How Cells Communicate With Each Other (College Board AP Biology)

• For effective function of tissues, organs and systems, cells must communicate with each other
• As organisms evolved from single-celled prokaryotes and eukaryotes into multicellular organisms, the need to cellular communication grew to enable sophisticated adaptations
  • This communication often takes the form of chemical changes
• Cell communication is often a means of responding to changes in external stimuli
  • Stimuli are converted to responses within the cell
  • A good example is the adrenaline 'flight or fight' response when an animal spots a potential predator
    • A chain or hormone-induced events occurs that leads to a noticeable response eg. a sprint to evade the predator's charge
• Cell communication can take place:
  • By direct contact, cell-to-cell 
  • Over short distances by employing local regulators
  • Over longer distances by employing messenger compounds such as hormones

Cell signaling

• Cell signaling is the process by which messages are sent to cells
• Cell signaling is very important as it allows multicellular organisms to control and coordinate their bodies and respond to their environments
• Cell signaling pathways coordinate the activities of cells, even if they are far apart within the organism
• The basic stages of a cell signaling 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 Signaling Flow Chart

Cell signaling involves the detection of a stimulus by a receptor, the transmission of a signal, and the response of an effector

Direct Cell-to-Cell Communication

• Some cellular communication happens between one cell and its immediate neighbors
  • This is referred to as juxtacrine signaling
• This can cause a cascade of communication

Plasmodesmata Between Plant Cells

• In cells of the phloem, channels between cells exist called plasmodesmata (singular: plasmodesma)
• These allow cytoplasm, and all its contents, to move between neighboring cells in the phloem tissue
• They can be thought of as 'cytoplasmic bridges'
• They form important channels for molecules such as growth regulators and nutrients, such as the many and varied products of photosynthesis

Plasmodesmata Diagram Showing Cell-to-Cell Contact

Plasmodesmata allow direct cell-to-cell contact and the transmission of cell signaling compounds

Horizontal Gene Transfer in Prokaryotes

• Neighboring bacterial cells can exchange DNA via a conjugation tube
• This is called horizontal gene transfer
  • Genetic material is passed from one individual cell to another within the same generation ('horizontally')
• This enables bacteria to exchange DNA rapidly
• Horizontal gene transfer can account for rapid changes in the population's genome eg. in developing resistance to antibiotic drugs

Horizontal Gene Transfer by Cell-to-Cell Contact Diagram

Horizontal gene transfer between bacterial cells by conjugation

Cell-to-Cell Contact in Immunity

• Antigens and antibodies provide a good example of cell-to-cell direct contact
• Each type of antibody molecule has specificity to their respective antigen
  • This means there is a direct molecular fit, like two pieces of a jigsaw puzzle fitting together

Direct Contact Between Antibodies and Antigens Diagram

Cell-to-cell contact between an antigen-presenting cell and a lymphocyte produces an immune response effect

• Other aspects of the immune response where cell-to-cell contact is important include:
  • The action of helper T cells
  • The action of killer T cells

Direct Cell-to-Cell Contact in the Immune System

T helper cells and T killer cells have cell-to-cell contact in immunity

Communication Between Cells Using Local Regulators

• Local regulators are molecule released from cells to affect other cells in cells in the nearby area
• This is known as paracrine signaling
• The chemical signals are referred to as ligands

Ligands

• Signaling 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 signaling 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, eg. 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 in a nearby cell, initiating a response inside that 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 is 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 colonize 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 catalyzes an oxidation reaction which releases energy as bioluminescence
• 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