1. Cell Membrane Function
The purpose of cells membranes is to?

2. A. Passive transport (simple and facilitated diffusion)
Large and small substances move across cell membranes in fundamentally different ways.
Small molecules-
A. Passive transport (simple and facilitated diffusion)
B. Active transport
Large molecules(endo and exocytosis)

3. A membrane’s molecular organization results in selective permeability
Membrane permeability is influenced by size, chemical composition and charge/polarity of the molecule trying to cross the membrane
a. Membranes are more permeable to small molecules than larger ones
b. Membranes are more permeable to hydrophobic molecules
c. Membranes are most permeable to uncharged/nonpolar molecules

4. Simple Diffusion
Defined-the spontaneous net movement of a substance from an area of its higher concentration to an area of its lower concentration until an equilibrium is achieved
Diffusion occurs because of the second law of thermodynamics

5. Diffusion of one solute
LE 7-11a
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
Diffusion of one solute

6. Diffusion of two solutes
LE 7-11b
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
Diffusion of two solutes

7. Osmosis Osmosis is a special case of diffusion
It involves the diffusion of water across a differentially permeable membrane
Cell and tissues can gain or lose water by osmosis depending on the type of environment they exist in

8. Effect of solute on cell solutions
The solute concentration of the environment determines whether a cell gains or loses water
The addition of solute lowers the concentration of water (makes it less than 100%)
Three terms describe the tendency of one solution to gain or lose water to another solution
Hypertonic (salty) solutions tend to gain water from hypotonic solutions (less salty)
Isotonic solutions gain and lose water to one another at the same rate.

9. Lower Higher Same concentration concentration concentration of sugar
LE 7-12
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
Osmosis

10. Environment “Cell” 0.01 M sucrose 0.03 M sucrose 0.01 M glucose
LE 7-UN140
Environment
“Cell”
0.01 M sucrose
0.03 M sucrose
0.01 M glucose
0.02 M glucose
0.01 M fructose

11. Cell survival depends on balancing water uptake and loss
Plant and animal responses to being placed in
A. hypertonic solution
B. hypotonic solutions
C. Isotonic solutions

12. Hypotonic solution Isotonic solution Hypertonic solution Animal cell
LE 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal
cell
H2O
H2O
H2O
H2O
Lysed
Normal
Shriveled
Plant
cell
H2O
H2O
H2O
H2O
Turgid (normal)
Flaccid
Plasmolyzed

13. LE 7-14
50 µm
Filling vacuole
50 µm
Contracting vacuole

14. Traffic across membranes
A membrane’s molecular organization results in selective permeability
Passive transport is diffusion across a membrane
Osmosis is the passive transport of water
Cell survival depends on balancing water uptake and loss
The solute concentration of the environment determines whether a cell gains or loses water
Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion)
Active transport is the pumping of solutes against their gradients
Some ion pumps generate voltage across membranes
In cotransport, a membrane protein couples the transport of one solute to another
Exocytosis and endocytosis transport large molecules

15. How do small molecules move across cell membranes?
Passive Transport is diffusion across a membrane
A. Simple diffusion-membrane is permeable; highlow concentration; no energy required
B. Facilitated diffusion-diffusion-membrane is impermeable (carrier molecule required) highlow concentration; no energy required

16. Facilitated diffusion
LE 7-17a
Passive transport
Diffusion
Facilitated diffusion

17. Facilitated Diffusion
Specific proteins facilitate the passive transport of selected solutes (facilitated diffusion)
Hydrophilic channels
Rotating carriers (conformational changes)

18. LE 7-15a
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM

19. LE 7-15b
Carrier protein
Solute

20. Active Transport
Active transport is the pumping of solutes against their gradients
A. Membrane is impermeable (carrier required); movement from low concentration to high concentration; energy required
B. Sodium/potassium pump (neurons)
C. Plants often actively transport nutrients from soil into the root cell (advantage of doing this?)

21. LE 7-17b
Active transport
ATP

22. A B Both A and B Neither A nor B
Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose . Which solution is hypertonic? A B
Both A and B
Neither A nor B

23. A B Both A and B Neither A nor B
Solution A (.2M glucose) is separated from solution B (.4 M glucose) by a membrane which is impermeable to glucose . Which solution will have a net gain of water? A B
Both A and B
Neither A nor B

24. In the Na+/K+ pump, the ATPase enzyme is activated by
Release of K+
Binding of K+
Binding of Na+
Release of Na+
phosphorylation

25. Figure 8.15 The sodium-potassium pump: a specific case of active transport

26. Cytoplasmic Na+ bonds to the sodium-potassium pump
LE 7-16
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
[K+] high
ATP P
Na+ P
CYTOPLASM
ADP
Cytoplasmic Na+ bonds to
the sodium-potassium pump
Na+ binding stimulates
phosphorylation by ATP.
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside. K+ K+ K+ K+ K+ P P K+
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.

