C2.1 Chemical signalling HL Flashcards
1
Q
¨èExplain why cell signalling is important for organisms.
A
In order to communicate and coordinate responses.
2
Q
What is the purpose of chemical signalling in humans?
A
In humans, chemical signalling helps in maintaining homeostasis, development, immune response, neural function and metabolic regulation.
3
Q
State the name of the molecules where signalling molecules can bind.
A
Receptors
4
Q
State an alternative name for a signalling molecule.
A
Ligand
5
Q
Name the step that occurs after a a ligand has bound to a receptor, but before the response has been brought about.
A
Signal transduction pathway
6
Q
Describe the difference between autocrine, paracrine and endocrine signalling.
A
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.
7
Q
State two examples of responses.
A
Transcription of DNA and opening of ligand-gated channels in neurones.
8
Q
How do cells communicate with each other?
A
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.
9
Q
What are ligands?
A
A ligand is a chemical signalling molecule which selectively bind to a specific site on another molecules.
10
Q
What are the examples of ligands?
A
They include hormones, neurotransmitters, cytokines and growth factors.
11
Q
What are receptors?
A
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.
12
Q
What is a ligand-binding site?
A
The site on a receptor to which the signalling chemical binds is its ligand-binding site
13
Q
How do receptors and ligands bind?
A
-Weak interaction at distance
-Conformational change increases molecular complimentarity
-Bonding of molecules
14
Q
What are the similarities between enzymes and receptors?
A
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.
15
Q
What are the differences between enzymes and receptors?
A
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.
16
Q
What is chemical signalling?
A
Chemical signalling is a process by which cells, tissues and organisms communicate with each other through the use of signalling molecules.
17
Q
What are the two different cell to cell interactions?
A
Cell-to-cell interactions can be either direct or indirect.
18
Q
What is direct cell-to-cell interactions?
A
Direct interactions involve cell-to-cell contact.
19
Q
What is indirect cell-to-cell interactions?
A
Indirect interactions occur through the secretion of molecules by one cell that are transported to the target cells.
20
Q
What are the stages of chemical signalling?
A
- 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.
21
Q
What methods have been developed to asses whether a population is large enough for group activity?
A
Other methods have evolved to assess whether a population is large enough for a group activity, for example, quorum sensing.
22
Q
What is quorum sensing based on?
A
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.
23
Q
What is quorum sensing?
A
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.
24
Q
How is the quorum established?
A
-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.
25
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.
26
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.
27
What is the role of autoinducers?
These autoinducers bind to regulators and induce or repress gene expression.
28
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.
29
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.
30
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.
31
What is quorum sensing an example of?
Quorum sensing is an example of interaction, because signalling molecules pass from cell to cell.
32
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.
33
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.
34
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.
35
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.
36
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.
37
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.
38
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.
39
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.
40
What is the role of hormones?
Hormones regulate activities of the target cells by promoting or inhibiting specific processes.
41
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.
42
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.
43
What are examples of hormones?
Insulin, thyroxin and testosterone are examples of hormones.
44
What are neurotransmitters?
Neurotransmitters (NTs) are chemicals that transmit signals across a 20-40nm wide synapse between two neurons in the nervous system.
45
What is a synapse?
A synapse is a junction between 2 neurons in the nervous system.
46
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.
47
What are the two types of neurotransmitters and what do they do?
-Excitatory NTs stimulate nerve impulses.
-Inhibitory NTS have the opposite effect.
48
What is the influence of the binding of neurotransmitters?
The binding influences whether a nerve impulse is initiated in the postsynaptic neuron.
49
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.
50
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.
51
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.
52
What is a cytokines?
Cytokines are small proteins which act as signalling chemicals.
53
Which cells secrete cytokines?
The same cytokine may be secreted by many different types of cells and one cell may secrete several different cytokines. Certain cytokines can be secreted by almost all cells in the body. They are mainly secreted by white blood cells such as macrophages and lymphocytes.
54
What is the distance that cytokines are transported?
Cytokines are not usually transported as far as hormones. Instead, they act either on the cell that produced them or on a nearby cell.
55
How to cytokines react with target cells?
Cytokines cannot enter cells, so they bind to receptors in the plasma membrane of a target cell.
56
What is the effect of cytokines binding to target cell?
This binding causes cascades of signalling inside the target cell, leading to changes in gene expression and thus in cell activity.
57
Can cytokines can bind to several types of receptors?
One type of cytokine can bind to several types of receptor and so have several effects.
58
What is the role of cytokines in inflammation?
