The Plasma
Membrane
and
Cellular
Transport
Maintaining a Balance
 Maintain
HOMEOSTASIS:
internal balance
 Selective
permeability: allows
some materials to pass
through membrane
while rejecting others.
Fluid-Mosaic Model
 This description is of a plasma
membrane that is made up of molecules
that are free to flow among one another.
 The kinds and arrangements of proteins
and lipids vary from one membrane to
another and give each type of membrane
specific permeability properties.
Structure of the Plasma Membrane
 Two-layered structure:
– Lipid bilayer
 Made of lipid
molecules with protein
molecules in the lipid
layer.
 PROTEINS aid in the
movement of materials
through the
membrane.
Structure of the Plasma Membrane
 The Lipids have two
fatty acids attached to
glycerol and a
phosphate group
(phospholipid).
 The Polar “head” of a
Phospholipid molecule
contains a polar
phosphorus group,
and two “tails” are
long, nonpolar carbon
chains.
Structure of the Plasma Membrane
Structure of the Plasma Membrane

F = Phospholipid bilayer

Phospholipid
– D = Hydrophilic
head
– E = Hydrophobic
tails

I = Cholesterol

Proteins
– J = transport

G = Cytoskeletal
filaments

A = Carbohydrate chain
C = Carbohydrate
Glycolipid
Glycoprotein


Proteins of the Plasma Membrane
1) Transport Proteins
2) Receptor Proteins
1) Transport Proteins
Channel Proteins –
channel for lipid
insoluble molecules
and ions to pass freely
through
Carrier Proteins – bind
to a substance and
carry it across
membrane, change
shape in process
2) Receptor Proteins
– Bind to chemical
messengers (Ex.
hormones) which
sends a message into
the cell causing
cellular reaction
How do materials move into and
out of the cell?
• Materials must move
in and out of the cell
through the plasma
membrane.
• Some materials
move between the
phospholipids.
• Some materials
move through the
proteins.
Plasma Membrane Transport
• Molecules move across the plasma
membrane by:
What are three types of
passive transport?
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
ATP energy is not
needed to move the
molecules through.
Passive Transport 1: Diffusion
• Also called Brownian Motion: random
motion of molecules.
• Molecules can move directly through the
phospholipids of the plasma membrane
What is Diffusion?
•
•
Diffusion is the net movement of molecules from a high
concentration to a low concentration until equally distributed.
Diffusion rate is related to temperature, pressure, state of
matter, size of concentration gradient, and surface area of
membrane.
 Dynamic Equilibrium:
a continuous movement
of molecules, but no
change in concentration.
Concentration Gradient
 The difference in
concentration of a
substance across a
space.
Passive Transport 2: Facilitated
Diffusion
• Molecules can move through the
plasma membrane with the aid of
transport proteins
This is called …
What is Facilitated Diffusion?
• Facilitated diffusion
is the net movement
of molecules from a
high concentration to
a low concentration
with the aid of
channel or carrier
proteins.
Passive Transport 3: Osmosis
• Water Molecules can move directly
through the phospholipids of the
plasma membrane
This is called …
What is Osmosis?
• Osmosis: diffusion of WATER
molecules through a membrane
from an area of higher water
concentration to lower water
concentration.
Osmosis in action
• What will happen in
the U-tube if water
freely moves through
the membrane but
glucose can not pass?
• Water moves from side
with high concentration of
water to side with lower
concentration of water.
Movement stops when
osmotic pressure equals
hydrostatic pressure.
Osmosis and Tonicity


Tonicity refers to the total solute
concentration of the solution outside the
cell.
What are the three types of tonicity?
1) Isotonic
2) Hypotonic
3) Hypertonic
Isotonic
 Solution in which the concentration of water (solvent)
outside the cell is the same as the concentration inside
the cell.
 What will happen to a cell placed in an Isotonic solution?
 The cell will have no net movement of water and will stay
the same size.
 Ex. Blood plasma has high concentration of albumin
molecules to make it isotonic to tissues.
Hypotonic (Less Concentration)
 Solution in where concentration of water (solvent) outside
the cell is higher than the concentration inside the cell.
 What will happen to a cell placed in a Hypotonic solution?
 The cell will gain water and swell.
 If the cell bursts, then we call this lysis. (Red blood cells =
hemolysis)
 In plant cells with rigid cell walls, this creates turgor
pressure.
Hypertonic (More Concentrated)
 Solution in which concentration of water (solvent)
outside the cell is lower than the concentration
inside the cell.
 What will happen to a cell placed in a Hypertonic
solution?
 The cell will lose water and shrink.
 In plant cells, the central vacuole will shrink and
the plasma membrane will pull away from the cell
wall causing the cytoplasm to shrink called
plasmolysis.
Review Tonicity
 What will happen to a red blood cell in a hypertonic solution?
 What will happen to a red blood cell in an isotonic solution?
 What will happen to a red blood cell in a hypotonic solution?
What are three types of
Active transport?
1) Active Transport
2) Exocytosis
3) Endocytosis
– Phagocytosis
– Pinocytosis
ATP energy is
required to move
the molecules
through.
Active Transport
 Molecules move from areas of low
concentration to areas of high concentration
with the aid of ATP energy.
The Importance of Active Transport
 Bring in essential molecules
 Rid cell of unwanted molecules (Ex.
sodium from urine in kidneys)
 Maintain internal conditions different
from the environment
 Regulate the volume of cells by
controlling osmotic potential
 Control cellular pH
 Re-establish concentration
gradients to run facilitated diffusion.
(Ex. Sodium-Potassium pump and
Proton pumps)
Active Transport 2:
Exocytosis (Exo – out)
 Movement of large
molecules bound in
vesicles out of the cell
with the aid of ATP
energy. Vesicle fuses
with the plasma
membrane to eject
macromolecules.
 Ex. Proteins,
polysaccharides,
polynucleotides, whole
cells, hormones, mucus,
neurotransmitters, waste
Active Transport 3:
Endocytosis (Endo – In)
 Movement of large
molecules into the cell
by engulfing them in
vesicles, using ATP
energy.
 Two types of
Endocytosis:
– Phagocytosis
– Pinocytosis