Organisms Respond To Changes In Their Environment: - The Role Of Hormones In Osmoregulation

- Regulation of the water potential of the blood -

Osmoreceptors in the hypothalamus monitors the water potential in the blood.

If the water potential decreases in the blood, water would move out of the osmoreceptor cells by osmosis // cells decrease in volume as a result of this // signal is sent to other cells in the hypothalamus // signal then sent to the posterior pituitary gland. - this secretes the antidiuretic hormone (ADH) into the blood.

ADH makes the walls of the distal convoluted tubule and collecting duct more permeable to water.
More water is reabsorbed from these tubules into the medulla and into the blood by osmosis // small concentration of urine is produced = reduction of water loss. 

Specific protein receptors on cell surface membrane of these cells binds to ADH molecules // activation of phosphorylase (enzyme) within cell // vesicles within the cell move and fuse with the cell surface membrane.

Vesicles contain plasma membrane and have numerous water channel proteins (aquaporins) so when they fuse with the membrane the number of water channels is considerably increased - making cell-surface membrane more permeable to water. 

ADH increases permeability of collecting ducts to urea // passes out // lowering water potential of fluid around the duct.

Combining effect is more water leaves the collecting duct urea // osmosis // down a concentration gradient // re-enters the blood.

Reabsorbed water came from the blood will prevent the water potential lowering // Osmorecpetors send nerve impulses to the thirst centre of the brain encouraging the individual to seek out and drink more water.
Hypothalamus detects the rise in water potential and send fewer impulse to the pituitary gland
Pituitary gland reduces the release of ADH // permeability of the ducts return to its former state // Homeostasis and Negative Feedback.

A fall in the solute concentration of blood raises its water potential. Due to either:
1) Large volumes of water being consumed.
2) Salts used in metabolic or excreted not being replaced in the diet.


The body responds to rise in water potential:
Osmoreceptors in hypothalamus detects rise in water potential // increase frequency of the of nerve impulses to the pituitary gland to reduce its release of ADH

Less ADH // via blood // decrease in permeability of the collecting duct to water and urea

Less water is re-absorbed into the blood from the collecting duct ⇒ more dilute urine = water potential falls in the blood.

In the blood the water potential has returned to normal // osmoreceptors cause the pituitary gland to raise ADH // the levels go back to normal ⇒ Negative Feedback.

Organisms Respond To Changes In Their Environment: - Role Of The Nephron In Osmoregulation

- Nephron filtering the blood -

1. Blood from the renal artery enters smaller arterioles in the cortex of the kidney.
2. Arterioles splits into a glomerulus // bundle of capillaries looped inside a Bowman's capsule // Ultrafiltration takes place.
3. Efferent arterioles that takes blood into each glomerulus // afferent arteriole take the filtered blood away from the glomerulus. 
4. Efferent arterioles are smaller in diameter than afferent arterioles // blood is under high pressure. 
5. High pressure forces liquid and small molecules in the blood out of the capillary and into the Bowman's capsule. 
6. Liquid and small molecules pass through 3 layers get into Bowman's capsule enter the nephron tubules - capillary wall // basement membrane and epithelium of Bowman's capsule.
7. Larger molecule - protein and blood cells can't pass through - stay in blood. Substances enter Bowman's capsule // glomerular filtrate 
8. Glomerular filtrate passes along the rest of nephron - useful substances are reabsorbed along the way. 
9. Filtrate flow through the collecting duct and passes out of the kidney along the ureter. 

Re-absorption of glucose and water by the proximal convoluted tubule

The proximal convoluted tubule is adapted to reabsorb substances: 

Its adaptations include:

- micovilli = large surface area to reasorb substances from filtrate 

- infolding of bases = large surface area to transfer reabsorbed substances back into the blood capillaries 

- high density of mitochondira to provide ATP // active transport

Process of re-absorbtion 

1. Sodium ions actively transported out of the cells lining proximal convoluted tubule into blood capillaries // sodium concentration drops 
2. Sodium ions diffuse down a concentration gradient from lumen of proximal convoluted tubule into epithelial lining cells // only through carrier proteins // facilitated diffusion. 
3. Carrier proteins are specific types to carrier other specific molecules// co-transport // molecules which have been co-transported into the cell of proximal convoluted tubule then diffuse into the blood // All glucose and other valuable minerals are reabsorbed 

Maintenance of gradient of sodium ions by the loop of Henle 

Loop of Henle = responsible for water being reabsorbed from the collecting duct // concentrating the urine so it has lower water potential than blood. 