Cytokines have cell signalling roles in inflammation and in other responses of the immune system. They also have roles in the control of cell growth and proliferation and in the development of embryos.
59
What are examples of cytokines?
Examples include erythropoietin (EPO), interferon and interleukin.
60
How calcium ions use in cell signalling?
Calcium ions are used for cell signalling in both muscle fibres and neurons.
61
How are calcium ions used in muscle fibres?
In muscle fibres, calcium ions are pumped into a specialised form of endoplasmic reticulum called the sarcoplasmic reticulum, generating a high concentration.
62
What happens when muscle fibres receive nerve impulse?
When the muscle fibre receives a nervous impulse, calcium channels open in the membrane of the SR and the ions can diffuse out.
63
Where do the calcium ions bind in muscle fibres?
They bind to proteins (troponin) that block muscle contraction, causing the proteins to change position which allows the muscle contraction to occur.
64
What happens if muscle fibres do not receive multiple impulses?
If the muscle fibre does not receive more impulses, these changes are reversed and the calcium is pumped back into the SR.
65
How are calcium ions used in nerve impulses?
In neurons, the arrival of a nerve impulse at a presynaptic membrane causes calcium ion channels to open, allowing inward diffusion. Inside the presynaptic neuron, calcium ions cause secretion of NT into the synaptic gap by exocytosis. The calcium ions are rapidly pumped back out into the synaptic cleft.
66
What are three main types of hormone groups?
- Amines or amino acid-derived hormones
- Peptide hormones
- Steroids or lipid-derived hormones
67
What are amines/amino acid-derived hormones?
These are small molecules derived from the amino acids tyrosine and tryptophan. For example, epinephrine (adrenaline) and norepinephrine (noradrenaline) secreted by the medulla of the adrenal gland, and thyroxin released by the thyroid gland. Another amino acid-derived hormone is melatonin, secreted by the pineal gland situated in the brain. It helps to maintain the circadian rhythm. These are water-soluble hormones.
68
What are peptide hormones?
These are in the form of polypeptide chains, small proteins, glycoproteins, etc. For example, insulin secreted by the pancreas in response to blood glucose level. It promotes the uptake and metabolism of glucose. Oxytocin, follicle-stimulating hormone and growth hormone are also examples of peptide hormones.
69
What are the similarities between peptide and amino-acid derived hormones?
Both peptide and amino acid-derived hormones are water-soluble and therefore cannot pass through the plasma membrane on their own. They require specific receptors on the surface of the target cells.
70
What are steroids or lipid-derived hormones?
These are derived from cholesterol (parent molecule). For example, oestradiol released by female reproductive organs and testosterone released by male reproductive organs. Other examples include cortisol and aldosterone released by the cortex of the adrenal gland. Steroid hormones are insoluble in water and thus, require carrier proteins to be transported via blood. They remain in circulation for a longer duration. They are lipid-soluble hormones.
71
What is epinephrine other name? And what is special about it's function?
Epinephrine is also called adrenaline because it is secreted from the medulla of the adrenal glands situated on top of the kidneys. It has a dual function, as a hormone and as a neurotransmitter.
72
Why do hormones act in different ways?
Hormones act in different ways depending on the location of the receptors.
73
What happens when hormones bind to the receptor?
When hormones bind to the receptors of target cells, they can induce different cellular changes. These range from changes in plasma membrane permeability, activation of protein synthesis, activation or deactivation of enzyme systems to promoting mitosis.
74
What are the two ways that hormones can act?
They can activate second messengers, which either activate or deactivate enzymes in the cells. They can also activate genes and cause them to be expressed or switched off.
75
What are the characteristics of hormones?
Hormones can be small, non-polar, hydrophobic molecules that diffuse through the cell membrane to reach receptors in the nucleus or cytoplasm, such as testosterone or progesterone. Hormones can also be water-soluble molecules that bind to receptors in the plasma membrane, such as insulin or glucagon, or epinephrine (adrenaline).
76
Where are neurotransmitters synthesised?
Neurotransmitters are synthesised in the neurons and stored in thin-walled sacs called synaptic vesicles.
77
Where are neurotransmitters released? And why?
They are released into the synaptic cleft in response to an action potential.
78
What happens when a neurotransmitter is in the synaptic cleft? And why is it important?
Once in the synaptic cleft, the neurotransmitter binds to a specific receptor on the postsynaptic membrane, causing a change in the electrical potential of the neuron and the transmission of the signal. These messages help us to move our limbs, feel sensations, keep our heart beating, and respond to all information our body receives from other internal parts of the body and our environment.
79
What are the different types of neurotransmitters?
