1. Cell Membrane: Structure and Function 6.2-6.3
The cell membrane
is the
gateway
into the cell.

2. Key words you should know:
Phospholipids Solution Pinocytosis
Polar Solute
Hydrophilic Solvent Exocytosis
Hydrophobic Partially permeable Phospholipid bilayer
Fluid mosaic model Surface area
Glycoproteins Turgid
Plasmolysis Passive transport
Cholesterol Plasmolysed
Proteins Selectively permeable
Transport proteins Active transport
Enzymes Carrier protein
Receptor molecules Bulk transport
Diffusion Endocytosis
Concentration gradient Phagocytosis
Facilitated diffusion Phagocytes
Osmosis Crenated

3. Cell Membrane Structure
All living cells are surrounded by a membrane.
A cell membrane is also known as plasma membrane.
To understand the function of anything in biology, you must study the structure first!
Cell Membranes from Opposing Neurons
(TEM x436,740).
Nerve cell
Gap between cells
Cell membrane {
} cell membrane
Nerve cell

4. Functions of the Plasma Membrane
Protective barrier
Regulate transport in & out of cell
Allow cell-cell recognition
Provide anchoring sites for filaments of cytoskeleton
Provide a binding site for enzymes and hormones
Interlocking surfaces bind cells together (junctions)
Contains the cytoplasm (fluid in cell)

5. Structure of Cell Membrane
The Plasma Membrane
Structure of Cell Membrane
4/19/2017
Phospholipids
Proteins (peripheral and integral)
Cholesterol
Carbohydrates
G. Podgorski, Biol. 1010

6. PHOSPHOLIPID REVIEW
Phospholipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO4-) replaces one of the three fatty acids normally found on a lipid.
The addition of this group makes a polar "head" and two nonpolar "tails".

7. HYDROPHILIC HEAD
At one end of the phospholipid is a phosphate group and several double bonded oxygens. The electrons at this end of the molecule are not shared equally. This end of the molecule has a charge and is attracted to water. It is POLAR
HYDROPHOBIC TAILS
The two long chains coming off of the bottom of this molecule are made up of carbon and hydrogen. Because both of these elements share their electrons evenly these chains have no charge. They are NON POLAR.
Molecules with no charge are not attracted to water; as a result water molecules tend to push them out of the way as they are attracted to each other. This causes molecules with no charge not to dissolve in water.

8. Phospholipids can form: BILAYERS
-2 layers of phospholipids with
hydrophobic tails protected inside by the hydrophilic heads.
The PHOSPHOLIPID BILAYER is the basic structure of membranes.

9. Polar heads are hydrophilic “water loving”
Nonpolar tails are hydrophobic “water fearing”
Phospholipids make membranes
Selectively Permeable- able to control what crosses

10. Nonpolar,
Hydrophobic
Fatty acid “tails”
Polar,
Hydrophilic
“head”
Polar,
Hydrophilic
“head”

11. FLUID MOSAIC MODEL
Cell membranes also contain proteins within the phospholipid bilayer.
This ‘model’ for the structure of the membrane is called the:
FLUID MOSAIC MODEL
FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid.
MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above.

12. Proteins Are Critical to Membrane Function
The Plasma Membrane
4/19/2017
Proteins Are Critical to Membrane Function
G. Podgorski, Biol. 1010

13. Proteins can float in the membrane or be fixed and also have hydrophobic and hydrophilic portions. Proteins may be embedded in the outer layer or in the inner layer and some span the two layers.
Hydrophobic and Hydrophilic parts of the protein molecules sit next to the Hydrophobic and Hydrophilic portions of the phospholids of the membrane. This ensures the proteins stay in the membrane.

14. Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS.
Glycoproteins act as chemical id tags.
Blood types are the result different glycoproteins.

15. The membrane also contains molecules of CHOLESTEROL
regulates the fluidity of the membrane
gives mechanical stability
helps to prevent ions from passing through the membrane.

16. The fluid mosaic model was developed using Freeze Fracture Studies.
The fracture occurs between the two phospholipid layers.
You can clearly see the exposed proteins sticking out of the two layers.
Individual phospholipids are too small to see.

17. Cell Membrane Permeability
Hydrophobic pass easily
Hydrophilic DO NOT

18. Semipermeable Membrane
Small molecules and larger non-polar molecules move through easily.
e.g. O2, CO2, H2O

19. Semipermeable Membrane
Ions
Hydrophilic molecules larger than water
Large molecules such as proteins
do not move through the membrane on their own.