This has 2 regions 

1. Descending limb - narrow - thin walls that are highly permeable to water. 
2. Ascending limb - wider - thick walls that impermeable to water.

1. Na+ ions are pumped out into the medulla using active transport // ascending limb impermeable therefore water does not move out lowering the water potential in the medulla, because there's a high concentration of ions. 
2. Due to there being a lower water potential in the medulla than in descending limb, water moves out the descending limb, into medulla by osmosis // filtrate is more concentrated. The water in the medulla is reabsorbed into the blood through the capillary network. 
3. Bottom of the ascending limb // Na+ ions diffuse out into the medulla, lowering the water potential further in the medulla. The ascending limb is impermeable to water, so it stays in the tubule. 
4. Water moves out of distal convoluted tubule by osmosis and is reabsorbed into the blood.
5. First three stages massively increase the ion concentration in the medulla, lowers the water potential. Water then moves out of the collecting duct by osmosis. The water in the medulla is reabsorbed into the blood through the capillary network. 

Organisms Respond To Changes In Their Environment: - Control of Blood Water Potential

- Structure Of The Nephron - 

Osmoregulation = the homeostatic control of the water potential of the blood. 

Structure of the mammalian kidney

In the mammal there are two kidneys found at the back of the abdominal cavity. 

A section through the kidney 

Fibrous Capsule = outer membrane that protects the kidney 

Cortex = lighter coloured outer region made up of renal (Bowman's) capsule, convoluted tubles and blood vessels. 

Medulla = darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels 

Renal pelvis = funnel-shaped cavity that collects urine into the ureter 

Ureter = tube that carries urine to the bladder 

Rental artery = supplies the kidney with blood from the heart via the aorta

Renal vein = returns blood to the heart via the vena cava 

Structure of the nephrons 

Renal (Bowman's) capsule = closed end at the start of the nephron. It cup-shaped and surrounds a mass of blood capillaries - Glomerulus. The inner layer of the renal capsule made of specialised cells - podocytes 

Proximal convoluted tubules = series of loops surrounded by blood capillaries. It's walls made epithelial cells - have microvilli 

Loop of Henle = long hairpin loop extends from cortex into medulla of kidney and back again. Surrounded by blood capillaries 

Distal convoluted tubule = series of loops surrounded by blood capillaries. Walls are made of epithelial cell - surrounded by fewer capillaries than proximal tubule. 

Collecting duct = tube number of distal convoluted tubules from a number of nephrons empty. Lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of kidney. 

Associated with each nephron there are number of blood vessels:

Afferent arteriole = tiny vessel arise from renal artery - supplies the nephron with blood // Afferent arteriole enters the renal capsule of the nephron 

Glomerulus = many-branched knots of capillaries from which fluid is forced out of the blood. Glomerulus capillaries recombine.

Efferent arterioles = tiny vessel leaves the renal capsule // smaller diameter than afferent arteriole causes an increase in blood pressure within the glomerulus. Efferent arterioles carries blood away from the renal capsule. 

Blood capillaries = concentrated network of capillaries surrounding the proximal convoluted tublue, loop of Henle mineral salts, glucose, and water. These capillaries merge to form venules, merging to form the renal vein. 

Organisms Respond To Changes In Their Environment: - Diabetes And Its Control

- Diabetes and its control - 

Types of sugar diabetes 

Type 1 - insulin dependent // body unable to produce insulin 

The body attacks its own cells - 𝛽 cells - result of autoimmune response. 

This develops quickly over a few weeks once the individual is born. Usually develops at a very young age. 

Type 2 - insulin independent // body glycoproteins receptors have lost or losing their responsiveness to insulin // body producing an insufficient amount of insulin. 

This develops in people over age of 40 - increases due to obesity and poor diet. 

Symptoms are less severe and may go unnoticed. 

