Homeostasis and Equilibrium Flashcards
What is homeostasis?
It is the self regulated process by which a living organism maintains internal stability whilst adjusting to external conditions
What are examples of homeostasis?
Regulation of body temp
Balance between acidity and alkalines
What is negative feedback?
It is when some exrernal variable triggers a counteracting response by the body to maintain homeostasis. In essence, it reverses a derivation from the set point, thus maintaining body parameters within their normal range
What is an example of negative feedback?
In hot weather, sensors detect the heat and sends it to the brain –> sends signals to counteract this –> sweat increases, and you cool down as sweat evaporates –> lower temp
Another example is blood glucose levels, if they are too high the body releases chemicals such as insulin to assist in controlling it –> -ve feedback
What are the 3 main components to a negative feedback loop?
A sensor
A control centre
An effector
What is the function of the sensor in the negative feedback loop?
Also referred to as a receptor, it monitors a physiological value. This value is reported to the control centre
What is the function of the control centre in the negative feedback loop?
The control centre compares the value given by the sensor to the normal range. If the value deviates too much from the set point, the control centre activates an effector
What is the function of the effector in the negative feedback loop?
The effector is the component in a feedback system that causes a change to reverse the situation and return the value to the normal range
WHat is the process of the stimulus –> response in a negative feedback loop?
Stimulus –> sensor –> control –> Effector –> response
I.e.
Body temp exceeds 37 degrees –> nerve cells in skin and brain –. temp regulation in brain –> sweat gland throughout the body activated, and increased respiration to exhale heat from lungs 00> increased heat loss
What is positive feedback loop?
This is when instead of getting a counteracting response to some variable, the change is instead intensified
Here, a deviation from normal range results in more changes.
This is normal in the body, as long as there is a definite end point
What are 2 examples of positive feedback loops?
An example is seen following a pentrating wound, where the most immediate threat is excessive blood loss. This could lead to shut down of vital organs –> death. Thus, the body responds to this through releasing substances in the injured blood vessel wall that begins the process of blood clotting. As each step of clotting occurs, it stimulates more blood clotting substances –> accentuates process of clotting and seals off damaged areas –> continues on until fully repaired
Another example is seen during childbirth/child labour. Here, the head of the baby moves against the cervix –> nerve impulses from cervix transmitted to brain –> brain stimulates pituitary gland to secrete oxytocin –> oxytocin carried in bloodstream to uterus –> Oxytocin stimulates uterine contractions and passes baby towards cervix –» continues on and on (positive feedback loop)
What is the endocrine system?
It assists with homeostasis through utilising hormones to control and coordinate the body’s metabolism, energy level, growth, reproduction etc. It is responsible to make hormones and release them directly into the bloodstream, so that they can travel to a target tissue and / or organ across the body.
When the chemicals reach cells in our body, they tell those cells how to behave and act
WHat is endocrine?
An internal secretion that pertains to a gland that secretes directly into the bloodstream
What is the endocrine gland?
A ductless gland that produces internal secretion discharged into the blood or lymph, and then circulated to all parts of the body. These make the actual hormones
What are hormones?
Hormones are a chemical/organic regulatory molecule messenger made by endocrine cells. These hormones are secreted into the bloodstream and travel to various target organs on tissues where they extend their effect
They are critical in coordinating growth + development, metabolism, immune function etc.
They act as signalling molecules, transmitting info between cells and tissues to regulate certain physiological processes by transmitting signals like glucose metabolism
What is a receptor?
It is a molecule, such as proteins where a signal molecule/messenger can bind. We can find receptors on the surface of a cell membrane or in the cytoplasm/nucleus
What is the process of cell signalling? (3)
RECEPTION: Typically a signal molecule/ligand binds to a receptor. This receptor should have high specificity - only responds to that one ligand, and it should have high affinity with the ligand. It is the process by which a cell detects a signal in the environment.
TRANSDUCTION: Receptor gets activated by the binding, which means the receptor could change shape, or even invoke a whole series of molecules/proteins changing their conformation in something called a ‘signal transduction pathway’, which can amplify the original signal. The process of activating a series of proteins inside the cell from the cell membrane. It is also a series of events that converts the signal to something the target cell can respond to
CELLULAR RESPONSE: A response occurs in response to the transduction, such as transcribing a protein or DNA. This step is when the target cell responds to the signal.
Ligand means?
That it is the smaller molecule that bonds to a larger molecule
What are the 2 different types of hormones?
