C2.1 Chemical Signalling Flashcards
[C2.1.1] Define ligand, and outline how a ligand binds to a receptor protein
Cells interact with each other by sending and receiving signals by using chemical substances. Ligand is a broad term for signalling chemicals that bind to a specific ligand-binding site on protein receptors. The binding changes shape and chemical properties of the receptor. Stimulating a response to the signal by the target cell.
[C2.1.1] Distinguish between an enzyme and a receptor
The key difference between enzymes and receptors are that enzymes catalyses substrates into its products while ligands bind to receptors to convey message to a cell, then ligand is released.
[C2.1.2] Describe quorum sensing by using an example
Quorum sensing is used by bacteria, where a specific activity or behaviour is triggered when the population density rises above a certain threshold. For example, marine bacteria called Vibrio fischeri uses quorum sensing. When population density rises, more signalling molecules are secreted, all binding to receptors in each cell, which triggers a change in gene expression that allow bioluminescence.
[C2.1.3] Describe the 4 types of signalling chemicals with examples
- Hormones
○ Produced by glands (specialised organs for secretion) and released into the blood capillaries
○ Transported via the bloodstream but only targets specific cells which have receptors for the specific hormone
○ Regulates activities of the target cell, and the effect is long lasting
○ Ex. Insulin, testosterone- Neurotransmitters
○ Transmit signals across synapses
○ Secreted when a nerve impulse reaches the end of pre-synaptic neuron, then binds to receptors on the plasma membrane on post-synaptic neuron to continue nerve impulse
○ Broken down and reabsorbed by presynaptic neuron rapidly, thus effect is short lived
○ Ex. Acetylcholine, dopamine - Cytokines
○ Small proteins secreted by many different cell types, and plays role in inflammation or other responses of the immune system, controlling cell growth, proliferation and embryo development
○ Is not transported as far as hormones, but instead binds to receptors on the plasma membrane of nearby cells to change gene expression and thus, cell activity
○ 1 type of cytokine can bind to multiple receptors
○ Ex. EPO - Calcium ions
○ Used for cell signalling in both muscle fibres and neurons
○ When muscle fibre receives nerve impulse, calcium bind to proteins that block muscle contraction, which changes conformation of this protein, hence allowing muscles to contract
○ In neurons, when the nerve impulse reaches pre-synaptic neuron, this causes calcium ions to diffuse into the cell, causing neurotransmitters to be secreted into synapse via exocytosis
- Neurotransmitters
[C2.1.4] Outline the 2 characteristics that all signalling chemicals must have
Have a distinctive shape and chemical properties so the receptor can distinguish the different chemicals
Be small and soluble enough to be transported
[C2.1.4] Provide examples of 3 types of hormones and 4 types of neurotransmitters
Hormones:
Amines - Melatonin, thyroxin, epinephrine
Peptides - Insulin, glucagon, ADH
Steroids - Oestradiol, Progesterone, Testosterone
Neurotransmitters:
Amines - Dopamine
Gases - Nitrous oxide
Amino acids - Glutamate
Esters - Acetylcholine
[C2.1.5] Distinguish between the effects and distance travelled by neurotransmitters and hormones
Neurotransmitters only travel a short distance from pre-synaptic neuron to post-synaptic neuron, thus having localised effects.
In contrast, hormones travel long distances via bloodstream from the glands to the target cells located anywhere around the body, thus having distant effects.
[C2.1.6] Distinguish between transmembrane and intracellular receptors
Transmembrane receptors: on the plasma membrane of the target cells, thus, signalling chemicals that do not enter the cell can bind on the sites exposed to the exterior
Intracellular receptors: in the cytoplasm or nucleus of the target cell since they are hydrophilic, thus signalling chemicals that enter the cell can bind
[C2.1.7/ C2.1.12] Describe the signal transduction pathway of transmembrane and intracellular receptors, by providing a definition of signal transduction pathway
Hydrophobic signalling chemicals like proteins, bind to transmembrane receptors, while hydrophilic chemicals like steroids, bind to intracellular receptors. Binding of these chemicals causes a sequence of interactions in the cell, called a signal transduction pathway, and the pathway can vary depending on the type of receptor.
Intracellular: ligand binds to receptor in cytoplasm and the complex act as a transcription factor to regulate gene expression. The complex enters the nucleus and binds to a speific site on DNA to inhibit or promote transcription
Transmembrane: Ligand binds to receptor on the plasma membrane, which changes the structure of the receptor to trigger a secondary messenger that results in a cascade of reactions
[C2.1.8/ C2.1.9/ C2.1.10/ C2.1.11] Describe the 4 occasions when transmembrane receptors are used
- Neurotransmitters
○ Acetylcholine is used to trigger action potential- G-protein coupled receptors (GPCR)
○ GPCRs are a large, diverse group of transmembrane receptors that convey signals into cells using a G protein (composed of α, β and γ subunits)
○ GPCR is maintained inactive when Guanosine Diphosphate (GDP) is bound to the G-protein
○ When a ligand binds to the transmembrane receptor, GDP is replaced by GTP, activating GPCR. This results in the subunits to separate into α & GTP and β & γ
○ The 2 groups act as secondary messengers to independently convey signals to effectors in the cell to trigger a response. - Epinephrine
○ Epinephrine binds to receptors on target cells which results in a change in shape and activates G-protein
○ This activated G-protein activates the enzyme that converts ATP in cytoplasm into cyclic AMP (cAMP)
○ cAMP is used as a secondary messenger that triggers a cascade of reaction:
○ cAMP activates protein kinase → protein kinase activates phosphorylase kinase → phosphorylase kinase activates glycogen phosphorylase → glycogen phosphorylase converts glycogen to glucose - Tyrosine kinase activity
○ Kinase is an enzyme that adds a phosphate group from ATP to a specific molecule through phosphorylation
○ Ex. Tyrosine kinase transfers phosphate from ATP to the amino acid tyrosine
○ Phosphorylation of tyrosine is triggered by the binding of insulin
- G-protein coupled receptors (GPCR)
[C2.1.13/ C2.1.14] Describe the effects of oestradiol, progesterone and testosterone on target cells and outline how cell signalling pathways can be regulated by positive and negative feedback
Females:
Oestradiol (either inhibit or promote GnRH) → hypothalamus secrete Gonadotropin-Releasing hormone (GnRH) → anterior pituitary secrete Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) that triggers ovulation→ FSH travel through bloodstream to ovaries → ovaries secrete oestradiol (therefore, increase in oestradiol, increae LH level - positive feedback) and progesterone (binds to receptors in endometrium cells (uterus lining) to affect various gene expression such as thickening uterus lining and cell proliferation)
Males:
Hypothalamus secrete Gonadotropin-Releasing hormone (GnRH) → anterior pituitary secrete Luteinizing Hormone (LH) → leydig cells in testes secrete testosterone (as testosterone increases, this signals LH to decrease and GnRH to stop - negative feedback)