27. – EXTRACELLULAR + FLUID ATP – + H+ H+ Proton pump H+ – + H+ H+ – +
LE 7-18 –
EXTRACELLULAR
FLUID +
ATP – + H+ H+
Proton pump H+ – + H+ H+ – +
CYTOPLASM H+ – +

28. Co-transport
In co-transport, a membrane protein couples the transport of one solute to another
In plants, transport of sucrose into cells is coupled to the active transport of H+ ions out of the cell

29. Sucrose-H+ cotransporter
LE 7-19 – +
ATP H+ H+ – +
Proton pump H+ H+ – + H+ – + H+
Diffusion
of H+
Sucrose-H+
cotransporter H+ – + – +
Sucrose

30. Movement of large molecules/cells into and out of cells
Exocytosis and endocytosis transport large molecules into and out of cells
Exocytosis-out
Endocytosis-in
Pinocytosis
Phagosytosis
Receptor-mediated endocytosis

31. PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming
LE 7-20b
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis
vesicles forming
(arrows) in a cell
lining a small
blood vessel
(TEM).
Vesicle

32. PHAGOCYTOSIS CYTOPLASM 1 µm Pseudopodium Pseudopodium of amoeba
LE 7-20a
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM)

33. RECEPTOR-MEDIATED ENDOCYTOSIS
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm

34. Familial Hypercholesterolemia
Symptoms/consequences
Causes
Genetics

36. Membrane Structure and Function
Membrane models have evolved to fit new data (science as a process)
A membrane is a fluid mosaic of lipids, proteins and carbohydrates
There is a lot of experimental evidence that favors the fluid mosaic model of membrane structure.

37. History of Membrane Models
Overton (1875) –Membranes contain lipids (like dissolve like)
Langmuir(1917)-Membranes have amphipathic lipids (phospholipids)
Gorter and Grendel(1925)-Phospholipid bilayer
Davson and Danielli (1935)-Phospholipids and proteins (sandwich)

38. Figure 8.1 Artificial membranes (cross sections)

39. LE 7-2
WATER
Hydrophilic
head
Hydrophobic
tail
WATER

40. Figure 8.2 Two generations of membrane models

41. History of Membrane Models (continued)
Robertson (1950)-Electron micrographs showing “trilaminate” structure
Problems with current models
Singer and Nicholson (1975)-Fluid mosaic model

42. Figure 8.19 The three types of endocytosis in animal cells

43. Fluid Mosaic Model
Consistent with all observations of membrane properties to date

44. Figure 7-01

45. LE 7-5
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Movement of phospholipids
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
Membrane fluidity
Cholesterol
Cholesterol within the animal cell membrane

46. Facilitated diffusion ATP hydrolysis A proton pump 1 and 3
In sucrose co-transport in plants, the active transport of sucrose into plant cells is couple to
Facilitated diffusion
ATP hydrolysis
A proton pump
1 and 3

47. Gorter and Grendle Davson and Danielli Singer and Nicholson Overton
This model of membrane structure consisted of a phospholipid bilayer sandwiched between 2 layers of protein:
Gorter and Grendle
Davson and Danielli
Singer and Nicholson
Overton
Robertson

48. Hypertonic environments Cooling temperatures Warming temperatures
An increased synthesis of phospholipids containing unsaturated fatty acids may be an adaptation by plants to:
Predators
Decreasing sunlight
Hypertonic environments
Cooling temperatures
Warming temperatures

49. Extracellular layer Proteins Knife Plasma membrane Cytoplasmic layer
LE 7-4
Extracellular
layer
Proteins
Knife
Plasma
membrane
Cytoplasmic
layer
Extracellular layer
Cytoplasmic layer

50. Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell
LE 7-6
Membrane proteins
Mouse cell
Mixed
proteins
after
1 hour
Human cell
Hybrid cell

51. Figure 8.9 Some functions of membrane proteins

52. EXTRACELLULAR SIDE N-terminus C-terminus CYTOPLASMIC SIDE a Helix
LE 7-8
EXTRACELLULAR
SIDE
N-terminus
C-terminus
CYTOPLASMIC
SIDE
a Helix