The human cell - 101
The Human Cell 101 - Simple to understand
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
spanning the membrane. Solutes, such as charged
ions in solution (E.g. Na+, K+ and Cl- as solutes)
require a carrier or channel to cross the membrane to avoid
coming into contact with the water-hating core of the membrane lipid-bilayer. (A
carrier or channel is made of protein molecules with a water hating, lipid loving
exterior which "happily" spans the membrane, and a water-loving center
through which water and small water soluble molecules can pass). Some substances,
such as 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
(E.g. K+ , Na+
Cl- , Ca2+ ),
but 1000x faster than by carrier protein (Ions move single file at
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:
(1)
Chemically-gated (also called ligand-gated).
Opened/closed by certain binding molecules (e.g.neurotransmitters,
hormones).
(2)
Voltage-gated. Opened/closed by a change in the
cell membrane voltage - K+ and Na+
ion-channels are voltage-gated. The voltage is mainly determined by the balance
of K+ and Na+
ions either side of the membrane. The transferral of ions further changes the cell
membrane voltage.
(3)
Electromagnetic Gate. Opened/closed
by specific-frequency electromagnetic signals produced by the body or received
from the external environment.
(4) 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 )
opposing the movement of an ion or molecule across the membrane, a special
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 uses that Na+ gradient to transport
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).