The plasma membrane is a phospholipid bilayer with embedded proteins that separates the interior of cells from the external environment. It regulates the passage of substances through passive transport mechanisms like diffusion and facilitated transport, as well as active transport processes requiring ATP. The membrane also uses endocytosis and exocytosis to transport larger particles and molecules into and out of cells through phagocytosis, pinocytosis, and receptor-mediated endocytosis. Osmosis governs the movement of water across the membrane, causing cells to swell or shrink in hypotonic or hypertonic solutions respectively.

1. UNIT 4 - MEMBRANES

2. THE PLASMA MEMBRANE
STRUCTURE
A phospholipid bi-layer (lipids 50% by weight)
Contains embedded proteins (50% by weight)
Fluid consistency (movement)
Mosaic pattern of proteins
Main FUNCTION
Separation of the internal cell environment from its
surroundings

4. MEMBRANE STRUCTURE
EXPLAINED
Phospholipids
Hydrophilic Head – Polar head containing a phosphate group
Hydrophobic Tails – two fatty acid chains with one or more
double bonds
Proteins
Peripheral – protein to protein associations. Are not
embedded into the membrane
Integrated – proteins that are embedded into the membrane

5. MEMBRANE STRUCTURE
EXPLAINED
Cholesterol (animal) and Sterols (Plants)
Major membrane constituent
Stops membranes from freezing at low temp.
Interferes with fluidity at high temp.
Carbohydrates
Component of Glycoproteins and Glycolipids
Only found on cell exterior
Forms the glycocalyx
Cell to cell interations and cell surface protection

6. MEMBRANE STRENGTH
There are 4 components that aid in the strengthening
of the plasma membrane.
1. Polar and Non-polar interactions
2. Embedded Proteins
3. Cholesterol
4. Cytoskeleton

7. PLASMA PROTEINS -
FUNCTIONS
Transport – Some proteins act to move substances
between the internal and external cell environment
Reception – Some proteins receive specific
substances, such as hormones, from the external
environment.
Enzymes – Some proteins associated with the plasma
membrane will act as enzymes in various ways.

8. CHANNEL PROTEIN
•FUNCTION: Allow molecules or ions of
appropriate size to cross freely.
•Disease: Cystic fibrosis (CF) gene mutation on
chromosome 7; produces faulty chloride channel
protein.
•Results in thick mucus forming in airways and
ducts.

10. CARRIER PROTEIN
•FUNCTION:
•Selectively interacts with specific molecules.
•Attaches to and assists the molecule across the
membrane.
•Disease: Diabetes mellitus-faulty carrier for glucose.

12. GLYCOPROTEINS
In animal cells, the carbohydrate chains of cell recognition
proteins are collectively called the glycocalyx. “sugar-coated”
The glycocalyx can function in cell-to-cell recognition,
adhesion between cells, and reception of signal molecules.
The diversity of carbohydrate chains is enormous,
providing each individual with a unique cellular
“fingerprint”.

13. CELL RECOGNITION
PROTEIN
•Major Histocompatibility Complex (MHC)
glycoproteins are specific for each and every person.
•Makes transplants difficult as tissue is considered a
foreign body and attacked by blood cells.

15. RECEPTOR PROTEIN
•FUNCTION: The receptor is shaped in such a way
that it is specific to only one molecule.
•Disease: Pygmies are short because their GH
receptor proteins are faulty and do not interact with
growth hormone.

17. ENZYMATIC PROTEIN
•FUNCTION: Carry out specific metabolic
reactions.
•Disease: Cholera bacteria release a toxin that
interferes with an enzyme that regulates Na+ ions in
the cell. Severe diarrhea results.

20. MEMBRANE
PERMEABILITY
The plasma membrane is differentially permeable
Small, uncharged molecules may pass freely through
membranes (CO2, O2) via their concentration gradient.
Larger, uncharged molecules may not pass
freely(glucose).
Charged molecules (H+, Na+) pass via transmembrane
proteins (channel proteins).

21. MEMBRANE TRANSPORT
Passive Transport
Movement in the direction of the positive electrochemical
gradient
No energy is required (diffusion, facilitated transport)

22. MEMBRANE TRANSPORT
Active Transport
Movement in an energy unfavourable direction (requires
ATP)

23. MEMBRANE TRANSPORT
Vesicle Transport
Movement of macromolecules
Endocytosis – Extracellular material enters the cell
Exocytosis – Intracellular material exits the cell

24. DIFFUSION AND OSMOSIS
Diffusion
Passive movement of molecules past the plasma membrane
Moves from high to low concentration
Ceases after equilibrium has been met
Osmosis
Passive movement of water past the plasma membrane
from high to low concentration
Concentration can be altered by the cell with active
transport of certain ions

25. OSMOSIS IN CELLS
A solution contains a solute (solid) and a solvent
(liquid).
Cells are normally isotonic to their surroundings, and
the solute concentration is the same inside and out of
the cell.
“Iso” means the same as, and “tonicity” refers to the
strength of the solution.

26. OSMOSIS IN PLANT AND
ANIMAL CELLS

27. OSMOSIS IN CELLS
Hypotonic solutions cause cells to swell and
possibly burst.
“Hypo” means less than.
Animal cells undergo lysis in hypotonic solution.
Increased turgor pressure occurs in plant cells in
hypotonic solutions.
Plant cells do not burst because they have a cell wall.

