Homeostasis

Question 1

Why do organisms need to maintain a constant internal environment?

Answer 1

Enzymes the organism relies on only work under specific conditions including:

• pH.
• Temperature.
• Water availability.
• Absence of toxins and inhibitors.

Question 2

What is a stimulus?

Answer 2

A change in the external environment an organism is in which puts stress on the body of an organism.

Question 3

What is a response?

Answer 3

A change in the behaviour or physiology of an organism which decreases the stress caused by the stimulus.

Question 4

How can the internal environment of an organism change without a change in the external environment?

Answer 4

As cells metabolise chemicals, they produce various products that may be toxic to the cell itself or other cells in the body.

Question 5

How is the internal environment of an organism maintained?

Answer 5

• Toxins are secreted by cells, they diffuse into the bloodstream via tissue fluid.
• The level of these toxins can then be monitored by special receptors away from the site of production.
• Toxins are removed by special organs or other physiological changes.
• Concentration of substances need to be monitored closely an maintained in order to maintain constant internal environment.

Question 6

Why are multicellular organisms more efficient than single celled organisms?

Answer 6

Cells in multicellular organisms are differentiated and specialised to performing certain jobs and work together in order to perform certain processes (Division of labour). Single celled organisms need to perform all functions.

Question 7

What are the features of a good communication system?

Answer 7

• Whole body coverage.
• Systems enabling intercellular communication.
• Specificity of signals.
• Rapid transfer of signals.
• Potential for short and long term responses.

Question 8

What are the two methods of cell signalling?

Answer 8

1. Nervous system consists of a network of neurones all over the body which can quickly relay information to target cells and cause rapid but short term responses.
2. Endocrine system consists of a number of glands all over the body that secrete hormones into the bloodstream which take longer to reach target cells, but can cause long term responses.

Question 9

What is homeostasis?

Answer 9

Maintenance of a constant internal environment despite changes in the external environment.

Question 10

What factors can be kept constant in the internal environment of an organism via homeostasis?

Answer 10

1. Body temperature.
2. Blood glucose.
3. Blood salt concentration.
4. Blood water potential.
5. Blood pressure.
6. Blood CO2 concentration.

Question 11

What is negative feedback?

Answer 11

Process by which when a change is detected, the response counteracts the change to restore the optimum state. This allows for the maintenance of a constant internal environment.

Question 12

What are the features of a negative feedback system?

Answer 12

1. Input: Level of the factor being controlled.
2. Receptor: Detects the level of the factor being controlled.
3. Regulator: Compares actual level of factor against optimum. If difference detected, effector signal generated.
4. Communication system: Carries information from the regulator to the effector.
5. Effector: Acts to return current level of factor to optimum level.
6. Output: New level of factor (optimum if system working properly).

Question 13

What is a positive feedback system?

Answer 13

Process by which when a change is detected, the response amplifies the change which in effect, amplifies the response etc. This results in the rapid acceleration in the rate of a change.

Question 14

When is a positive feedback system used?

Answer 14

During situations such as childbirth and cervix contractions, where the rapid acceleration of a change is desirable.

Question 15

What is an ectotherm?

Answer 15

An organism that relies on external sources of heat in order to regulate its internal temperature.

Question 16

What are the advantages of being an ectotherm?

Answer 16

• They use less food for respiration so eat less.
• They can survive longer periods of time on little/no food.
• A greater proportion of energy can be used for growth.

Question 17

What are the disadvantages of being an ectotherm?

Answer 17

• They are less active in colder temperatures and need to warm up before becoming active, a period of time when they are vulnerable to predition.
• They cannot be active during winter and so must find enough food before winter to survive.

Question 18

How do ectotherms regulate their internal temperature?

Answer 18

• When they are cold, they will change physiologically or behaviourally in order to increase their absorption of heat from the environment.
• When they are hot, they will change physiologically or behaviourally in order to decrease their absorption of heat from the environment.

Question 19

What techniques and adaptation do ectotherms have to regulate internal temperature?

Answer 19

• Exposing body to sun increases eat absorption.
• Orienting body towards sun increases surface area exposed and heat absorption.
• Orienting body away from sun decreases surface area exposed and heat absorption.
• Hiding in shade/burrow decreases exposure to sun and thus heat absorption.
• Altering body shape increases/decreases surface area to increase/decrease heat absorption.
• Increasing breathing rate/movement increases rate of water evaporation and heat loss.

Question 20

What is an endotherm?

Answer 20

An organism that uses internal sources of heat, such as heat generated from metabolism in order to maintain a constant internal temperature.

Question 21

What are the advantages of being an endotherm?

Answer 21

• Constant body temperature regardless of external temperature.
• Can be active even during periods of time when external temperature is low. E.g. winter, night…
• Can inhabit colder areas of the planet.

Question 22

What are the disadvantages of being an endotherm?

Answer 22

• Large amounts of energy used to maintain body temperature.
• More food required in order to satisfy demand in energy.
• Less energy is used for growth as more is required for maintaining body temperature.

Question 23

How is a change in body temperature detected?

Answer 23

• Peripheral thermoreceptors detect changes in the external environment that may potentially affect core temperature and sends messages to the hypothalamus to hasten main response when core temperature is affected.
• Thermoreceptors in the thermoregulatory centre of the hypothalamus constantly measures temperature of the body and blood in order to detect changes that bring about the appropriate response.

Question 24

What changes occur when the body experiences a decrease in external temperature?

