The Cell Membrane
General Function
• Separates cell from outside world
• Separates compartments inside cell to
protect important processes and events
Other functions
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Protective
Regulates transport in or out of cells or organelles
Allows cell recognition
Allows selective receptivity and signal transduction
Provides anchoring sites for cytoskeletal filaments; helps
maintain shape
Compartmentalizes cell
Provides stable site for enzyme catalysis
Regulates fusion with other cells via junctions
Allows cell or organelle motility
History of Research
• !895 Overton – Membranes are lipids
• 1917 Langmuier – made artificial membranes
• 1925 – Gorter and Grendel – Phospholipid
boundary 2 layer
• 1935 - Davson and Danielli – “fat sandwich”
• 1950 – Robertson – triple layered
• 1966 – Leonard and Singer – analyzed proteins
• 1972 – Singer and Nicholson – Fluid Mosaic
Model
Fluid Mosaic Model
• Proposed by Singer
and Nicolson in 1972
• Described cell
membrane as a “sea
of lipids with proteins
floating like
icebergs…”
Description of cell membrane
.
Phospholipid molecules oriented so that hydrophilic (water loving )
heads directed outward and hydrophobic (water hating)tails
directed inward.
Proteins embedded in two layers of lipids (lipid bilayer).
Proteins allow substances to pass into and out of the cell.
Diagrams:
Prokaryotic Cell, Mariana Ruiz
Membrane: NIST
Phospholipids
• Phosphate group is
polar
• Connected to two
fatty acid tails; one
saturated and the
other with a double
bond
• Lipid bilayer gives
membrane it’s fluid
characteristics
Cholesterol
• Makes lipid bilayer
less deformable and
decreases
permeability to water.
• At 37 degrees,
membranes are less
fluid, but it also helps
prevent the
phospholipids from
solidifying
Proteins
• Integral –
imbedded in
membrane
• Transmembrane
– span
membrane
completely
• Peripheral –
loosely bound to
surface
Role of Proteins
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Channel Proteins ie. Aquaporins
Transport ie. Sodium Pumps
Adhesion
Receptor Proteins ie. Hormones
Enzymes
Carbohydrates
• Usually branched
oligosaccharides with
<15 sugar units
• Crucial in cell to cell
recognition (immune
response)
• Tissue Differentiation
Membrane is semi-permeable
• Hydrophobic core impedes passage of
ions and polar molecules
• Non-polar molecules such as
hydrocarbons, carbon dioxide and oxygen
dissolve in phospholipid bilayer
• Transport Proteins allow hydrophilic
substances to pass through
• Aquaporins accelerate transport of water
(3 billion water molecules / second!!!)
Diffusion
Primary function of plasma
membrane → regulate movement
of molecules entering or leaving
cell.
PASSIVE TRANSPORT
Movement of molecules is passive if
no energy sources of the cell are
expended.
Diffusion = when molecules move
from a higher to a lower
concentration.
What type of things might affect
the rate of diffusion?
Diffusion Animation: biologycorner.com
Facilitated Diffusion
PASSIVE TRANSPORT (Continued)
Facilitated Diffusion
Proteins assist in diffusion of molecules across plasma membrane.
Movement only occurs in the presence of a concentration gradient.
Some molecules move across the membrane more quickly if
diffusion is facilitated by a carrier molecule. (Example: Glucose)
Diagram:
Facilitated diffusion, Mariana Ruiz
Osmosis
Osmosis
Diffusion of water across the
plasma membrane.
Environment surrounding cells
may contain amounts of
dissolved substances
(solutes) that are…
equal to
less than
greater than
…those found within the cell.
Diagrams:
Osmosis - www.scienceaid.co.uk/biology/plants/osmosis.html
Blood Cells: Mariana Ruiz
Tonicity
Isotonic : no net
movement of water between
cell and environment
Hypertonic : a higher
concentration of solute.
Hypotonic: a lower
concentration of solute.
Water will always move
toward a hypertonic
environment!!
Active Transport
ACTIVE TRANSPORT
How most molecules move across
the plasma membrane.
Requires energy .
Analogous to a pump moving water
uphill.
Types of active transport are
classified by type of energy
used to drive molecules across
membranes.
ATP Driven Active Transport
Energy from adenosine
triphosphate (ATP) drives
substances across the plasma
membrane with the aid of
carrier molecules.
Diagram:
Source unknown
Endocytosis
• Movement into cell
by formation of
vacuole
• Requires energy
• Includes
phagoctosis
(particles) and
pinocytosis
(liquids)
Lab: Osmosis and Diffusion
• Exercise 1: Diffusion
In this activity, you fill a dialysis bag with a
sugar/starch solution and immerse the bag in a
dilute iodine solution.
• Exercise 2: Osmosis
In this activity you investigate the relationship
between solute concentration and water
movement by filling six different dialysis bags
with increasing concentrations of sucrose and
placing the bags into distilled water.
• Exercise 3: Water Potential of Potato
Cores
This activity is very similar to Exercise 2, except that
you use cores from potatoes instead of dialysis bags.
You submerge the cores in solutions of varying
sucrose concentrations. When you calculate the
percent change in mass, some of the cores will have
gained weight while others will have lost weight,
depending on the movement of water. You then graph
this data and determine which concentration of the
sucrose solution is in equilibrium with the cores.
Since you know that the pressure potential of the
surrounding solution in an open beaker is zero, you
can now calculate the water potential.
• Exercise 4: Elodea Cell Plasmolysis
In this activity, you watch the effect of
placing a living cell into a solution that has
a lower or higher concentration of water
than the cell.
Water Potential
• water potential - the tendency of water to
leave one place in favor of another. Water
always moves from an area of higher
water potential to an area of lower water
potential.
• Water potential is affected by two factors:
pressure and the amount of solute.
Calculating Water Potential
• Water potential ( ) =
pressure potential ( ) + solute potential ( )
• Pressure potential ( ): In a plant cell, pressure exerted by
the rigid cell wall that limits further water uptake.
• Solute potential ( ): The effect of solute concentration.
Pure water at atmospheric pressure has a solute
potential of zero. As solute is added, the value for solute
potential becomes more negative. This causes water
potential to decrease also.
• In sum, as solute is added, the water potential of a
solution drops, and water will tend to move into the
solution.
• In this laboratory we use bars as the unit of measure for
water potential; 1 bar = approximately 1 atmosphere.
Factors Affecting Water Potential
• The water potential of pure water in an
open container is zero because there is no
solute and the pressure in the container is
zero. Adding solute lowers the water
potential. When a solution is enclosed by a
rigid cell wall, the movement of water into
the cell will exert pressure on the cell wall.
This increase in pressure within the cell
will raise the water potential.