29. OSMOSIS IN CELLS
Hypertonic solutions cause cells to lose water.
“Hyper” means more than; hypertonic solutions
contain more solute.
Animal cells undergo crenation (shrivel) in hypertonic
solutions.
Plant cells undergo plasmolysis, the shrinking of the
cytoplasm.

31. HYPO/HYPER/ISO TONIC
10% Salt
15% Salt

32. HYPO/HYPER/ISO TONIC
0.9% Fructose
0.9%
Fructose

33. HYPO/HYPER/ISO TONIC
20% Salt
15% Salt

34. HYPO/HYPER/ISO TONIC
Pure
Water 2% Salt

35. HYPO/HYPER/ISO TONIC
3%
Salt 95%
water

36. TRANSPORT BY CARRIER
PROTEINS
Used to move biologically significant molecules past
the plasma membrane.
Each carrier protein interacts with a specific
molecule
Carrier proteins move molecules with both
Facilitated Transport and Active Transport methods.

37. FACILITATED
TRANSPORT
Does not require energy
Movement in the direction of the concentration gradient
takes place.
Molecules bind with the carrier protein and the protein
undergoes a conformational change
Allows for polar and charged molecules to pass through
the plasma membrane

38. GLUCOSE – FACILITATED
DIFFUSION

39. ACTIVE TRANSPORT
Requires energy (ATP) to be carried out.
Movement against the concentration gradient takes
place. (low to high concentration)
Binding of the target molecule to the protein initiates
ATP hydrolysis and a conformational change of the
protein.

40. ACTIVE TRANSPORT

41. NA+/K+ PUMP
Carrier proteins involved in active transport are called pumps.
Step 1 – Na+ binds to the protein inside the cell
Step 2 – ATP hydrolyzes and Na+ is released externally
Step 3 – K+ binds to the protein outside the cell
Step 4 – Hydrolysis of phosphate bound to the pump releases
K+ into the cytoplasm
Each ATP hydrolysis of ATP pumps 3 Na+ ions outside the cell
and 2 K+ ions into the cell.

42. NA+/K+ PUMP
Draw Steps on Board

43. Exocytosis
During exocytosis, vesicles fuse with the plasma
membrane for secretion.
Some cells are specialized to produce and release
specific molecules.

44. EXOCYTOSIS

45. EXAMPLES OF
EXOCYTOSIS
The release of digestive enzymes from cells of the
pancreas.
Secretion of the hormone insulin in response to
rising blood glucose levels.
Called regulated secretion since it only happens when
insulin is needed to reduce blood glucose.

46. ENDOCYTOSIS
During endocytosis, cells take in substances by
invaginating a portion of the plasma membrane, and
forming a vesicle around the substance.
Endocytosis occurs as:
Phagocytosis – large particles
Pinocytosis – small particles
Receptor-mediated endocytosis – specific particles

47. PHAGOCYTOSIS
• Phagocytosis occurs when the
substance to be transported into
the cell is large.
• Amoebas ingest food by
phagocytosis.

48. PHAGOCYTOSIS

49. PHAGOCYTOSIS
White blood cells are amoeboid and
engulf worn-out cellular debris or
bacteria using phagocytosis.

50. PHAGOCYTOSIS

51. PINOCYTOSIS
Pinocytosis occurs when a macromolecule, such as
a polypeptide, is to be transported into the cell.
The macromolecule or polypeptide is still
considered small when compared to things
brought in by phagocytosis--the resulting vesicle
or vacuole is also small.

52. PINOCYTOSIS

53. PINOCYTOSIS
Pinocytosis occurs continuously and this uses up
the plasma membrane.
What keeps the relative size of the plasma
membrane constant?

54. RECEPTOR-MEDIATED
ENDOCYTOSIS
Receptor-mediated endocytosis is a form of
pinocytosis.
The substance to be taken in binds with a specific
receptor protein, which migrates to a pit or is already
in a coated pit.

55. RECEPTOR-MEDIATED
ENDOCYTOSIS

56. RECEPTOR-MEDIATED
ENDOCYTOSIS
Receptor-mediated endocytosis is responsible for
cells taking up low-density lipoprotein (LDL)
when LDL receptors gather in a coated pit.
Disease - Hypercholesterolemia
LDL receptors are unable to properly bind to the
coated pit. Cholesterol builds up in arteries.

57. DIFFUSION – FACTORS
AFFECTING RATE
Concentration
Temperature
Ionic/Molecular Size
Shape of Ion/Molecule
Viscosity
Movement of Medium

58. DIFFUSION – FACTORS
AFFECTING RATE

59. CHAPTER SUMMARY
The structure of the plasma membrane allows it to
be differentially permeable.
The fluid phospholipid bilayer, its mosaic of
proteins, and its glycocalyx make possible many
unique functions of the plasma membrane.

60. SUMMARY CONTINUED…
Passive and active methods of transport regulate materials entering and exiting cells.
Osmosis describes the movement of water (solvent) as opposed to the solute.
Hypotonic solutions cause increased turgor pressure or even cause lysis.

61. SUMMARY CONTINUED…
Hypertonic solutions cause crenation (animals) or
plasmolysis (plants) in cells.

62. SUMMARY CONTINUED…
Hypertonic solutions cause crenation (animals) or
plasmolysis (plants) in cells.
Passive transport includes simple diffusion or
facilitated transport with carrier proteins--no
energy.
Active transport involves ATP with carrier proteins.
Exocytosis and endocytosis round out the final
mechanisms by which materials pass through the
plasma membrane.
Can you name and explain the different variations
of endocytosis?