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Biology A2 UNIT 5 > Homeostasis > Flashcards

Flashcards in Homeostasis Deck (88):

How can organisms limit the external changes to cells?

By maintaining a relatively constant internal environment for their cells.



The maintenance of a constant internal environment in organisms.


What is the internal environment made up of?

Tissue fluids that bathe each cell, supplying nutrients and removing wastes.
Maintaining the features of this fluid at the optimum levels protects the cells from changes in the external environment, thereby giving the organism a degree of independence.


What does homeostasis involve and ensure?

Involves maintaining the chemical makeup, volume and other features of the blood and tissue fluid within restricted limits.

Ensures that the cells of the body are in an environment that meets their needs and allows them to function normally despite external changes.


Why is homeostasis essential for the proper functioning of organisms?

-The enzymes that control the biochemical reactions within cells, and other proteins, such as channel proteins, are sensitive to changes in pH. Any changes to these factors reduces the efficiency of enzymes or may even prevent them working altogether, eg denaturing them. Even the smallest fluctuations in temperature or pH can impair the ability of enzymes to carry out their roles effectively.
-Changes to the water potential of the blood and tissue fluids may cause cells to shrink and expand as a result of water leaving or entering by osmosis. In both instances the cells cannot operate normally. The maintenance of constant blood glucose conc. is essential in ensuring a constant water potential. A constant blood glucose conc. also ensures a reliable source for resp. by cells.
-Organisms with the ability to maintain a constant internal environment are more independent of the external environment. Have wider geographical range and therefore greater chance of finding food, shelter, etc.


What does maintaining a constant internal environment mean?

Reactions take place at a constant and predictable rate.


What are the series of stages of control of a self regulating system?

THE SET POINT: which is desired level at which system operates.
RECEPTOR: which detects any deviation from the set point and informs the...
CONTROLLER: which coordinates information from various receptors and sends instructions to an appropriate...
EFFECTOR: which brings about the changes needed to return the system to the set point. This return to normality creates a...
FEEDBACK LOOP: which informs the receptor of the changes to the system brought about by the effector.


How are control mechanisms coordinated?

Systems normally have many receptors and effectors. It is important to ensure that the information provided by receptors is analysed by the control centre before action is taken. Receiving information from a number of sources allows a better degree of control.


Example of how control mechanisms are coordinated?

Temperature receptors in the skin may signal that the skin itself is cold and that the body temperature should be raised. However, information from the temperature centre in the brain may indicate that blood temperature is already above normal.

This situation could arise during strenuous exercise when the blood temperature rises but sweat cools the skin.

By analysing info from all detectors, the brain can decide the best course of action.


What else must the control centre do?

Coordinate action of the effectors so that they operate harmoniously.
E.g. sweating would be less effective in cooling the body if it were not accompanied by vasodilation.


Why must body temperature be regulated?

If too low- rate at which enzyme controlled reactions take place may be too slow for the organism to function properly.
If too high- Enzymes may be denatured and the organism may cease to function altogether.


What are two methods of gaining heat?

-Production of heat by the metabolism of food during respiration.
-Gain of heat from the environment by conduction (e.g. from ground), convection (e.g. from surrounding air or water) and radiation.


What are two methods of losing heat?

-Evaporation of water (e.g. during sweating)
-Loss of heat to the environment by conduction (e.g. to the ground), convection (e.g. to the surrounding air or water) and radiation.



Occurs mainly in solids and is the transfer of energy through matter from particle to particle. Heat causes the particles to vibrate and gain kinetic energy. These particles cause adjacent particles to vibrate and so the kinetic energy is transferred through the material.



Occurs in fluids (gases and liquids) and is the transfer of heat as a result of the movement of the warmed matter itself. The heat causes the fluid to expand and move, carrying with it the heat that it has absorbed.



Energy transferred by electromagnetic waves. When these waves hit an object they normally heat up.



Gain most of their heat from the environment, so their body temperature fluctuates with that of the environment.