Water soluble hormones
Lipid soluble hormones.
What are the characteristics of a water soluble molecule. Examine the limitations of the structure, and the consequence on the signalling process
These are hydrophillic –> polar or charged. These are soluble in water.
However, because it is hydrophillic, they are repelled by the hydrophobic tails in the phospholipid bilayer. Basically, the cell membrane doesn’t allow for water soluble molecules to come in.
Thus, because it can’t go in, we need a method to still transfer the signal across the membrane, this can be achieved through a TRANS MEMBRANE RECEPTOR
What is a trans membrane receptor
These are reeptors that are embedded in the cell membrane. They act in cell signalling by receiving extracellular, water soluble hormones. This then binds to the cell surface receptors on the outside, which triggers an interceullar response
What is the first messenger?
This is the intial hormone that binds to the cell surface receptor, which triggers intracellular events leading to a cellular response.
This first messenger is typically an extracellular signalling molecule, because it only happens because it is a water soluble and can’t penetrate the membrane
What is the second messenger?
Second messengers are small molecules and ions that relay signals received by cell-surface receptors to effector proteins.
The primary function of a second messenger is to amplify the signal of a first messenger such as a hormone.
To be a second messenger, there are several characterisics it must have:
Low amounts in resting state
Regulated Synthesis
Regulated destruction
Acts through other proteins
What is a common example of a second messenger? That we study
cyclic AMP (cAMP)
What is a common example of a cell membrane receptor? That we study
G protein coupled receptors (GPCR)
Explain how water soluble hormones work, with reference to GPCR and cAMP.
The hormone comes and binds itself to the cell surface receptor (GPCR). This triggers the release of a G protein which bonds with an enzyme ‘adenylate cyclase’, and activates it.
This catalyses a reaction when ATP in the cell is converted into cAMP, which acts as a second messenger. The cAMP then acts as a catalyst for intracellular activity or biolofical effects in response to the info from the first messenger. cAMP is a common second messenger.
It could have effects such as:
Activiates proteins + enzymes
Inhibits proteins + enzymes
Altering gene expression etc.
What are lipid soluble hormones?
These are hydrophobic hormones, and they can cross the cell membrane easily. Thus, unlike water soluble hormones, they don’t need a cell surface receptor
Explain how a lipid soluble hormone works.
Their receptors are in the cytoplasm or nucleus. They are typically called ‘nuclear receptors’ . The binding of the hormone to the nucleus receptor forces ‘hormone reception complex’ which goes into the nucleus and binds with the DNA.
they DON’T need a second messenger, because they can just cross the bilayer
How do hormones allow for communication across cells?
Hormones released down bloodstream, and travel throughout the body, carrying info between cells to regulate physiological processes.
Each hormone is designed to interact with specific target cells that possess receptors for that particular hormone. These target cells can be locate d in distant organs or tissues. Thus, it allows for communication despite long distances
What are steroid hormones?
It is a steroid that acts as a hormone. They are cyclical chemical compounds made up of rings of carbon atoms that play an essential roles such as through looking at metabolism, immune response and reproductive. They are derived from cholestrol
Give an example of how lipid soluble hormones would work
For example, the steroid hormone testosterone passes through the plasma membrane, which then binds to a receptor protein in the cytoplasm, activating it. The hormone receptor complex enters the nucleus and binds to specific genes. The blood protein stimulates the transcription of the gene into mRNA, which can be translated into a specific pattern
What is the pituitary gland?
It is a small, pea sized gland at the base of the brain. It tells other glands in your body what to do. It does this through making, storing and releasing hormones. Also known as the ‘master gland’
Which two hormones are produced by the posterior pituitary that we studying?
Oxytocin and antidiuretic hormone (ADH)
What is oxytocin and how does it work?
It is a peptive (a compound consisting of two or more amino acids linked in a chain) hormone released by the posterior pituitary, and plays a significant role in childbirth and assisting with breast milk
During labour, the cervix stretching will cause increased oxytocin released, which increases uterine contractions, further helping with childbirth, further causing the cervix to stretch, forming a positive feedback loop, as it keeps going until the cervix is stretched far enough for childbirth to occur
Additionally, suckling at a lactating breast will send a signal to release oxytocin, which stimulates milk injections by contracting the cells which live in the breast. Thijs is a further positive feedback loop
What is ADH?