90% of people have type II diabetes 

Control of diabetes 

Diabetes can be successfully treated which depends on the type of diabetes 

Type 1 diabetes = controlled by injections of insulin - cannot be taken orally as it is an enzyme and would be digested. Typcially taken 3-4 times a day. Dose of insulin would match the dose intake of glucose // glucose concentration is monitored using biosensors. 

Type 2 diabetes = controlled by regulating the intake of carbohydrates in the diet // exercise is also taken into account. Supplemented by injections of insulin by the use of drugs that stimulate insulin production. 

Organisms Respond To Changes In Their Environments: - Hormones And The Regulation Of Blood Glucose Concentration

- Hormones and their mode of action - 

Hormones differ chemically but there are characteristics which would be the same also. 

Hormones :

- Produced in glands // secrete hormones directly into blood

- Carried in blood plasma to cells which are acted on // target cells // specific receptors on their cell-surface membrane // complementary to specific hormones.
- Effective in very low concentrations // widespread // long-lasting.

 Second Messenger Model 

2 hormones are used - Adrenaline and glucagon 

1. Adrealine binds to the transmembrane protein receptor within the cell-surface membrane of the liver cell. 
2. Binding of the adrealine causes the protein to change the shape on the inside of the membrane
3. Change in shape of the protein leading to the activation of an enzyme - adenly cyclase // activated adenly cyclase converts ATP to cyclic AMP (cAMP) 
4. cAMP acts as second messenger that binds to the protein kinase enzyme, its activiated by changing its shape.
5. Active protein kinase enzyme catalyses the conversion of glycogen to glucose which moves out of the liver cell // facilitated diffusion // in to the blood through channel proteins. 

Role of the Pancrease in regulating blood glucose 

It produces enzymes - 

Protease, amylase and lipase for digestion and hormones. 

Insulin and glucagon for regulating blood glucose concentration. 

There are groups of hormone-producing cells - islets of Lnagerhans - cells of the islets of Langerhans:

- ɑ cells, larger and produce the hormone glucagon

- 𝛽 cells, smaller and produce the hormone insulin 

Role of the liver in regulating blood sugar 

There are three important processes involved with regulating blood sugar which takes place in the liver:

Glycogenesis - this is the conversion of glucose into glycogen. 

This occurs when the blood glucose concentration is higher than normal. The liver removes the glucose from the blood and converts it into glycogen. 

Glycogenolysis - this is the breakdown of glucagon into glucose.

This occurs when the blood glucose concentration is lower than normal. The liver converts glycogen into glucose which is then diffused into the blood. 

Gluconeogenesis - this is production of glucose from sources other than carbohydrates. 

This occurs when glycogen supply is exhausted. The liver produces glucose from non-carbohydrate sources - amino acids and glycerol. 

Regulation of blood glucose concentration 

Factors that influence blood glucose concentration

1. Directly from the diet = glucose is absorbed from the hydrolysis of carbohydrates 
2. From the hydrolysis of glycagon in the small intestine = glycogenolysis // stored in liver and muscle cells 
3. From gluconeogenesis 

Insulin and the 𝛽 cells of the pancreas 

when combined with receptors insulin brings about:

- Change in tertiary strucute of the glucose transport carrier proteins - changing shape and open - allows more glucose into the cells by facilitated diffusion.

- Increase in number of carrier proteins responsible for glucose transport in the cell-surface membrane. Protein from which these channels are made is part of the vesicles. // rise in insulin concentration the vesicles fusing with the cell-surface membrane increasing the number of glucose transport channels. 

- Activation of enzyme that convert glucose to glycogen and fat. 

Blood glucose is lowers through the following ways:

- Increasing the rate of absorption of glucose into the cells - muscle cells

- Increasing rate of respiration of the cells // uses more glucose // increasing the uptake from the blood

- Increasing glycogenesis in liver and muscle cells

- Increasing the rate of conversion of glucose to fat. 

The processes taking place = Negative feedback // removal of glucose reduced the secretion of insulin from 𝛽 cells. 

Glucagon and the ɑ cells of the pancreas 

ɑ cells detect the fall in blood glucose concentrations releasing the hormone glycogen directly into the blood plasma. 

