1. Life on the Edge – Cell Membranes

2. Cellular Membrane & Transport

3. Look at all that stuff!!!!
And it’s only 10nm thick!

4. Cell Membrane Structure – Who is hungry?
Davson and Danielli (1935)
Proposed the “sandwich model” to explain membrane structure
Lipid bilayer sandwiched between two protein layers
Problems
Later research showed:
Proteins highly variable in size and shape
Many too big for a 10nm thick structure
Membrane was thin and uniform
Many have non-polar regions, can’t interact with water

5. Cell Membrane Structure – Fluid Mosaic Model
1972
Singer and Nicolson propose the fluid mosaic model
Mosaic because a variety of components are embedded in the phospholipid bilayer
Fluid because the phospholipids and some components can drift laterally, like it’s a liquid

6. TEM picture of a real cell membrane.
About Cell Membranes
All cells have a cell membrane
Functions:
Controls what enters and exits the cell to maintain an internal balance called homeostasis
Provides protection and support for the cell
TEM picture of a real cell membrane.

7. About Cell Membranes (continued)
Structure of cell membrane
Lipid Bilayer -2 layers of phospholipids
Phosphate head is polar (water loving)
Fatty acid tails non-polar (water fearing)
Proteins embedded in membrane
Phospholipid
Lipid Bilayer

8. Fluid Mosaic Model of the cell membrane
Polar heads love water & dissolve.
Membrane movement animation
Non-polar tails hide from water.
Carbohydrate cell markers
The Fluid Mosaic model – molecules can move about allowing the membrane to adjust/change.
Proteins

9. Fluidity of the Cell Membrane
For cells to function, fluidity must be optimal
Too Fluid: membrane structure is weakened
Too Rigid: many functions, such as transport, cease
As temperatures cool, membranes switch from fluid to solid
The temperature at which this takes place depends on:
Type of fatty acids
Unsaturated: Double bonds prevent hydrocarbon chains from interacting via Van der Waals forces
Kinks form, phospholipids more widely spaced, more fluid
Saturated: hydrocarbon chains interact
More viscous

10. Fluidity of the Cell Membrane
2) Presence of cholesterol: slightly amphipathic
Low temperatures: maintains fluidity acting as a spacer between hydrocarbon chains
High temperatures: maintains rigidity by interacting with phospholipid heads, restricting motion
Cholesterol

11. About Cell Membranes (continued)
4. Cell membranes have pores (holes) in it
Selectively permeable: Allows some molecules in and keeps other molecules out
The structure helps it be selective!
Pores

12. Structure of the Cell Membrane
Outside of cell
Carbohydrate
chains
Proteins
Lipid
Bilayer
Consists of a mixture of proteins , phospholipids and cholesterol
Cholesterol adds flexibility in the phospholipids
Due to differences in behaviour based on temperature, can also provide support
Transport
Protein
Phospholipids
Inside of cell
(cytoplasm)
Animations
of membrane structure
Go to Section:

13. Proteins Receptor proteins (communication) Recognition proteins
Structure:
Proteins with attached sugar molecules.
Glycoproteins -- Function:
Attachment sites for molecules needing to enter, or for messenger molecules such as hormones.
These are very specific to each person and play a role in recognizing our own cells (organ transplants).
Proteins
Receptor proteins (communication)
Recognition proteins
Transport proteins – carrier or channel

14. That’s it!

15. Types of Cellular Transport
Animations of Active Transport & Passive Transport
Passive Transport
cell doesn’t use energy
Diffusion
Facilitated Diffusion
Osmosis
high
low
Weeee!!!

16. This is gonna be hard work!!
Active Transport
cell does use energy
Protein Pumps
Endocytosis
Exocytosis
high
low
This is gonna be hard work!!

17. Passive Transport (HighLow) cell uses no energy
molecules move randomly
Molecules spread out from an area of high concentration to an area of low concentration.
(HighLow)
Three types:

18. 3 Types of Passive Transport
Diffusion
Facilitative Diffusion – diffusion with the help of transport proteins
Osmosis – diffusion of water

19. Simple Diffusion Animation
Diffusion: random movement of particles from an area of high concentration to an area of low concentration.
(High to Low)
Diffusion continues until all molecules are evenly spaced (equilibrium is reached)-Note: molecules will still move around but stay spread out.

