Transport Across
Cell Membranes
Cell Membrane Structure
• The cell membrane functions to:
• form a barrier between the internal and external environment of
the cell.
• Regulate what is allowed to enter and exit the cells internal
environment.
• Provides protection and support.
Fluid mosaic model
• The fluid mosaic model was developed to explain how the cell
membrane is structured.
• Phospholipid bilayer with proteins floating in and on it.
Proteins in the cell membrane
• Water is able to travel through pores in proteins found in the
cell membrane
• Proteins have polar and non-polar regions, this way they can
span the membrane
• There are both peripheral (on the outside) proteins and
integral (part of the membrane) proteins
• Some proteins within
the membrane have
carbohydrates
attached. These are
called glycoproteins
• Lipids in the bilayer
can also have
carbohydrates
attached. These are
called glycolipids.
• Carbohydrate chains
only occur on the
outside of cells and
function to identify the
cell
• Proteins perform several functions in the cell membrane:
• Channel proteins: allow a specific molecule to cross the
membrane freely
• Carrier protein: combine with a specific molecule to move it
across the membrane
• Recognition proteins: allow the cell to be recognized by other
cells (glycoproteins)
• Ex. Tissue rejection in organ donation
• Receptor protein: have a special shape to be able to bind with
other molecules
• Enzymatic proteins: carry out metabolic reactions
Selective Permeability
• Permeable- the ability to pass through a membrane
• The cell membrane is a selectively permeable membrane.
• This means that it allows some things through and prevents
others from entering or exiting.
• http://www.youtube.com/watch?v=Pt4Ch-YW-xs
• Transport across a membrane is classified as either passive or
active
• Why would cells be selectively permeable?
• Solution- a liquid with one or more substances
dissolved in it
• Solvent- the liquid that the solute is dissolved in
• Solute- the substance dissolved in a solution
• Concentration- how strong it is… the
solute/volume (percentage)
Jigsaw Activity
• Form groups of 3
• Each member of the group find information on
one type of passive transport (p.71-74)
• 1. Diffusion
• 2. Osmosis (ignore “osmosis in cells” section)
• 3. Facilitated transport
Transport through the
Membrane
• Simple Diffusion
• Facilitated
Diffusion
• Active
Transport
Diffusion
• Particles moving from an area of greater concentration
toward an area of lesser concentration until it is equally
distributed
• Diffusion refers to the process by which the molecules of
two different substances intermingle (mix) as a result of
their kinetic energy of random motion.
•
•
•
•
•
Does not use a transport protein
Moves with the concentration gradient (high to low)
Does not require additional cell energy
Passive
Examples:
• Water
• Carbon dioxide
• oxygen
• The rate of diffusion can be increased by:
• Increased temperature
• Increased surface area
• Change in shape of molecules
• Change in concentration
• (Why for each?)
Osmosis
• The net movement of water molecules from the
area of greater concentration of water to the
area of lesser concentration of water until it is
evenly distributed
• Must be across a selectively permeable membrane
• Water passes through pores in membrane proteins
• Osmotic pressure can work against physical pressure
Review
• https://www.youtube.com/watch?v=JShwXBWGMyY
Facilitated Transport
• An assisted form of diffusion
• Moves molecules that would not be able to cross the
membrane alone
• Moves toward the concentration gradient
• Requires carrier proteins
• No energy is needed
Tonicity
• Water molecules travel freely through protein
channels to try to maintain an environmental solute
concentration.
• Osmotic pressure is a measure of the number of
collisions water molecules make with the cell
membrane surface.
• If there is a high concentration difference between
the inside and outside of the cell there will be a high
osmotic pressure… more collisions as water tries to
get across the membrane
• Turgor pressure is the force the cytoplasm
exerts against the inside of the cell
membrane.
• Large and prolonged changes in turgidity can
have effects on cell function.
• What the solution concentration outside the
cell looks like has effect on the cells turgor
pressure.
Isotonic
• If the cell is placed in a solution that
has equal solute concentration to
inside the cell the solution is called
isotonic
• The cell will not shrink or swell
because the osmotic pressure on
each side of the cell membrane is
equal
• The concentrations of solution on
both sides of the membrane are
equal
Hypertonic
• When a cell is exposed to an environment with
a greater solute concentration, water will leave
the cell to try to bring the inside and the
outside solutions into equilibrium.
• The cell will shrivel as water leaves and the
solute inside the cell will concentrate so that it
is more similar to the concentration outside
the cell.
