The Human Cell 101 - Simple to understand
The Human Cell 101
How do Substances Cross the Cell Membrane
Substances can cross the cell or
either a passive or active transport mechanism.
– Does not require energy
(through membrane lipid
a substance moves freely across
the membrane lipid
a concentration gradient
diffuses from an area of high
concentration to an area of low concentration )
Generally limited to small,
non-polar substances -
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,
(polarized) can also diffuse slowly through the
lipid by-layer -
Water is a
special case, passing through the membrane by osmosis.
(carrier or channel mediated) –
transported down an
electrical or chemical concentration gradient via a carrier-protein
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, which are too large to pass directly through
the membrane, 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+,
An ion-channel –
is a tiny
pore generally used to transport
only inorganic ions
but 1000x faster than by carrier protein (Ions move single file at
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.
4 types of gating mechanisms:
Chemically-gated (also called ligand-gated)
– Opened/closed by certain binding molecules
Voltage-gated – opened/closed by a change in the cell membrane voltage -
ion-channels are voltage-gated. The voltage is mainly determined by the balance
ions either side of the membrane. The transferral of ions further changes the
cell membrane voltage.
Electromagnetic Gate – Opened/closed by specific-frequency electromagnetic
by the body or received from the external environment.
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.
(Using a pump)
ATP energy is needed to run a “Switch-Operated” Enzyme Pump to transport
through a protein channel against a gradient
Direct Active Transport Mechanism -
is an electrical or chemical gradient (called
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
Active transport also
supports the vital imbalance of ions -
such as K+,
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’s “battery”
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.
Na+/glucose indirect active transport mechanism,
in particular, uses the
the first step, generating a strong Na+ gradient across the cell membrane. Then
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).