14.1.114-15 · Homeostasis and Control
Homeostasis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 9 parts Lesson 2 of 31 Unit quiz
Lesson summary
14.1.1 Homeostasis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Define homeostasis and explain how negative feedback maintains a stable internal environment.Definition — regulation of the internal conditions of a cell or organism to maintain optimum conditions fo…
- Importance
- Core body temperature
- Blood glucose concentration
- Water potential of the blood
- Respiratory gases
- Metabolic waste
Part 1 of 7What homeostasis is
Definition
- Definition regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
Internal environment
- Internal environment the tissue fluid that bathes every cell.
Set point
- Set point the normal value or narrow range at which a factor is held.
Importance
- Importance it keeps conditions optimal for enzyme action and cell function.
Part 2 of 7Factors controlled in mammals
1Core body temperature
- Core body temperature held near 37 °C.
2Blood glucose concentration
- Blood glucose concentration kept within limits.
3Water potential of the blood
- Water potential of the blood regulated by the kidneys.
4Blood pH
- Blood pH kept stable.
5Respiratory gases
- CO2 and O2 concentrations in the blood.
6Metabolic waste
- CO2 and urea.
Part 3 of 7Why control matters
Temperature
- Temperature low slows enzyme reactions and diffusion; high denatures enzymes and damages membranes.
Water potential
- Water potential too low and cells lose water; too high and animal cells may burst.
Blood glucose
- Blood glucose too low and neurones cannot respire; too high lowers blood water potential.
pH
- pH changes alter ionic and hydrogen bonds, changing the active site shape.
Part 4 of 7Negative feedback loop
1
A stimulus changes the controlled factor from the set point.
2
A receptor detects the change.
3
A coordination system (nervous, endocrine or both) transfers the information.
4
An effector (muscle or gland) carries out a response.
5
The response reverses the change and the factor returns to the set point.
Two directions
- Two directions separate mechanisms for a rise and a fall give finer control.
Continuous monitoring
- Continuous monitoring if the factor rises it is lowered, and if it falls it is raised.
Part 5 of 7Nervous and endocrine coordination
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Information carried by | Electrical impulses along neurones | Hormones in the blood |
| Speed | Fast | Slower |
| Duration | Short-lived | Longer-lasting |
Part 6 of 7Positive feedback
Definition
- Definition a response that amplifies the original change.
Effect
- Effect moves the system away from its starting point, so it does not maintain a set point.
Examples
- Examples oxytocin strengthening uterine contractions in birth, sodium channels opening in a neurone, and platelet activation in clotting.
Ending
- Ending each stops when an outside event ends the process.
| Feature | Negative feedback | Positive feedback |
|---|---|---|
| Response | Reverses the change | Amplifies the change |
| Result | Stability around a set point | Rapid movement away from the starting state |
Part 7 of 7More from the specification
1Negative feedback loop triggers
- Negative feedback loop triggers deviation from a physiological set point causes compensatory effector mechanisms to restore dynamic internal equilibrium.
2Basement membrane
- Collagen and glycoprotein filter layer.
3Target cells
- Cells that respond to a particular hormone.
4Glomerulus
- A knot of capillaries inside the Bowman's capsule.
5Cause
- A factor that directly produces an effect through a known mechanism.
6Gradient
- A difference in concentration.
7Epithelial cell
- A cell forming a covering or lining layer of tissue.
8Selective reabsorption
- Return of useful substances from the filtrate to the blood.
9Glomerular filtrate
- Fluid that enters Bowman's capsule.
10Luminal membrane
- Cell surface membrane facing the lumen.
11Glucose oxidase
- Enzyme oxidising glucose to gluconic acid and hydrogen peroxide.
12Filtrate
- The liquid collected in the Bowman's capsule.
13Capsule
- Protective outer layer on some bacteria.
14Capillary
- Vessel one cell thick where exchange takes place.
15Reabsorption
- Return of useful substances from the filtrate to the blood.
16Insulin
- A pancreatic hormone that lowers blood glucose.
17Nephron
- Kidney tubule that filters blood and forms urine.
18Plasma
- The liquid part of blood.
19Protein
- A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.
20Glucagon
- Glucagon its enzyme cascade works only in liver cells, because muscle cells have no glucagon receptors.
Key terms
| Term | Meaning |
|---|---|
| Homeostasis | Maintaining a stable internal environment. |
| Set point | Normal value or range of a controlled factor. |
| Negative feedback | Response that reverses a change. |
| Positive feedback | Response that amplifies a change. |
| Receptor | Structure detecting a stimulus. |
| Effector | Muscle or gland carrying out a response. |
| Tissue fluid | Fluid surrounding body cells. |
Avoid these mistakes
1
Saying negative feedback means a harmful response — it reverses the change and is beneficial.
2
Saying homeostasis keeps conditions constant — conditions fluctuate around a set point.
3
Leaving out the receptor or effector from a loop — all components are needed.
4
Describing positive feedback as increasing something good — it amplifies whatever the change is.
Last updated: