5.1.2: Homeostasis

Question 1

What may be maintained by homeostasis?

Answer 1

• body temperature
• blood glucose concentration
• blood salt concentration
• water potential of blood
• blood pressure
• carbon dioxide concentration

Question 2

Any response to changes in the environment requires a complex mechanism, which may involve a series of tissues and organs that coordinate through cell signalling. What is the standard response pathway?

Answer 2

stimulus –> receptor –> communication pathway (cell signalling) –> effector –> response

Question 3

Where can sensory receptors be found? Give examples.

Answer 3

-Sensory receptors such as temperature receptors may be on the surface of the skin (monitor changes in the external environment).
-Other receptors are internal to monitor conditions inside the body (e.g. temp receptors in the brain).
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Question 4

What happens when a sensory receptor detects a change?

Answer 4

It will be stimulated to send a message to an effector.

Question 5

What do the communication systems such as the hormonal and neuronal systems do?

Answer 5

They act by signalling between cells. It is used to transmit a message from the receptor cells to the effector cells via the coordination centre which is usually in the brain.

Question 6

What are the messages from the receptor to the coordination centre known as?

Answer 6

The input.

Question 7

What are the messages sent to the effector known as?

Answer 7

The output.

Question 8

What will the effector cells such as liver or muscles do once they receive the message?

Answer 8

Bring about a response that will change the conditions inside the body.

Question 9

Describe the process of feedback.

Answer 9

-When effectors respond to the output from the coordination centre, they bring about a response that will change the conditions inside the body.
-Such changes will be detected by the receptors.
-This will have an effect on the response pathway.
(In effect, the pathway will change)

Question 10

What is negative feedback?

Answer 10

The mechanism that brings the conditions back to optimum.

Question 11

What happens when conditions move away from the optimum?

Answer 11

• receptors detect this stimulus.
• receptors send input to coordination centres.
• coordination centres send output to effectors.
• effectors respond to output and reverse initial change.
• the system moves closer to optimum and the stimulus is reduced.
• receptors detect a reduction in stimulus and reduce input to coordination centre.
• Output to effectors is reduced and effectors reduce their activity.

Question 12

For negative feedback to work, which three processes must occur?

Answer 12

1. A change to the internal environment must be detected.
2. The change must be signalled to other cells.
3. There must be an effective response that reverses a change in conditions.

Question 13

Describe how temperature is controlled by negative feedback when core temperature rises.

Answer 13

• thermoregulatory centre in the hypothalamus detects change.
• nervous and hormonal systems carry signals to skin, liver and muscles.
• less heat is generated and more heat is lost.
• temperature falls.

Question 14

Describe how temperature is controlled by negative feedback when core temperature falls.

Answer 14

• thermoregulatory centre in the hypothalamus detects change.
• nervous and hormonal systems carry signals to skin, liver and muscles.
• more heat is generated and less heat is lost.
• temperature rises.

Question 15

A negative feedback mechanism can maintain a reasonably constant set of conditions. However, there will be some variation around the optimum condition. Why is this?

Answer 15

• When a stimulus is detected it may take some time to respond and the response may cause a slight ‘overshoot’.
• However, as long as the variation is not too great, the conditions will remain acceptable.

Question 16

Why do the conditions within a living organism need to remain within a narrow range?

Answer 16

To remain warm enough to allow enzymes to function efficiently and cool enough to not cause damage to the body’s many other proteins.

Question 17

What is positive feedback?

Answer 17

When positive feedback occurs, the response is to increase the original change.
This destabilises the system and is usually harmful.

Question 18

Give an example of positive feedback causing harm.

Answer 18

• body temperature falls.
• enzymes become less active.
• the exergonic reactions that release heat are slower.
• less heat is released.
• the body cools further and slows enzyme controlled reactions even more.
• body temperature spirals downwards.

Question 19

Outline the positive feedback cycle.

Answer 19

• Ther is a change away from optimum.
• receptor detects this change and sends an input to the coordination centre.
• coordination centre sends an output to the effector.
• effector increases this change.
• There is more change away from the optimum.

Question 20

Give an example where positive feedback is beneficial.

Answer 20

• at the end of a pregnancy, there is a dilation of the cervix.
• as the cervix begins to stretch there this causes the posterior pituitary gland to secrete the hormone oxytocin.
• oxytocin increases the uterine contractions which stretch the cervix more.
• this causes the secretion of more oxytocin.
• once the cervix is fully dilated, the baby can be born and the birth ends the production of oxytocin.

Question 21

What is another example of positive feedback?

Answer 21

The activity of neurones.