How do ectotherms control their body temperature?

By adapting their behaviour to changes in the external temperature.


Give an example of an ectotherm and explain why it cannot warm up by exercising.

Reptiles- e.g. lizards cannot warm up by exercising because, if their body temperature is low, they cannot respire fast enough to provide the energy for rapid movement.


What methods can ectotherms use to control their body temperature?

-EXPOSING THEMSELVES TO THE SUN. In order to gain heat lizards orientate themselves so that the maximum surface area of their body is exposed to the warming rays of the sun.

-TAKING SHELTER. Lizards will shelter in the shade to prevent over-heating when the Sun's radiation is at its peak. At night they retreat into burrows in order to reduce heat loss when the external temperature is low.

-GAINING HEAT FROM THE GROUND. Lizards will press their bodies against areas of hot ground to warm themselves up. When the required temperature is reached, they raise themselves off the ground on their legs.

-GENERATING METABOLIC HEAT. Although not the main source of heat, respiration still provides a proportion of a lizard's body heat.

-COLOUR VARIATIONS. Darker colours absorb more heat while lighter colours reflect more heat. Lizards in colder environments are generally darker in colour that those in warmer areas.


How do endotherms regulate their temperature?

Gain most of their heat from internal metabolic activities. Their body temperature remains relatively constant despite fluctuations in the external temperature.

Use behaviour to maintain a constant body temperature.
Shelter from cold wind or hot sunshine.
Curl up when its cold and spread out more when it is hot.

Also use a wide range of physiological mechanisms to regulate temperature.


What is the core body temperature of endotherms?

This range is a compromise between having a temperature at which enzymes work more rapidly and the amount of energy needed to maintain that higher temperature.


What are the short term adaptations that mammals and birds in cold climates have evolved to have in order to survive?

-Body with a small surface area to volume ratio.
Within volume that heat is produced and from surface area that heat is lost.
Small SA:vol means the easier it is to maintain a high body temperature.

Mammals in cold climates are therefore relatively large, they also have smaller extremities, such as ears, and thick fur or fat layers for insulation.

Some of these features can be varied over a period of time, e.g. more fat in winter.


How do mammals make rapid changes to body temperature? (6)

VASOCONSTRICTION: The diameter of arterioles near the surface of the skin made smaller. This reduces the volume of blood reaching the skin surface through capillaries. Most of the blood entering the skin therefore passes beneath the insulating layer of fat and so loses little heat to environment.

SHIVERING: The muscles of the body undergo involuntary rhythmic contractions that produce metabolic heat.

RAISING OF HEAT: The hair erector muscles in the skin contract, thus raising the hairs on the body. This enables a thicker layer of still air, which is a good insulator, to be trapped next to the skin, thus improving insulation and conserving heat in mammals with thick fur.

INCREASED METABOLIC RATE: In cold conditions more of the hormones that increase metabolic rate are produced. As a result metabolic activity, including respiration, is increased and so more heat is generated.

DECREASE IN SWEATING: Sweating is reduced, or ceases altogether in cold conditions.

BEHAVIOURAL MECHANISMS: Sheltering from the wind, basking in the sun and huddling together all help animals to maintain their core body temperature.


What are long term adaptations to life in a warm climate?

Large SA:volume ratio (large extremities, such as ears)

Lighter coloured fur to reflect heat.


What are the rapid responses that enable heat to be lost when environmental temperature is high?

VASODILATION: The diameter of the arterioles near the surface of the skin becomes larger. This allows warm blood to pass close to the skin surface through the capillaries. The heat from this blood is then radiated away from the body.

INCREASED SWEATING: To evaporate water from the skin surface requires energy in the form of heat. In relatively hairless mammals, such as humans, sweating is highly effective means of losing heat. In mammals with fur, cooling is achieved by the evaporation of water from the mouth and tongue, which occurs when air is rapidly passed over these surfaces during panting.

