Unit 8: Cell communication
Cells can communicate through two systems; intracellular and cell surface receptors.
Cells need to communicate to interact with other cells. Within the cell, they need to coordinate the rate
of development and growth throughout the entire cell. There is a specific signaling pathway that is
followed. First the signaling molecules are made in the signaling cell. It then travels to the target cell,
where it binds to the receptor protein on the target cell. When the signaling molecules are banded in
signal transduction, the target cell changes in behavior.
The same signal can cause different responses, depending on which target cell it went in. So for
example, a signaling molecule can create one signal. This one signal, say acetyl chorine, can hit three
target cells. These three cells can be a heart cell, salivary gland cell, or skeleton cell. For the heart and
skeleton muscle cell, it increases the amount of contractions. For the salivary gland cell, it increases
secretion of saliva.
Cell signaling is a faster way to make proteins than the normal DNA>RNA> PROTEIN dogma. This is
because in cell signaling, you alter the function of a protein. In normal protein synthesis, an altered
protein synthesis is made. The alter protein synthesis and the function both change the cytoplasm
machinery. The change in machinery in the cytoplasm changes the cell behavior.
In animal cells, there are long and short range communication responses. There are four main types;
endocrine, peregrine, neuronal, and contact-dependent. Endocrine is communications through
hormones, and peregrine is through a mediated signal cell. Both are long range communication as they
require an intermediate. Examples of short range communication would be neuronal or contact
dependent. Neuronal is when you have a neurotransmitter that transmits signals through axons and
synapses. Contact dependent is when you have a membrane bound signal molecule such that the
signaling cell touches the target cell.
The chemistry of the signal molecule determines the location of the receptor. There are two types of
receptors; cell surface receptors, and intracellular receptors.
Cell surface receptors are on the outer plasma membrane of the cell. Large hydrophilic signal molecules
attach to these receptors.
Intracellular receptors are inside the cell, for ex. within the nucleus. Small hydrophobic signal molecules
which can enter the cell attach to these receptors. These small hydrophobic signal molecules usually
enter the cell, and they send signals for gene expression. Steroids are related as their receptors are
similar to intracellular receptors.
An example of a steroid would be cortical. A cortical steroid hormone passes through the plasma
membrane. The cresol attaches to a nuclear receptor protein which changes its conformation, activating
the receptor protein. It then enters the nucleolus. In the nucleus, the activated cortical/receptor
complex binds to DNA and starts transition. Intracellular signaling cascade is when there is a cascade of signaling sequences. One thing leads to five
other things. For instance, there is a primary transduction. Then there is relay, and it transducers and
amplifies. There are second messengers. Second messengers are small, non protein molecules that relay
signals from the cell surface receptors to instead target molecules within the cell. These second
messengers are then integrated and distributed across a cell.
Intracellular signal molecules sometimes act as a molecular switch. There are two ways for signaling, by
protein phosphorylation, or by a GTP-binding protein.
In protein phosphorylation, a phosphate is added to turn the signal molecules on. It is turned off when a
phosphosphate taken off by protein phosphates. Each step in a cascade needs to be inactivated. The
activities between kinesis and phosphates determines the activity of a protein regulated by
phosphorylation. There are proteins regulated by phosphorylation. These proteins are called kinesis,
where the process of phosoplyation causes a cascade. This is when one phosphorylation creates an
inactive protein kinas, which eventually keeps going until an inactive protein is made active. The
phosphyloation of proteins is not random. The ___ kinas phosphoylates the OH groups of a _______
group sequence of a protein. Ex. tyrosine kinesis phosphotrlates the OH group on tyrosine.
In GTP-binding protein, the molecule contains a GDP protein. When turned on, a GTP is formed, adding
phosphate creating GTP on the signal. This causes intrinsic GTP-ase activity. When turned off, this
phosphate is taken off through hydrolysis, reducing it back to GDP. Therefore this is a cycle of replacing
GDP with new GTPs. When the signal is turned on, and it contains GTP, a cascade of steps will follow.
There are multiple classes of cell-surface receptors.
A ligand is an ion or molecule that binds to a central metal atom. There are four types of receptors on
the surface: Ion channel-couple, G-protein coupled, enzyme coupled receptors,
Ion channel-coupled receptors are when you have a signal molecule which binds to a signal molecule
(ligand) opening or closing of an ion channel. This changes the flow of voltages across the membrane.
Ion channel coupled receptors control the flow of ions, they can also be called neurotransmitter-gated
or ligand-gated channels.
G-protein coupled receptors ( GPCRs) are when you bind a signal molecule (ligand), it activates GTP
binding proteins, or shorter G proteins. When the GTP proteins are activated, they activate enzymes
that start the cascade. Most drugs work this way.
Enzyme Coupled receptors are when you have two enzymes, both of which are inactive catalytic
domains. When a signal molecule is paired up to form a dimmer, it can attach to these two inactive
domains. Another way it does this is when the signal molecule pulls one enzyme on one side and
another on the other side. In this case the enzymes themselves are associated with the enzymes. Upstream targets
The trans-membrane protein weaves in and out of the plasma membrane. It does this though the G
protein coupled receptor ( GPCR). When the G protein is inactive (i.e. it is in GDP form), it just stays on
the plasma membrane away from the receptor protein. When a signal molecule attaches to the receptor
protein, the GDP moves towards to signal molecule. The protein attaches to the inactive GDP. Then, the
GDP becomes activated through phosphyloation. The activated G protein splits up into a and b subunits
so that they line up across the plasma membrane. In between each su