20. Facilitated diffusion (Channel Protein) Diffusion (Lipid Bilayer)
2. Facilitated diffusion: diffusion of specific particles through transport proteins found in the membrane
Transport Proteins are specific – they “select” only certain molecules to cross the membrane
Transports larger or charged molecules
Facilitated diffusion (Channel Protein)
Diffusion (Lipid Bilayer)
Carrier Protein

21. Passive Transport: 2. Facilitated Diffusion
Glucose
molecules
Cellular Transport From a-
High
High Concentration
Channel Proteins animations
Cell Membrane
Low Concentration
Protein
channel
Low
Transport Protein
Through a 
Go to Section:

22. Passive Transport: 3. Osmosis
Osmosis animation
3.Osmosis: diffusion of water through a selectively permeable membrane
Water moves from high to low concentrations
Water moves freely through pores.
Solute (green) to large to move across.

23. Active Transport (Low  High) cell uses energy
actively moves molecules to where they are needed
Movement from an area of low concentration to an area of high concentration
(Low  High)
Three Types:

24. Types of Active Transport
Sodium Potassium Pumps (Active Transport using proteins)
1. Protein Pumps -transport proteins that require energy to do work
Example: Sodium / Potassium Pumps are important in nerve responses.
Protein changes shape to move molecules: this requires energy!

25. A QUESTION!
HOW DO THE REALLY LARGE MOLECULES (Hormones, polysaccharides etc.) move in and out of cells??

26. An Answer!! By two processes called ENDOCYTOSIS AND EXOCYTOSIS.
Both methods require the use of vesicles and ATP!

27. Types of Active Transport
2. Endocytosis: taking bulky material into a cell
Uses energy
Cell membrane in-folds around food particle
“cell eating”
forms food vacuole & digests food
This is how white blood cells eat bacteria!
Three Types…
Phagocytosis
(Cell Eating):
-Used by white blood cells and amoeba.
-The cell moves out and surrounds the solid particle.
2. Pinocytosis (Cell drinking)
-Same process as phagocytosis except the cell
is moving liquids.
3. RME – Receptor Mediated Endocytosis
-Special receptor molecules on the membrane bind with the large molecule to be moved.
-When enough receptor molecules have gathered in one place, pinocytosis occurs.
-E.g. Moving cholesterol into the cell.

28. Types of Active Transport
3. Exocytosis: Forces material out of cell in bulk
membrane surrounding the material fuses with cell membrane
Cell changes shape – requires energy
EX: Hormones or wastes released from cell
Endocytosis & Exocytosis animations

29. Effects of Osmosis on Life
Osmosis- diffusion of water through a selectively permeable membrane
Water is so small and there is so much of it the cell can’t control it’s movement through the cell membrane.

30. Osmosis Animations for isotonic, hypertonic, and hypotonic solutions
Hypotonic: The solution has a lower concentration of solutes and a higher concentration of water than inside the cell. (Low solute; High water)

31. Result: Water moves from the solution to inside the cell): Cell Swells and bursts open (cytolysis)!

32. Osmosis Animations for isotonic, hypertonic, and hypotonic solutions
Hypertonic Solution
Hypertonic: The solution has a higher concentration of solutes and a lower concentration of water than inside the cell. (High solute; Low water)
shrinks

33. Result: Water moves from inside the cell into the solution: Cell shrinks (Plasmolysis)!

34. Osmosis Animations for isotonic, hypertonic, and hypotonic solutions
Isotonic Solution
Isotonic: The concentration of solutes in the solution is equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and the cell remains same size! (Dynamic Equilibrium)

35. B C A What type of solution are these cells in? Isotonic Hypotonic
Hypertonic

36. How Organisms Deal with Osmotic Pressure
Paramecium (protist) removing excess water video
Bacteria and plants have cell walls that prevent them from over-expanding. In plants the pressure exerted on the cell wall is called tugor pressure.

37. How Organisms Deal with Osmotic Pressure
Paramecium (protist) removing excess water video
A protist like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over-expanding. .

38. How Organisms Deal with Osmotic Pressure
Paramecium (protist) removing excess water video
Salt water fish pump salt out of their specialized gills so they do not dehydrate.
Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water.

39. Done and Done!