
Eg. hydrocarbons, CO2, O2, H2O

27. Diffusion 2nd Law of Thermodynamics governs biological systems
universe tends towards disorder (entropy)
Movement from high concentration of that substance to low concentration of that substance.
Diffusion
movement from HIGH  LOW concentration

28. Simple Diffusion Move from HIGH to LOW concentration movement of water
“passive transport”
no energy needed
movement of water
diffusion
osmosis

29. Facilitated Diffusion
Diffusion through protein channels
channels move specific molecules across cell membrane
no energy needed
facilitated = with help
open channel = fast transport
HIGH
LOW
Donuts!
Each transport protein is specific as to the substances that it will translocate (move).
For example, the glucose transport protein in the liver will carry glucose from the blood to the cytoplasm, but not fructose, its structural isomer.
Some transport proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel through the membrane -- simply provide corridors allowing a specific molecule or ion to cross the membrane.
These channel proteins allow fast transport.
For example, water channel proteins, aquaporins, facilitate massive amounts of diffusion.
“The Bouncer”

30. Facilitated Diffusion
2/19/20192/19/20192/19/20192/19/20192/19/20192/19/20192/19/20192/19/20192/19/2019
Facilitated Diffusion
Transport proteins (channel or carrier proteins) help hydrophilic substances cross
Two ways:
Provide hydrophilic channel
Loosely bind/carry molecule across
Eg. ions, polar molecules (H2O, glucose)

31. conformational change
Active Transport
Cells may need to move molecules against concentration gradient (L  H)
conformational shape change transports solute from one side of membrane to other
protein “pump”
“costs” energy = ATP
conformational change
Some transport proteins do not provide channels but appear to actually translocate the solute-binding site and solute across the membrane as the protein changes shape. These shape changes could be triggered by the binding and release of the transported molecule. This is model for active transport.
“The Doorman”

32. Electrogenic Pumps: generate voltage across membrane
Na+/K+ Pump
Proton Pump
Pump Na+ out, K+ into cell
Nerve transmission
Push protons (H+) across membrane
Eg. mitochondria (ATP production)

33. Eg. sucrose-H+ cotransporter (sugar-loading in plants)
Cotransport: membrane protein enables “downhill” diffusion of one solute to drive “uphill” transport of other
Eg. sucrose-H+ cotransporter (sugar-loading in plants)

34. Active transport Many models & mechanisms ATP ATP antiport symport
Plants: nitrate & phosphate pumps in roots.
Why?
Nitrate for amino acids
Phosphate for DNA & membranes
Not coincidentally these are the main constituents of fertilizer
Supplying these nutrients to plants
Replenishing the soil since plants are depleting it
antiport
symport

35. Bulk Transport Endocytosis: take in macromolecules, form new vesicles
Moving large molecules into & out of cell
through vesicles & vacuoles
Transport of proteins, polysaccharides, large molecules
Type of Active transport  requires energy (ATP)
Endocytosis: take in macromolecules, form new vesicles
Exocytosis: vesicles fuse with cell membrane, expel contents

36. Receptor-Mediated Endocytosis:
Types of Endocytosis
Phagocytosis:
“cellular eating” - solids
Pinocytosis:
“cellular drinking” - fluids
Receptor-Mediated Endocytosis:
Ligands bind to specific receptors on cell surface

37. Endocytosis phagocytosis “cellular eating” pinocytosis
Cell membrane enfolds the food particle, forms a vesicle, which fuses with lysosome for digestion
Used by protists, immune system macrophages
phagocytosis
“cellular eating”
pinocytosis
Cell membrane enfolds around liquid (or dissolved solutes)
“cellular drinking”
receptor-mediated endocytosis
triggered by molecular signal

39. Passive vs. Active Transport
Little or no Energy
High  low concentrations
DOWN the concentration gradient
eg. diffusion, osmosis, facilitated diffusion (w/transport protein)
Requires Energy (ATP)
Low  high concentrations
AGAINST the concentration gradient
eg. pumps, exo/endocytosis

40. Transport Summary Passive Transport Active transport Simple diffusion
diffusion of nonpolar, hydrophobic molecules
lipids
HIGH  LOW concentration gradient
Facilitated transport
diffusion of polar, hydrophilic molecules
through a protein channel
Active transport
diffusion against concentration gradient
LOW  HIGH
uses a protein pump
requires ATP
ATP

41. Transport summary simple diffusion facilitated diffusion
ATP
active transport

42. The Special Case of Water Movement of water across the cell membrane

43. Amoeba Sisters!
Osmosis and Water Potential

44. Osmosis is just diffusion of water
Water is very important to life, so we talk about water separately
Diffusion of water from HIGH concentration of water to LOW concentration of water
across a semi-permeable membrane

45. Concentration of water
Direction of osmosis is determined by comparing total solute concentrations
Hypertonic - more solute, less water
Hypotonic - less solute, more water
Isotonic - equal solute, equal water
hypotonic
hypertonic
water
net movement of water

46. Managing water balance
Cell survival depends on balancing water uptake & loss
freshwater
balanced
saltwater

47. Managing water balance1
Managing water balance
Hypotonic
a cell in fresh water
high concentration of water around cell
problem: cell gains water, swells & can burst
example: Paramecium
ex: water continually enters Paramecium cell
solution: contractile vacuole
pumps water out of cell
ATP
Called osmoregulation
plant cells
turgid = full
cell wall protects from bursting
KABOOM!
ATP
No problem, here
freshwater

48. Pumping water out
Contractile vacuole in Paramecium
ATP

49. Managing water balance2
Managing water balance
Hypertonic
a cell in salt water
low concentration of water around cell
problem: cell loses water & can die
example: shellfish
solution: take up water or pump out salt
plant cells
plasmolysis = wilt
can recover
I’m shrinking, I’m shrinking!
I will survive!
saltwater

50. Managing water balance3
Managing water balance
Isotonic
animal cell immersed in mild salt solution
no difference in concentration of water between cell & environment
problem: none
no net movement of water
flows across membrane equally, in both directions
cell in equilibrium
volume of cell is stable
example: blood cells in blood plasma
slightly salty IV solution in hospital
That’s perfect!
I could be better…
balanced

51. Aquaporin: channel protein that allows passage of H2O

52. Do you understand Osmosis…
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell

53. Understanding Water Potential

54. Water potential equation:
H2O moves from high ψ  low ψ potential
Water potential equation:
ψ = ψS + ψP
Water potential (ψ) = free energy of water
Solute potential (ψS) = solute concentration (osmotic potential)
Pressure potential (ψP) = physical pressure on solution; turgor pressure (plants) = pressure from the cell wall
Pure water: ψP = 0 MPa
Plant cells: ψP = 1 MPa

55. Calculating Solute Potential (ψS)
ψS = -iCRT
i = ionization constant (# particles made in water)
C = molar concentration
R = pressure constant ( liter bars/mole-K)
T = temperature in K ( C)
The addition of solute to water lowers the solute potential (more negative) and therefore decreases the water potential.

56. Where will WATER move? From an area of:
higher ψ  lower ψ (more negative ψ)
low solute concentration  high solute concentration
high pressure  low pressure

57. Which chamber has a lower water potential?
Which chamber has a lower solute potential?
In which direction will osmosis occur?
If one chamber has a Ψ of kPa, and the other kPa, which is the chamber that has the higher Ψ?

59. Sample Problem -4.95 Into the cell
Calculate the solute potential of a 0.1M NaCl solution at 25°C.
If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed in the 0.1M NaCl solution?
-4.95
Into the cell