Cell Membrane and
Transport
The Cell Membrane
Cell membranes are
composed of two
phospholipid layers.
(bilayer)
The cell membrane has two
major functions.
1. Forms a boundary
between inside and
outside of the cell
2. controls passage of
materials
Present in all cells
•Phospholipid is a specialized lipid (fat) that has one
of it’s fatty acid tails removed so a phosphate can
be added.
•This causes a normal lipid that is ALWAYS
hydrophobic, to become partially hydrophilic.
•Terms that describe this are; hydrophilic/polar,
hydrophobic/non-polar
•Why do we want to have modified lipids (fats) in
the cell membrane?
Properties of the membrane
The fluid mosaic model describes the membrane.
There are other molecules embedded in
the membrane – this makes the surface
bumpy cell membrane
carbohydrate
chain
protein
cholesterol
protein
protein channel
5 specialized proteins
Adhesion
Communication (cell to cell)
Receptor
Recognition
Passive Transport
Active Transport
The cell membrane is selectively
Some molecules can cross the
permeable.
membrane while others cannot.
Chemical signals are
transmitted across the
cell membrane.
Receptors bind with
ligands and change
shape.
A ligand is a specific
molecule that binds
with a receptor to
make something
happen.
There are two types of receptors.
1) intracellular receptor
Ligand moves through the cell
membrane to reach receptor
on inside of cell.
Example: hormones
2) membrane receptor
Ligand on outside of cell binds
with a receptor within the cell
membrane.
Ex: Neurotransmitters, growth
factors
This is the end of the membrane lecture
that will be done in class on 9/16.
The remaining slides are the transport
lecture that will be done in class on 9/20.
Basic movement across cell membrane:
Materials move across the membrane in a variety of ways:
 Passively: (no energy required) moved based on concentration
gradient.
 Actively: moving items against a concentration gradient, energy is
required
 Facilitated diffusion: No energy, just using helpers (like proteins) to move
things in or out of a cell that are large.
 The two most common passive movement processes are diffusion
(solutions) and osmosis (water)
 Solutions are substances (solutes) dissolved in a solvent (water) to form
a solution.
Diffusion
 No solute likes to be crowded. Therefore, they will move from area that is
very saturated (crowded) to one that is not so saturated (uncrowded).
This process of widely spreading out is called diffusion.
 Who has been in a room where someone sprays perfume? Does it stay in
just one area?
 NO!! The perfume is moving from high concentration to low concentration
 Diffusion takes no energy, so it is passive.
 Osmosis is the passive movement of water.
Terms to describe solutions
Hypo=less than
Hyper=more than
Tonic =solute
Osmotic = Water
Iso = equal
Try these:
more solution (higher concentration of solute)
less solution (lower concentration of solute)
Equal amounts of solute
So if there is more solute then what does that say about the amount of
water?
Now try these:
More solute = less water = ?
Less solute = more water = ?
Remember, you are comparing 2 environments! Inside the
cell(intracellular) and outside the cell (extra-cellular)
Effects of water movement:
 If a cell loses to much water it will shrink and die! That’s called crenate or
plasmolyze.
 If a cell gains to much water it will burst and die! This is called cytolysis.
(lysis for short)
 Both are bad!
 Cells try to maintain homeostasis when it comes to water and solutes
and solutions. Homeostasis means a stable internal environment.
(Biological balance)
Active Transport
 But what happens if we want solutes to go from an area of low
concentration to high concentration?
 Must use ENERGY!
 It’s like putting the smell back in the can! (Active transport-going from
low concentration to highly concentrated)
Movement by vesicle
What is a vesicle?
Endocytosis: Moving things into the cell using a vesicle
Exocytosis: Moving things out of the cell using a vesicle
Pinocytosis: Bringing in of or removing liquids
Phagocytosis: Bringing in or removing of solids or large particles.
These are not exclusive! You can combine them to describe the type of movement!!