Principles Of Homeostasis And Negative Feedback

Question 1

What is homeostasis?

Answer 1

• maintenance of a constant internal environment (blood and tissue fluid that bathes the cells)

Question 2

Why is homeostasis important?

Answer 2

• ensures that changes to water levels, glucose levels, temperature levels, pH etc. are controlled within set limits

Question 3

Why is it important that core body temperature and blood pH are kept optimal?

Answer 3

• If temperature too high/pH not optimal…
• hydrogen/ionic bonds in enzymes will break
• The tertiary structure of the enzyme will change
• The shape of the active site will change
• The substrate will no longer fit into the active site (not complementary with each other)
• No enzyme-substrate complexes will form
• Metabolism stops / organism cannot survive

Question 4

How does homeostasis affect an organisms range of geographical habitats?

Answer 4

• range of geographical habitat will be greater the better homeostasis is

Question 5

What are the homeostasis steps (generalised)?

Answer 5

• set point
• receptor
• coordinator
• effector
• feedback loop

Question 6

What is set point?

Answer 6

• a system such as the body which operates at a set level

Question 7

What is a receptor?

Answer 7

• cells or organs that detect changes or deviations from this set point

Question 8

What is a coordinator?

Answer 8

• an inbuilt system that connects each receptor with an appropriate effector, designed to bring the system back to normal

Question 9

What is an effector?

Answer 9

• muscle or gland that responds to the changes and returns the system back to normal

Question 10

What is the feedback loop?

Answer 10

• control is by negative feedback, the body corrects deviations from a set point eg. When water/temperature levels become too high or low

Question 11

What is negative feedback?

Answer 11

• control mechanism where the body corrects deviations from set point (an optimum level), but part of negative feedback is turning off the corrective measures, as the system gets closer to its normal range

Question 12

Negative feedback example for sweat glands

Answer 12

• The hypothalamus in this example triggers impulses to be sent to the effectors (e.g., sweat glands) to respond to the deviation from set point
  when body temperature rose
• When the cooler blood now passes over the hypothalamus, receptors ( thermoreceptors) respond by reducing the activity of sweat glands

Question 13

Why is it important as part of negative feedback to turn off the corrective measures?

Answer 13

• If information is not fed back once an effector has corrected any deviation, then the receptor will continue to stimulate the effector and an over-correction will cause a deviation in the opposite direction to the original one
• This could lead to potentially damaging consequences in a system (i.e. human)

Question 14

What is the advantage of having separate negative feedback mechanisms to control deviations away from normal?

Answer 14

• provides a greater degree of homeostatic control – the body ensures that information from all receptors is analysed by the coordinator before action is taken

Question 15

can you think of a specific example in the body where ‘mixed messages’ as such, could be dangerous?

Answer 15

• during strenuous exercise, the blood temp rises but sweating cools the skin – body would detect skin being cooled, but then would not want to raise overall temp to compensate

Question 16

Examples of negative feedback

Answer 16

• human body temperature
• human blood pressure
• human metabolism
• regulation of blood sugar in humans
• production of human red blood cells

Question 17

Human body temperature

Answer 17

• if the temperature drops, the body shivers to bring up the temperature and if it is too warm, the body will sweat to cool down due to evaporation

Question 18

Human blood pressure

Answer 18

• when blood pressure increases, signals are sent to the brain from the blood vessels
• Signals are sent to the heart from the brain, and heart rate slows down, thus helping blood pressure to return to normal

Question 19

Human metabolism

Answer 19

• when a human is hungry, metabolism slows down to conserve energy and allows the human to continue living with less food

Question 20

Regulation of blood sugar in humans

Answer 20

• when blood sugar rises, insulin sends a signal to the liver, muscles, and other cells to store the excess glucose
• Some is stored as body fat, and other glucose is stored as glycogen in the liver and muscles

Question 21

Production of human red blood cells

Answer 21

• decrease in oxygen is detected by the kidneys and they secrete erythropoietin
• This hormone stimulates the production of red blood cells

Question 22

Positive feedback

Answer 22

• mechanism whereby feedback (after a deviation from set point) causes the corrective measures to remain on. This makes the system deviate even more from its original level

Question 23

What are some examples of positive feedback?

Answer 23

• Influx of sodium into a cell during build up of action potential – sodium entry increases the permeability of the neurone, triggering entry of more ions
• Release of the hormone oxytocin during pregnancy causes an action that stimulates more of its own release

Question 24

More detail for release of oxytocin during pregnancy positive feedback

Answer 24

• baby pushes against cervix causing it to stretch
• stretching of cervix causes nerve impulses to be sent to brain
• brain stimulates posterior pituitary to release oxytocin
• oxytocin causes smooth muscle lining the uterus to contract

Question 25

Osteoblasts are cells that form bone tissue. The secretion of osteocalcin (in an inactive form) by osteoblasts is controlled by positive feedback. Use information from the diagram to explain why this is positive feedback.

Answer 25

• Osteocalcin causes more release of insulin
• more insulin causes more (inactive) osteocalcin release

Question 26

Osteoblasts are cells that form bone tissue. The secretion of osteocalcin (in an inactive form) by osteoblasts is controlled by positive feedback. The acidic pH conditions created by osteoclasts cause the inactive form of the protein hormone osteocalcin to change into the active form of osteocalcin. Suggest how.

Answer 26

• change in pH changes / breaks ionic or hydrogen bonds
• changes tertiary structure

Question 27

The scientists concluded that drinking a sucrose solution had a positive feedback effect on the rats’ desire to eat. How do these data support this conclusion?

Answer 27

• Positive correlation between sucrose and dopamine concentrations / higher concentration of sucrose, more dopamine
• So (dopamine) makes them want to drink / eat more (sucrose)
• Positive feedback because drinking / eating leads to wanting to drink / eat (even) more

Question 28

In humans, when the stomach starts to become full of food, receptors in the wall of the stomach are stimulated. This leads to negative feedback on the desire to eat. Suggest why this feedback is important

Answer 28

• Negative feedback stops desire / wish to eat / appetite / stops dopamine release / makes them feel full
• limits amount eaten / stops eating / prevents constant eating
• Prevents / reduces risk of obesity / too much energy intake / prevents vomiting