Glycogen action include: 

- Attaching to specific protein receptors on cell-surface membrane of liver cells 

- Activating enzymes convert glycogen to glucose 

- Activating enzymes involved in conversion of amino acids and glycerol into glucose = glyconeogenesis 

This returns the blood glucose concentration to the optimum. 

Role of adrenaline in regulating the blood glucose level 

Adrenaline raises the blood glucose concentration by:

- Attaching to protein receptors on the cell-surface membrane of target cells 

- Activating enzymes that causes the breakdown of glycogen to glucose in the liver. 

Adrenaline is produced when there is a period of excitement or stress. 

Hormone interaction in regulating the blood glucose

Insulin and glycagon act in opposite directions = antagonistically 

These two hormones are sensitive to control of the blood glucose concentration // blood glucose concentration is not constant there are fluctuations around the optimum temperature. 

When writing about negative feedback its important to mention that the secretion of a hormone - insulin - would result in the reduction of its own secretion. 

Organisms Respond To Changes In Their Environments: - Feedback Mechanisms

- The control mechanisms -

Control of any self regulating system involves a series of stages

- Optimum point
- Receptor
- Coordination
- Effector
- Feedback mechanism

Homeostatic systems detect change and respond by negative feedback.

1) The receptor, communication system and effectors is the foundation of the homeostatic system. 

2) Receptors are there to detect when there is a change in temperature - this would be when temperature is too high or too low. Via the nervous system or hormonal system the information is passed on to the effector. 

3) The effectors are suppose to counteract the change // bring the level back to an optimum. 

4) Negative feedback = process of bring the level to an optimum temperature. 

5) Negative feedback keeps the body's temperature at an optimum which is above 0.5℃ or below 37℃.

6) However if they change is too drastic then the Negative feedback would be unable to counteract the change. This can be when there has been prolonged exposure to cold weather. 

There are multiple Negative feedback mechanisms ⇒ more control. By only having one negative feedback system ⇒ there is a slower response and less control. 

Positive feedback mechanisms can amplify a change from the normal level. 

1) some changes can trigger a positive feedback // amplifying the change. 

2) Increasing the level further away from the normal level.

3) Positive feedback rapidly activates something // Blood clot after a deep cut. 

4) Positive feedback can occur when homeostatic system breaks down e.g. Being too cold for too long.

Positive feedback is not involved in homeostasis because it does not keep the internal environment stable.

- Coordination of control mechanisms -

Control system normally have receptors and effectors allows them to have separate mechanisms that produce a positive movement towards an optimum. Allows greater degree of control of the particular factor being regulated. 

Organisms Respond To Changes In Their Environments: - Principles Of Homeostasis

- Principles Of Homeostasis -

What is Homeostasis?

This is the maintenance of an interval environment within restricted limits in organisms.

This is to ensure that the chemical make-up, volume and other features of blood and tissue fluid within restricted limits are maintained.

Homeostasis is the maintenance of a stable internal environment.

Any changes that are made to the external environment can affect the internal environment of an organism.

To prevent this, homeostasis is involved in control systems that would maintain the constant internal environment.

This allows virtual cells to function normally and prevent damage to them and they are kept in a stable environment.

Factors that would be maintained ⇒ the core body temperature and blood pH.

This is because both pH and temperature are factors that effect enzyme activity and enzymes control the rate of metabolic reactions.

Temperature: if it is too high or at an extreme the enzymes would become denatured.
This is due to the hydrogen bonds breaking that holds them in their 3D shape. Thus changing the active site, which leads to an ineffective catalyst.

If the temperature is too low the enzyme activity is reduced and it slows down the rate of metabolic reactions.

In order for the rate of enzyme activity to remain at a constant the enzymes have an optimum temperature at which they work best at. In humans, this would be 37oC.

pH: if the pH is too high or too low // at either ends of extremes// the enzymes denature. Hydrogen bonds that hold the shape of the enzymes are broken. Therefore, the active site of the enzyme is changed which makes it ineffective catalyst. The metabolic reactions are less efficient.

The highest rate of enzyme activity remains at an optimum pH // usually remains around pH 7.

