Summary
BIOL1000 Block 1 Summary: Cellular Communication
- Institution
- University Of The Witwatersrand (wits)
A summary of cellular signalling and cell communication
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CELLULAR MESSAGING - COMMUNICATION AT A CELLULAR LEVEL● Cell-cell communication is essential for multicellular and unicellular organisms
● Cells most often communicate via chemical signals
○ E.g. fight-or-flight response is triggered by epinephrine molecule
The fight or flight response (EXAMPLE)
● Is a cascade of events brought about by chemical signals - this is called
chemical signalling
● Epinephrine is the signal that starts the event (a receptor protein in cell
membrane)
○ This molecule activates the rest of the events that complete this
response.
● The signal is then passed onto a target
Key concepts of cellular communication
1. External signals: these are converted into responses into the cell
2. Reception: a signalling molecule binds to a receptor and changes its shape
3. Transduction: the change in the molecular shape of the receptor results in a
cascade of events/interactions/signals between receptors until it reaches the
target molecule
4. Response: once the target molecule is reached, the response is carried out
EVOLUTION OF CELL SIGNALLING
● Sacchromyces cerevisia communicates about sex in yeast cells
○ It has an a and an alpha mating type
, ○ Cells of different mating types locate each other via secreted factors
specific to each type
1. They exchange mating factors to see if they are compatible - each yeast cell has
receptors that are specific for its partner’s alpha or a mating factor
2. If a yeast with the opposite mating factor is found, mating commences
3. The cells fuse and exchange DNA, forming a cell with both a and alpha genes
4. The binding of the mating factors initiates a response called signal transduction
pathway
a. This is similar to what happens in multicellular organisms
● Bacterial cells in biofilm communities can communicate with each other through
chemical signals
○ A bacterial cell secretes molecules that are detected by other bacterial
cells
○ A concentration of these signaling molecules allows bacteria to sense the
cellular environment in neighbouring cells (i.e. how many other
microorganisms are present, what they are like)
○ They can put together a specific target response for these conditions, and
coordinate changes in the bacterial community
■ Quorum sensing
○ Clumps of bacteria communicate this way
● Mycobacteria communicate about nutrient availability
○ When they are short on nutrients, they pass on a message to their
neighbours
○ This signal causes neighbouring cells to aggregate
, ○ Cells form a fruiting body that produces spores
■ Spores have thick walls that allow the bacteria to survive, until the
environment is favourable again
Local and long distance signalling
● Cells in multicellular organisms are not always close together and need to pass
messages over long distances
○ They do this by using signalling molecules
In multicellular organisms, cells communicate by:
1. Direct contact
● Cells are passed through gap junctions/plasmodesmata that connect the
cytoplasms of adjacent cells
○ This way chemical signals can be passed without needing to cross the
plasma membrane
● Some cells have cell protrusions which can connect to and bind to each other
and pass messages this way
2. Local signalling
● Signals are transferred between neighbouring cells that are a short distance
apart
A. Paracrine signalling
● Occurs between a gland and a neighbouring cell
B. Synaptic signalling
● Observed with neurons
● Passing of chemical messages from one neuron to another
3. Long distance signalling
● Observed with hormones
, ● Hormones are secreted into the bloodstream - the blood helps carry hormones
(which carry the messages/signals) across long distances in the body
● Neural signals also sometimes have to travel the length of a long nerve cell
SIGNAL TRANSDUCTION PATHWAY
= series of steps by which a signal on a cell’s surface is converted into a specific
cellular response
● A chemical signal is received and needs to be converted into a response which
has many steps
● This is a standard set of step involved in converting a signal into a response
The three stages of cell signalling:
1. Reception: chemical signal is detected outside the cell
2. Transduction: chemical signal is converted into a format that is able to be
passed onto several relay molecules (sometimes it can occur in one step, but
often over many molecules)
3. Response: the pathway has communicated the signal to where it is supposed to
reach, and can now activate a cellular response
1. RECEPTION
● A signalling molecule (AKA Ligand) binds to a receptor protein and causes it to
change shape
○ The shape change is the initial stage of transduction
● This is very specific, only a specific chemical molecule can bind to a specific
receptor
● Receptors are mostly proteins bound to the membrane:
, 1. Plasma membrane receptors
2. Cytosol ic receipts
3. Nuclear receptors
1. Plasma membrane receptors
A. G protein-coupled receptors (GPCRs)
● The larges family of transmembrane receptors
● Name is acquired from their association with G proteins
○ G proteins are cytoplasmic proteins named after their ability to bind to
energy rich GTP (an analog of ATP)
Steps of GPCR signalling:
1. G protein is attached to the inside of the cell membrane - it is still able to slide
around on the cytoplasmic side
a. The G protein acts as a switch: when GDP is attached, it is inactive. When
GTP is attached, it is active
2. When a chemical signalling molecule attaches to the receptor, inactive G protein
attaches to the receptor
3. GTP replaces GDP (via phosphorylation activity) and the the G protein is now
activated
4. The activated GTP uncouples from the receptor, causing it to become
deactivated
5. The GTP moves along the plasma membrane and couples with an enzyme,
causing it to become activated
6. The active enzyme can drive the next step
7. The G protein hydrolyses the GTP molecule, forming an inactive GDP.
8. The G protein can now be reused
, Activities driven by GPCRs:
● Human vision, smell and taste
● Sharing of mating factors in yeast
● The fight or flight response
● Many bacterial diseases and infections interfere with the GPCR system to
weaken the host
B. Receptor tyrosine kinases (RTKs)
● Membrane receptors that also use an energy-rich molecule, ATP
● They pick up phosphate groups from ATP and transfer them to another protein
in the system
○ In the process they pass on a message
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