The plasma membrane is a thin layer of fat and protein
covering the surface of every cell; it acts as a selective barrier that
regulates the passage of substances in and out of cells. These substances are
involved in the metabolic processes in the cell, some of them crosses the barrier
without difficulties while others passes slowly with much difficulties, yet
others are entirely stopped from entering.
PROCESSES BY WHICH
MATERIALS MAYBE EXCHANGED ACROSS A MEMBRANE
DIFFUSION: - This is the movement of molecules
from an area of higher concentration to that of a lower concentration through
concentration gradients with the aim of balancing concentration on both sides
of a membrane.
A Concentration
Gradient is a region of high concentration of solute on one side of a
membrane and a region of low concentration of solute on the other side.
During diffusion, molecules spread
into an available space; this is due to thermal energy present in molecules at
temperature above absolute zero. The rate of diffusion of substances is
affected by membrane permeability since membrane selectively permeable.
2. DIFFUSION CHANNEL: - Water and dissolved ions being
charged cannot diffuse through the phospholipids component of the plasma membrane;
rather they pass through specialized pore of channels created by transmembrane
protein.
Ion channel
allows specific ions of certain size and charge to diffuse at all time or they
maybe gated channels that need signal to open. Gated channels may open or close when a
signaling molecule binds to specific binding site on transmembrane protein.
3. CARRIER MEDIATED TRANSPORT: - Nutrients such as sugar, waste
products and amino acid are transported across a cell by special membrane
protein known as transporters or carriers. There are three types of mediated transport,
a – Active transport:- Energy is used to move molecules in direction
opposite to a concentration gradient. The energy used is called Proton Motive
Force and its gotten from Adenosine Triphosphate(ATP) or co enzyme.
b – Passive diffusion: - This is the movement of substance from region
of higher to lower concentration at a rate proportional to the concentration
difference of the membrane without the use f energy or special molecule.
Passive transport is a spontaneous process that takes place because of entropy
of a system.
b(1) Facilitated diffusion: - is a kind of passive where proteins
(carrier molecules) help substances to diffuse across a membrane that it
ordinarily cannot pass through. This
type of transport process increases the speed of molecules by making it move rapidly,
the protein spurn the membrane and selectively increase the permeability of the
membrane for specific molecules. Sodium ion, chlorine and glucose cannot cross
the phospholipids bilayer guarding a cell, but with the help of carrier
proteins, and ion channels these substances are moved across the cell.
OSMOSIS: - Water molecule can move across any
membrane without resistance if there is a high concentration on one side and a
low concentration on the other side, water molecule will diffuse across the membrane
from a solution of lower concentration (hypotonic) to that of higher osmotic
concentration (hypertonic) through a semi permeable membrane.
Osmosis is a selective diffusion
driven by internal energy of solvent molecule.
When a solution of sugar is separated from
pure water by a membrane permeable to water but not sugar, (semi permeable
membrane), water crosses into the sugar solution, the movement of water is what
illustrates osmosis. If external pressure is applied to the sugar solution, the
movement of water will be opposed. The pressure needed to prevent the flow of
water is called osmotic pressure, and
the more concentrated the solution the greater it is.
The amount of osmotic pressure
produced by solution in standard condition depends on the number of dissolved
solute particles, molecules ions relative to their sizes in a given amount of
water.
Osmosis plays an important role in
controlling the distribution of water in living organism.
b(3) –Filtration – This uses
hydrostatic pressure to transport solute, solvent molecules and ions across a
membrane.
Other ways materials can be move across a cell are mainly
used to carry solid materials.
1.
ENOCYTOSIS
:- the term describes three related processes,
a – Phagocytosis :- This means cell ‘eating’, in this process,
an area of the plasma membrane coated externally with actin and actin binding
proteins forms a pocket that engulfs the solid material eg bacteria, the
membrane enclosed vesicle or phagosome then detaches from the cell surface and
move into the cytoplasm where it fuses with lysosome and it contents are
digested.
b – Pinocytosis :- This is similar to
phagocytosis except that it involves engulfing liquid, very small invaginations
are formed on the surface if the cell membrane, these then close to form fluid filled vesicles.
c – Exocytosis
:- The membrane of vesicle can fuse with
the plasma membrane which then rupture
and discharge its contents. Exocytosis is the exchange of material across
Plasma membrane where by secretory granules or droplets are released from a
cell.
CITATIONS
Integrated principle of Zoology 14th edition by Hickman, Robberts,
Keenn, Larson, Panson and Elsenchour.
Wikipedia the free encyclopedia.
Cell biology, by Stephen Ojo.
Dictionary of biology, By Uvarov, Chapman and Isaacs.
Stedman’s medical dictionary for health professions,by Dirckk, university of Dayton Student Health Centre Dayton, Ohio.
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