UNIT 1 - KA4 Flashcards
Where do multicellular organisms signal between
Multicellular organisms signal between cells using extracellular signalling molecules
What are examples of extra cellular signalling molecules
Steroid hormones, peptide hormones, and neurotransmitters are examples of
extracellular signalling molecules.
What are receptor molecules of target cells
Receptor molecules of target cells are proteins with a binding site for a specific signal molecule
What effect does binding have on the receptor
Binding changes the conformation of the receptor, which initiates a response within the cell
What do different cell types produce
Different cell types produce specific signals that can only be detected and responded to by cells with the specific receptor
What effects may signalling molecules have on different target cell types
Signalling molecules may have different effects on different target cell types due to
differences in the intracellular signalling molecules and pathways that are involved.
What may different cell types show to the same signal
In a multicellular organism, different cell types may show a tissue-specific response to
the same signal
What is a tissue specific response to a signal molecule
With a multicellular organism the response may be tissue specific (ie the same signal molecule may bring about a different response depending on the cell type it binds)
When does signal transduction occur
Occurs when an extracellular signalling molecule (e.g hormone) activates a specific receptor located on the cell surface or inside the cell. The receptor triggers a chain of events inside the cell, creating a response
What do hydrophobic signals bind to and why
Hydrophobic signals can diffuse directly through the phospholipid bilayer of membranes. They therefore bind to intra cellular receptors
Explain why hydrophobic signals are capable of diffusing through the plasma membrane (2)
- Hydrophobic signals can diffuse directly through the phospholipid bilayer of membranes. They therefore bind to intra cellular receptors
- they can do this because the tails of the phospholipids in the plasma membrane are also hydrophobic and allow the molecules to pass across
What are the receptors for hydrophobic signalling molecules
The receptors for hydrophobic signalling molecules are transcription factors
Describe what transcription factors are and their function
Transcription factors are proteins that when bound to DNA can either stimulate or inhibit initiation of transcription
What can transcription factors do to binding of RNA polymerase
Transcription factors can enhance or block the binding of RNA polymerase to specific genes. Thereby controlling whether the gene is transcribed and therefore expressed
Examples of hydrophobic signals are
- the thyroid hormone thyroxine
- steroid hormones such as oestrogen for testosterone
What are receptor proteins for steroid hormones
The receptor proteins for steroid hormones are transcription factors
What do steroid hormones form
Steroid hormones bind to specific receptors forming a hormone receptor complex
Where are the receptors for steroid hormones
Steroid hormones bind to specific receptors in the cytosol or in the nucleus. The hormone receptor complex moves to the nucleus where it binds to specific sites on DNA
What is a hormone response element (HRE)?
The hormone receptor complex moved to the nucleus where it binds to specific sites on DNA. The specific DNA sequences that the hormone receptor complex binds to are called hormone response elements
What does binding at the HRE influence the rate of
Binding at these sites influence the rare of transcription, with each steroid hormone affecting the gene expression of many different genes
Summary of the pathway of steroids (4)
- Steroid hormone passes through the plasma membrane into the nucleus
- the bromine binds to the receptor (transcription factor) forming a hormone- receptor complex
- the hormone - receptor complex binds to specific sites on DNA called hormone response elements (HREs)
- rate of transcription and gene expression is affected
What is thyroxine
Thyroxine is a hydrophobic hormone produced by the thyroid gland. It is involved in regulating the Rate of metabolism.
As thyroxine is small and hydrophobic it is capable of diffusing through the phospholipid bilayer and binding an intracellular receptor
What happens when thyroxine is not present
When thyroxine is not present it’s receptor protein is found in the nucleus bound to DNA. It sits on the DNA and inhibits the transcription of the gene for Na/K-ATPase (sodium potassium pump)
Explain how the presence of thyroxine leads to the transcription of Na/K-ATPase (sodium - potassium pump)(2)
- when thyroxine binds it causes a conformational change in the receptor. This means the receptor protein can no longer bind the DNA and the gene is transcribed
- binding of thyroxine to its receptor leads to transcription of the gene for the sodium -potassium pump. This increases metabolic rate.
