The cell membrane regulates what enters and exits the cell. It is a phospholipid bilayer with proteins embedded. Materials move across the membrane through passive diffusion, facilitated diffusion, or active transport using protein channels and pumps. Water moves across the membrane through osmosis to equalize its concentration gradient. Large particles enter through endocytosis using vesicles formed from the membrane.

1. Cell
membrane

3. Plasma Membrane
• Boundary that separates the living cell from it’s
non-living surroundings.
• Phospholipid bilayer
• Amphipathic - having both:
❖ hydrophilic heads
❖ hydrophobic tails
• ~8 nm thick
• Is a dynamic structure
phosphate
lipid
“attracted to water”
“repelled by water”

4. Cell (plasma) membrane
• cell membrane is the boundary
IN
1. food
- sugars
- proteins
- fats
2. Salts
3. O2
4. H2O
OUT
1. waste
- ammonia
- salts
- CO2
- H2O
2. products
- proteins
cell needs materials in & products or waste out

5. Function
• Regulates the movement of materials from
one environment to the other.
• Transports raw materials into the cell and
waste out of the cell.
• Prevents the entry of unwanted matter and
the escape of needed materials.
• Maintain a steady environment:
Homeostasis

6. Davson-Danielli model
In 1935, Hugh Davson and James Danielli proposed a model of
the cell membrane in which the phospholipid bilayer lies between two
layers of globular protein.

7. Why Davson-Danielli model of cell
membrane was rejected????
• This model assumes that all membranes are
identical this was known to be false
• The membrane proteins would be exposed to
hydrophilic environments on all sides (from the
phospholipids and from the water of the
cytoplasm). This is not a stable configuration.

8. Fluid mosaic bilayer model
S.J. Singer and G.L. Nicolson advanced the fluid mosaic
model in 1972.

9. The Fluid-Mosaic Membrane Model
• Membranes are not static; they have a fluid
consistency.
• Most membrane lipids and proteins can drift
about laterally in the plane of the membrane.
• Cholesterol enhances membrane fluidity, allows
animal membranes to function in a wide range of
temperatures and also makes the membrane
less permeable to biological molecules.

10. Functions of Membrane
Macromolecules
• Lipids
– Phospholipid bilayer
• Forms boundary to isolate cell contents from
environment
• Restricts passage of hydrophilic substances
across the membrane
– Cholesterol
• Increases bilayer strength, flexibility
• Reduces membrane fluidity
• Reduces permeability to water-soluble
substances

11. • Carbohydrates:
– Usually branched molecules of 15 or less
sugar units.
– Some are bonded to lipids: Glycolipids.
– Most are bonded to proteins: Glycoproteins.
➢Function: Cell-cell recognition.

13. Membrane Structure
Phospholipids are the major
structural component of
membranes.

14. All membranes are phospholipid bilayers
with embedded proteins.
Phospholipid Bilayer

15. • Hydrophilic end, which is insoluble in
water (water hating) and is known as non-
polar end
• Hydrophobic end, which is soluble in water
(water-loving) and is known as polar end

16. • Phospholipids in the plasma membrane can move
within the bilayer
• Most of the lipids, and some proteins, drift laterally
• Rarely does a molecule flip-flop transversely
across the membrane

17. Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
Movement of phospholipids

18. Membrane Proteins
• Proteins determine membrane’s specific functions
– cell membrane & organelle membranes each have unique
collections of proteins
• Membrane proteins:
– EXTRINSIC/Peripheral proteins
• loosely bound to surface of membrane
• cell surface identity marker (antigens)
– INTRINSIC/Integral proteins
• penetrate lipid bilayer, usually across whole membrane
• transmembrane protein
• transport proteins
– channels, permeases (pumps)

19. • Six major functions of membrane proteins:
– Transport
– Enzymatic activity
– Signal transduction
– Cell-cell recognition
– Intercellular joining
– Attachment to the cytoskeleton and
extracellular matrix (ECM)

20. ➢Transport of specific solutes into or out of cells.
➢Enzymatic activity, sometimes catalyzing one of a
number of steps of a metabolic pathway.
➢Signal transduction, relaying hormonal messages to
the cell.
➢Cell-cell recognition, allowing other proteins to
attach two adjacent cells together.
➢Intercellular joining of adjacent cells with gap or
tight junctions.
➢Attachment to the cytoskeleton and extracellular
matrix, maintaining cell shape and stabilizing the
location of certain membrane proteins.

