Membrane Transport
Reasons For Membrane Transport
Cells need membrane
transport to undergo
cellular processes:
-- get water and
nutrients into the cell
-- remove waste from
the cell
-- communicate with
other cells
-- maintain
homeostasis
Crossing the Cell Membrane
The cell membrane is mostly phospholipids
Only non-polar (lipid soluble) substances can cross the
membrane
-- oxygen molecules, carbon dioxide, other small
non-polar molecules
-- occasionally, water molecules can find gaps
Large, polar molecules like water, glucose and ions need to
cross through protein channels.
By regulating what goes in and out, cells maintain the
proper concentrations for metabolism
We say that cell membranes are selectively permeable –
choosing what to let in or out
The Concentration Gradient
Concentration – the amount of ions per unit volume of a
substance
The concentration gradient is the comparison of the
concentration of a substance inside the cell to the
concentration of a substance outside the cell.
Substances usually move freely “down (with) the gradient”
Down (or with) the Gradient – moving from an area of high
concentration to an area of low concentration
Against the Gradient – moving from an area of low
concentration to an area of high concentration
-- to move against the gradient usually requires an
input of energy and does not happen freely
Drawing of a Concentration Gradient
Movement “Down” or
“with” the gradient
Movement “against”
the gradient
High
concentration
Low
Concentration
Cell
Membrane
Diffusion
Diffusion – the movement of molecules down or with the
concentration gradient
-- this is how non-polar substances move easily
Factors that affect diffusion rates:
-- size of the gradient
-- bigger gradient  faster diffusion
-- temperature
-- higher temp  faster diffusion
-- electric charge of the molecules
-- pressure
Diffusion Into a Cell
Transport Mechanisms
Molecules like glucose, water,
etc. . . that cannot pass through
the phospholipid bilayer need
another way of entering and
exiting the cell.
-- proteins act as transporters
for these molecules
Transporters open and close
when molecules bind
-- only open to one side of the
membrane at a time
Transport can be either passive or active (some proteins
do both and are called cotransporters)
Passive Transport
Passive Transport – diffusion across a membrane
through a transport protein
-- depends only on concentration or charge gradient
-- always goes “with” or “down” the gradient
-- also called “facilitated diffusion”
Active Transport
Active Transport – the pumping of molecules against the
gradient through proteins
-- process requires energy (from ATP)
Why would a cell do this?
-- need to maintain both a concentration and charge
gradient for cells to function properly
-- muscle and nerve cells need an electric
charge potential to contract
Examples of Active Transport Mechanisms
-- Calcium pump
-- Sodium/Potassium Pump
Osmosis
Osmosis – the diffusion of water across a cell membrane
-- more water crosses the membrane than anything else
The diffusion gradient for water depends on the number of
solute molecules on both sides of the cell membrane (called
tonicity)
Hypotonic Solution – the solution outside the cell has a
lower concentration of solutes than inside the cell
Hypertonic Solution – the solution outside the cell has a
greater concentration of solutes than inside the cell
Isotonic Solution – the concentration of solution inside and
outside the cell are equal
Solutions tend to flow from hypotonic to hypertonic fluids
A Cell in Hypotonic Conditions
The concentration of solutes inside the cell is greater than
the concentration of solutes outside the cell
-- water rushes into the cell
-- cells expand and burst
Distilled Water
2%
sucrose
0% sucrose
A Cell in Hypertonic Conditions
The concentration of solutes outside the cell is greater
than the concentration of solutes inside the cell
-- water rushes out of the cell
-- the cell shrivels up
Solution of
2%
sucrose
10% sucrose
A Cell in Isotonic Conditions
Since the concentrations both inside and outside the cell
are equal, there is no net movement of water
-- cell remains the same
-- most body fluids are isotonic
-- cells also have built-in mechanisms to adjust to
changes in tonicity
Solution of
Solution of
2%
sucrose
2%
sucrose
2% sucrose
2% sucrose
Avoiding Changes in Fluid Pressure
Animal cells transport
solutes in and out of
cells to prevent bursting
and shrinking
Plant cells continue to
expand because of
their cell wall
-- water pressure builds up in the cell (called
turgor pressure)
-- cell membrane pulls away from cell wall
-- eventually, turgor pressure reaches a point
called the osmotic pressure, where the force of the
pressure prevents further osmosis
Taking In Large Molecules
Cells can only move
small molecules through
the membrane.
To move large molecules,
cells use vesicles, which
are small, membranebound sacs to move
molecules to and from
the cell surface
This process is called either exocytosis or endocytosis
Exocytosis
In exocytosis, the cell moves large molecules out of the cell
A vesicle containing material to leave the cell forms in the
cytoplasm
The vesicle reaches the cell membrane and fuses with it
The contents of the vesicle are released outside the cell
Endocytosis
In endocytosis, cells take in
large molecules
When large molecules are near
the surface, the cell membrane
balloons inward and traps the
molecules
The membrane pinches off to
form a vesicle inside the cell
Phagocytosis
Phagocytosis is a specialized form of endocytosis, in which
cells take in very large molecules
-- looks more like a whole section of cell forming
around a molecule than the small vesicle of endocytosis
-- this is how amoebas (and other protists) get food,
and how our white blood cells destroy viruses
-- actually means “cell eating”