20. Types of Transport
Across
Cell Membranes

21. Two Forms of Transport Across the Membrane
The Plasma Membrane
Two Forms of Transport Across the Membrane
4/19/2017
G. Podgorski, Biol. 1010

22. Passive Transport Simple Diffusion Doesn’t require cell energy
The Plasma Membrane
4/19/2017
Passive Transport
Simple Diffusion
Doesn’t require cell energy
Molecules move from high to low concentration
Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.
G. Podgorski, Biol. 1010

23. Molecules move from area of HIGH to LOW concentration,
down their concentration gradient.

24. Diffusion is a PASSIVE process which means no cell energy is used to make the molecules move, they have a natural KINETIC ENERGY

25. Diffusion of Liquids

26. Diffusion through a Membrane
Cell membrane
Solute moves DOWN the concentration gradient from HIGH to LOW concentration

27. Osmosis Diffusion of water across a membrane
Moves from HIGH water potential (low solute) to LOW water potential (high solute)

28. Osmosis-Diffusion of H2O Across A Membrane
Low solute concentration
High solute concentration

29. CELL 10% NaCL 90% H2O ENVIRONMENT NO NET MOVEMENT 10% NaCL 90% H2O
What is the direction of water movement?
equilibrium
The cell s at _______________.

30. CELL 10% NaCL 90% H2O 20% NaCL 80% H2O
What is the direction of water movement?

31. CELL 15% NaCL 85% H2O ENVIRONMENT 5% NaCL 95% H2O
What is the direction of water movement?

32. Cells in Solutions

33. Isotonic Solution Hypotonic Solution Hypertonic Solution
NO NET MOVEMENT OF H2O
(equal amounts entering & leaving)
CYTOLYSIS
(water diffuses into the cell)
CRENATION
(water diffuses out of the cell)

34. Cytolysis & Crenation
Cytolysis
Crenation

35. Osmosis in Red Blood Cells
In Isotonic
Solution
In Hypertonic Solution
In Hypotonic Solution

36. hypotonic hypertonic hypertonic isotonic hypotonic (Crenated cell)
Indicate the diffusion gradient of water with an arrow.
hypotonic
hypertonic
hypertonic
isotonic
hypotonic
(Crenated cell)
hypertonic
isotonic
hypotonic
(Plasmolyzed cell)
(Turgid cell)

37. Transport Proteins
Channel proteins are embedded in the cell membrane & have a pore for materials to cross
Carrier proteins can change shape to move material from one side of the membrane to the other

38. Facilitated Diffusion
The Plasma Membrane
4/19/2017
Passive Transport
Facilitated Diffusion
Doesn’t require energy
Uses transport proteins to move high to low concentration
Moves materials with the concentration gradient.
Examples: Glucose or amino acids moving from blood into a cell.
G. Podgorski, Biol. 1010

39. Facilitated Diffusion
Molecules will randomly move through the pores in Channel Proteins.

40. Facilitated Diffusion
Some Carrier proteins do not extend through the membrane.
They bond and drag molecules through the lipid bilayer and release them on the opposite side.

41. Carrier Proteins
Other carrier proteins change shape to move materials across the cell membrane

42. Active Transport Requires energy or ATP
The Plasma Membrane
4/19/2017
Active Transport
Requires energy or ATP
Moves materials from LOW to HIGH concentration
AGAINST concentration gradient
G. Podgorski, Biol. 1010

43. Active Transport Examples:
Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.
Called Na+-K+ Pump

44. Sodium-Potassium Pump
(+)
(-)
3 Na+ pumped out for every 2 K+ pumped in; creates a membrane potential

45. Moving the “Big Stuff” Out
Exocytosis- moving things out.
Molecules are moved out of the cell by vesicles that fuse with the plasma membrane.
This is how many hormones are secreted and how nerve cells communicate with one another.

46. Exocytosis Large molecules that are manufactured in the cell are released through the cell membrane.
Inside Cell
Cell environment

47. The Plasma Membrane
Exocytosis
4/19/2017
Exocytic vesicle immediately after fusion with plasma membrane.
G. Podgorski, Biol. 1010

48. Moving the “Big Stuff” In
The Plasma Membrane
Moving the “Big Stuff” In
4/19/2017
Large molecules move materials into the cell by one of three forms of endocytosis.
Pinnocytosis
Receptor-mediated EndoCytosis
Phagocytosis
G. Podgorski, Biol. 1010

49. 1. Pinocytosis Most common form of endocytosis.
The Plasma Membrane
1. Pinocytosis
4/19/2017
Most common form of endocytosis.
Takes in dissolved molecules as a vesicle.
G. Podgorski, Biol. 1010

50. Pinocytosis Cell forms an invagination
Materials dissolve in water to be brought into cell
Called “Cell Drinking”

51. Example of Pinocytosis
The Plasma Membrane
Example of Pinocytosis
4/19/2017
mature transport vesicle
pinocytic vesicles forming
Transport across a capillary cell (blue).
G. Podgorski, Biol. 1010

52. 2. Receptor-Mediated Endocytosis
The Plasma Membrane
4/19/2017
2. Receptor-Mediated Endocytosis
Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc.
G. Podgorski, Biol. 1010

53. Receptor-Mediated Endocytosis
The Plasma Membrane
Receptor-Mediated Endocytosis
4/19/2017
Coated pit
Vesicle
G. Podgorski, Biol. 1010

54. The Plasma Membrane
4/19/2017
3. Phagocytosis
Used to engulf large particles such as food, bacteria, etc. into vesicles
Called “Cell Eating”
G. Podgorski, Biol. 1010