LOWERING BODY HAIR: The hair erector muscles in the skin relax and the elasticity of the skin causes them to flatten against the body. This reduces the thickness of the insulating layer and allows more heat to be lost to the environment when the internal temperature is higher than the external temperature.

BEHAVIOURAL MECHANISMS: Avoiding the heat of the day by sheltering in burrows and seeking out shade help to prevent the body temperature from rising.


What are the two parts of the thermoregulatory centre in the hypothalamus?

-Heat gain centre- which is activated by a fall in blood temperature. This controls the mechanisms that increase body temperature.

-Heat loss centre- which is activated by a rise in blood temperature. This controls the mechanisms that decrease body temperature.


How is temperature regulation an example of homeostasis? (steps)

STIMULUS: a change in body temperature
Detected by RECEPTORS: Thermoreceptors
which pass the information to a
COORDINATOR: Hypothalamus in the brain, which then causes an EFFECTOR: the skin to produce the appropraiate RESPONSE: increase or decrease in core temperature.



Region of the brain adjoining the pituitary gland that acts as a control centre for the autonomic nervous system and regulates body temperature and fluid balance.


Describe the regulation of body temperature.

The hypothalamus monitors the temperature of the blood passing through it. In addition the thermoreceptors in the skin measure skin temperature. These thermoreceptors send impulses along the autonomic nervous system to the hypothalamus. They provide information on the environmental temperature and so give advanced warning of potential changes in the core body temperature. The animal can therefore take measure to conserve or lose heat as appropriate, before core temperature is affected.

The two sets of thermoreceptors (in the hypothalamus and the skin) interact to control temperature.

Of the two it is the core temperature, as measured in the blood passing through the hypothalamus, which is most important.


How does problems with the thyroid cause inability to regulate body temperature?

An overactive thyroid can cause a person to become too hot whereas an under active thyroid can cause people to become too cold.

The thyroid produces hormones that are able to influence how much blood vessels dilate, which in turn affects how much heat can escape from the body.


What is the regulation of blood glucose an example of?

How different hormones interact in achieving homeostasis.


What characteristics do hormones have in common?

-produced by glands, which secrete the hormone directly into the blood (endocrine glands).
-Carried in the blood plasma to the cells on which they act- known as target cells- which have receptors on their cell-surface membranes that are complementary to the hormone.
-are effective in very small quantities, but often have widespread and long-lasting effects.


How does the second messenger model of hormone action work?

-The hormone is the first messenger. It binds to specific receptors on the cell-surface membrane of target cells to form a hormone-receptor complex.
-The hormone-receptor complex thus produced activates an enzyme inside the cell that results in the production of a chemical that acts as a second messenger.
-The second messenger causes a series of chemical changes that produce the required response. In the case of adrenaline, this response is the conversion of glycogen to glucose.



Large, pale coloured gland that is situated in the upper abdomen, behind the stomach.


What does the pancreas produce?

Enzymes (protease- breaks down protein, amylase- starch and lipase- fats) for digestion and hormones (insulin and glucagon) for regulating blood glucose.


What is the pancreas largely made up of?

When examined microscopically, the pancreas is made up largely of the cells that produce its digestive enzymes.
Scattered throughout these cells are groups of hormone producing cells known as islets of LAngerhans.


What are the two types of cells of the islets of Langerhans?

-alpha cells, which are the larger and produce the hormone glucagon.
-beta cells, which are smaller and produce the hormone insulin.


What is the main substrate for respiration?

Glucose- provides the source of energy for almost all organisms.


Why is it essential that the blood of mammals contains a relatively constant level of glucose?

For respiration-
Glucose broken down during glycolysis and, if oxygen is present, the Kreb's cycle and electron transport chain, to provide ATP (the energy currency of cells).


What happens if levels of glucose fall too low?

Cells will be deprived of energy and die- brain cells are especially sensitive in this respect because they can only respire glucose.


What happens if glucose levels rise too high?