But this varies for some enzymes // enzymes for the stomach work best at a low pH.

Because cells need glucose for energy it is important to maintain the right concentration of glucose in the blood.

Blood glucose concentration also affects the water potential of blood - there is the potential that water molecules diffuse in and out of or into a solution.

If blood glucose concentration is high ⇒ water potential of blood is reduced // water molecules would then diffuse out of the cell // osmosis // cell will shrivel up and die.

If blood glucose concentration is too low ⇒ cells cannot carry out normal activities there isn't enough glucose for respiration to provide energy.

- The control mechanisms -

Control of any self regulating system involves a series of stages

- Optimum point
- Receptor
- Coordination
- Effector
- Feedback mechanism

Homeostatic systems detect change and respond by negative feedback.

1) The receptor, communication system and effectors is the foundation of the homeostatic system. 

2) Receptors are there to detect when there is a change in temperature - this would be when temperature is too high or too low. Via the nervous system or hormonal system the information is passed on to the effector. 

3) The effectors are suppose to counteract the change // bring the level back to an optimum. 

4) Negative feedback = process of bring the level to an optimum temperature. 

5) Negative feedback keeps the body's temperature at an optimum which is above 0.5℃ or below 37℃.

6) However if they change is too drastic then the Negative feedback would be unable to counteract the change. This can be when there has been prolonged exposure to cold weather. 

There are multiple Negative feedback mechanisms ⇒ more control. By only having one negative feedback system ⇒ there is a slower response and less control. 

Positive feedback mechanisms can amplify a change from the normal level. 

1) some changes can trigger a positive feedback // amplifying the change. 

2) Increasing the level further away from the normal level.

3) Positive feedback rapidly activates something // Blood clot after a deep cut. 

4) Positive feedback can occur when homeostatic system breaks down e.g. Being too cold for too long.

Positive feedback is not involved in homeostasis because it does not keep the internal environment stable.

- Coordination of control mechanisms -

Control system normally have receptors and effectors allows them to have separate mechanisms that produce a positive movement towards an optimum. Allows greater degree of control of the particular factor being regulated. 

Organisms Respond To Changes In Their Environment: - Control Of Heart Rate.

- Control Of Heart Rate -

The Autonomic Nervous system

It has two subdivisions:

1. Sympathetic nervous system - stimulates effectors so speeds up any activity. Acts rather like an emergency controller // allows us to cope with stressful situations heightening our awareness and prepare us for the activity. 
2. Parasympathetic nervous system -  inhibits effectors so it slows down any activity. It controls activities to a normal resting conditions // conserves the energy and the body's reserves are replenished. 

The system oppose each other - antagonistic. If one contracts a muscle the other relaxes a muscle. Each doing the opposing job 

- Control Of Heart Rate -

Cardiac muscle = myogenic // able to contract without receiving signals.

1. Sinoatrial node (SAN) is in the wall of the right atrium, recieves an electrical activity which spreads across both atria casuing them to contract. SAN is like a pacemaker, setting the rhythm of the heart. 
2. Layer of non-conductive collagen tissue // atrioventricular septum // stops waves of electrical activity from being passed directly to the ventricles. 
3. The waves are then passed to/enters the atrioventricular nodes (AVN) This is in between the atria 
4. There is a slight delay when the AVN passes the electrical activity to the bundle of His // a series of muscle fibres (Purkyne tissue) which collectively make up the bundle of His // the atria would have had to emptied before the AVN reacts. 
5. Bundle of His grouped of muscle fibres responsible for conducting the waves of electrical activity between ventricles to apex. // bottom of the heart. 
6. Purkyne tissue carries the waves of electrical activity into a muscular walls of both ventricles, both contracting at the same time // from the bottom up. 

- Modifying the resting heart rate - 

Adult human hearts has a resting heart beat of 70BPM // important to be able to change this heart rate in order to meet the varying demands for oxygen.
During exercise the heart rate is likely to increase to double the original resting heart rate.

