Chapter 11: Cell Communication

11.1 - External Signals are converted into Responses within the Cell

Evolution of Cell Signaling

• Cells can use cellular signaling to identify their mates
  • Secrete mating factors that bind to the receptors of the other cell (exchange of mating factors)
  • Those factors binding to the receptors induce fusion to the cells (mating)
  • This happens through transduction in a series of phases called the signal transduction pathway
  • Signaling first existed in ancient prokaryotes and single-celled eukaryotes but was evolved and adapted to their descendants
  • The daughter cell’s nucleus contains genetic information from both of the parents (new cell)
• Other uses of the signaling pathway: bacteria use it to monitor the density of local cells
  • Allowed them to synchronize their behaviors accordingly

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Local and Long Distance Signaling

• Cells use the signal transduction pathway for communication especially the cells of multicellular organisms (may or may not be adjacent to one another)
• Animal and plant cells have junctions where the cytoplasm from adjacent cells can connect for communication
  • Cell junctions: plant and animal cells have junctions that allow molecules to pass through freely with adjacent cells without crossing plasma membranes (plasmodesmata in plant cells)

Local signaling methods

• Communication with direct contact is called cell-cell recognition in animal cells
  • Used in embryonic development and immune responses
  • Interact through molecules protruding from their surfaces
  • These might work like the lock-and-key model → they seem to have specific active site-looking areas in each junction to only fit a specific type of other molecules
• Paracrine signaling: just in animals, sending messenger molecules from one cell to another to get a certain response
  • Growth factors are used in this way → stimulate nearby cells to grow and divide (cell cycle)
  • A secreting cell targets another cell by releasing certain local regulators
• Synaptic signaling: Juste dans animaux
  • Electrical signals along nerve cells trigger the secretion of neurotransmitters → these neurotransmitters act as chemical messengers
  • Diffuse along the synapse (space between the nerve cell and the target cell) to trigger a response in the target cell
  • Target cells can include nerve cells or other body cells like muscle cells
• Plant local signaling methods/regulators/molecules are different from animals due to their cell walls

Long Distance Signaling Methods

• Use chemicals known as hormones
  • Hormones are secreted chemicals that are formed in specific cells that travel in body fluids to act on specific target cells to change the target cell’s functioning
• Endocrine signaling:
  • In animals, hormones are released from specialized cells and travel through the bloodstream (circulatory system) to reach a target cell where the target responds to the hormone
  • Hormones may travel through most bodily fluids but mostly do so through blood. They can reach virtually all cells and are only bound by some.
  • In plants, hormones known as plant growth regulators travel through cells or by diffusing in the air as gases

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The 3 Stages of Cell Signaling

• Hormones don't directly act with the enzymes within the cell → an internal signal transduction must happen
• The plasma membrane is necessary for the transmission of any signal from the outside of the cell

1. Reception: occur a the plasma membrane as a signaling molecule interacts with the receptor

• The cell’s recognition of the signal. A chemical signal is detected by the cell when it binds to the receptor on the surface of the cell (or sometimes inside the cell)

1. Transduction: happens in the cytoplasm, a pathway of several steps with each relay molecule bringing upon a change in the next molecule. The final molecule is what triggers the response

• The changing of the receptor protein due to being bound to the signal molecule triggers the transduction pathway
• It can happen in a single step but most often than not, happens in a series of steps known as the signal transduction pathway (a series of steps linking a mechanical, chemical, or electrical stimulus to a specific response)

1. Response: may be any imaginable cellular activity

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11.2 - Reception: a signaling molecule binds to a receptor protein causing it to change shape

• Signal molecules are called ligands i.e. molecules that bind to other molecules, usually bigger ones
  • All substrates are ligands
• The results of the receptor changing shape:

1. The receptor is directly activated allowing it to interact with other molecules
2. Two or more receptor molecules are aggregated together and cause molecular processes to start within the cell

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Receptors in the Plasma Membrane

G Protein-Couple Receptors (GPCRs)

• The most common cell surface receptors
• GCPRs work with the help of G Proteins that are activated when GCPRs respond to signal molecules → ONLY ACTIVATE 1 SIGNAL TRANSDUCTION PATHWAY
• When there's no signal molecule bound yet (inactive state), there's a G Protein+GDP unit on the inside of the plasma membrane not yet attached to the GCPR
• When a signal molecule binds itself to a GCPR, the receptor activates. A G protein binds itself on the cytoplasmic side of the GCPR. At this moment, a GTP replaces the GDP and thus the G protein is activated.
• The activated G protein leaves the receptor and joins an enzyme (still in the plasma membrane). This changes the shape of the enzyme and kickstarts a signal transduction process.
• Since its temporary job with the enzyme is done, the G protein leaves the enzyme and goes back to its place in the plasma membrane. Since the protein also functions as an enzyme, it hydrolyzes the GTP (triphosphate), leaving the GDP (diphosphate) attached to it and a phosphate group floating around.

Receptor Tyrosine Kinases (RTKs)

• Receptor proteins whose job is to attach phosphates to tyrosines
• They have 3 tyrosine kinases attached to them
• They also have extracellular ligand-binding sites
• 1 RTK may activate more than 10 signal transduction pathways and cell responses
• Can be associated with cancer due to how many STPs (signal transduction pathways) they activate
• How do RTKs work:
  • Originally, the RTKs exist as monomers or single units in the plasma membrane
  • The binding of a signaling molecule (ligands) causes two RTKs to join together and form units known as dimers in a process called dimerization
  • In some cases: may form large clusters instead of dimers
  • Dimerization causes phosphate groups from 6 ATP molecules to bind to the 6 (3 from each OG RTKs) tyrosine kinases
  • The 6 ATP molecules thereby become ADP molecules
  • Relay proteins attach to each phosphorylated tyrosine → relay proteins undergo structural changes → signal transduction pathway triggered that leads to a cellular response

Ion Channel Receptors

• Instead of structural changes, these receptors function as “gates”
  • When ligands bind onto these receptors, the gate opens or closes to allow the movement of specific ions like Na+ or Ca2+
• These ions flow to change the concentration of that particular ion in the cell → affecting the cellular activity
• When the ligand disassociates (separates) from the receptor, the gated ion channels in the receptor open/close once again to their original position
• Often found in the nervous system
• Some ion channel receptors may be triggered by electrical signals instead of ligands

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Intracellular Receptors

• Found in the cytoplasm or nucleus of target cells
• Each signaling molecule still comes from outside the cell but has to come through the plasma membrane to reach the receptor
• Hormone properties like being hydrophobic or hydrophilic really matter since the molecule needs to pass through the membrane

A signaling molecule/hormone’s path:

• Pass through the plasma membrane
• Bind to an intracellular receptor in the cytoplasm (or nucleus) → activate it
• Then the signaling molecule+receptor unit triggers a signal transduction pathway and the result is a specific cellular response
  • Ex: hormone+recptor complex enters the nucleus and binds to a specific gene. Then they stimulate the transcription of genes into mRNA. Lastly, protein synthesis
• Signaling molecules that are not water soluble will probably pass through the plasma membrane better → MIGHT BE ON TEST
  • Look for lipid-soluble molecules