C2.1 Chemical signalling HL Flashcards
¨èExplain why cell signalling is important for organisms.
In order to communicate and coordinate responses.
What is the purpose of chemical signalling in humans?
In humans, chemical signalling helps in maintaining homeostasis, development, immune response, neural function and metabolic regulation.
State the name of the molecules where signalling molecules can bind.
Receptors
State an alternative name for a signalling molecule.
Ligand
Name the step that occurs after a a ligand has bound to a receptor, but before the response has been brought about.
Signal transduction pathway
Describe the difference between autocrine, paracrine and endocrine signalling.
Autocrine is when a cell signals to itself, paracrine is signalling by diffusion between two cells close to each other and endocrine is communication over a large distance.
State two examples of responses.
Transcription of DNA and opening of ligand-gated channels in neurones.
How do cells communicate with each other?
Cells communicate with each other by sending and receiving chemical signalling molecules called ligands. Typically, one cell produces chemical substances as messengers, and another one (often called the target cell) receives them using a receptor.
What are ligands?
A ligand is a chemical signalling molecule which selectively bind to a specific site on another molecules.
What are the examples of ligands?
They include hormones, neurotransmitters, cytokines and growth factors.
What are receptors?
A receptor is a protein with a site to which the signalling chemical can bind. The binding causes changes in the receptor which stimulates a response to the signal.
What is a ligand-binding site?
The site on a receptor to which the signalling chemical binds is its ligand-binding site
How do receptors and ligands bind?
-Weak interaction at distance
-Conformational change increases molecular complimentarity
-Bonding of molecules
What are the similarities between enzymes and receptors?
The selectivity or specificity of binding is similar to enzyme-substrate specificity in enzymes:
In both enzymes and receptors, binding of the ligand occurs at a specific site.
The shape and chemical properties of the ligand-binding site match those of the ligand, preventing other substances from binding.
Both enzymes and receptors are unchanged by the binding of a ligand, even if there are temporary changes to induce fit.
What are the differences between enzymes and receptors?
There are also key differences between enzymes and receptors:
When a substrate binds to the active site of an enzyme, the substrate changes. It is converted chemically into the product and released. Another substrate can then bind to the active site, and this cycle can repeat many times per second. Binding is very brief.
In contrast, a signalling chemical may remain bound to a receptor for a long time because the ligand-binding site does not act as a catalyst and does not convert the signal chemical into a product. The signalling chemical is eventually released unchanged.
What is chemical signalling?
Chemical signalling is a process by which cells, tissues and organisms communicate with each other through the use of signalling molecules.
What are the two different cell to cell interactions?
Cell-to-cell interactions can be either direct or indirect.
What is direct cell-to-cell interactions?
Direct interactions involve cell-to-cell contact.
What is indirect cell-to-cell interactions?
Indirect interactions occur through the secretion of molecules by one cell that are transported to the target cells.
What are the stages of chemical signalling?
- Synthesis and release of a ligand from a signalling cell.
- Transport and diffusion – ligands travel through the bloodstream or by diffusion through the extracellular fluid to reach the target cells.
- Receptor binding – the signalling molecules bind to specific receptors on the surface of the target cell (called cell surface receptors) or, in the case of intracellular signalling, within the cytoplasm or nucleus of the target cell. Receptors are specific to ligands. For example, hormones such as insulin and glucagon have specific receptors on cells to which they bind.
- Signal transduction – when the ligand binds to its receptor, it causes a conformational change in the receptor, initiating a cascade of biochemical reactions allowing it to bind to other molecules.
- Cellular response – the activated signalling pathways lead to specific cellular responses, such as changes in gene expression, activation or inhibition of enzymes, alteration of ion channel activity, or modulation of cellular metabolism.
- Signal termination – the signalling molecule is either degraded or removed from the extracellular space, and the receptor is inactivated, bringing the signal transduction process to an end.
What methods have been developed to asses whether a population is large enough for group activity?
Other methods have evolved to assess whether a population is large enough for a group activity, for example, quorum sensing.
What is quorum sensing based on?
This is based on intercellular communication and has been observed in a wide range of bacteria. A switch in activity or behaviour is triggered when the population density rises above a certain threshold. Bacteria have been found to have a quorum before an action (e.g. bioluminiscence, pathogenicity,…) can take place.
What is quorum sensing?
Quorum sensing is a form of cell signalling in bacteria based on the number of cells, which allows communication and a common decision on a specific action.
How is the quorum established?
-This quorum is perceived with the help of signalling molecules (autoinducers).
-These molecules diffuse freely between cells and bind to receptors in each cell.
-When there has been sufficient binding of the signalling molecules to receptors in a cell, gene expression is changed. This causes a switch in activities.
-At a low rate (due to the low number of cells) this will only result in individual behaviour to that chemical signal.
-As the population density rises, all cells receive more of the signalling chemical from other cells.
-At high concentrations (above a certain density), every cell in the population receives enough to cause the change in gene expression and the resulting switch in activity – they have sensed there is a quorum.
what happens with and increase of population density?
With an increase of population density all cells receive more signals from other molecules, resulting in an a group behaviour which is much more powerful than an individual action.
What do bacteria involved in quorum sensing release?
Bacteria involved in quorum sensing release small signalling molecules called autoinducers which diffuse and accumulate in their environment.