Medulla Oblongata = controls the changes to the heart rate in the a region of the brain // sub-divided into two centres:

1. Centre that increases heart rate - linked to sinoatrial node - sympathetic nervous system (SNS)
2. Centre that decreases heart rate - linked to sinoatrial node - parasympathetic nervous system (PNS) 

- Control by chemoreceptors -

Pressure receptors = baroreceptors in the aorta & carotid arteries // stimulated by high & low blood pressure. 

Chemical receptors = chemoreceptors in the aorta, carotid arteries, and medulla // monitor oxygen levels in the blood and carbon dioxide and pH - both indicators of O2 levels. 

This works by: 

1. Blood has a higher than normal concentration of carbon dioxide // pH is lowered. 
2. Chemorecpetors in walls of carotid arteries // aorta detect & increases the frequency of nerve impulses to the centre in medulla oblongata - increases heart rate. 
3. This centre increases the frequency of impulses via SNS to SAN increasing the rate of production of electrical waves by SAN thus increasing the heart rate
4. Increased blood flow that causes leads to lungs removing the carbon dioxide, CO2 concentration return to normal in the blood. 
5. pH of the blood rises to normal // the chemoreceptors in the walls of the carotid arteries & aorta reduce frequency of nerve impulse to the medulla oblongata 
6. Medulla oblongata reduces frequency of impulses to SAN, leads to a reduction in the heart rate. 

- Control by pressure receptors - 

The pressure receptors = occurs within heart walls // carotids arteries & aorta // sequence of that follows changes in activity levels:

• When blood pressure is higher than normal = receptors transmits more nervous impulses to the centre in medulla oblongata = decreases heart rate. Impulses sent via PNS (Parasympathetic) to SAN from centre. 
• When blood pressure is lower than normal = receptors transmit more nerve impulses to centre in medulla oblongata = increases heart rate. Impulses sent via SNS (Sympathetic) to SAN from centre. 

Organisms Respond To Changes In Their Environment: - Receptors

- Receptors - 

A Pacinian corpuscle is a sensory neruone :

1. Specific to single type of stimulus - responds to mechanical pressure 
2. Produces a generator potential, acting as a transducer which converts the change in form of energy by stimulus into nerve impulses, that can be understood. Nerve Impulse = energy 

Receptors in nervous system convert energy of the stimulus into a nervous impulse = generator potential. 

- What does the structure of the Pacinian corpuscle look like? -

The sensory neurone ending at the centre of the Pacinian corpuscle has special sodium channel in the plasma membrane = stretch-mediated sodium channel. 

The permebility to sodium changes when it is deformed = i.e. stretching the pacinican corpuscle. 

- What is the function of the Pacinian corpuscle? -

1. Pacinian corpuscle contain the end of the sensory neurone - wrapped in many layers of connective tissue = lamellae
2. When it is stimulated the lamallae is deformed and presses on the sensory nerve endings. 
3. This causes the sensory neruone's cell membrane to stretch, defroming the stretch-mediated sodium ion channels. // channels open and sodium ions diffuse into the cell 
4. Influx of the sodium ions changes the potential of the membrane - becomes depolarised - a generator potential is created. 
5. If the generator potential reaches the threshold an action potential is triggered. 

- Differences between Rods and Cones -

Rods = sensitive // firing action potential in dim light. Many rods join to only one neurone, this means that a weak generator potential needed, as well as, a weak threshold to trigger the action potential. 

Rods have low visual acuity // many rods attached to one neurone 

Cones = less sensitive // enables to cones to differentiate whether an image is of two objects or one object. 

Cones have high visual acuity // one cone is attached to one neurone 

when the light hits two cones from two points two action potentials are produced and go to the brain. 

In other words, this enables the two points that were close together as two separate points. 

Organisms Respond To Changes In Their Environment: - A Reflex Arc

- Reflex Arc -

The nervous system has two main devisions:

1. Central Nervous System = made up of the spinal cord and the brain ~ (CNS)
2. Peripheral Nervous System = made up nerves branching from either the brain or the spinal cord. ~ (PNS) 

Peripheral System the divides into subdivisions: 

1. Sensory neurons = carries electrical impulses AWAY from receptors TO CNS 
2. Motor neurons = carries nerve impulses AWAY from CNS TO effectors 

Motor neurons are sub-divided into:

1. Voluntary nervous system = carries nerve impulses to body muscle // conscious control = voluntary 
2. Autonomic nervous system = carries nerve impulses to glands // smooth muscle & cardiac muscle under subconscious control = involuntary. 

