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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
Lipids
 Proteins
 Question: How are the materials
arranged?

Membrane Models
Davson-Danielli Model 1935
Lipid bilayer.
 Proteins coat the surfaces.
 Sometimes called the “sandwich” model.

Evidence
Biochemical work.
 TEM pictures show the membrane as a
double line.

Problems
Not all membranes in a cell were the
same.
 How could the proteins stay in place?
 Result - the model was questioned and
tested by scientific process.

Fluid Mosaic Model 1972
New model to fit the new evidence with
membranes.
 Example of “Science as a Process”.

Fluid Mosaic Model

Refers to the way the lipids and proteins
behave in a membrane.
“Fluid”
Refers to the lipid bilayer.
 Molecules are not bonded together, so are
free to shift.
 Must remain "fluid" for membranes to
function.

Ways to keep the membrane
“fluid”
Lipid changes or shifts:
 Cold hardening of plants (shift to
unsaturated fatty acids).
 Hibernating animals (Cholesterol increase).

“Mosaic”
Proteins: float in a sea of lipids.
 Proteins form a collage or mosaic pattern
that shifts over time.

Evidence
TEM pictures of fractured membranes.
 Cell fusion studies.
 Tagging of membrane proteins by
antibodies.

Protein Function in Membranes

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
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
Transport.
Enzymatic activity.
Receptor sites for signals.
Cell adhesion.
Cell-cell recognition.
Attachment to the cytoskeleton.
Types of Membrane Proteins
Integral - inserted into the lipid bilayer.
 Peripheral - not embedded in the lipid
bilayer, but are attached to the membrane
surface.

Question?
How do the integral proteins stick to the
membrane?
 By the solubility of their amino acids.

Hydrophilic
Amino Acids
Hydrophobic
Amino Acids
Hydrophilic
Amino Acids
Membranes are Bifacial
The lipid composition of the two layers is
different.
 The proteins have specific orientations.
 Carbohydrates are found only on the outer
surface.

Carbohydrates
Membrane Carbohydrates
Branched oligosaccharides form
glycolipids and glycoproteins on external
surface.
 Function - recognition of "self" vs "other”.

Question

How do materials get across a cell's
membrane?
Problems
Lipid 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 Effect Diffusion
1.
2.
3.
4.
5.
Concentration
Temperature
Pressure
Particle size
Mixing
Osmosis
Diffusion of water.
 Water moving from an area if 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
Isosmotic solution.
 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
Hypoosmotic solution
 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
Hyperosmotic solution
 Cell's water is higher than the outside
water (less solutes)
 Water moves out of the cell.
 Cell shrinks or plasmolysis occurs.

Facilitated Diffusion
Transport protein that helps materials
through the cell membrane.
 Doesn't require energy (ATP).
 Works on a downhill concentration
gradient.

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
3. Receptor Mediated - uses receptors to
"catch" specific kinds of molecules.
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.
