The human cell - 101
The Human Cell easy to understand. The Cell Membrane - The Cell's Gateway and Barrier
The cell membrane - The cell's gateway and barrier
The cell membrane is made mostly of a double layer
of phospholipids, which envelopes the cell and separates its interior from its
environment. A phospholipid molecule has a hydrophilic (water-loving) phosphate head
and two hydrophobic (water-hating) fatty-acid tails. By forming a bilayer with
their tails intertwining, they are able to form a separation between the watery
inside and outside of the cell. At body temperature, membranes are a liquid with a consistency similar to
cooking oil.
Cell Membrane
All human cells
acquire the nutritional molecules and ions they need from their surrounding
extracellular fluid. This is also where the cell "dumps" its waste. Thus, there is an unceasing
traffic of molecules and ions in and out of the cell through the cell membrane
(E.g. glucose, Na+ , Ca2+ ), and also through its organelle
membranes (E.g. proteins, mRNA, Ca2+ ,
ATP) . Typically, the cells are located in close proximity to the
blood vessels, which supply nutrients and oxygen and pick up waste to and from
the cells (substances are transferred by diffusion between the capillaries and
the extracellular fluid).
The cell membrane
serves as both a gateway and a barrier for the cell and is said to be
"semi-permeable". It can either
allow a molecule or ion from the extra-cellular fluid to pass through
freely, pass through to a limited extent or not pass through at all, thus
regulating its interaction with its environment. The cell membrane also contains
cholesterol, which serves to "waterproof" the cell.
How do substances cross the cell membrane?
Substances can cross the cell or organelle membrane by either
a passive or active transport mechanism.
Passive Transport
By diffusion - does NOT require energy
(a) Simple diffusion - through membrane lipid bi-layer
A substance moves freely across
the membrane lipid bilayer down
a concentration gradient (i.e. diffuses from an area of high concentration
to an area of low concentration )
Generally limited to small, non-polar substances. Molecule dissolves in phospholipid
bilayer, diffuses across it, then dissolves in the aqueous solution on the other
side. E.g. Lipids; steroids; lipid-soluble molecules; Uncharged molecules, such
as O2, N2; ; Alcohol; Anaesthetic gases (ether); Pesticides;
and non-polar solutes, such as CO2 , urea. Water
(polarized) can also diffuse slowly
through the lipid by-layer. Water is
a special case, passing through the membrane by osmosis .
(b) Facilitated diffusion - carrier-protein or
ion-channel mediated
Substances are transported
down an electrical or chemical concentration gradient
facilitated by a carrier-protein or ion-channel
Solutes, such as charged
ions )
require a carrier or channel to cross the membrane glucose , are too large to pass directly
through the membrane, but are able to cross when provided a carrier large enough
to accommodate them. Some molecules, such as starch and proteins, are just simply
too large to cross the cell membrane.
A carrier-protein. Binds a selective substance on one side
of the membrane, and then following a change of shape, the protein releases the
substance on the other side of the membrane. Each carrier protein is designed to
respond to only one substance. Used to transport small organic molecules (E.g. glucose,
sucrose, amino acids) and some inorganic ions(E.g. K+,
Na+).
An ion-channel.
A tiny pore generally used to transport only inorganic ions 108 /second). Most ion-channels are selective
of the type of ion they allow to pass.
"Switch-Operated" GATES for Ion-channels.
Most ion-channels contain a "switch-operated" GATE that may be
opened or closed, such that a channel only transports when the gate is opened in
response to an external chemical, electrical or mechanical stimulus or by conditions
within the cell.
There are 4 types of gating mechanisms:
Chemically-gated .
Opened/closed by certain binding molecules (e.g.neurotransmitters,
hormones).
Voltage-gated.
Electromagnetic Gate.
Mechanical-Gate.
E.g. Opened / closed by sound waves bending the hair cells of
the inner ear leading to the creation of nerve impulses, which the brain translates
into sound.
Active Transport
ATP energy is needed to run a "Switch-Operated"
Enzyme Pump to transport through a protein channel against a gradient
(a) Direct active transport mechanism
When there is an electrical or chemical gradient (called an
electrochemical gradient
transport protein pump is needed to convey the ion or molecule.
Active transport enables the cell to receive essential nutrients from the extracellular
fluid - even when the nutrients
are more concentrated inside the cell
Active transport enables the cell to remove waste products from the inside of the
cell into the extracellular fluid.
Active transport also supports the vital imbalance of ions - such as
K+, Na+, Ca++
and H+ across the membrane.
The most important example of direct active transport is the
sodium/potassium (Na/K) pump
The Na/K pump pumps sodium
ions (Na+) out of the cell and potassium ions (K+ )
into the cell, across the membrane against the electrochemical forces, in
order to maintain the
"cell battery"
voltage.
(b) Indirect active transport mechanism
E.g. Used to transport
glucose
Involves using energy to establish a gradient across the cell membrane,
and then utilizing that gradient to transport a molecule against its concentration
gradient. The Na+/glucose indirect active transport mechanism, in particular, uses
the Na/ K
pump as the first step, generating a strong Na+ gradient across the cell
membrane. Then the glucose /Na+
symport protein glucose into the cell. (Symport means binding
two molecules at a time and using the gradient of one solute's concentration to
force the other molecule against its gradient).
