Signal Transduction (College Board AP® Biology): Study Guide

Responding to the environment

• Signal transduction pathways influence how the cells respond to their environment

Example: quorum sensing

• Bacteria constantly release chemical signals into their environment

• Individual bacteria can detect the levels of these chemical signals and use them as a measure of population density, i.e. the higher the level of chemicals, the larger the local bacteria population

• Bacteria can adjust their cellular processes on the basis of population density, e.g. they may increase expression of chemicals that make them more effective competitors at high population density

Example: epinephrine and glycogen

• A hormone called epinephrine (also known as adrenaline) can act to increase blood glucose concentration

• Epinephrine binds to receptors on the outside of a cell and brings about an intracellular response

  1. Epinephrine binds to specific receptors on the membrane of liver cells

  2. This activates the enzyme adenylyl cyclase by changing its shape

  3. Active adenylyl cyclase catalyses the conversion of ATP to the secondary messenger cyclic AMP (cAMP)

  4. cAMP binds to and activates protein kinase A enzymes

  5. Active protein kinase A enzymes phosphorylate phosphorylase kinase enzymes

  6. Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes

  7. Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose

Types of cellular response

• Signal transduction may result in different types of cellular response, e.g.

  • changes in gene expression

  • changes in cell function

  • programmed cell death (apoptosis)

• Specific examples that demonstrate different cellular responses include:

  • Cytokines:

    • Cytokines are proteins that are important in cell signalling

    • They regulate gene expression to allow for cell replication and division, e.g. during coordination of the immune response

  • Mating pheromones in yeast:

    • Yeast cells release chemicals called pheromones into their environment

    • Pheromones bind to G protein-coupled receptors, causing a signaling cascade that leads to the transcription of mating-specific genes

  • The SRY gene:

    • This is a gene located on the Y chromosome in animals

    • Expression of this gene results in production of the SRY protein which initiates transcription of genes that lead to testis development in males

  • Ethylene:

    • Ethylene (ethene) gas is a plant hormone

    • It binds to receptors on the surface of fruit cells, initiating a signaling cascade that results in altered expression of genes that regulate fruit ripening

  • Hox genes:

    • Hox genes code for proteins which regulate transcription during the development of an embryo

    • They ensure that different body parts develop in the right part of an organism

    • The proteins regulate expression of genes involved with cell division, differentiation and apoptosis

Changes in signal transduction pathways

• Changes to signal transduction pathways can have a significant impact on cells

• Changes may occur due to the effect of mutations on components of the signaling pathway, e.g.:

  • mutations in genes that code for the ligand-binding domain of receptor proteins may result in receptors to which ligands can no longer bind

  • mutations in genes that code for adenylyl cyclase may prevent the formation of secondary messenger cAMP, preventing signals from extracellular receptors reaching the inside of a cell

• Factors in the environment may also affect the functioning of signal transduction pathways, e.g.:

  • the cholera toxin binds to G-protein-coupled receptors and disrupts the signalling cascade that regulates chloride ion channels in the cell membranes of intestinal cells, resulting in severe diarrhoea

  • lead is a metal that can imitate the action of calcium ions in signal transduction pathways, affecting many cellular processes