-amino acids
-peptides
-amines
-nitrous oxide
80
What are examples of amino acid neurotransmitters?
Glycine, glutamate and GABA (Gamma-aminobutyric acid) belong to the amino acid neurotransmitter class. They are involved in fast synaptic transmission.
81
What are examples of peptide neurotransmitters?
Neuropeptide Y is an example of peptide neurotransmitter. It is responsible for a number of physiological and homeostatic processes. It increases the motivation to eat food.
82
What are examples of amines neurotransmitters?
Biogenic amines are modified amino acids. For example, serotonin regulates the mood while dopamine is involved in reward and movement regulation in the brain, and norepinephrine (noradrenaline) controls the fight or flight response.
83
How does nitrous oxide acts as a neurotransmitters?
Nitrous oxide acts as a modulator of neuronal function.
84
What are the mechanism of action of neurotransmitters?
Neurotransmitters are stored in vesicles in the neuron. When a neuron gets stimulated, these vesicles fuse with the plasma membrane and release the neurotransmitters into the synaptic cleft. They diffuse across the synapse and bind to the receptors on the target cells.
85
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
86
What are the mechanism of action of calcium ions?
Ca2+ can also act as a second messenger in the signal transduction pathway. Physiological processes like muscle contraction, nerve impulses and fertilisation among others, use calcium signalling. High levels of cytoplasmic Ca2+ can also cause the cell to undergo apoptosis. Other biochemical roles of calcium include regulating enzyme activity, permeability of ion channels, activity of ion pumps, and components of the cytoskeleton.
87
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.
88
What is an example of local signalling?
Neurotransmitters diffusing across the synapse (a gap of only 20 – 40nm) are an example of localized effects of signalling molecules.
89
What does distance signalling occur?
Some signalling molecules are transported long distances in the body from the cells that secrete them to the target cells.
90
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.
91
How does a signalling molecule enter the target cell?
Depending on whether signalling molecules can enter the target cell or not, they can bind either to receptors which are embedded within the plasma membrane (transmembrane receptors) – these have a band of hydrophobic amino acids on their surface that is attracted to the apolar tails of phospholipids in the core of the membrane. On either side of this band, there are hydrophilic amino acids which are in contact with aqueous solutions inside and outside the cell.
receptors which are found inside the cytoplasm or nucleus (intracellular receptors) – these have hydrophilic amino acids so they remain dissolved in the aqueous fluids of the cytoplasm or nucleus.
92
How do signalling molecules enter through transmembrane receptors?
Signalling molecule is hydrophilic and/or polar and therefore cannot enter the cell membrane
93
How do signalling molecules enter through intracellular receptors?
Signalling molecule is hydrophobic and/or non-polar and therefore can enter the cell.
94
What does the binding of a signalling chemical cause?
Binding of a signalling chemical to a receptor causes a sequence of interactions in the cell, called a signal transduction pathway.
These pathways are very varied as they have evolved repeatedly, rather than having a common origin.
95
What happen when a ligand bind to a receptor through transmembrane receptors?
In many cases ligands (e.g. peptide hormones such as insulin) bind to a receptor in the membrane.
This causes a change in shape/structure of the receptor.
The inner side of the receptor becomes catalytically active and causes production of a secondary messenger within the cell.
This conveys the signal to effectors within the cell that carry out the responses.
96
What are the features of a ligand binding site for transmembrane receptors?
The ligand binding site of the integral membrane protein is hydrophilic (which is attracted to the polar tails of phospholipids in the core of the membrane) while the parts that traverse the membrane are hydrophobic so that the molecules can be anchored. The areas near the phosphate heads of the protein are hydrophilic again.
97
What happen when a ligand bind to a receptor through intracellular receptors?
Intracellular receptors such as steroid receptors are present as soluble proteins in the cytoplasm.
Steroid hormones are lipid soluble, so they diffuse from the blood stream across the plasma membrane and then bind to the receptor molecule forming an active ligand-receptor complex.
In most cases this complex regulates gene expression by binding to DNA at specific sites, promoting or inhibiting transcription of particular genes.
98
What are characteristics of intracellular receptors?
Intracellular receptors have hydrophilic amino acids so they remain dissolved in the aquaeous fluids of the cytpoplasm or nucleus.
99
What is a signal transduction pathway?
When a signalling chemical binds to a receptor, a sequence of interactions in the cell is triggered. This is called a signal transduction pathway.
100
What is the response of signal transduction pathways?
-Regulation of protein activity (e.g. by opening or closing an ion channel)
-Regulation of protein synthesis through gene expression
-Regulation of enzyme activity
-Rearrangement of the cytoskeleton of the cell
-Death of the cell (apoptosis)
101
What are the different types of transmembrane receptors?