It is a chemical produced which causes kidneys to release less water, increase uptake of water and decrease the amount of urine produced (to maximise amount of water)
Main function is to increase H2O absorption
What is plasma osmolarity
electrolyte/sodium - water balance
How does ADH work - why and when is it released?ra
ADH is released as a response to an increase in plasma osmolarity, or a decrease in blood volume or a decrease in blood pressure. When this happens, ADH stimulates an increase in the amount of aquaporins in the kidney, to increase absorption of water, which thereby prevents loss of water –> prevents osmolarity increase again, and releases the water back into the body.
This allows for decrease in plasma osmolarity and increase in blood volume and blood pressure
Calcium
Calcium concentrations are so low that a dramatic increase will be considered a ‘call to action’
It has profound impacts on protein activity, interactions and conformation
Calcium FLOODS into cells very quickly. In turn, changes cellular actions on all levels.
What is the importance of circulating glucose?
Glucose is the main source of energy for the brain (neurons). Additionally, glycogen stores in the brain is very little. Thus, we need glucose to be constantly circulating so that it can get to the brain –> brain can continue to function.
When there is low blood glucose, brain doesn’t have enough energy –> loss of brain function –> coma
What is the form of transport for glucose to enter cells?
Utilises passive transport through diffusion to enter cells.
Cell membranes are semi permeable, however glucose can’t cross membranes, and thus needs the assistance of glucose transporters
What are the main types of glucose transporters?
SGLT transporters
GLUT transporters
What are SGLT transporters?
These are typically seen in the small intestine & kidney. The small intestine is responsible for uptake of glucose from the blood. Meanwhile the kidney functions to reuptake the sodium and glucose from the blood
Overall, these exist to assist in transporting sodium and glucose across a cell membrane
Uses enerfy from the negative sodium ion gradient by the ATPase pump to transport glucose across membrane
Essentially involves transport of sodium which allows for glucose to be transported as well?!
What are the 2 types of GLUT transporters?
GLUT2 and GLUT4
What are GLUT2 transporters?
They are mainly in the liver and pancreatic beta cells, and they don’t rely on insulin for facilitated diffusion
What are GLUT4 transporters?
They are mainly in the muscle and fat cells. These respond to insulin.
What is the problem with relying solely on the diffusion of glucose
Problem is that glucose can reach high peaks or low throughs without additional glucose control.
Thus if we only rely on glucose diffusion, it will provide a bad control of glucose. Thus, we need glucagon and insulin to add further control
What does insulin do?
Insulin generally promotes glucose uptake within muscle and fat cells, and also promote glycogen storage in the liver
What does glucagon do?
Glucagon targets the liver to promote glycogen breakdown and thereby promote glucose release into the body. This is an important function during the fasting state of the body
What is the fed state
This state occurs essentially after eating, and the nutrients and blood sugars have been absorbed in the system and are circulating
During this state, blood sugar increases
Describe the mechanisms that maintain blood sugar in the fed state
During the fed state, mainly the mechanisms that attempt to maintain a stable blood sugar focus on lowering sugar through increasing its uptake into cells, to be stored. I.e. to the liver, fat and muscle cells. This is to prevent an extremely high blood sugar level and also allows for the cells to have enough glucose to function properly.
What happens to muscle cells during the fed state?
Insulin is secreted, which increases the amount of GLUT4 transporters on the cell membranes of cells. This increased GLUT4 results in more glucose entering the muscle cell –> enabling them to get the glucose required.
However, if it was just reliant on simple diffusion, glucose entering the muscle will stop at a certain point because [glucose] in the cell = [glucose] outside the cell. Thus the idea of storing the glucose is important to disturb this equilibrium and ensure more and constant glucose entering the muscle cells –> glycogen etc.
Why is storing the incoming glucose as glycogen important?
Keeps glucose coming in the cell via passive transport
Allows for storage of glucose into glycogen to be used for exercise or energy later on
What happens to fats during the fed state
During the fed state, fats take up glucose through GLUT4 and uses fatty acids to form triglycerides (fats) which can be used as an energy store
Fat cells can just keep getting bigger with a continuous intake of glucose
Once again, insulin increases amount of glucose absorbed
What happens to the liver during the fed state
During the fed state, liver builds up energy stores in the form of glycogen and triglycerides
What is the fasting state?
Typically occurs a few hours after eating and its the period when the body isn’t receiving any dietary glucose or nutrition through food intake.
