1. Membrane Structure & Function
Chapter 8
Membrane Structure & Function

3. Membrane Structure Selective permeability Known as the plasma membrane
Controls traffic
Known as the plasma membrane
Amphipathic - hydrophobic & hydrophilic regions
Singer-Nicolson developed the fluid mosaic model

5. Membranes are Fluid

6. Structures related to properties & function
Membrane is usually about as fluid as salad oil.

8. Fluid Mosaic Model “Mosaic” Structure due to: Proteins: Lipids
Phospholipids - membrane fluidity
Cholesterol - membrane stabilization
“Mosaic” Structure due to:
Proteins:
Integral proteins - transmembrane proteins
Peripheral proteins - surface /appendages
Attachments-framework for animal cells
Membrane carbohydrates -~ cell to cell recognition; oligosaccharides (cell markers); glycoproteins; glycolipids; ABO blood typing

9. Sidedness of Plasma Membranes
Carbohydrates only on the outside surface
Proteins may be anchored
Inside to cytoskeleton
Outside to the extracellular matrix

10. Molecules that start on the inside face of Golgi Complex end up on the outside face of the plasma membrane

11. Know positioning of these structures; polar and nonpolar regions of membrane

13. Membrane Structure Membrane protein functions: Transport
Enzymatic activity
Signal transduction
Intercellular joining
Cell-cell recognition
ECM attachment

15. Traffic Across Membranes
Easily
hydrophobic
With Assistance
Hydrophilc: Polar, charged
Channels
Shuttles

16. Transport Passive Active No energy expenditure
Diffuse through membrane
Diffuse aided by protein
Facilitated diffusion
Active
Energy expenditure—
ATP
Usually against concentration Gradient
Pumps
cotransport
Bulk Transport
Endocytosis
Exocytosis

17. Passive Transport
diffusion of a substance across a biological membrane
No energy exerted
Diffusion - tendency of any molecule to spread out into available space
Concentration gradient – moves from high to low

18. Osmosis - the diffusion of water across a selectively permeable membrane; DOWN the concentration gradient.
Direction determined only by a difference in total solute concentration
Rate influenced by
Temperature
Steepness of conc. gradient

20. Water Balance Osmoregulation - control of water balance
Comparison of 2 solutions:
Hypertonic - higher concentration of solutes
Hypotonic - lower concentration of solutes
Isotonic - equal concentrations of solutes

21. Water Balance Cells with Walls (plants, bacteria, fungi):
Require hypotonic external environments to keep their turgor pressure (water pressure pushing cell membrane out against cell wall)
Become limp or flaccid when lose turgor pressure
Plasmolysis - plasma membrane pulls away from cell wall

22. Water Balance Cells without Walls (animals, most protist):
Require isotonic external environments
Hypertonic environments – cells swell & may burst with too much water pressure (Cytolysis)
May have contractile vacuoles (some protists; paramecium also have less porous membrane) to control internal water pressure

23. Contractile vacuoles & Osmoregulation in Paramecium

26. Specialized Transport
UtilizingTransport proteins (with or without channels)
Facilitated diffusion - passage of molecules and ions with transport proteins across a membrane down the concentration gradient.
Specific for its substrate

27. Facilitated diffusion
Transport of water and certain hydrophilic solutes across; down conc. gradient.
Transport Proteins
Most-very specific
2 types pf proteins
1. Channel Proteins
2. Transport Proteins

28. Facilitated diffusion 1. Channel Proteins
Permits rapid flow across membrane.
1. Aquaporins: plants and animals. Discovered in plants in 1994
2. Ion Channels
Many of these are Gated Channels-
Stimulus to open
Electrical or Chemical Stimulus
Example: Nerve cell stimulated by a neurotransmitter molecules (chemical) , opens gate, allows Na+ into the cell.

29. Diseases linked to Ion Channels A multitude of human and animal diseases are caused by dysfunction of ion channels. This may be genetic, i.e. caused directly by mutations in genes coding for ion channels. Such diseases are called ‘channelopathies’. Examples of channelopathies are cystic fibrosis, epilepsy, and arrhythmias, e.g. the long QT syndrome. Also, diseases may result from defects caused by mutations in genes coding for regulatory proteins.

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

31. Facilitated diffusion 2. Transport Proteins
Undergoes subtle change in shape.
Alternates between 2 conformations, moving molecule across as changes.

32. Facilated Diffusion
2. Carrier Proteins: Undergo conformational change, solute is transported as the protein changes shape.,

33. Active transport
Pump substance across membranes, against its concentration gradient, through a a Carrier Protein, with the help of cellular energy (ATP)
Example: How cells maintain a higher concentrations of K+ inside cell

35. Active Transport Systems
1. The sodium-potassium pump.
Specific kind of Active Transport
Esp. important in animals
Exchanges Na+ for K+
Transports
3 Na+ out for every 2 K+ into the cell.
Restores normal electrochemical gradient following an action potential.

36. 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.

37. Maintenance of Membrane Potential by Ion Pumps
Membrane Potential: The voltage (charge separation) across a membrane
Cytoplasm of a cell is negative
Extracellular fluid is positive
Ranges from mV
Favors diffusion of cations into the cell, and anions out of cells .

38. The electrochemical gradient: a combination of 2 forces that influence the movement of ions across membranes.
A chemical force (the ion’s concentration gradient)
An electrical force (the effect of the membrane potential on the ion’s movement).
SO…….ions move across membranes down their electrochemical gradient.

39. An electrogenic pump is a transport protein that generates voltage across a membrane.
The main electrogenic pump of animals is a sodium-potassium pump.
The main electrogenic pump of plants, fungi, and bacteria is a proton pump—actively transports H+ out of cells.

40. – 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+ – +

41. Cotransport
An ATP powered pump indirectly drives the Active Transport of several other solutes.
Example: Sucrose H+ Cotransporter
Sucrose loading in plants

42. CO-TRANSPORT... movement of 2 solutes together -
                                                
Often moves 1 solute passively & other actively       Ex:   1)  H+ pump coupled with sucrose transport ( H+symport* )               2)  epithelial transport* Na+glucose model ( glucose absorption* )
Replenish ions, sugar, fluid following strenuous exercise, diarrhea

46. – 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+ – +

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

48. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles.

49. 1. Exocytosis In exocytosis: cell secretes macromolecules.
Vessicle buds from Gogi.
Many secretory cells use this—
Examples: Pancreas secretes insulin
Neurons secrete neurotransmitters

50. 2. Endocytosis Reverse process.
The cell takes in macromolecules by forming vesicles from the plasma membrane.

51. Three types of endocytosis:
1. Phagocytosis (“cellular eating”): Cell engulfs particle in a vacuole. Not specific.
2. Pinocytosis (“cellular drinking”): Cell creates vesicle around fluid. Not specific.
3. Receptor-mediated endocytosis:
Very Specific
Binding of ligands to receptors triggers vesicle formation

52. 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

53. Familial hypercholesteremia
Defective gene/genes required for regulation, synthesis, transport, recycling, or turnover of LDL receptors.

54. Receptor-mediated endocytosis (ligands)
Exocytosis - secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis - import of macromolecules by forming new vesicles with the plasma membrane
Phagocytosis –cell “eating”
Pinocytosis – cell “drinking”
Receptor-mediated endocytosis (ligands)

55. Bulk transit across membranes

58. Cotransport (symport) involves more than one type of particle being transported by in the same direction at the same time by the same mechanism From