Signal transduction (also known as cell signaling) is the transmission of molecular signals from a cell's exterior to its interior. Signals received by cells must be transmitted effectively into the cell to ensure an appropriate response. This step is initiated by cell-surface receptors.

1. SIGNAL TRANSDUCTION
OLAREWAJU BLESSING
(LION O.B.O)

2. Definition:
Signal transduction is the process by which a chemical or physical
signal is transmitted through a cell as series of molecular events, most
commonly protein phosphorylation catalysed by protein kinases, which
ultimately results in a cellular response.
It is the process by which a physiological signal is transformed
into a specific cellular responses. Proteins responsible for detecting
stimuli are generally termed receptors, although in some cases the
term sensor is used.
The signal transduction pathway is as follows;

3. Membrane receptors transfer information
from the environment to the cell’s interior.
• A message also referred to as ligand is picked up by the receptor
extending outside the cell from the cell membrane. The message will
dock with the receptor and changes the shape of the receptor in
order for the messenger to be able to enter the cell.
• Transduction is the process of switching the signal message from the
outside of the cell to a message within the cell.
• First messengers are the signalling molecules (hormones,
neurotransmitters, etc.) that reach the cell from the extracellular fluid
and bind to their specific receptors.

4. Secondary Messengers
• Secondary messengers are the substances that enter the cytoplasm
and act within the cell to trigger a response. It serves as a chemical
relays from the cell membrane to the cytoplasm, thus carrying out
intracellular signal transduction.
During this process of signal transduction, the secondary
messenger are often free to diffuse to other components of the cell like
the nucleus. The signal is also amplified during this process.
• An example of secondary messenger is cyclic AMP that can target
cells for example cells in the liver to release glucose from glycogen.

5. Protein Phosphorylation
• Many secondary messengers elicit responses by activating protein
kinases. Protein kinase is a kinase enzyme that modifies other
proteins by chemically adding phosphate groups to them.
Phosphorylation usually results in a functional change of the target
protein by changing enzyme activity, cellular location, or association
with other proteins.

6. Signal Termination
• Protein phosphatases are one mechanism for the termination of a
signalling process. After signalling process has been initiated and the
information has been transduced, the signal is terminated by the
phosphatase.

7. Example – Liver Cell:
• Epinephrine (a ligand) is the messenger given off from the adrenal gland. It
binds with a receptor called a G-protein receptor on the outside of the cell
and causes a conformational change to the protein for it to be able to pass
through the lipid bilayer of the cell membrane.
• Underside of the receptor inside the cytoplasm is three alpha subunits.
• One of the alpha subunit detaches and bind to a protein that activate an
enzyme that converts ATP to cyclic AMP (secondary messenger) that will
spread throughout the cell and target protein kinase.
• Protein kinase is activated by cAMP and becomes phosphorylated picking
up energy from ATP. They can then act as enzyme within the cell.
• The phosphate becomes phosphorylase that releases glucose from
glycogen within the cell.