Signalling Molecules (HL) (HL IB Biology)

• There are many different types of chemical signalling 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, i.e. 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 release by endocrine glands, e.g. the pancreas, and the bind to receptors on target cells

• Examples of hormones might include
  • Insulin
  • Glucagon
  • Thyroxine
  • Testosterone

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 localised 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 cytokines 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 signalling 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 signalling 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 fibres within muscle tissue
    • Calcium ions are pumped back out of the muscle fibres, 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 signalling molecule binds to an external membrane receptor

• Within the categories of hormones and neurotransmitters, there are several different chemical configurations
• Each configuration is fundamental in ensuring the efficacy of the molecule as a signalling molecule
• Chemicals must be:
  • Able to bind to receptors; this is dependent on the shape and chemical properties of the molecule
  • Small and soluble so that they can move around the body

Hormones

• Hormones fit into three categories: amines, peptides or steroids
• Different categories of hormones have different properties which influence how they interact with their target cells, e.g.
  • Amines and proteins are hydrophilic, which makes it difficult to cross phospholipid bilayers, so they function by binding to external membrane receptors
  • Steroid hormones are hydrophobic so can cross cell membranes and bind to receptors inside cells

Hormone category examples table

Amines	Proteins	Steroids
Melatonin	Insulin	Oestradiol
Thyroxin	Glucagon	Progesterone
Epinephrine	ADH	Testosterone

Neurotransmitters

• Neurotransmitters categories include: amines, gases, amino acids and peptides
• Most neurotransmitters are hydrophilic, so they function by binding to receptors on cell surface membranes
• Some neurotransmitters also act as hormones, e.g. epinephrine, also known as adrenaline
• Some neurones only produce one type of neurotransmitter, others produce multiple which can be released simultaneously
  • If multiple neurotransmitters are released at once they can stimulate several different outcomes at the same time

Neurotransmitter categories examples table

Amines	Gases	Amino acids	Peptides
Dopamine	Nitrous oxide	Glutamate	Endorphins
Epinephrine		Glycine

Comparing the effects of hormones and neurotransmitters

• Neurotransmitters have a localised effect
  • There is a very short distance for them to diffuse from the presynaptic to the post synaptic membrane
  • This is an average distance of 20 nanometres
• Hormones may have a more distant effect
  • They travel much longer distances as they travel in the blood to cells which have the correct receptors
  • These cells could be located very close to the gland or could require transport to the furthest point of the body