What is the role of autoinducers?
These autoinducers bind to regulators and induce or repress gene expression.
What do Gram-positive and Gram-negative bacteria use?
Gram-positive bacteria use processed oligopeptides while Gram-negative bacteria use acylated homoserine lactones (acyl-HSLs) to communicate.
What is an example of quorum sensing?
An example of quorum sensing with a distinct biological purpose is found in the Hawaiian bobtail squid, where the bacterium Vibrio fischeri displays an act of quorum sensing.
How does Vibrio fischeri use quorum sensing as a purpose?
In response to a rise in population density, V. fischeri releases N-acyl homoserine lactone (an autoinducer). It binds to regulators and induces the lux operon, allowing the bacteria to regulate its luminescence. The lux operon is a group of genes that encode regulatory proteins and the production of luminescent proteins. Luciferase produces light when it oxidises its substrate, luciferin. This is the light that can be seen during bioluminescence. The greater the concentration of autoinducer produced, the brighter the glow. The more bacteria that are present will also mean more autoinducer will be released and more glow. It works in a positive feedback system.
What is quorum sensing an example of?
Quorum sensing is an example of interaction, because signalling molecules pass from cell to cell.
What is the activity promoted by quorum sensing an example of?
The activities promoted by quorum sensing are examples of interdependence, because they are only effective if more than one cell participates.
What is an example of quorum sensing in bacteria?
For example, high densities of bacteria on teeth secrete glue-like chemicals onto the tooth surface. Bacteria adhere (stick) to these chemicals in a thin layer called a biofilm.
What is an example of quorum sensing with bacteria bioluminescence?
In other bacteria bioluminescence is only switched on when there is a high population density capable of producing bright light.
What is a method to overcome quorum sensing?
While quorum sensing is a promising tool in the degradation of industrial waste and other environmental contaminants, methods designed to overcome quorum sensing, known as quorum quenching, are of particular interest.
What are example of uses of quorum quenching?
In medicine, quorum quenching can be used in the treatment of bacterial infections without the use of antibiotics. In the food industry, N-acyl homoserine can be targeted toward bacteria that cause food spoilage and biofilm formation. There are also potential applications in the fields of bioelectricity generation and fermentation.
What are hormones?
Hormones are chemical signalling molecules produced in small amounts by endocrine glands in the body. They are secreted into blood capillaries and transported in the bloodstream.
What does the exocrine gland contain to aid in the secreted hormones?
Exocrine glands have a duct leading out of the organ to transport the secretion.
What is the bloodstreams relationship with hormones?
The bloodstream transports hormones to all parts of the body. However, they only have effects on target cells which have receptors for the hormone.
What is the role of hormones?
Hormones regulate activities of the target cells by promoting or inhibiting specific processes.
How long do hormones work after being secreted?
They can persist in the body for hours after being secreted so the activities of target cells can be affected for much longer than with nerve impulses.
Why is the transportation of hormones in the bloodstream advantageous?
Transport in the bloodstream means the secreting and target cells can be far apart and one hormone can have very widespread effects.
What are examples of hormones?
Insulin, thyroxin and testosterone are examples of hormones.
What are neurotransmitters?
Neurotransmitters (NTs) are chemicals that transmit signals across a 20-40nm wide synapse between two neurons in the nervous system.
What is a synapse?
A synapse is a junction between 2 neurons in the nervous system.
Where is neurotransmitters released?
The NT is released from the presynaptic neuron (when a nerve impulse reaches it), diffuses across the gap and binds to receptors in the plasma membrane of the postsynaptic neuron.
What are the two types of neurotransmitters and what do they do?
-Excitatory NTs stimulate nerve impulses.
-Inhibitory NTS have the opposite effect.
What is the influence of the binding of neurotransmitters?
The binding influences whether a nerve impulse is initiated in the postsynaptic neuron.
How long does it take for a neurotransmitter to be released?
This happens in a fraction of a second, so NTs convey their signal far more quickly than hormones.
Why are the neurotransmitters effects short-lived?
NTs are rapidly broken down in the synaptic gap or reabsorbed into the presynaptic neurone, so they only persist for a fraction of a second – their effects are short-lived.
How is neurotransmitters ensured they only affect on neuron?
Rapid removal of NTs from the synaptic gap ensures it only affects 1 specific postsynaptic neuron; it does not usually diffuse out of the synapse to have more widespread effects.
What is a cytokines?
Cytokines are small proteins which act as signalling chemicals.
What are three main types of hormone groups?
- Amines or amino acid-derived hormones
- Peptide hormones
- Steroids or lipid-derived hormones
What are the mechanism of action of cytokines?
Cytokines bind to the specific receptors on the membrane of target cells, triggering signal transduction pathways that ultimately alter gene expression in the target cells
How does local signalling occur?
Local signalling occurs by:
-one cell secreting molecules on nearby target cells (e.g. a growth factor) in the extracellular fluid, or
-synaptic signalling by neurons which release neurotransmitter into the synaptic gap or to stimulate a target cell.
What is an example of distance signalling?
Hormones are transported in the blood from the gland that produces them to all parts of the body. E.g. luteinising hormone (LH) is secreted by the pituitary gland in the brain and transported to the ovaries in females and testes in males.