21. Enzymes
Signal
Receptor
ATP
Transport Enzymatic activity Signal transduction

22. Glyco-
protein
Cell-cell recognition Intercellular joining Attachment to the
cytoskeleton and extra-
cellular matrix (ECM)

23. Getting through cell membrane
1. Passive Transport
– Simple diffusion
• diffusion of Nonpolar, hydrophobic molecules
– lipids
– high → low concentration gradient
– Facilitated transport
• diffusion of polar, hydrophilic molecules
• through a protein channel
– high → low concentration gradient
2. Active transport
– diffusion against concentration gradient
• low → high
– uses a protein pump
– requires ATP
ATP

24. 3. Bulk Transport
✓ Endocytosis:
The cell membrane folds inward, traps and encloses a small amount
of matter from the extracellular fluid.
o Pinocytosis:
The intake of a small droplet of extracellular fluid. This occurs in
nearly all cell types.
o Phagocytosis:
The intake of a large droplet of extracellular fluid. This occurs in
specialized cells.
o Receptor-assisted endocytosis:
The intake of specific molecules that attach to special proteins in
the cell membrane. These proteins are uniquely shaped to fit the
shape of a specific molecule.
✓ Exocytosis :
Substance is expelled after being enclosed in a vesicle within the cell.
Used to move large molecules out of the cell

25. Cell membrane channels
inside cell
outside cell
sugaraaH2O
saltwaste
• Need to make “doors” through membrane
– protein channels allow substances in & out
• specific channels allow specific material in & out
• H2O channel, salt channel, sugar channel, etc.

26. – move from HIGH to LOW concentration
– passive transport
– no energy needed

27. • Move from HIGH to LOW
inside cell
outside cell
Which way
will fat move?fat
fat
fat
fat
fat
fat fat
fat
fat
fat
fat
fat
fat
fat
LOW
HIGH
Simple Diffusion

28. Simple Diffusion
• Nonpolar, hydrophobic molecules diffuse
directly through the lipid bilayer
– Examples: O2, CO2, steroids
• Polar, hydrophilic substances cannot pass
directly through the lipid bilayer
– Examples: water, ions, carbohydrates

29. Simple Diffusion
small, nonpolar molecules
(ex. O2, CO2)
Polar molecules
(ex. Glucose, water)
ions
(ex. H+, Na+, K+)
LIPID-SOLUBLE WATER-SOLUBLE
LIPID-SOLUBLE

30. Facilitated Diffusion
• Passive movement of a substance into or out of the cell by
means of carrier proteins or channel proteins.
• Moves molecules from high to low regions of concentration.
• Carrier proteins: Transports non-charged molecules with a
specific shape.
• Channel proteins: Tunnel shape that transports small
charged molecules.
• No energy needed
• Example: glucose enter/leaves cells through facilitated
diffusion

31. Facilitated Diffusion
• Move from HIGH to LOW through a channel
protein
inside cell
outside cell
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
sugar
Which way
will sugar
move?
sugar
sugar
LOW
HIGH

32. Summary of Diffusion
• Move from HIGH to LOW concentration
– directly through membrane
• simple diffusion
• no energy needed
– help through a protein channel
• facilitated diffusion
• no energy needed
HIGH
LOW

33. Factors Affecting Diffusion Rate
• Steepness of concentration gradient
– Steeper gradient, faster diffusion
• Molecular size
– Smaller molecules, faster diffusion
• Temperature
– Higher temperature, faster diffusion

34. Simple vs. facilitated diffusion
inside cell
outside cell
lipid
inside cell
outside cell
H2O
simple diffusion facilitated diffusion
H2O
protein channel

35. Active transport
• Cells may need molecules to move
against concentration “hill”
– need to pump “uphill”
• from LOW to HIGH using energy
– protein pump
– requires energy
• ATP
ATP

36. Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP

37. Osmosis
• Osmosis – A special case of diffusion of
water across a selectively permeable
membrane
• water travels from an area of higher
concentration to an area of lower water
concentration

38. Osmosis Terms
▪ Isotonic: Water inside the cell equals the water outside
the cell and equal amounts of water move in and out of
the cell.
▪ Hypotonic: Water outside the cell is greater than that
inside the cell, water moves into the cell, may cause cell
to burst (lysis)
▪ Hypertonic: Water inside the cell is greater than outside.
Water moves out of the cell, may cause the cell to shrink
(plasmolysis)

39. Animal
cell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlant
cell
Turgid (normal) Flaccid Plasmolyzed

40. Endocytosis
• Active process for movement of large
molecules and organisms
– Substance is taken in by vesicle formed from
cell membrane
• Phagocytosis: (cellular eating) solid substance in
vesicle
• Pinocytosis: (cellular drinking) liquid droplets in
vesicle.
– Receptor-Mediated Endocytosis: incoming
substance binds to receptor