Thyroxine is NOT present summary
- Thyroxine is not present
- thyroid hormone receptor protein binds to DNA
- transcription of Na/K-ATPase gene is inhibited
- less Na/K-ATPase is transcribed
- metabolic rate is reduced
Thyroxine is present summary
- Thyroxine is present
- thyroxine binds to thyroid hormone receptor
- receptor undergoes a conformational change meaning it can no longer bind to DNA
- gene for Na/K-ATPase is transcribed
- metabolic rate increases
Where do hydrophilic signalling molecules bind
Hydrophilic signalling molecules bind to transmembrane receptors and do not enter the cytosol
Examples of hydrophilic signals are
- peptide hormones (eg insulin and ADH)
- neurotransmitters (eg acetylcholine)
This is because they are not capable of passing through the hydrophobic plasma membrane
3 key steps of hydrophilic signalling
1 - reception - signalling molecule binds to transmembrane receptor
2 - transduction - signal is passed through the cell
3 - response - will vary depending on the signal
When do transmembrane receptors change confirmation (2)
- Transmembrane receptors change
conformation when the ligand binds to the
extracellular face; the signal molecule does
not enter the cell, but the signal is transduced
across the plasma membrane - transmembrane receptors act as signal transducers by converting the extra cellular ligand binding event into intracellular signals, which alters the behaviour of the cell
Transduced hydrophilic signals often involve
- G proteins
- Cascade of phosphorylation by kinase
Describe the role of G-proteins transduction of an extracellular signal
G-proteins relay signals from activated receptors (receptors that have bound a signalling molecule) to target proteins such as enzymes and ion channels. Activated enzymes will then catalyse reactions within the cell. Ion channels will then either open or close to control ion movement
Name the type of enzyme involved in the phosphorylation of a protein
Kinases
Describe the role of phosphorylation cascades in the transduction of an extracellular signal
Phosphorylation cascades involve a series of events with one kinase activating the next in the sequence and so on. Phosphorylation cascades can result in the phosphorylation of many proteins as a result of the original signalling event
How many pathways do phosphorylation cascades allow
Phosphorylation cascades allow more than one intracellular signalling pathway to be activated.
Glucagon and Insulin
The levels of glucose in the blood must be controlled. There are two hormones involved in this:
• insulin
• glucagon
What type of molecule is insulin?
insulin is a hydrophilic peptide hormone which promotes the conversion of excess glucose in the blood into glycogen for storage
Describe the role of insulin in the body.
A rise in blood sugar level is detected by cells in the pancreas (islets of langerhans)
which produce insulin. Insulin promotes the storage of excess glucose in various parts of the body such as the liver, fat tissue and muscle tissue
Name three types of tissue which are affected by the action of insulin
- liver
- fat tissue
- muscle tissue
How does glucose pass into cells
Glucose passes into cells by travelling through a transporter protein.
Name the type of transporter protein.
GLUT4
Name the type of transport bywhich glucose moves into cells.
facilitated diffusion
The process which occurs when insulin binds to its receptor is as follows:
- Insulin binds to its receptor
- Receptor undergoes a conformational change that trigger’s phosphorylation of the receptor
- À phosphorylation cascade is started inside the cell
- Vesicles containing GLUT 4 are transported to the cell membrane
- Glucose passes through the GLUT4 transporters
Which types of tissue use GLUT4 transporters in the human body
GLUT 4 is a glucose transporter found only in fat/ muscle cells. In the liver it is a different glucose transporter which is recruited to the cell surface (GLUT 2/ GLUT1)
Diabetes
if there is a failure in the process. the person suffers from diabetes mellitus
There are two types:
- Туре 1
- Type 2
Describe the cause of Type 1 diabetes and how this is treated.
Type 1 diabetes is caused by a failure to produce insulin in the pancreas.
It is Treated with regular insulin injections throughout the day
Describe the cause of Type 2 diabetes and how this is treated.
Type II is caused by loss of insulin receptor function This therefore means that recmitment of GLUT 4 to the membrane fails.
Why is exercise a form of treatment for Type 2 diabetes?
Type II diabetes is usually associated with obesity. Exercise also triggers the recruitment of GLUT4. so can improve uptake of glucose to fat and muscle cells in subjects with type 2 diabetes
What difference do all cells have
All cells have an electrical potential difference (voltage) across their plasma membrane
What is the membrane potential
This voltage is called the membrane potential. This is when there is a difference in electrical charge on the two sides of the membrane
What is the resting potential
The membrane potential of a neuron that is not transmitting signals is called the resting potential.
Describe the membrane potential of a neuron when at rest
The membrane potential of a neuron that is not transmitting signals is called the resting potential. Resting membrane potential is a state where there is no net flow of ions across the membrane
What is a nerve transmission
The transmission of a nerve impulse requires changes in the membrane potential of the neurons plasma membrane. Nerve transmission is a wave of depolarisation of the resting potential of a neuron.
