intracellular communication & signal transduction

explain the relationship between the signaling cell and the responding cell

• signaling cell = releases signaling molecules

• responding cell = has receptor proteins that bind to the signaling molecule

what are the 4 types of intracellular signaling molecules

• Paracrines

• NTs

• hormones

• neurohormones

describe Paracrines (2) and give an example

• Local chemical messengers

• Exert effect only on neighbouring cells in immediate
  environment of secretion site

• example = histamine

describe NTs (2) and give an example

• Short-range chemical messengers

• Diffuse across narrow space (like the synaptic cleft) to act locally on adjoining target cell (another neuron, a muscle, or a gland)

• acetylcholine

describe hormones (3) and give an example

• Long-range messengers

• Secreted into blood by endocrine glands in response to appropriate signal

• Exert effect on target cells some distance away from release site

• insulin

describe neurohormones (2) and give an example

• Hormones released into blood by neurosecretory neurons

• Distributed through blood to distant target cells

• ADH - antidiuretic hormone expl

explain the steps in cell signaling (4)

• receptor activation - The signal binds to a receptor, which is then activated.

• process of signal transduction - The signal is transmitted to the interior of the cell by a signal transduction pathway

• response - The cell responds, for example by activating an enzyme or turning on transcription of a gene.

• termination - The response is terminated so that new signals can be received

what is a transducer and give an example

• converts one form of energy into another form of energy

• (e.g. Finger pulse pressure transducer converts the pressure wave into an electrical signal)

what is signal transduction (3)

• an incoming signal (from intercellular messengers) are transmitted through a cell membrane in to the cell’s interior to produce a cellular response

• The multiple steps of a second messenger system have a cascading, or multiplying effect, which greatly amplifies the original signal

• The amplitude of the output is greater than the input

what are the 2 responses caused by the binding of the extracellular messenger (1st messenger) to its receptors:

• FAST - opening or closing ion channels, causing EPSPs or IPSPs

• SLOW - activating a second messenger system

what are the 2 types of hormones

• Hydrophilic hormones - not soluble in lipids / soluble in water

• Lipophilic hormones - are soluble in lipids / insoluble in water

• each hormone type reacts with its target cell in different ways

what is the location of the hormones receptor determined by

by the hormones lipid solubility

what type of receptor does a lipophilic hormone have and why

• intracellular receptors

• can easily pass through the (lipid soluble) cell membrane - small and non polar

explain the action of lipophilic hormones and duration and time

• binds to an activated/receiving receptor inside the cell

• these molecules pass directly into nucleus and activate specific genes to cause the formation of new intracellular proteins - has an effect on gene transcription and protein synthesis in the cell

• Time to response is several hours, i.e. time taken to synthesize new protein

• Duration of response, after hormonal trigger, is depend on how quickly protein is degraded

give 2 examples of lipophilic hormones and where they are derived from

• Thyroid hormone, an amine hormone derived from the amino acid tyrosine

• Steroid hormones, oestrogen and testosterone, derived from cholesterol

what type of receptors do hydrophilic hormones bind to and why

• cell surface receptors - G protein coupled receptors

• as cannot pass through the cell surface membrane - as lipid insoluble and water soluble - polar

explain the action of hydrophilic hormones, duration and time (4)

• Combine with a protein receptor on outside of membrane & activate a second messenger system

• altering the activity of pre-existing intracellular proteins, usually enzymes

• Time to response is a few minutes, i.e. time taken to activate & release the enzyme

• Duration of response, after hormonal trigger, depends on how quickly the enzyme is inactivated

give 2 examples of hydrophilic hormones and where they are derived from

• Insulin (a peptide hormone)

• Catecholamines, noradrenalin (NA) and adrenalin (Adr), amine hormones derived from the amino acid tyrosine

Give 3 examples each of peptide hormones and steroid hormones

• peptide: insulin, glucagon, thyroid stimulating hormone (TSH)

• steroid: testosterone, oestrogen and cortisol

what are the 2 different ways by which a hormone can bring
about a response in its target cell:

• Direct gene activation of steroid hormones - lipid soluble hormone

• Activation of a G-protein coupled receptor - hydrophilic hormone

• remember this is dependent on the lipid solubility of the hormone

explain the action of G protein coupled receptors (4)

• These receptors have GTPases enzymic activity

• They bind to the nucleotide guanosine triphosphate (GTP) and hydrolyze it to guanosine diphosphate (GDP) - GTP to GDP

• They are involved in transmitting signals from a variety of stimuli outside a cell to its interior

• When they are bound to GTP, they are 'on', and, when
  they are bound to GDP, they are 'off'.

what are the 2 G protein coupled receptor pathways: second messenger systems

• Cyclic AMP second messenger system – involving the activation of a G- protein

• Intracellular Ca2+ second messenger system – also involving the activation of a G-protein

explain the major components of the cyclic AMP second messenger system (4) including diagram

• hydrophilic hormone (this is the first messenger) binds to the G protein coupled receptor

• = activates a cascade pathway that leads to activation of intracellular proteins, enzymes (protein kinase A)

• = activates many different cellular responses  

• parts of that cascade pathway: GDP - adenylate cyclase = activates cAMP - protein kinase A - addition of phosphates

explain the major components of the intracellular Ca2+ second messenger system (3) including diagram

• Hydrophilic hormone (1st messenger) binds to G protein activates it and gains GTPase activity

• = activates a cascade pathway that leads to a different enzyme being activated

• leads to rising levels of IP3 – which can act on intracellular stores of Ca2+ - causes Ca2+ channel to open and increase of Ca2+ in the cell

which of the membrane proteins in the 2 second messenger pathways acts as a signal transducer

the G protein - link between 1st messenger and 2nd messenger

Why are reactions initiated by second-messenger mechanisms called cascades?

• Each step in the pathway has a huge amplification effect and the number of product molecules increases dramatically at each step

• a single signal has a cascade effect.

Describe the events of direct gene activation of steroid hormones and give an example (4)

• steroid hormone binds to receptor chaperonin complex which moves it to the DNA

• steroid hormone is directing process of transcription in nucleus - makes mRNA

• mRNA exits nucleus and we have process pf translation and protein synthesis - forming new proteins

• An example is the binding of aldosterone to an intracellular receptor

What is the crucial difference between the signaling mechanism depicted in the direct activation of genes and the second messenger systems?

• In the direct activation of genes the hormone can enter the cell (because it is soluble in the cell membrane) and directly stimulates the desired effect by acting on DNA to cause transcription

• Whereas in the activation of the second messenger systems, the hormone cannot enter the cell because it is insoluble in the cell membrane

what is up-regulation in terms of receptors

Sometimes target cells can form more receptors in response to
rising blood levels of that specific hormone

what is down-regulation in terms of receptors

• Sometimes prolonged exposure to high concentrations of a
  hormone desensitizes the target cells so that the cell responds less vigorously

• This involves the loss of receptors

comparison of nervous and endocrine system - table - arrangement, types of chemical messenger, distance of action, speed, duration, specificity and major functions

• both major communication systems of the body