Cell Signalling (HL) (HL IB Biology)

Cell signalling

• Cell signalling is the process by which messages are sent to cells
• Cell signalling is very important as it allows multicellular organisms to control and coordinate their bodies and respond to their environments
• Cell signalling pathways coordinate the activities of cells, even if they are far apart within the organism
• The basic stages of a cell signalling pathway are:
  • A stimulus or signal is received by a receptor
  • The signal is converted to a signal that can be passed on – this process is known as transduction
  • The signal is transmitted to a target (effector)
  • An appropriate response is made

Cell signalling diagram

Cells signalling involves the detection of a stimulus by a receptor, the transmission of a signal, and the response of an effector

Ligands

• Signalling molecules are often called ligands
• Examples of ligands include:
  • Proteins and amino acids
  • Nucleotides
  • Steroids
  • Amines
• Ligands are involved in the following stages of a cell signalling pathway:
  • Ligands are secreted from a cell (the sending cell) into the extracellular space
  • The ligands are then transported through the extracellular space to a target cell
  • The ligands bind to surface receptors (specific to that ligand) on the target cell
    • These receptors may be proteins with binding sites, e.g. a glycoprotein
  • The message carried by the ligand is relayed through a chain of chemical messengers inside the cell, triggering a response

Ligand binding diagram

Ligands bind to specific binding sites, initiating a response inside the cell

• Bacteria communicate with each other using ligands
• This allows a bacterial colony to respond to changes in population size by altering gene expression
• The term quorum sensing has been applied to this concept as it suggests that a bacterial colony will monitor its size to assess when a threshold has been reached
  • The term quorum is used to describe the minimum number of individuals who much be present for a process to take place
• It works through the following mechanism:
  • Ligands released by bacteria bind to receptors on the surface of other bacteria
  • The more bacteria are present in a population, the more ligands are released
  • When a threshold number of receptors are occupied, a change in gene expression is triggered
  • A change in gene expression leads to a change in activity which signals that a quorum has been met in the bacterial colony

Vibrio fischeri 

• Vibrio fischeri is a species of bacterium found in marine environments, where they form mutualistic associations with some species of squid, e.g. the bobtail squid
  • The benefit to the squid is increased camouflage
    • The bacteria emit light by bioluminescence, lighting up the underside of the squid and making it less visible against the bright sky from underneath
  • The benefit to the bacterial colony is the provision of amino acids and sugar from the squid's metabolic processes
• Vibrio fischeri enable the squid to produce light by bioluminescence as follows:
  • Vibrio fischeri colonise a structure inside the squid called the light organ and release a ligand called an autoinducer into the extracellular environment
    • The more bacteria are present, the more autoinducer is released
  • The autoinducer enters other bacterial cells and binds to a receptor called LuxR in the cytoplasm
  • When enough autoinducer-LuxR complexes have formed, a threshold is reached, resulting in transcription of DNA that leads to the synthesis of the enzyme luciferase
  • Luciferase catalyses an oxidation reaction which releases energy as bioluminescence
• Production of bioluminescence only occurs in bacteria when the colony is large enough to switch on the synthesis of luciferase

CC BY-SA 3.0, via Wikimedia Commons

Vibrio fischeri allow bobtail squid to emit bioluminescence, helping the squid to avoid predation