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Transport across the plasma membrane is essential to the life of
a cell.
·
Certain substances must move into the cell to support metabolic
reactions.
·
Certain substances produced by the cell for export and cellular
waste materials must move out.
Two types of
transport processes are
·
Passive transport
o
Substances move down its concentration or electrical gradient to
cross the membrane using its own kinetic energy.
o
Kinetic energy is intrinsic to the particles that are moving.
o
No
input of energy from the cell.
o
Eg:
Diffusion ,Osmosis.
o
In
facilitated diffusion substances must bind to specific proteins
to cross a cellular membrane.
·
Active transport
o
Cellular energy is used to drive the substance uphill against
its concentration or electrical gradient.
o
Cellular energy in the form of ATP is used here.
Passive
Transport
Diffusion.
Diffusion
is the net movement of material from an area of high
concentration of that material to an area with lower
concentration. The difference of concentration between the two
areas is often termed as the concentration gradient, and
diffusion will continue until this gradient has been eliminated.
Since diffusion moves material from area of higher concentration
to the lower, it is described as moving solutes "down the
concentration gradient" (compared with
active transport, which
often moves material from area of low concentration to area of
higher concentration, and therefore referred to as moving the
material "against the concentration gradient").
If and
when the concentration gradient has been eliminated, no net
exchange of material occurs. Although material may move forth
from one area to the other, it will be balanced by movement of
the same amount of material to the opposite direction.
Diffusion is biologically important because it enables the
abolishment of concentration gradients in the body. For example,
metabolic activity will consume
oxygen, which will reduce
its concentration in the bloodstream; diffusion of oxygen in the
alveoli of the lungs allows
it to be replenished.
Is a passive
process in which the random mixing of particles in a solution
occurs because of particles own kinetic energy.
·
Here
both the solutes(dissolved substance) and the solvent(Liquid
that does the dissolving) undergo diffusion.
o
If a
particular solute is present in high concentration in one area
of a solution and in low concentration in another area,solute
molecules will diffuse toward the area of lower concentration.ie
they move down their concentration gradient.Later partilcles
become evenly distributed throughout the solution and the
solution is said to be at equilibrium.The particles will
continue to move about randomly due to their kinetic energy but
without change in concentration.
·
Substance may also diffuse through a membrane if the membrane is
permeable to them.
o
Factors which influence the rate of diffusion across plasma
membrane
§
Steepness of the concentration gradient.
·
The
greater the difference between the two sides of the membrane,the
higher the rate of diffusion.
§
Temperature.
·
The
higher the temperature,the faster the rate of diffusion.
§
Mass
of the diffusing substance.
·
The
larger the mass of the diffusing particle,the slower its
diffusion rate.
§
Surface area.
·
The
larger the membrane surface area available for diffusion the
faster the diffusion rate.
o
Eg:
Air sacs of the lungs have a large surface area available for
diffusion of oxygen from the air into the blood.Some lung
diseases,such as emphysema,reduce the surface area.This slows
the rate of oxygen diffusion and makes breathing more difficult.
§
Diffusioin distance.
·
The
greater the distance over which diffusion must occur,the longer
it takes.
o
Diffusion across a plasma membrane takes only a fraction of
second because the membrane is so thin.In pneumonia fluid
collects in lung;the additional fluid increases the diffusion
distance because oxygen must move through both the built-up
fluid and the membrane to reach the blood stream.
o
Diffusion through the lipid bilayer
§
The
basic structural framework of the plasma membrane is the lipid
bilayer.
§
Non
polar,hydrophobic molecules diffuse freely through the lipid
bilayer of the plasma membrane of cells without the help of
membrane transport proteins.Eg: O2,CO 2,Nitrogen
gases,fatty acids,steroids,fat soluble vitamins.
§
Eg:
Movement of oxygen and carbon dioxide between blood and body
cells.Movement of oxygen and carbondioxide between blood and air
during breathing.
o
Diffusion through membrane ion channels.
§
Most
membrane channels are ion channels.
§
Each
ion can diffuse across the membrane only at certain sites.
§
In
plasma membrane the most numerous ion channels are selective for
Potassium or Chloride ions.Fewer channels are available for
Sodium and Calcium ions.
§
Diffusion of ions through channels is generally slower than free
diffusion through the lipid bilayer.However more than a million
potassium ions can flow through a K channel in one second!
