2.4 Cell
Membranes
and Transport
2.4 Cell Membranes and Transport

The cell membrane consists of structures called
phospholipids


Phospholipids form bilayers in water due to the
amphipathic properties of phospholipid molecules.
Phospholipids: a lipid molecule comprised of a polar
(hydrophilic) phosphate head and a nonpolar(hydrophobic) fatty acid tail.

Hydrophilic: polar, water loving head made of
phospholipid.

Hydrophobic: nonpolar, water fearing tails made of
2 fatty acid chains
 Cholesterol
is a component of animal cell
membranes.
 The cholesterol makes the phospholipid bilayer less
fluid at moderate temperatures, but slightly more
liquid than it normally would be at cold
temperatures. This helps maintain its functionality
and strength over a wide range of temperatures.
Davson-Danielli Model
 NOTE:
this model is not correct but shows
the progression of the current model
 Lipid bilayer composed of phospholipids



Hydrophobic tails inside
Hydrophilic heads outside
This forms two separate water-interacting
surfaces
 Proteins


coat outer surface
This forms a protein-lipid sandwich
Proteins do not permeate the lipid bilayer
Problems with this model:


This model assumes that all membranes are
identical - this was known to be false
(composition varies from cell to cell)
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.
Fluid Mosaic Model of Cell membranes

A model conceived by S.J. Singer and Garth
Nicolson in 1972 to describe the structural
features of biological membranes.

The plasma membrane is described to be
fluid because of its lipids and membrane
proteins that move laterally or sideways
throughout the membrane. That means the
membrane is not solid, but more like a 'fluid'.

It is a mosaic because it is made of many
different parts like such as integral proteins,
peripheral proteins, glycoproteins,
phospholipids, glycolipids, and in some cases
cholesterol, lipoproteins. Because it is fluid,
the membrane's shape may be altered.
http://www.youtube.com/watch?v=Qqsf_UJ
cfBc
Proteins Found within the Plasma Membrane
Examples of Membrane Proteins:

Membrane proteins are diverse in terms of
structure, position in the membrane and function.
1. Transport Channel Proteins (D):
 Proteins pores (w/ 'lock & key gate') through
which ions and macromolecules may pass.
2. Enzymes (I):
 Membranes provide a convenient surface for
enzymes to be embedded. Enzymes for related
reactions are organized next to each other,
organizing the reactions for greater efficiency.
(ex. Electron transport chain)
3. Cell Surface Receptors

Many proteins have 'lock & key' surfaces, which will
only fit to specific substances. When the chemical
binds to the protein, it is brought into the cell (via
facilitated diffusion). This is how some large food
macromolecules, chemical messengers, or even
viruses enter cells.

responsible for cell-cell signaling

Receptors communicate by binding to
neurotransmitters, peptide hormones, and growth
factors)
4. Cell Surface Identity Markers (E):

Glycoproteins act as ID markers which identify
the cell as belonging to that individual organism.

Ex. Antigens – responsible for antibody response
5. Cell Adhesion Proteins (A):
 Adjacent cells stick together via interlocking
proteins on their membranes.

Ex. Desmosomes-a cell structure specialized for
cell-to-cell adhesion.
Desmosome:
Membrane Permeability and
Transport
 Membrane
permeability can be(ability to allow
chemicals to pass / not pass across it):
 permeable- holes in membrane are large & all
molecules pass through.
 semi-permeable- smaller holes in membrane
allow only small molecules to pass through.
 non-permeable - no holes in membrane.
Nothing passes through.
http://www.youtube.com/watch?v=y31DlJ6
uGgE – Bozeman science
Examples of Cell Transport
 Passive
Transport - Movement of
molecules from high to low
concentration. Needs no energy input.
 Active Transport - Movement of molecules
from low to high concentration (opposite
the flow of diffusion). Needs input of
energy (ATP).
Concentration
Passive
Active
Examples of Passive Transport:
1. Diffusion: Random movement of any
molecules from higher to lower
concentration.
2. Osmosis: Diffusion of water across a
membrane. Occurs from high
concentration of H2O to low conc. H2O
http://www.youtube.com/watch?v=w3_8FSrqc-I
Terms Associated with Osmosis in
Cells:
 Hypertonic:
solute)

(hyper = more / tonic =
a) Any solution having more solute (less
H2O) than inside cell. More water flows
out of cell so cell shrinks.
 Can
shrivel the cell - crenation
Terms Associated with Osmosis in
Cells:
Hypotonic: (hypo = less; tonic
= solute)
 a)
Any solution having less
solute (more H2O) than the
cell contents. Water diffuses
into the cell (which has more
solute, less H2O) so cell
expands.
 Can burst the cell -hemolysis
Terms Associated with Osmosis in
Cells:
Isotonic: (iso = equal/tonic = solute)
2
solutions with equal
concentrations of solute and
water. Net flow of water between
the 2 is equal. We say both are in
equilibrium. H2O diffusion still
occurs in both directions at
equilibrium
Facilitated Diffusion:
3. Facilitated Diffusion: passive transport (no
energy required).
 Passage of molecules from high to low
concentration (diffusion) across membrane via
binding to a membrane protein which carries it
across.

Animation: [Facillitated Diffusion]
Ion Channels
Facilitated diffusion with Ion channels:
Special channel proteins allow ions to
pass through membrane w/out interacting
with hydrophobiclipid tails from high to
low concentration.
Active Tansport
Active Transport: requires large amounts
of energy (ATP) to 'pump' materials
against force of diffusion (low to high).
 Includes endocytosis (phagocytosis,
receptor mediated endocytosis and
pinocytosis); exocytosis; ion channels (low
to high concentration)
 The fluidity of membranes allows materials
to be taken into cells by endocytosis or
released by exocytosis. Vesicles move
materials within cells.

http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/fre
e/0072437316/120068/bio02.swf::Endocytosis%20and%20Ex
ocytosis
Types of Endocytosis:

1) Phagocytosis: Endocytosis (bulk
passage): move large polar molecules. This
action requires energy input (ATP) by cell.
2) Pinocytosis: cell drinking, liquid material
brought into cell by vacuole formation.
Animations: [Pinocytosis] [Pinocytosis]
 B.
Exocytosis: materials discharged from cell
(vacuole passes plasma membrane to dump
contents out of cell).
 http://www.youtube.com/watch?v=U9pvm_
4-bHg
C. Sodium-potassium pump. Moves 3 Na + out of
cell & 2 K+ into cell. Both Na+ and K+ are moved
from low to high concentration.

http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pum
p_works.html
Documentary
 https://www.youtube.com/watch?v=FFrK
N7hJm64&list=PL5BgsIwbuBEjQn8IbS4gyfhT
iaji__gc2 –BBC Secret Universe