Membrane Structure and Function
“The function of the cell membrane is to control what goes in and out”
“Selectively permeable / Semi-permeable”
I.
Structure of the Cell Membrane
The “Fat Sandwich Model” vs. The Fluid Mosaic Model
A.
Fluid - Phospholipids
B.
Mosaic - Proteins
Wrong
Fat Sandwich Model
Fluid Mosaic Model
ExtraCellular Face
Cytoplasmic Face
Functions:
1.
Phospholipid - Hydrophilic head, polar,
interacts
2.
Phospholipid – Hydrophobic tail,
nonpolar, separates
3.
Cholesterol : Maintains Fluidity
at different temperatures
Transport
Intracellular
joining
Enzymes
Cell-cell
recognition
4. Glycolipids / Glycoprotein / Carbohydrates
Cell Markers, ID’s, Recognition,
Communication
5.
Peripheral Proteins: Cytoskeleton
attachment
6.
Integral Proteins Many Functions
Signal
transduction
Attachment
II. Molecules can pass through Membranes if:
A. Small enough
Example: Fast - O2, CO2, H2O
Slow - C6H12O6
Can’t – polypetides (proteins) polysaccarides (starches)
B. Non-Polar enough
Example: O2, CO2, Steroids, Lipids
Note: Ions do not move through membranes very well (Na+, Cl-, H+, ) due
hydration shell
C. Concentrated enough
Example: H2O, All living things are 70-80% water
D. Helped by Transport Proteins Enough
Transport Proteins provide unique
environments in the membrane to allow
passage of specific molecules
E. Any combination of the above
Example - Water
III. What energy “makes" molecules move through membranes?
A. Molecular motion (Kinetic Energy) Diffusion
1. Definition: Movement of molecules from a high concentration
to a low concentration
2. Characteristics
a. Follows a concentration gradient
b. Does not require additional energy
c. “Passive Transport”
3. How cell uses diffusion
Environment
High Oxygen
Cell
Cell Respiration
Low Carbon Dioxide
IV. Osmosis The Special Case of Water
A. Definition: The diffusion of water across a semi-permeable membrane
B. Why does water have its own unique word to describe how it diffuses?
1. All organism are 70-80% water - The highest of all concentration gradients
2. Membranes are completely permeable to water (The membrane can not “just say no”)
Environment
Cell
Environment
H2O – 98%
DS -- 2%
Cell
H2O – 90%
Cell
Environment
Cell
H2O – 98%
Cell
DS -- 10%
Cell
DS -- 2%
Water Flow
H2O – 80%
Tonicity
DS -- 20%
H2O – 95%
Tonicity
DS -- 5%
Environment: Hypotonic
Environment: Hypertonic
Cell: Hypertonic
Cell: Hypotonic
H2O – 98%
Tonicity
DS -- 2%
Isotonic
Jargon
Plant: Turgor Pressure
Plasmolysis
Flaccid
Animal: Cytolysis
Crenation
Osmotic Balance
“Hydrostatic Skeleton”
Summary
LAB DATA
Elodea in Freshwater
Elodea in Salt and/or Glucose
Elodea in Acetone
V.
Facilitated Diffusion
A.
Definition: Movement of specific molecules from a high
to a low concentration with the help of specific transport
proteins
B.
Characteristics of transport protein
1.
No Extra energy requirement High  Low gradient
2.
Can change shape
3.
Shape change may open or close channels
4.
Works like an enzyme
Examples: Ion Channels, Glucose transport, Aquaporins
Glucose
Insulin
Ion Channel
Transport
Glucose Transport
Transport
Protein
Inside Cell
Open
Channel
VI. Active Transport - Moving Molecules against the Gradient
A. Characteristics
1. Requires ATP
2. Moves molecules from Low to High concentration
3. Requires specific transport proteins
B. Active Transport of Small Molecules
1. Usually involves the transport of Ions (why?)
2. Examples
a) Sodium potassium pump
How the pump actually works
b) Proton (Hydrogen) pump Electrogenic pumps
c) Co-Transport pumps - Active transport of H+ coupled
with the passive transport of a different molecule
C. Active Transport of Large Molecules (Bulk Transport)
-All require fusion of membranes
1. Exocytosis - Bulk transport outside the cell
a) Golgi Body Secretions
b) Contractile Vacuoles “The Protozoan Sump Pump”
Cell
80% H20
Environment 99% H20
Contractile Vacuole
H20
H20
Paramecium
Active transport of water to
the outside of the cell
2. Endocytosis
a) Phagocytosis “Cell Eating”
Cell engulfs particle by wrapping pseudopodia (false feet) around the particle
Food particle becomes a food vacuole to be digested by lysosome
Amoebas
White Blood Cells
b) Pinocytosis “Cell Drinking”
Small pockets in cell membrane pinches off and pulls in extra-cellular fluids
c) Receptor Mediated Endocytosis
Membrane bound protein receptor sites bind to specific “ligands” outside
of cell
Receptors accumulate in a membrane “pit” and are then pulled in by
pinocytosis
(Summary)
A Small Phospholipid Drop In Water
“Miscelle”
“Amphipathic Molecule”
Phospholipid bilayer forms when larger
quantities are immersed in water
Slide 2
Proton Pump
Slide 10
Proton Pump (active transport) coupled with a cotransport protein (passive)
Slide 10
Slide 2
Proof of the Fluidity
of Membranes
Movement of Phospholipids
Flip-flop rare
Lateral movement common
Factors Determining Membrane Fluidity
Unsaturated
Saturated
(Plants)
(Less Dense)
(Animals)
(More Dense)
How do animals maintain membrane fluidity?
Cholesterol
Slows phospholipid movement at higher temperatures
yet hinders solidification at lower temperatures
Fluid
Viscous
Slide 2
Phagocytosis
Pinocytosis
Receptor
Mediated
Endocytosis
Slide 9
Slide 1
“Porin” an Example of a Transport Channel
Slide 3
Slide 8
Slide 9
Slide 9
Receptor-mediated Endocytosis (pinocytosis)
Slide 9
Slide 5
Aquaporins – Protein Water Channels
Water molecules
outside cell
Cell Membrane
Water molecules
inside cell
Aquaporin I
Aquaporin II
Slide 8
The Sodium Potassium Pump in Action
Inside the Cell
Outside the Cell
Slide 10