Lowers water potential of the blood and creates osmotic problems that can cause dehydration and be equally dangerous.


What is the normal level of glucose?

90mg in each 100cm3 of blood.


Where does blood glucose come from?

-DIRECTLY FROM THE DIET. In the form of glucose resulting from the breakdown of other carbohydrate.
-FROM THE BREAKDOWN OF GLYCOGEN (GLYCOGENOLYSIS) stored in the liver and muscle cells. A normal liver contains 75-100g of glycogen, produced by converting excess glucose from the diet in a process called glycogenesis.
-FROM GLUCONEOGENESIS, which is the production of new glucose (from a source other than carbohydrate). The liver, for example, can make glucose from glycerol and amino acids.


Why does level of glucose fluctuate in animals?

Do not eat continuously and diet varies.
Also glucose is used up at different rates depending on the level of mental and physical activity.


What do the beta cells of the islets of Langerhans in the pancreas do when they detect a rise in blood glucose level?

Respond by secreting the hormone insulin directly into the blood plasma.



A globular protein made up of 51 amino acids.


What do almost all body cells have that bind with insulin molecules?

GLYCOPROTEIN RECEPTORS on their cell surface membrane.


What does insulin bring about when it combines with glycoprotein receptors?

-A change in the tertiary structure of the glucose transport protein channels, causing them to change shape and open, allowing more glucose into the cells.

-An increase in the number of carrier molecules in the cell-surface membrane.

-Activation of the enzymes that convert glucose to glycogen and fat.


How is blood glucose lowered by the binding of insulin to glycoprotein receptors?

-By increasing the rate of absorption of glucose into the cells, especially in muscle cells.
-By increasing the respiratory rate of cells, which therefore use up more glucose, thus increasing their uptake of glucose from the blood.
-By increasing the rate of conversion of glucose into glycogen (glycogenesis) in the cells of the liver and muscles.
-by increasing the rate of conversion of glucose to fat.


What is the effect of the processes brought about by insulin binding to glycoprotein receptors?

To remove glucose from the blood and so return its level to normal. This lowering of blood glucose level cause the beta cells to reduce their secretion of insulin (=negative feedback)


What do the alpha cells in the islets of Langerhans do?

Detect a fall in blood glucose and respond by secreting the hormone glucagon directly into the blood plasma.


Where are the receptors that bind to glucagon?

Only the cells of the liver have receptors that bind to glucagon, so only liver cells respond.


What do liver cells do once glucagon has been bound to their receptors?

-Activate an enzyme that converts glycogen to glucose.
-Increasing the conversion of amino acids and glycerol into glucose (gluconeogenesis).


What is the overall affect of glucagon binding to receptors on liver cells?

Increase the amount of glucose in the blood and return it to its normal level. The raising of blood glucose causes the alpha cells to reduce glucagon secretion (=negative feedback).


When is adrenaline produced?

At times of excitement or stress.


Where is adrenaline produced?

By the adrenal glands that lie above the kidneys.


How do adrenaline raise glucose levels?

-It activates glycogenolysis (glycogen to glucose)
-It inhibits glycogenesis (glycogen from glucose)
-Activates glucagon secretion and inhibits insulin secretion.


When is glucagon produced?

Released overnight and between meals.


What does it mean that insulin and glucagon act antagonistically?

They act in opposite directions, insulin lowers blood glucose level and glucagon increases it.


How is the insulin/ glucagon system self regulating?

It is the level of glucose in the blood that determines the quantity of insulin and glucagon produced.

This allows highly sensitive control of blood glucose level.


What does glucose level fluctuate around?

The set point- negative feedback.


Describe how blood glucose level is controlled by negative feedback.

Only when blood glucose level falls below a set point is the insulin secretion reduced, leading to rise in blood glucose.
In the same way, only when the level exceed the set point is glucagon secretion reduced, causing a all in the glucose level.


What is diabetes?

A chronic disease in which a person is unable to metabolise carbohydrate, especially glucose, properly.

