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

Fat Sandwich Model Fluid Mosaic Model

Wrong

Functions:

1. Phospholipid - Hydrophilic head, polar, interacts

2. Phospholipid – Hydrophobic tail, nonpolar, separates

3. Cholesterol : Maintains Fluidity

at different temperatures

4. Glycolipids / Glycoprotein / Carbohydrates

Cell Markers, ID’s, Recognition, Communication

5. Peripheral Proteins: Cytoskeleton attachment

6. Integral Proteins Many Functions

Cytoplasmic Face

ExtraCellular Face

Transport

Enzymes

Signal transduction

Intracellular joining

Cell-cell recognition

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 CellHigh Oxygen

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 Environment EnvironmentCell CellCellH2O – 98%

DS -- 2%

H2O – 90%

DS -- 10%

H2O – 98%

DS -- 2%

H2O – 80%

DS -- 20%

H2O – 95%

DS -- 5%H2O – 98%

DS -- 2%

Water Flow

Tonicity Environment: Hypotonic

Cell: Hypertonic

JargonPlant: Turgor Pressure

Animal: Cytolysis

“Hydrostatic Skeleton”

Environment: Hypertonic

Cell: Hypotonic

Isotonic

Plasmolysis

Crenation

Summary

Flaccid

Osmotic Balance

Tonicity Tonicity

Cell Cell Cell

Elodea in Freshwater Elodea in Salt and/or Glucose

LAB DATA

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

Ion Channel Transport Glucose Transport

Inside Cell

GlucoseInsulin

Transport Protein

Open Channel

Examples: Ion Channels, Glucose transport, Aquaporins

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

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”07-16-EndomembraneSystem.mov

Cell 80% H20 Environment 99% H20

H20

Contractile VacuoleH20

Paramecium

Active transport of water to the outside of the cell

How the pump actually works

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”

Phospholipid bilayer forms when larger quantities are immersed in water

“Amphipathic Molecule”

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

Lateral movement common Flip-flop rare

Factors Determining Membrane FluidityUnsaturated Saturated

(Less Dense) (More Dense)

Fluid Viscous

(Plants) (Animals)How do animals maintain membrane fluidity?

Cholesterol

Slows phospholipid movement at higher temperatures yet hinders solidification at lower temperatures

Slide 2

Phagocytosis

Pinocytosis

Receptor Mediated Endocytosis

Slide 9

Slide 1

Slide 3“Porin” an Example of a Transport Channel

Slide 8

Slide 9

Slide 9

Slide 9

Receptor-mediated Endocytosis (pinocytosis)

Slide 5

Aquaporins – Protein Water Channels

Water molecules outside cell

Water molecules inside cell

Aquaporin I

Cell Membrane

Slide 8Aquaporin II

The Sodium Potassium Pump in Action

Slide 10

Inside the Cell

Outside the Cell