During fasting, the body relies on stored energy sources, such as glycogen in the liver and the muscles
Generally, during the fasting state, insulin will decrease but glucagon increases
What happens to the muscle cells during the fasting state
Here, during fasting state, glycogen which was previously stored is used for energy to allow for exercise.
Releases lactate from exercising muscles which can be then taken by the liver and used for gluconeogensis, which can then be released into the bloodstream to maintain blood glucose levels during fasting
What happens to the fats during the fasting state
Fat cells release stored triglycerides into glycerol and free fatty acids which can be used for energy
What happens to the liver during the fasting phase
During fasting state, the liver aims to generate glucose to increase blood sugar levels (prevent hypoglycaemia). When there is decreased blood sugar levels on the outside of the cell, the GLUT2 transporters assist in transporting glucose formed through assistance of glucagon to catalyse the reaction from glycogen to glucose.
The liver responds to the increased glucagon levels to up the levels of blood glucose
What is the function of the pancreas?
Produce enzymes to digest food
Produce hormones insulin & glucagon to help maintain blood sugar control
What is the essential part of the pancreas which creates insulin?
The Pancreatic islets, also known as islets of Langerhans
What is the pancreatic islets made up of?
Alpha cells (important)
Beta cells (important)
Delta cells
Pancreatic polypeptide
What do alpha cells do?
Synthesise glucagon
What do beta cells do?
Synthesise insulin
What are some various factors which beta cells respond to?
Glucose, amino acids, hormones, neuronal input etc.
What is beta cell excitability?
Refers to the ability of beta cells to respond to various signals by generating electrical activity, leading to the secretion of insulin. This is critical for regulation of blood glucose levels in the body
WHat happens when the beta cell isn’t being stimulated yet
They maintain a relatively stable resting membrane potential, at around ~70 millivolts (mv). This resting potential is primarily determined by the activity of the potassium channels, which move K+ ions outside of the cell –> inside of the cell is more -ve
List the important parts of beta cell excitability? (i.e. the components)
GLUT2, ATP, K-ATP channel, VGCC (voltage gated calcium channels), Ca2+, insulin granules
Explain the process of beta cell excitability
Beta cells contain GLUT2, when blood glucose levels rise, glucose enters beta cells through these GLUT2 transporters.
Results in increased production of ATP through glucose metabolism. This increased ATP level inhibits K ATP channels, causing them to close. As a result, K+ ions can’t exit the cell –> membrane depolarisation (inside of cell becomes less negative)
Membrane depolarisation then triggers the opening of voltage gated calcium channels (VGCC), which are sensitive to the membrane potential. Calcium ions then flow into the cell through these channels due to their electrochemical gradient
The influx of Ca2+ into the beta cell cytoplasm triggers various intracellular signaling processes. It also involves activation of Ca2+ sensitive proteins, which in turn activate other proteins involved in the exocytosis of the insulin containing vesicles
After insulin is secreted, the beta cell membrane repolarises –> returning to the resting membrane potential, which is facilitated by closure of Ca2+ channels and opening of K+ channels –> K+ ions leave cells –> repolarisation
What is membrane depolarisation
Depolarization is the process by which the membrane potential becomes less negative, facilitating the generation of an action potential.
How many secretory insulin granules do we have?Why?
Around 10000 granules, in beta cells, however we only excrete 100 granules –> most likely because the rest is a backup, as this is an essential part of the biological process
Is membrane potential proportional to glucose levels?
Yes
What is GLP one?
It is a hormone coming from cells lining the small intestine, and acts as a key regulator of beta cells
When eating, the body triggers secretion of GLP one which then gets into circulation as a hormone, and is picked up by beta cells –> priming beta cells for release of insulin from glucose digestion
What is type 1 diabetes?
This is an autoimmune disorder. In this case, the body recognises itself as being an invasive element and targets itself as part of the immune system and tries to destroy part of it.
More specifically, it recognises beta cells as a hostile cell –> generates antibody against proteins that are present in beta cells including insulin
Here, killer T cells go to kill all B cells in pancreatic islets
Median age is 12 years old, and typically occurs in children. Includes symptoms such as weight loss, fatigue, frequent urination and excessive thirst
What is type 2 diabetes
This is where beta cells dont work as well (could be due to prolonged exposure to high levels of glucose) –> reduced production of insulin
Insulin resistance can also occur in cells –> less response –> high blood glucose due to inability for storage of glucose
Symptoms include:
slow healing of wounds, blurred vision, constant hunger, fungal infection, frequent urination, numbness of hands and feet
