CELL MEMBRANE
Cell Membrane
•
•
•
•
Boundary that separates the living cell
from it’s non-living surroundings.
Phospholipid bilayer
Amphipathic - having both:
hydrophilic heads
hydrophobic tails
Phospholipid
~8 nm thick
Cell Membrane - cont.
• Controls traffic into and out of the cell with
phospholipids and transport proteins.
• Selectively permeable
Transport protein
Selective Permeability
• The property of biological
membranes which allows some
substances to cross more easily
than others.
Fluid Mosaic Model
• 1972 - Singer and Nicolson
called the membrane a “Fluid
Mosaic Model”.
• Mosaic: different proteins
embedded in the phospholipids.
• Fluid: proteins and
phospholipids can move freely
in the membrane.
Fluid Mosaic Model- cont.
• Components of a phospholipid
bilayer.
1. phospholipids
2. proteins - enzymes, receptors,
transport.
3. glycolipids
4. glycoproteins
5. carbohydrates
6. cholesterol
Animal Cell’s Cell Membrane
What are typical roles of proteins
in the cell membrane?
Some Functions of Membrane Proteins
H
What is Diffusion?
Diffusion
• The net movement of a substance
(molecules) down a concentration
gradient from an area of high
concentration to an area of low
concentration.
• passive transport: NO energy is expended.
• facilitated diffusion: type of passive transport
which uses transport proteins.
•What is osmosis?
Osmosis
• The movement of water across
selectively permeable membranes.
• The water moves from a high
concentration to low concentration.
Question:
What’s in a Solution?
Answer:
• solute
+
• NaCl +
H20
solvent  solution
 saltwater
Hypertonic
• A solution with a greater solute
concentration compared to
another solution.
3% NaCl
97% H2O
Red Blood Cell
solution
5% NaCl
95% H2O
Hypotonic
• A solution with a lower solute
concentration compared to another
solution.
3% NaCl
97% H2O
Red Blood Cell
solution
1% NaCl
99% H2O
The contractile vacuole of Paramecium: an
evolutionary adaptation for osmoregulation
Movement of H2O
• Water will “ALWAYS” diffuses
down a concentration gradient
from a HYPOTONIC solution to a
HYPERTONIC solution.
“ALWAYS REMEMBER”
• HYPOTONIC 
HYPERTONIC
• Water flows towards the solutes!!
Isotonic
• A solution with an equal solute
concentration compared to another
solution.
3%NaCl
97% H2O
solution
3%NaCl
97% H2O
Red Blood Cell
Do Water Molecules Stop
Moving in Isotonic Conditions?
• No.
• They continue to diffuse, however
there is no net movement!
• In general, which way does water
move?
• From hypotonic to hypertonic!
Water Balance in Cells
Elodea in Distilled Water
Elodea in Salt Water
?
?
Which way
will the
water
move?
WHY?
Plant Cells
• Firmness or tension (vacuole full) that
is found in plant cells (cell wall) that are
in a hypotonic environment is called
TURGID.
• This process is called TURGOR
PRESSURE.
Water
Cell
Wall
Water
Central
Vacuole
Water
Plant Cells
• When the plasma membrane pulls away
from the cell wall (vacuole empty) in a
hypertonic environment (loss of water) is
called PLASMOLYSIS.
Water
Water
plasma membrane
Cell
Wall
Water
A watered tomato plant regains its
turgor
Animal Cells
• Animal cells placed into a hypotonic solution
will HEMOLYSIS (EXPLODE).
• Animal cells placed into a hypertonic solution
will CRENATE (SHRIVEL).
Hemolysis
Crenation
Red
Blood
Cells
Facilitated Diffusion
• Diffusion of solutes with the help of
transport proteins. (passive
transport)
• Example: How glucose enters cells
• Why do these solutes need a protein
to facilitate their diffusion?
That’s right!
Because they are too polar or
too large to pass through the
lipid bilayer.
Two Models for Facilitated Diffusion
Channel
Protein
Carrier
Protein
Active Transport
• The movement of molecules (small or
large) across the plasma membrane in
which energy (ATP) is required.
• Examples:
1.
2.
3.
Sodium (Na) - Potassium (K) Pump
Exocytosis
Endocytosis
Review of Passive and Active Transport:
• A transport protein that
generates voltage across a
membrane is called an
electrogenic pump.
• One example is the sodium
potassium pump
The Sodium-Potassium Pump: a Specific
Case of Active Transport
Proton pumps are the main electrogenic
pumps of bacteria, fungi and plants.
Transport Proteins
• Transports molecules or ions
across biological membranes using
active transport.
• 3 types of transport proteins:
1. uniport
2. symport
3. antiport
Uniport Transport Protein
• Carries a single solute across the
membrane.
extracellular
fluid
intracellular
fluid
Symport Transport Protein
• Translocates 2 different solutes
simultaneously in same direction.
• Ex: amino acids that enter the
intestine require simultaneous
binding of Na and an amino acid to
the same transport protein.
extracellular
fluid
intracellular
fluid
Antiport Transport Protein
• Exchanges 2 solutes by
transporting them in opposite
directions.
extracellular
fluid
intracellular
fluid
Sodium-Potassium Pump
• This antiport uses energy (active
transport) released from splitting ATP to
transport Sodium (Na+) out of and
Potassium (K+) into cells.
extracellular
fluid
intracellular
fluid
K+
K+
Na+
Na+
What is cotransport ?
Question:
• How are large molecules
transported into and out of
the plasma membranes?
Answer:
• Exocytosis and Endocytosis
Exocytosis
• Cell secretes macromolecules
(proteins and other
biochemicals) out of cell.
Examples:
• Part of the Endomembrane System: the
fusion of transport vesicles with plasma
membrane.
• Nervous System: How neurotransmitters are
secreted into the synapse between neurons
Endocytosis
• The energy requiring movement
of particles (foreign or natural)
into the cell.
• 3 types of endocytosis:
A. Phagocytosis
B. Pinocytosis
C. Receptor-mediated
endocytosis
A. Phagocytosis
• Cell eating: cells engulf particles
• Example:
White Blood Cells
Food
Vacuole
Bacteria
White Blood Cell
B. Pinocytosis
• “Cell drinking”: droplets of
extracellular fluid are absorbed
into the cell by small vesicles.
• Examples:
1. How mammalian embryonic cells take in
nutrients from surrounding cells.
2. How endothelial cells take in nutrients
from blood capillaries and move them into
surrounding tissues
C. Receptor-Mediated Endocytosis
• Importing specific
macromolecules (hormones)
into the cell by the inward
budding of vesicles formed from
coated pits (receptors). Liver Cell
Hormones
Receptors
The Three Types of Endocytosis in Animal
Cells
1)Describe how facilitated diffusion
differs from “regular” diffusion and
osmosis.
2) Give one similarity between the
two.
3) Describe how active transport
differs from passive transport and
give an example of active transport.
3) Describe how active transport
differs from passive transport and
give an example of active transport.
4) Why did you calculate percent
changes in mass during the lab
yesterday?
5) What condition results in plants
as a result of them being placed in a
hypotonic solution? (this was in
the video)
How is osmosis different than
diffusion?
If the solution on the outside of
the cell has more solute than the
solution inside the cell the
outer solution is said to be
__________ to the cell.
Why don’t cells in large organisms
burst?
Movement of H2O
• Water will “ALWAYS” diffuses
down a concentration gradient
from a HYPOTONIC solution to a
HYPERTONIC solution.
“ALWAYS REMEMBER”
• HYPOTONIC 
HYPERTONIC
• Water flows towards the solutes!!