Answer 24

1. Decrease in external temperature detected by peripheral thermoreceptors and causes decrease in core body temperature which is detected by hypothalamus.
2. Less/no sweat is secreted to reduce heat loss as latent heat of evaporation for water.
3. Hairs on skin erect to trap a layer of insulating air above the skin.
4. Arterioles leading towards skin surface constrict during vasoconstriction. This increases insulation and decreases amount of heat radiated from blood via skin.
5. Rate of metabolism increased in hepatocytes which increases heat generated. This is spread around body via blood.
6. Muscle cells shiver, generating heat from friction and increased respiration,
7. Behavioural changes such as moving into sunlight or increased activity to increase rate of respiration.
8. Changes increase body temperature to optimum.

Question 25

What changes occur when the body experiences an increase in external temperature?

Answer 25

1. Increase in external temperature detected by pertpheral thermoreceptors and hypothalamus.
2. More sweat is secreted by sweat glands. Water steals heat from skin and blood to supply latent heat of vaporisation.
3. Hairs on skin flatten to decrease insulation and increase heat loss.
4. Arterioles leading towards skin surface dilate during vasodilation. This increases blood flow in capillaries near surface of skin and increases radiation of heat from skin.
5. Rate of metabolism in hepatocytes reduced to decrease amount of heat produced.
6. Behavioural changes such as moving into shade or minimising surface area of body exposed ti sun.
7. Changes decrease body temperature to optimum.

Question 26

Which communication system is primarily used for thermoregulation in endotherms?

Answer 26

Nervous system.

Question 27

What is the difference between endocrine and exocrine glands?

Answer 27

• Endocrine glands are ductless glands that release hormones directly into the blood.
• Exocrine glands are ducted glands that have their secretion sites away from the gland itself, so the duct is required to carry substances from the gland to its site of use.

Question 28

Why is the endocrine system specific?

Answer 28

Only certain cells in certain tissues have complementary receptors to certain hormones and will bind. This means that hormones can be carried all over the body but will only affect certain target cells in target tissues.

Question 29

What are the 2 different types of hormones?

Answer 29

• Protein/peptide hormones are large, fat insoluble molecules that usually require secondary messengers to cause an effect in target cells.
• Steroid hormones are small, fat soluble hormones that travel directly through the phospholipid bilayer to cause an effect in target cells.

Question 30

What is the actions of adrenaline?

Answer 30

• Adrenaline is an amino acid-based hormone so cannot pass through plasma membrane.
• Adrenaline binds onto complementary shaped receptors on surface of target cells.
• Receptors associated with the enzyme adenyl cyclase.
• Adrenaline is the primary messenger as it binds to the part of the receptor outside the cell.
• The binding action activates the adenyl cyclase which begins converting ATP into cyclic AMP inside the cell.
• Cyclic AMP acts as the secondary messenger as it triggers changes inside the cell associated with adrenaline by activating enzymes and pathways.

Question 31

What is the structure and function of the adrenal gland?

Answer 31

• Adrenal cortex (outside part of gland) is associated with production of the hormone adrenaline.
• The adrenal medulla (inside part of gland) is associated with production of steroid hormones including:
  1. Mineralocorticoids that regulate inorganic ion concentration in the body (Na+, K+).
  2. Glucocorticoids help regulate blood glucose by controlling metabolism in liver.

Question 32

What are the 2 functions of the pancreas?

Answer 32

1. Secretion of digestive enzymes and sodium hydrogencarbonate as pancreatic juice into the duodenum via the pancreatic duct (Exocrine function).
2. Monitoring and secretion of enzymes associated with controlling glucose levels in the blood.

Question 33

What substances do the enzyme-secreting cells in the pancreas produce?

Answer 33

• Amylase.
• Trypsinogen (precursor to trypsin).
• Lipase.
• Sodium hydrogencarbonate.

Question 34

What cells make up the islets of Langerhans?

Answer 34

1. Alpha cells that are associated with secretion of glucagon.
2. Beta cells that are associated with secretion of insulin.

Question 35

What happens when there is a rise in blood glucose above normal?

Answer 35

1. Rise in blood glucose detected by beta cells in the islets of Langerhans in the pancreas.
2. Beta cells secrete the hormone insulin into the bloodstream.
3. Insulin travels via the bloodstream to target cells with complementary receptors, including muscle cells and hepatocytes.
4. Insulin increases the rate of conversion of glucose into glycogen in hepatocytes (glycogenesis), which is stored.
5. Insulin increases the rate of conversion of glucose into glycogen in muscle cells, as well as increasing respiration rate.
6. Blood glucose drops to normal level.

Question 36

What happens when there is a drop in blood glucose below normal?

Answer 36

1. Drop in blood glucose detected by alpha cells in the islets of Langerhans.
2. Alpha cells secrete the hormone glucagon into the bloodstream.
3. Glucagon travels via the bloodstream to hepatocytes, target cells with complementary receptors.
4. Glucagon increases rate of conversion of glycogen into glucose (glycogenolysis), amino acids and fats into glucose (gluconeogenesis).
5. Glucose is released into the bloodstream.
6. Blood glucose rises to normal level.

Question 37

What are the steps involved in controlling insulin secretion?

Answer 37

1. Plasma membrane of beta-cells contain Ca2+ and K+ channels.
2. K+ channels are usually open and Ca2+ closed, K+ diffuses out of cell, down concentration gradient, making inside of cell more negative at -70mV.
3. An increase in blood glucose concentration results in more glucose diffusing into the cell.
4. More glucose is respired to produce more ATP.
5. Excess ATP causes K+ channels to close.
6. K+ concentration increases inside the cell so p.d. across membrane decreases.
7. Rise in p.d. opens voltage-gated Ca2+ channels.
8. Ca2+ ions diffuse into cell down concentration gradient and cause vesicles containing insulin to move to plasma membrane.
9. Insulin vesicle fuse with plasma membrane and release insulin outside of cell by exocytosis.
10. Insulin eventually diffuses into bloodstream.