1) Cells use signal transduction pathways to respond to their environment. Signals like hormones bind to receptors on the cell surface which triggers a transduction cascade of protein interactions inside the cell. 2) Phosphorylation and dephosphorylation by protein kinases is a key mechanism in signal transduction cascades, altering the behavior of proteins and turning pathways on and off. 3) Membrane receptors transfer information about external signals to the cell interior by undergoing a conformational change upon ligand binding, starting intracellular signaling cascades.

1. UNIT -I Ms.Smita
Shukla
All cells must know how to respond to their environment. They must be
able to divide, grow, secrete, synthesize, degrade, differentiate, cease
growth, and even die when the appropriate signal is given. This signal
invariably is a molecule which binds to a receptor, typically on the cell
surface. (Exceptions include light transduction in retinal cells when the
signal is a photon, and lipophilic hormones which pass through the
membrane.) Binding is followed by shape changes in transmembrane
protein receptors which effectively transmit the signal into the
cytoplasm.
To survive, an organism must constantly adjust its internal state to changes
in the environment. To track environmental changes, the organism must
receive signals. These may be in the form of chemicals, such
as hormones or nutrients, or may take another form, such as light, heat, or
sound.
A signal itself rarely causes a simple, direct chemical change inside the cell.
Instead, the signal sets off a chain of events that may involve several or
even dozens of steps. The signal is thereby transduced, or changed in form.
Signal transduction refers to the entire set of pathways and interactions by
which environmental signals are received and responded to by single cells.
Signal transduction systems are especially important in multicellular
organisms, because of the need to coordinate the activities of hundreds to
trillions of cells. Multicellular organisms have developed a variety of
mechanisms allowing very efficient and controlled cell-to-cell
communication.
Though we take it for granted, it is actually astonishing that our skin, for
example, continues to grow at the right rate to replace the continuous loss
of its surface every day of our lives. This tight regulation is found in every
tissue of our body all of the time, and when this fine control breaks down,

2. UNIT -I Ms.Smita
Shukla
cancer may be the result. Clearly the molecular mechanisms behind this
astounding level of control must be powerful, versatile, and sophisticated.
Signals, Receptors, and Cascades
The signals that cells use to communicate with one another are often small
amino acid chains, called peptides. Depending on the cell type that releases
them and the effect they have on the target cell, they may be called
hormones, growth factors, neuropeptides, neurotransmitters, or
cytokines. Other small molecules can also be signals, such as amino acids
and steroids such as testosterone. External signals such as odorants and
tastes can be carried to us in the atmosphere or in the fluids of our food and
drinks. Stretch, pressure, and other mechanical effects as well as heat, pain,
and light can also act as signals.
Given the huge variety of signals to which a cell is exposed, how does it
know which to respond to? The answer is that signals are received by
protein receptors made by the cell, and a cell is sensitive only to those
signals for which it has made receptors. For instance, every cell in the body
is exposed to estrogens circulating in the blood, but only a subset of them
make estrogens receptors, and are therefore sensitive to its influence.
Chemical signals such as hormones bind to their receptors, usually at the
surface of the cell (the plasma membrane), but sometimes within the cell.
This causes a conformation (shape) change in the receptor. The
conformation change typically alters the ability of the receptor to bind to
another molecule in the cell, modifying that molecule's conformation, or
triggering other actions.
This sequence of events triggered by the signal-receptor interaction is
called a transduction cascade. A transduction cascade involves a network
of enzymes that act on one another in specific ways to ultimately generate
precise and appropriate responses.

3. UNIT -I Ms.Smita
Shukla
The Importance of Phosphorylation &
Dephosphorylation
After a signal is received, signal transduction involves altering the
behaviour of proteins in the cascade, in effect turning them on or off
like a switch. Adding or removing phosphates is a fundamental
mechanism for altering the shape, and therefore the behaviour, of a
protein. Phosphorylation may open up an enzyme's active site, allowing
it to perform chemical reactions, or it may frequently generate a binding
site allowing a specific interaction (may make a bulge in one side
preventing the protein from fitting together) with a molecular partner.
Enzymes that add phosphate groups to other molecules are called kinases,
and the molecules the enzymes act on are called substrates.
Protein kinases are a family of enzymes that use ATP to add phosphate
groups on to other proteins, thereby altering the properties of these
substrate proteins.
Protein kinases themselves are frequently turned on or off by
phosphorylation performed by other protein kinases; thus a kinase can be
both enzyme and substrate.
Membrane receptors transfer information from the
environment to the cell's interior.
A few nonpolar signal molecules such as estrogens and other steroid
hormones are able to diffuse through the cell membranes and, hence, enter
the cell. Once inside the cell, these molecules can bind to proteins that
interact directly with DNA and modulate gene transcription.
Thus, a chemical signal enters the cell and directly alters gene-expression
patterns.
However, most signal molecules are too large and too polar to pass
through the membrane, and no appropriate transport systems are present.

4. UNIT -I Ms.Smita
Shukla
Thus, the information that signal molecules are present must be transmitted
across the cell membrane without the molecules themselves entering the
cell. A membrane-associated receptor protein often performs the function
of information transfer across the membrane.
Such a receptor is an intrinsic membrane protein that has both
extracellular and intracellular domains. A binding site on the extracellular
domain specifically recognizes the signal molecule (often referred to as
the ligand). Such binding sites are analogous to enzyme active sites except
that no catalysis takes place within them.
The interaction of the ligand and the receptor alters the tertiary or
quaternary structure of the receptor, including the intracellular domain.
These structural changes are not sufficient to yield an appropriate
response, because they are restricted to a small number of receptor
molecules in the cell membrane.
The information embodied by the presence of the ligand, often called
the primary messenger, must be transduced into other forms that can
alter the biochemistry of the cell.