Cell Communication (College Board AP® Biology): Study Guide

Direct contact

• For effective functioning of tissues, organs and systems, cells must communicate with each other

• Cell communication can be:

  • by direct, or cell-to-cell, contact

  • over short distances

  • over longer distances

• Cells communicate using chemical signaling molecules known as ligands

Communication by cell-to-cell contact

• Communication between cells that are in direct contact with each other is also known as juxtacrine signaling

Example: immune cells

• Direct contact occurs during binding of immune cells, such as T helper cells and T killer cells, to specific antigens displayed on the surface of antigen presenting cells

  • Antigen presenting cells can be:

    • phagocytes that have engulfed pathogens

    • pathogens themselves

    • infected body cells

  • This binding activates the immune cells and changes their activities, e.g. causing them to differentiate into memory cells

Example: plasmodesmata

• Plant cells can be connected by plasmodesmata (singular: plasmodesma); these are cytoplasmic bridges that allow materials to move between neighboring cells

  • Plasmodesmata provide channels for the direct movement of ligands between cells

Local signaling

• Cells that are a short distance apart communicate using ligands that travel only within the immediate area; these ligands are known as local regulators

  • This form of signaling is also known as paracrine signaling

• Local regulators diffuse through the spaces between cells and only affect cells in the immediate vicinity of their source

Example: neurotransmitters

• Neurotransmitters are released at the synapses

• They diffuse across the tiny gap between neurones, bind to receptors on the new cell and enable the continued transmission of a nerve impulse

Other examples of local regulators

• Quorum sensing

  • Bacteria release signaling molecules into their environment; the concentration of these chemicals allows individual bacteria to determine the population density and change their activities accordingly

• Morphogens

  • Morphogens are signaling molecules that provide information to cells during early development of an individual

  • E.g. the spinal cord in an embryo releases a ligand known as Shh, which diffuses to nearby cells; the concentration of Shh decreases with distance from its source, and it is this concentration that determines the development of different neurone types

• Plant immune responses

  • Pathogens release chemicals that are detected by receptors on the plant cell surface, triggering a cascade response within the cell

  • Signaling molecules are also passed to neighboring cells, warning them of infection

Distant signaling

• Cells that are a long distance apart communicate using ligands that can travel a long way from their source; these ligands are known as hormones

  • This form of signaling is known as endocrine signaling

• Hormones bind to specific receptors on the membranes of their target cells in order to bring about a response

Example: insulin

• Insulin is a hormone released by cells of the pancreas in response to increased blood glucose levels

• It travels in the blood to the liver, where it binds to receptors on the surface of liver cells in order to increase uptake of glucose from the blood

Other examples of hormonal communication

• Human growth hormone

  • Released by: the pituitary gland in the brain

  • Regulates: growth and metabolism

• Thyroid hormones

  • Released by: the thyroid gland in the neck

  • Regulates, e.g.:

    • metabolism

    • body temperature

    • bone health

    • digestion

• Testosterone

  • Released by: the testes or ovaries

  • Regulates, e.g.:

    • development of male secondary sexual characteristics

    • bone and muscle health

• Estrogen

  • Released by: the testes or ovaries

  • Regulates, e.g.:

    • development of female secondary sexual characteristics

    • the menstrual cycle

    • bone health