Metabolic disorder caused by the inability to control blood glucose levels due to lack of insulin or loss of responsiveness to insulin.


Which form of diabetes is more common?

Diabetes mellitus (sugar diabetes).


What is Type 1 (insulin dependent) diabetes caused by?

The body being unable to produce insulin.


When does Type 1 diabetes usually begin?



What could Type 1 diabetes the result of?

An autoimmune response whereby the body's immune system attacks its own cells, in this case the B-cells of the islets of Langerhans.
After eating, the blood glucose level rises and stays high- hyperglycaemia.
The kidneys can't reabsorb all the glucose, so some is excreted in urine.


How does Type 1 diabetes develop?

Quickly, over a few weeks, and symptoms are normally obvious.


What are the symptoms of diabetes?

-High blood glucose level.
-Presence of glucose in urine.
-Increased thirst or hunger.
-Need to urinate excessively.
-Genital itching or regular episodes of thrush.
-Weight loss.
-Blurred vision.


What is Type 2 diabetes usually due to?

The glycoprotein receptors on the body cells losing their responsiveness to insulin.
However, it may also be due to an inadequate supply of insulin from the pancreas.


When does Type 2 diabetes usually develop?

In people over the age of 40.
However, an increasing number of cases of obesity and poor diet leading to Type 2 diabetes in adolescents.


How does Type 2 diabetes develop?

Slowly and symptoms are usually less severe and may go unnoticed.


How is type 1 diabetes controlled?

By injection of insulin, typically either two or four times a day. The dose of insulin must be matched exactly to the glucose intake as glucose levels may drop dangerously low (hypoglycaemia).


What percentage of those with diabetes have type 2?



Why can't insulin be taken orally?

Being a protein, it would be digested in the alimentary canal.


What happens if a diabetic takes too much insulin?

They will experience a low blood glucose level that can result in unconsciousness.


How is the correct does of insulin ensured?

Blood glucose levels are monitored using biosensors.


How is type 2 diabetes controlled?

By regulating the intake of carbohydrate in the diet and matching this to the amount of exercise taken.
In some cases this may be supplemented by injections of insulin or by the use of drugs that stimulate insulin production.
Other drugs slow down the rate at which the body absorbs glucose from the intestine.


What is gestational diabetes?

Diabetes that arises in pregnant women.

Usually goes away once baby is born.


What causes gestational diabetes?

It is the hormonal changes (hormones made by the placenta that resist insulin) in the second and third trimesters of pregnancy, along with the growth demands of the foetus, that increase a pregnant woman's insulin needs by two to three times that of normal.
If your body cannot make this amount of insulin, sugar from the foods you eat will stay in your blood stream and cause high blood sugars.


How is gestational diabetes usually managed?

diet and exercise.
rarely need insulin injections


What is Type 3 diabetes?

Title that has been proposed for Alzheimer's disease which is results from insulin resistance in the brain.


What is double diabetes?

When someone with type 1 diabetes develops insulin resistance, a key feature of Type 2 diabetes.


What is the most common reason for developing double diabetes?



What is secondary diabetes?

Diabetes that results from a medical condition:
Cystic fibrosis
Chronic pancreatitis
Polycystic ovary syndrome (PCOS)
Cushing's syndrome
Pancreatic cancer


How is a blood clot rapidly activated after injury?

-Platelets become activated and release a chemical- this triggers more platelets to be activated.
-Platelets very quickly form a blood clot at the injury site.
-The process ends with negative feedback, when the body detects the blood clot has been formed.


How do both adrenaline and glucagon activate glycogenolysis inside a cell even though they bind to receptors on outside of cell? (Second messenger model)

-Adrenaline and glucagon bind to their specific receptors and activate an enzyme called adenylate cyclase.
-Activated adenylate cyclase converts ATP into a chemical signal called a "second messenger".
-The second messenger is called cyclic AMP.
-cAMP activates a cascade that breaks down glycogen into glucose.