-For neurotransmitters and changes to membrane potential
-That activate G protein
-Mechanism of action of epinephrine (adrenaline) receptors
-Transmembrane receptors with tyrosine kinase activity
-Intracellular receptors that affect gene expression
102
How do neurotransmitters transmembrane receptors function?
-NTs convey signals between neurons, and between neurons and muscles.
-NTs are released into the synaptic gap and diffuse to the membrane of the post-synaptic neuron or muscle fibre.
-There they bind to receptors in these membranes.
-The receptors are transmembrane proteins and are ligand-gated.
-Binding causes membrane channels to open and ions move through these channels by facilitated diffusion, changing the membrane potential.
-This change in potential is a signal that stimulates or inhibits either a nerve impulse in a postsynaptic neuron, or a contraction in a muscle fibre.
103
What is an example of neurotransmitter transmembrane receptor?
-Acetylcholine is used as a NT in many synapses, including those between neurons and muscle fibres.
-When acetylcholine binds to the binding site on an acetylcholine receptor, the conformation (shape) of the receptor changes.
-A channel opens, allowing sodium ions to pass into the cell (they flow down their concentration gradient).
-This leads to a local depolarisation that changes the membrane potential which triggers an action potential
104
What G proteins?
G proteins are membrane bound protein receptors which bind to GTP, usually activated by the binding of a hormone or other ligand.
105
What is GTP?
GTP is an energy-rich nucleotide similar to ATP.
106
What happens after the activation of the G protein?
After activation of the G protein a number of signals can be transmitted.
107
What are the characteristics of a G protein?
G protein coupled receptors have multiple helices that span the plasma membrane and that form hydrophobic interactions with the core of the plasma membrane. G proteins bind to the third and largest cytoplasmic loop of the receptor.
108
How is the G protein activated?
The G protein acts as an on/off switch.
If GDP is bound to the G protein, the G protein is inactive.
When activated, the G protein dissociates from the receptor and binds to an enzyme or other protein, activating it.
This leads to a signal transduction and cellular response.
109
What happens when the ligand binds to the G protein?
When a ligand binds to the binding site on the receptor, the receptor changes shape, and induces a shape change also in the coupled G protein.
110
What is the G protein composed of? and how does it link with its activation?
The G protein is composed of 3 subunits (𝛂, 𝛃 and 𝛄).
The GDP detaches from the 𝛂 subunit.
This gives place to GTP, which results in the activation of the G protein.
111
What is the purpose/importance of the G protein?
A broad range of receptor functions are mediated by G-protein-coupled receptors and their associated G proteins.
The ligands that bind to these receptors are diverse and include light-sensitive compounds, odours, pheromones, hormones and neurotransmitters.
112
What is epinephrine (adrenaline)?
Epinephrine (Adrenaline) is a hormone released from adrenal glands.
It circulates in the blood stream and binds to a class of G protein receptors (adrenergic receptors).
It acts as a peptide hormone which upon binding to the receptor activates a cascade of reactions mediated by a secondary messenger to amplify the strength of the signal.
113
What are the effects of epinephrine (adrenaline)?
- Preparation for vigorous activity
- Increase in heart- and breathing rate
- Enables increased delivery of oxygen and glucose to muscle cells.
- Vasoconstriction of blood vessels
114
What is the mechanism of epinephrine (adrenaline)?
- Epinephrine binds to a transmembrane receptor in the plasma membrane of a target cell.
- This changes the shape of the receptor, activating G protein within the membrane.
- Activated G protein activates the enzyme adenylyl cyclase in the membrane and this converts ATP in the cytoplasm into many cyclic AMP (cAMP).
cAMP is the most common second(ary) messenger used in cells.
- Secondary messengers start a sequence of responses within the cell, amplifying the signal until a large-scale process is triggered.
- This happens very rapidly e.g. liver cells break down glycogen and release glucose into the blood within seconds of receiving the epinephrine signal.
115
What does the activation of the adenylyl result in?
- The activation of the adenylyl cyclase results in a boost in the production of the secondary messenger cAMP.
- This in the end leads to the activation of an enzyme (glycogen phosphorylase) which breaks down glycogen.
- With more glucose around as a respiratory substrate muscle activity increases.
116
What is kinase?
A kinase is an enzyme that adds a phosphate group from ATP to a specific molecule to phosphorylate it.
117
What are the characteristics of kinase?
Kinases are inactively bound to transmembrane receptors. Upon binding they become active and can phosphorylate other molecules and trigger a chain of reactions.
