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Cell Signalling (HL) (HL IB Biology)

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Ruth

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Ruth

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Receptors & Ligands

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

Basic stages of cell signalling pathway, downloadable AS & A Level Biology revision notes

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 in cell signalling, downloadable AS & A Level Biology revision notes

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

Cell Signalling: Quorum Sensing in Bacteria

  • 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

 

bobtail-squid

CC BY-SA 3.0, via Wikimedia Commons

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

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Ruth

Author: Ruth

Expertise: Biology

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. She gained 10 years of teaching experience across the 3 key science disciplines and physical education. Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.