• The outside solution is called hypertonic
(hyper=high). It has a higher concentration of
solute than the cytoplasm.
• The shrinking of a cell in a hypertonic solution
is called plasmolysis. If it is a red blood cell it is
called crenation.
Hypotonic
• When a cell is placed in a solution that
has a lower concentration of solute
compared to the cell cytoplasm it is
called hypotonic (hypo=low)
• Water moves from outside the cell to
inside the cell to try to lower the
solute concentration inside the cell
• The cell swells and may even burst if
the osmotic pressure is great enough.
• When a red blood cell bursts it is
called hemolysis.
Practice Question
• Which set of conditions would cause the fluid levels to change
as shown after 10 minutes?
Solution in Side A
Solution in Side B
Distilled water
5% salt
2% glucose
5% glucose
5% protein
2% protein
2% salt
2% salt
Practice Question
• Is the solution hypertonic, isotonic, or hypotonic?
• Which way will water move?
• What will happen to the cell?
Active Transport
• Ions or molecules move across
the membrane and collect either
inside or outside the cell
• works in the opposite direction
as diffusion from low
concentration to high
concentration.
• Requires carrier proteins and
energy to move molecules
across the membrane
• Active transport uses carrier proteins like one form of
facilitated diffusion.
• Energy (in the form of ATP) is required for the protein to bind
with the molecule to move it across the membrane.
• Active transport is used by cells of the thyroid gland to
collect iodine from the body. Iodine moves against the
concentration gradient toward the more concentrated
area inside the cell.
• Up to 40% of a cells energy supply can be used up to
perform active transport. Cells that primarily gather
certain molecules by active transport have lots of
mitochondria close to their cell membrane to produce
energy.
• Proteins involved in
active transport are of
ten called pumps
Sodium-Potassium pump
• All animal cells, especially nervous cells, contain sodiumpotassium pumps.
• This protein moves sodium to the outside of the cell and
potassium into the cell.
• Both molecules are moved by the same protein: the sodiumpotassium pump.
• https://www.youtube.com/watch?v=GTHWig1vOnY
• The attachment and detachment of a phosphate group from
ATP causes a shape change in the protein that allows it to bind
sodium
NaCl
• Salt is very important in the functions of cells.
• Sodium is pumped across the membrane by carrier proteins
• Chloride ions flow across the membrane through protein
channels because they are attracted by the positively charged
sodium.
Exocytosis and Endocytosis
• The cell needs a way to get very large or macromolecules
across the cell membrane and these do not fit through cell
membrane proteins.
• These large molecules are transported by vesicle formation.
• Vesicle formation requires
energy, so this is a form of
Active Transport.
Exocytosis
• Is a process of the cell to get molecules out of the cell
(exo=exit)
• Vesicles are like small packages of molecules that need to be
released encased in cell membrane
• The contents are released to outside the cell when the vesicle
membrane fuses with the cell membrane.
• Vesicles are often formed by the Golgi apparatus and contain
proteins that the cell needs in it’s membrane or is getting out
of the cell.
• Vesicles are used by the cell as a way to get membrane bound
proteins from Golgi apparatus membrane to the cell
membrane surface where they adhere to the surface and
become part of the membrane.
• Some cells are
specialized to produce
molecules and excrete
them out of the cell
• Ex. Pancreatic cells
excrete digestive
enzymes.
• These cells wait for a
signal from outside the
cell before they
exocytosis takes place.
Endocytosis
• Endocytosis is the process cells use to get macromolecules
from outside the cell membrane to inside the cell membrane
• The cell membrane
Pushes in toward the cell
In one are
• The macromolecule is
Surrounded by the cell
Membrane and the
Membrane pinches off
Forming a vesicle inside the cell.
Phagocytosis
• When endocytosis is used to take
in a large object like a food
particle or cell the process is
called phagocytosis and the
vesicle formed is called a vacuole.
• Single cell organisms like amoebas
often perform phagocytosis to
obtain food.
• Some human white blood cells
also do phagocytosis to engulf
objects in the body that they see
as foreign
Pinocytosis
• Pinocytosis is when vesicles
form around a liquid or very
small particles.
• Blood cells, kidney cells,
and intestinal wall cells
perform pinocytosis
constantly
• To balance out the cell
membrane lost by
endocytosis, there is
exocytosis also happening.
Receptor-Mediated
Endocytosis
• This is a form of very specific pinocytosis.
• The cell wall is lined with receptors in one location called a
coated pit, that bind a specific molecule. When that molecule
is bound the vesicle forms and is moved into the cell.