Signal Transduction (College Board AP Biology)

• There are many different types of chemical signaling molecules in animals

Hormones

• A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs, ie. they are chemicals which transmit information from one part of the organism to another and bring about a change
• Endocrine glands produce hormones and secrete them into capillaries in the gland tissue
  
  • Collectively these glands are known as the endocrine system
• Hormones are transported in the blood to target cells
• Hormones only affect cells with target receptors to which the hormones can bind
  • These are either found on the cell surface membrane or inside cells
  • Receptors have to be complementary to hormones for binding to occur, so they are specific to a particular hormone 

Hormone action diagram

Hormones are released by endocrine glands, eg. the pancreas, and the bind to receptors on target cells

• Examples of hormones might include
  • Insulin
  • Glucagon
  • Thyroxine
  • Human Growth Hormone
  • Testosterone
  • Estrogen

Neurotransmitters

• Neurotransmitters are chemicals that transmit signals across the synaptic cleft from the presynaptic neurone to the post-synaptic neurone
• The neurotransmitters diffuse across the cleft and bind with receptor molecules on the postsynaptic membrane; this causes associated sodium ion channels on the postsynaptic membrane to open, allowing sodium ions to diffuse into the postsynaptic cell
• If enough neurotransmitter molecules bind with receptors on the postsynaptic membrane, then a nerve impulse is generated, which then travels along the postsynaptic neurone
• The neurotransmitters are then broken down to prevent continued stimulation of the postsynaptic neurone

Neurotransmitter diagram

Neurotransmitters are released from the presynaptic knob and bind to receptors on the postsynaptic membrane

• Signals from neurotransmitters are short lived and localized compared to hormones
• Examples of neurotransmitters include:
  • Acetylcholine
  • Norepinephrine
  • Dopamine

Cytokines

• Cytokines are proteins released by nearly all cells in the human body
• There are several different types of cytokine and each one plays a role in determining activity of another cell
• Cytokines interact with cells by binding to receptors on the cell surface membrane; they cannot enter the cytoplasm
• Binding of cytokines leads to a cascade of events inside the cell which impacts gene expression and, therefore, cell activity
• Cytokines are involved in signaling between white blood cells during an immune response, as well as regulating the cell cycle for cell growth and proliferation during embryonic development
• Examples of cytokines include
  • Interleukin
  • Erythropoietin
  • Interferon

Calcium ions

• Calcium ions (Ca²⁺) are involved in many of the signaling pathways of the human body, for example:
  • During muscle contraction an influx of Ca²⁺ initiates a change in shape of specific proteins which allow contraction of fibers within muscle tissue
    • Calcium ions are pumped back out of the muscle fibers, meaning that the response is rapid and short lived
  • At a synapse the arrival of a nerve impulse stimulates the movement of Ca²⁺ into the presynaptic knob from the synaptic cleft, which then triggers vesicles to release neurotransmitters into the synapse
    • Calcium ions are pumped back into the synaptic cleft, meaning that the response is rapid and short lived
  • Calcium ions sometimes act as second messengers, meaning that they are part of the cascade of reactions that occurs inside a cell after another signaling molecule binds to an external membrane receptor

Other Examples of Signal Transduction to Respond to the Environment

• The SRY gene, if expressed in animals, triggers the male sexual development pathway
  • The gene is carried on the Y chromosome
  • The absence of the SRY protein causes ovaries to develop into gonads, hence femalenes

• Mating pheromones in yeast trigger mating gene expression
  • Like the yeast mating gene, human vision, smell and taste responses all rely on G proteins (proteins that harness the energy of energy-rich GTP molecules)
• Ethylene (ethene) gas is commonly regarded as a plant hormone
  • Ethylene encourages the ripening of fruit
• HOX genes play a role in regulating the development of an animal's body versus its body plan eg. to ensure that bones grow to the right length, or internal organs grow to the correct size and shape