§
A
channel is said to be gated when part of the channel protein
acts as a “plug” or “gate”,changing shape in one way to open the
pore and in another way to close it.
§
When
the gates of a channel are open,ions diffuse into or out of
cells,down their electrochemical gradients.
§
The
plasma membranes of different types of cells have diffeent
numbers of ion channels and thus display different
permeabilities to various ions.
Osmosis
Osmosis is the
diffusion of a
solvent across a membrane
to a region of higher
solute concentration. (In
biological processes then, it usually is diffusion of water
molecules). Most
cell membranes are
permeable to water, and since the diffusion of water plays such
an important role in the biological functioning of any living
being, a special term has been coined for it -- osmosis.
·
Is a
net movement of solvent through a selectively permeable
membrane.
·
In
living systems,thesolvent is water,wich moves by osmosis across
plasma membranes from an area of higher water concentration to
lower concentration.
·
In
otherwords water move through a selectively permeable membrane
from an area of lower solute concentration to an area og higher
solute concentration.
·
Water
molecules pass through a plasma membrane in two ways
o
By
moving through the lipid bilayer
o
By
moving through aquaporins(integral membrane proteins that
function as water channels)
·
Osmosis occurs only when a membrane is permeable to water but
not to certain solutes.
·
Osmotic pressure
·
Hydrostatic pressure
·
Solution’s tonicity
o
A
solution’s tonicity is a measure of the solution’s ability to
change the volume of cells by altering its water content.
o
Isotonic
o
Hypertonic
o
Hypotonic
Medical
uses of Isotonic,Hypertonic and Hypotonic solutions.
·
Isotonic solution
o
RBC’s
and other body cells may be damaged or destroyed if exposed to
hypertonic or hypotonic solutions.So most IV fluds are isotonic.
·
Eg:
isotonic saline (0.9% NaCl)
·
D5W
(dextrose 5% in water)
·
Hypertonic solution
o
Mannitol is useful to treat patients with cerebral oedema where
there is excess interstitial fluid in the brain.This causes
osmosis of water from interstitial fluid to blood.kidneys
excrete the excess of water from blood into urine.
·
Hypotonic solution
o
Given
either orally or intravenously can be used to treat people who
are dehydrated.The water in the hypotonic solution moves from
the blood into the interstitial fluid and then into the body
cells to rehydrate them.
Facilitated diffusion
Facilitated diffusion is movement of
molecules across the
cell membrane via special
transport proteins that are
embedded within the cellular membrane. Many large molecules,
such as
glucose, are insoluble in
lipids and too large to fit
through the membrane pores. Therefore, it will bind with its
specific carrier proteins, and the complex will then be bonded
to a
receptor site and moved
through the cellular membrane. Bear in mind, however, that
facilitated diffusion is a passive process, and the solutes
still move down the
concentration gradient. The
alveoli are tiny grapelike
sacs located at the end of the
bronchial tubes. This is
where oxygen diffuses into the alveoli and is exchanged for
carbon dioxide.
·
Is
done by solutes that are too polar or highly charged to diffuse
through the lipid bilayer and are too big to diffuse through
membrane channels.
·
Here
a solute binds to a specific transporter on one side of the
membrane and is released to the other side after the transporter
undergoes a change in shape.
·
The
solute binds more often to the transporter on the side of the
membrane with a higher concentration of solute.
·
The
rate of facilitated diffusion is determined by the steepness of
the concentration gradient across the membrane.
·
The
number of transporters available in a plasma membrane places an
upper limit known as the transport maximum.Once all the
transporters are occupied,the transport maximum is reached, and
a further increase in the concentration gradient doesnot
increase the rate of facilitated diffusion.
·
Eg:
Hormone Insulin promotes the insertion of many copies of a
specific type of glucose transporter into the plasma membranes
of certain cells.Thus the effect of insulin to elevate the
transport maximum for facilitated diffusion of glucose into
cells.With more transporters available.body cells can pick up
glucose from the blood rapidly.
Filtration
Filtration is movement of water and solute molecules across the
cell membrane due to hydrostatic
pressure generated by the
cardiovascular system. Depending on the size of the membrane
pores, only solutes of a certain size may pass through it. For
example, the membrane pores of the
Bowman's capsule in the
kidneys are very small, and only
albumin, the smallest of
the proteins, have any chance of being filtered through. On the
other hand, the membrane pores of
liver cells are extremely
large, to allow a variety of solutes to pass through and be
metabolized. |
