Chapter 7 Membrane Structure
and Function
Plasma Membrane
The membrane at the boundary of every cell.
Functions as a selective barrier for the passage of materials in and out of cells.
Membrane Composition
phospholipids Proteins Cholesterol Question:
How are the materials arranged?
Phospholipid Bilayer
Phospholipids Hydrophilic
heads Hydrophobic
tails
Membrane Models
Fluid Mosaic Model 1972
New model to fit the new evidence with membranes.
Fluid Mosaic Model
The way the phospholipids and proteins behave in a membrane.
“Fluid”
Refers to the phospholipid bilayer.
Molecules are not bonded together, so are free to shift.
Must remain "fluid" for membranes to function.
“Mosaic”
Proteins: float in a sea of phospholipids.
Proteins form a collage or mosaic pattern that shifts over time.
Protein Function in Membranes
Transport. Enzymatic activity. Receptor sites for signals. Cell adhesion. Cell-cell recognition. Attachment to the cytoskeleton.
Types of Membrane Proteins
Integral - inserted into the phospholipid bilayer.
Peripheral - are attached to the membrane surface.
I or P?(mark these on the list above)
Transport –Channels and pumps (Integral)
Enzymes- I or P Receptors- P or sometimes I Cell adhesion- P or I Cell to cell recognition- P
(glycoproteins) Attachment to cytoskeleton- I
Question?
How do the integral proteins stick to the membrane?
By the solubility of their amino acids.
Non-polar will be in the hydrophobic area
Hydrophilic Amino Acids
Hydrophobic Amino Acids
Hydrophilic Amino Acids
Membranes are Bifacial
Inside layer is different than outer layer
The proteins have specific orientations.
Carbohydrates are found only on the outer surface.
Carbohydrates
Membrane Carbohydrates
Branched oligosaccharides form glycophospholipids and glycoproteins on external surface.
Function – cell to cell recognition
Cholesterol
Keeps membranes the right fluidity level Not too flimsy Not too rigid
End of Part 1 Create a Paper Model of a
membrane
Question
How do materials get across a cell's membrane?
Problems
phospholipid bilayer is hydrophobic. Hydrophilic materials don't cross easily.
Large molecules don't cross easily. Too big to get through the membrane.
Mechanisms
1. Passive Transport
2. Active Transport
Passive Transport
Movement across membranes that does NOT require cellular energy.
Types of Passive Transport
1. Diffusion
2. Osmosis
3. Facilitated Diffusion
Diffusion
The net movement of atoms, ions or molecules down a concentration gradient.
Movement is from: High Low
Equilibrium
When the concentration is equal on both sides.
There is no net movement of materials.
Factors that Affect Diffusion
1. Concentration
2. Temperature
3. Pressure
4. Particle size
5. Mixing
Osmosis
Diffusion of water. Water moving from an area of
its high concentration to an area of its low concentration.
No cell energy is used.
Tonicity
The concentration of water relative to a cell. 1. Isotonic (same)
2. Hypotonic (below)
3. Hypertonic (above)
Isotonic
Cell and water are equal in solute concentration.
No net movement of water in or out of the cell.
No change in cell size.
Hypotonic
Cell's water is lower than the outside water (more solutes).
Water moves into the cell. Cell swells, may burst or the
cell is turgid.
Hypertonic
Cell's water is higher than the outside water (less solutes)
Water moves out of the cell. Cell shrinks or plasmolysis
occurs.
Facilitated Diffusion
RequiresTransport protein that helps materials through the cell membrane.
Doesn't require energy (ATP).
Aquaporins
Newly found channels for osmosis.
GFP labeled Aquaporins
Active Transport
Movement across membranes that DOES require cellular energy.
Types of Active Transport
1. Carrier-Mediated
2. Endocytosis
3. Exocytosis
Carrier-Mediated Transport
General term for the active transport of materials into cells AGAINST the concentration gradient.
Movement is: low high
Examples
1. Na+- K+ pump
2. Electrogenic or H+ pumps
3. Cotransport
Na+- K+ pump
Moves Na+ ions out of cells while moving K+ ions in.
Electrogenic or H+ pumps
Also called Proton pumps. Create voltages across
membranes for other cell processes.
Used by plants, fungi and bacteria.
Cotransport
Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane.
Example - Sucrose transport
Exocytosis
Moves bulk material out of cells.
Example - secretion of enzymes.
Endocytosis
Moves bulk materials into cells.
Several types known.
Types
1. Pinocytosis - liquids
2. Phagocytosis – solids
Carbohydrates
Forming vesicles
Summary
Know membrane structure. Be able to discuss the various
methods by which cells move materials through membranes.
Be able to solve problems in osmosis.