118
What is an example of kinase?
e.g. the enzyme tyrosine kinase transfers phosphate from ATP to the amino acid tyrosine in a protein.
119
What are tyrosine kinases?
Some transmembrane receptors are composed of two protein tails extend into the cytoplasm which are tyrosine kinases.
120
What are once a signal molecule binds a tyrosine kinase?
Upon binding of a signalling molecule e.g. insulin the kinases move together to form a dimer, and attach phosphate groups to the tyrosine parts of the other tail. The phosphorylation triggers a chain of reactions – typically multiple of them (signal transduction).
121
What the mechanism for the insulin receptor and tyrosine kinase?
- Upon binding of insulin the kinases move together to form a dimer, which then is phosphorylated.
- The phosphorylation triggers a chain of events.
- Vesicles containing glucose transporters move to the plasma membrane and fuse with it, inserting transporters into the membrane. These transporters are channel proteins that allow glucose to enter the cell by facilitated diffusion. The glucose can then be used as a substrate in cell respiration.
122
What are the properties of intracellular receptors that affects gene expression?
Molecules which are hydrophobic are lipid-soluble and can therefore simply diffuse through the plasma membrane (e.g. steroid hormones). Once inside the cell they can bind to an intracellular receptor and move to the nucleus where they affect gene expression by regulating transcription.
123
What is an example of an intracellular receptor that affects gene expression?
e.g. the androgen receptor binds testosterone and the resulting complex increases production of the FADS1 gene. This in turn increases production of important fats in prostate cells.
124
What are oestradiol and progesterone?
The hormones oestradiol and progesterone are involved in reproduction. They are steroid hormones and therefore lipid soluble, so they can pass through the plasma membrane of target cells.
125
What happens once a a receptor binds within the cytoplasm?
Once bound to a receptor within the cytoplasm the hormone-receptor complex moves to the nucleus where it acts as a transcription factor enhancing the transcription of specific proteins.
126
What is the purpose/function of oestradiol?
- Oestradiol has a broad range of effects in the ovary and the uterus.
- It also acts on the brain, helping to regulate the release of reproductive hormones.
- Within the hypothalamus of the brain, the hormone gonadotropin-releasing hormone (GnRH) is produced and released. This hormone triggers the release of the sex hormones luteinising hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary.
- At different stages of the human menstrual cycle, oestradiol can either inhibit or promote the release of GnRH by the hypothalamus.
- Just before and during ovulation, oestradiol has a stimulating effect by binding to a receptor within the cytoplasm of the hypothalamus cell. Once bound, the hormone-receptor complex moves to the nucleus where it acts as a transcription factor, enhancing the transcription of GnRH mRNA.
127
What is the purpose/function of progesterone?
- The hormone progesterone is produced by the ovary and maintains the uterus lining so that it can support a developing follicle.
- Progesterone is a steroid hormone, able to diffuse through the plasma membrane of uterine cells and bind to a receptor in the cytoplasm.
- The hormone-receptor complex then enters the nucleus where it interacts with the DNA as a transcription factor. This affects gene expression.
- For example, one of the genes activated is insulin-like growth factor which contributes to the cellular proliferation necessary for maintaining the lining of the endometrium.
128
How can cell signalling be related to positive feedback loops?
Cell signalling can be positive, resulting in the end product amplifying the starting point so that more product is created.
129
How can cell signalling be related to negative feedback loops?
Cell signalling can also be negative, resulting in an increase of the end product shutting off the start of the signalling pathway so that less is produced.
130
How is Ip3 and calcium stores of calcium a positive feedback?
E.g. in muscle, the ER stores calcium.
Inositol triphosphate (IP3) is a (secondary) signalling molecule required in many calcium dependent processes.
- The binding of IP3 to an IP3 receptor causes a partial release of Ca2+ ions from the ER.
- The release of Ca2+ ions activates more IP3 receptors, which consequently releases more ions.
- This process is known as calcium-induced calcium release.
131
How is fruit ripening a positive feedback?
- Fruit ripening results in the production of the plant hormone ethylene.
- Ethylene triggers a signal cascade and causes nearby fruits to also start ripening.
132
How is insulin a negative feedback?
- The use of insulin in the control of the blood sugar level is an example of negative feedback.
- Binding of insulin to a transmembrane receptor leads to a signal cascade, which results in the expression of GLUT-4 membrane protein containing vesicles.
-The membrane proteins are embedded within the membrane and allow absorption of glucose. The decreased concentration of glucose in the blood inhibits insulin production.