Define the term polarisation
As the outside of the cell has an overall positive charge compared to the inside we say that the membrane is polarised
What happens if the change in membrane potential is big enough
If the change in membrane potential is big enough it may trigger an action potential
Define the term “action potential”
An action potential is a wave of electrical excitation along a neurons plasma membrane
How do neurotransmitters initiate a response
Neurotransmitters initiate a response by binding to their receptors at a synapse
What are neurotransmitter receptors
Neurotransmitter receptors are Ligand- gated ion channels
What happens after the neurotransmitter has bound
After the neurotransmitter has bound to the receptor, a series of events occurs which results in depolarisation of the plasma membrane
Define the term “depolarisation”
When a nerve impulse passes along a neuron it passes as a wave of depolarisation of the resting potential. This is when the charge across the membrane switches - the inside of the membrane becomes briefly positively charged in relation to the outside.
What happens to the wave of depolarisation after depolarisation of the plasma membrane
This wave of depolarisation will continue down the neuron, transmitting an electrical impulse
When does the initiation of a nerve impulse begin
Initiation of a nerve impulse begins with binding of a neurotransmitter to a transmembrane receptor on the surface of the next neuron
When is the opening of neighbouring voltage- gated sodium channels is triggered
If sufficient ion movement occurs, and the membrane is depolarised beyond a threshold value, the opening of neighbouring voltage-gated sodium channels is triggered and sodium ions enter the cell down their electrochemical gradient
Describe the effect of this rapid influx of sodium ions into the neuron
The influx of Na+ ions leads to a rapid and large change in the membrane potential. The wave of depolarisation which occurs along the neuron is called the action potential. A short time after opening, the sodium channels become inactivated
What happens after the action potential reaches the end of the neuron
When the action potential reaches the end of the neuron it causes vesicles containing neurotransmitter to fuse with the membrane - this releases neurotransmitter ehuch stimulates a response in a connecting cell
How is the resting potential of the membrane restored following a nerve impulse
To restore the membrane potential to a resting state, the sodium channels become inactivated. Voltage-gated potassium (K+) channels then open to allow potassium ions to move out of the cell
What does the restoration of the resting membrane potential allows
restoration of the resting membrane potential allows the inactive voltage-gated sodium channels to return to a conformation that allows them to open again in response
To another depolarisation of the membrane.
Describe the role of the sodium potassium pump in restoration of the resting membrane potential
Following repolarisation the sodium and potassium ion concentration gradients are reduced. The sodium-potassium pump restores the sodium and potassium ions back to restoring potential levels and it re-establishes ion concentration gradients. It actively transports excess ions in and out of the cell
Summary of the steps involved in a nerve transmission
- Neurotransmitters initiate a response by binding to their receptors at a synapse
- the neurotransmitter receptor is a ligand-gated Na+ channel so the binding cause the channel open and letting Na+ ions diffuse into the cell
- Positively charged sodium ions move in. This causes the inside of the neuron to become less negatively charged
- If sufficient Na+ ions enter the cell, it triggers the opening and neighbouring voltage -gated sodium channels and more Na+ ions enter the cell
- A wave of depolarisation moves down the neuron called the action potential
- A short time after opening, Na+ channels become inactivated
- To restore the resting potential, voltage-gated potassium (K+) channels then open to allow potassium ions move out the cell
- The sodium-potassium pump restores the sodium and potassium ions back to resting potential levels and it re establishes ion concentration gradients
What does the retina area within the eye do
The retina is the area within the eye that detects light. It converts light into electrical signals.
Animals have two types of photoreceptor cell in the retina
- Rod cells
- Cone cells
Describe the function of rod cells in the retina
Rod cells are sensitive to changes in light intensity (brightness of light) they are useful for vision in areas of low light intensity eg a dim room. However they do not allow colour perception
Describe the function of cone cells in the retina
Cone cells are responsible for colour vision and only function in bright light. They are particularly sensitive to specific colours (wavelengths) of light : green,red, blue and (in some animals) UV
Photoreceptors of the eye contain
- light sensitive molecule called RETINAL
- membrane protein called OPSIN
What is this name given to the retinal -opsin complex in rod cells?
In rod cells thus retinal - opsin complex is called RHODOPSIN
Describe photoreceptors in cone cells
Cells different forms of opsin combine with retinal to give different photoreceptor proteins each with a maximal sensitivity to specific wavelengths of light (red, green, blue or UV)
Describe the stages in the generation of a nerve impulse in response to light in rod cells
- Retinal absorbs a photon of light and becomes photoexcited rhodopsin. Photo excited rhodopsin activates G- protein called transduction. A single photo excited rhodopsin activates hundreds of molecules of transducin
- G- proteins (transducin) activate phosphodiesterase (PDE) each activated G-protein activates one molecule of PDE
- PDE catalyses hydrolysis of cyclic GMP (cGMP)l each active PDE molecule breaks down thousands of cGMP molecules per second
- Concentration of cGMP decreases causing a closure of ion channels in the membrane of rod cells. As no move Na+ ions enter cell, the membrane potential increases. Hyper polarisation of the membrane triggers nerve impulse in neurons in the retina