This is a summary of the nervous organisation. ↑

Spinal Cord

This is summarised in the picture // no need for additional information the spinal cord for the specification is pretty simple. 

Reflex Arc 

Involuntary response to a sensory stimulus = Reflex Arc.

For example, withdrawal reflex // a spinal reflex 

Spinal reflex has 7 main stages:

Stimulus > Receptor > Sensory Neurone > Coordinator > Motor Neurone > Effector > Response  

An example of this would be if you was to touch a hot surface -

Stimulus = heat from the surface

Receptor = temperature receptors on the skin generates a nerve impulse in sensory neurone

Sensory Neurone = passes impulse to spinal cord

Coordinator // Intermediate neruone = links sensory to motor neruone in spinal cord

Motor Neurone = carries the impulse from spinal cord to the muscle in the upper arm

Effector = muscle in the upper arm is stimulated to move

Response = pulling your hand away from the hot surface 

Organisms Respond To Changes In Their Environment:- Plant Growth Factors

- Plant Growth Factors -

Plants respond to many different things, a number of factors they do respond to would include:

1. Light 
2. Gravity 
3. Water 

Control of tropism by IAA.

IAA = Indoleacetic acid // an important auxin produced in the tips of the shoots.
IAA is moved around the plant to control of tropism - moving by:
short distance = diffusion or active transport
long distance = via the phloem

Different parts of the plants would have different concentrations of IAA.
Uneven distribution of IAA means there is uneven growth of the plant.

- How is Indoleacetic acid (IAA) involved in phototropism? - 

Light is detected by photorecpetors, which set off a chain of reactions leading to the redistribution of the auxin IAA. More IAA moves to the shaded side of the stem.

IAA causes the cells to elongate by loosening the structure of the cell wall. The mechanism, for this process is unknown, but is thought to involve hydrogen ions (H+).

Because the cells on the shaded slide have a higher concentration of IAA they stretch more than the cells in the light. This causes the shoot to bend towards the light. // = Positive phototropism.

Organisms Respond To Changes In Their Environment - Survival And Response.

- Survival and Response -

There are different types of responses in plants and organisms. 

1. Taxes 
2. Kineses
3. Tropisms

The sequence of events can involve a chemical control or nerve cells, which can be summarised as: 

stimulus ⇒ receptors ⇒ coordinator ⇒ effector ⇒ response. 

Taxes 

This is a simple response whose direction is determined by direction of the stimulus.

So think of earthworms and bacteria. These organisms often move as a cause of environmental change. They either move to a favourable or unfavourable stimulus.

Positive taxis = Movement TOWARDS the stimulus.

Negative taxis = Movement AWAY from the stimulus.

Using the examples given;

Earthworms move away from light // Negative photo-taxis // thus increasing their chance of survival. Their body moves them to an environment which has a much better suited condition for them to live. Soil = moist, dark conditions.

Bacteria ( some species ) will move towards regions of high concentration of glucose // Positive chemo-taxis // thus increasing their chance of survival.

Kinesis

This is a when an organisms changes its speed at which it moves and rate at which it changes its direction.

Example -  

A woodlice crosses sharp dividing line between favourable (moist)  and unfavourable (dry) environment the RATE OF TURNING INCREASES // in other words the organism moving towards favourable environment would be a QUICK RETURN once they MOVE FROM a unfavourable environment. // Increases chance of survival dry environment means the woodlice would dry out. 

A woodlice crosses divide and travels a considerable amount of distance in the unfavourable (dry) environment, the return to favourable environment would be SLOW. often moving in a straight line before turning. 

Tropism 

This is the growth of part of the plant in response to a directional stimulus.

There are two types of responses -

1. Positive response. 
2. Negative response.

The response to the stimulus is to ensure that the survival value is increased.

Positive phototropism & Negative gravitropism - Plant shoots growing towards the light // favourable position to capture light for photosynthesis. 

Negative phototropism & Positive gravitropism - Plant roots growing towards the soil // better able to absorb water and minerals.