Cell Membrane Function
• Formation of a cell
membrane that could
allow some materials to
pass while restricting the
movement of others was
a major step in the
evolution of the cell.
Such membranes can
regulate the contents of
the spaces they enclose
and thereby maintain
• The plasma membrane consists of a phospholipid bilayer
with embedded proteins.
o Some of the embedded proteins that span the entire membrane
permit passage of substances across the membrane; they are called
The shaded, globular objects
are embedded proteins.
Cell Membrane Structure
• All biological
• Some substances can pass into or out of the cell by diffusion.
Diffusion is also the way most substances move within the cell.
o Diffusion is the net movement of a substance from an area of higher
concentration to an area of lower concentration.
o Thus, a difference in concentration between two regions is required for
diffusion to occur.
• A gradient is a change in something as you move from one
location to another. So a concentration gradient is a change in
concentration of something from one place to another.
If we picture each individual molecule as a little blue dot, a constant
concentration of molecules (no gradient) would look like this:
A concentration gradient, with a higher concentration of molecules on
the left than on the right, would look like this:
Why does diffusion happen?
• The tiny particles (atoms, ions, and molecules) that make up all
matter are in constant motion, called thermal motion, and the
warmer they are, the faster they move. The energy of these
moving particles is called thermal energy.
• In liquids and gases, the overall
movement of these particles is
random and chaotic, each one
moving in a different direction.
o The direction of motion of any
one molecule can be changed by
collision with another molecule or
with the side of the container, but
once in motion the molecule will
travel in a straight line until a
o The probability of a molecule
moving in one direction is the
same as the probability of it
moving in the opposite direction.
• Now, imagine that a crystal of dye is added to a beaker full of
water and begins to dissolve. Each dissolved dye molecule
moves around randomly.
o Initially, more dye molecules will be moving away from the crystal than
toward it. However, the net movement will be away from the crystal.
o When the dye molecules become evenly distributed, their random
thermal motion in all directions will be equal, so their net movement
will stop. This condition is called dynamic equilibrium.
• A concentration gradient has potential energy because it has
the capacity to do work.
o If a substance has different concentrations on opposite sides of a
membrane, its diffusion through the membrane could do work.
o It’s like a dam that separates water at different elevations: the kinetic
energy of the water as it passes through a tunnel in the dam could do
work by spinning a water wheel or turbine.
• Substances are said to diffuse in the "direction of their
concentration gradient" or "down their gradient".
• Diffusion of one substance is independent of the diffusion of
all other substances.
• The rate of diffusion is affected by the size of the
concentration gradient, the size of the particles, and the
• Higher temperature → Diffuse fasterTemperature
• Higher gradient → Diffuse faster
• Smaller particles → Diffuse fasterParticle size
• The diffusion of solutes directly through the phospholipid
bilayer of a cell membrane is called simple diffusion.
• The size, charge, and
polarity of substances
they can pass through
• Substances diffuse into or
out of the cell down their
o Since most cells use oxygen
and produce carbon dioxide
during respiration, oxygen is
continually diffusing into the
cell and carbon dioxide is
continually diffusing out.
• The diffusion of water across a selectively permeable
membrane is so important in biology that it’s given a special
o Since it is always true that the concentration of water in a solution
decreases as the concentration of solute increases, water diffuses
toward the side of the membrane with the higher solute
o If the membrane is impermeable to the solute, then osmosis will cause
an increase in the volume of the solution with the lower water
concentration/higher solute concentration.
o The solution with the lower water concentration/higher solute
concentration is said to be hypertonic and the other hypotonic. Water
diffuses from hypotonic to hypertonic solutions.
o Two solutions with equal solute concentrations (and equal water
concentrations) are said to be isotonic. No osmosis occurs.
• Substances that cannot
diffuse through the
phospholipid bilayer may
diffuse through transport
proteins if they are
available. This process is
called facilitated diffusion.
o Substances that cross by
facilitated diffusion include
glucose, Na+, K+, and Cl−.
o Each transport protein is
effective for one specific
• Some transport proteins
use the kinetic energy of
through them to do work.
o For example, in the
chloroplast, the energy
released by the diffusion
of H+ through a transport
protein (ATP synthase) is
used to make ATP from
• Active transport occurs when a transport protein pumps a
solute across a cell membrane from an area of lower
concentration toward an area of higher concentration
using energy from the breakdown of ATP.
• For example, the sodium-potassium pump (Na+-K+ pump) is
an active transport protein that simultaneously transports
potassium ions (K+) into and sodium ions (Na+) out of the
animal cell. About 30% of the energy used by animal cells is
used just to run this pump (70% in their nerve cells).
Endocytosis and Exocytosis
• Moving substances into a cell or out of a cell by enclosing
them in vesicles is called endocytosis and exocytosis
o A vesicle is a small, spherical, membrane-bound sac within the cell
used for transport or storage.
o The processes are "uphill" because substances are being transported
in the direction opposite to their concentration gradient.
o The objects do not actually pass through a membrane on their way
into or out of the cell.
o In endocytosis, the vesicle is
formed by the plasma
membrane. Endocytosis is used
by cells to import water,
macromolecules, or larger
objects, such as cell fragments
or even whole cells.
o In exocytosis, the vesicle is
formed by an organelle within
the cell. Exocytosis is used by
secretory cells to export large
groups of molecules such as
hormones and other products.
Summary of Cellular Transport
"Uphill" ATP None