Cell Membranes and
Transport
B9 - Analyze the structure and
function of the cell membrane
Cell Walls



NB** Cell walls are
different from cell
membranes
Stiff, non-living
Made of complex
carbohydrates





Cellulose for plants
Chitin for fungi
Chitin-like frame for
bacteria
Used for support and
protection
Very porous; entry only
controlled by size
Cell Membranes
“gate keepers”

1. Isolate form
outside
 2. Control entry and
exit
 3. Communicate
with others
 4. Bare
identification (I’m
one of you!)
Which of these statements are
true comparing cell walls with
membranes?
A
B
C
Walls
Non-living
Membranes
Living
Plants and bacteria
only
Control entrance by
size only
Animals only
D
Made with cellulose
E
Contain pores
Control
entrance by
many factors
Made with
lipids
Contain
pores
Fluid Mosaic Model

A phospholipid
bilayer with proteins
scattered through it
 “fluid” because the
proteins seem to
“float” around the
bilayer
 Hydrophilic heads
on the outside
 Hydrophobic tails
on the inside

Hydrophobic layer is a barrier to H2O soluble
molecules (but makes it less fluid)

Cholesterol in the bilayer is even less
permeable to H2O soluble molecules (but
makes it less fluid)
“Protein Mosaic”

Membrane proteins will interact with the
hydrophobic and hydrophilic layers of the bilayer
 Some proteins will protrude into the cytoplasm,
some into the extracellular space, others into both
Glycoproteins

Membrane
proteins that
have a
carbohydrate
chain attached
 Often seen in
proteins that
protrude outside
the cell
Glycolipids

Membrane lipids
that have a
carbohydrate chain
attached

Both glycoproteins
and glycolipids
OFTEN function in
cell-to-cell
communication
and/or recognition
What does the “fluid” in “fluid
mosaic model” refer to?





A. The structure of the cell membrane
B. The structure of the cell wall
C. The fact that the membrane is made up
mostly of water
D. The fact that the membrane is always
changing, so it seems to be “fluid”
E. The fact that the membrane is made up
of lipids, and they tend to “flow”
What does “mosaic” mean?





A. a picture
B. a lipid
C. a bunch of different things clumped
together on a background
D. a type of protein that lets things into
the cell
E. No idea!
Which of the following is true
regarding this diagram?





A. 1a and 1b are
fatty acids
B. 3 is a phosphate
group
C. 5 is the
hydrophobic end of
the molecule
D. 6 is the
hydrophobic end of
the molecule
E. this is a type of
lipid
Which one is a:

1. Phospholipid
 2. Glycoprotein
 3. Cholesterol
3 major membrane Protein
Categories:

1. Transport proteins
 Regulated, fast method for specific molecules to
enter and exit


Channel proteins
Carrier proteins
2. Receptor Proteins

When activated, set off enzymatic
sequences inside the cell
3. Recognition Proteins

“identification tags”
Membrane Transport - RATE

Depends on:




Gradient (concentration, electrical or
pressure)
Size of molecule
Lipid solubility
# of transporters
Diffusion

The random net movement of molecules from an
area of high concentration to an area of low
concentration.
 (this is following the “concentration gradient”)
Osmosis

The diffusion of WATER across a selectively
permeable membrane
 (this is also following the “concentration gradient”
and does not require energy)
Osmotic Effects

Isotonic solution


Same solute
concentration
Cell is happy (no
net loss or gain of
water)
HYPERtonic
solutions

[Solute] is greater
outside the cell than
inside the cell
 Cell is not happy

It will crenate (shrink)
HYPOtonic solutions

Solute concentation
is less outside the
cell than inside
 Cell is not happy

Cell will lyse
Active transport

Often against the
concentration
gradient



Therefore,
REQUIRES
ENERGY
(ATP --> ADP + P)
Uses transporter
proteins
Endocytosis - 3 types
Phagocytosis
Large particles
2. Pinocytosis

Liquid and
 smaller particles
only
Receptor-mediated
Endocytosis

Uses receptors to
bind first to the
desired molecules,
then gathers them
together before
enclosing them in a
membrane