1. HOMEOSTASIS
Objectives – What you will need to know from this section
Define the term: homeostasis.
Homeostasis is the maintenance of a constant internal environment within the body.

2. 3.4.2 Necessity for HOMEOSTASIS
Objectives – What you will need to know from this section
Explain the need for homeostasis.
Homeostasis is necessary if an organism is to be independent of its surroundings, and if its metabolism is to function efficiently.

3. HOMEOSTASIS
Your body and its individual cells need just the right conditions to perform at their best.
A cell’s delicately balanced chemical reactions work best within narrow limits of temperature, pH, solute concentration etc.

4. Simple aquatic organisms (like Amoeba and seaweeds) obtain their food, water and oxygen directly from the water that surrounds them.

5. More complex organisms carry their own internal sea inside them (the tissue fluid), which is in contact with all the living cells of the body.
Tissue Fluid

6. Many organisms can control conditions of temperature, fluid balance and chemistry within themselves (their ‘internal environment’) even when conditions in their external environment are changing.
Homeostasis is the maintenance of a constant internal environment within the body.

7. LEARNING CHECK What is homeostasis? Name a simple aquatic organism
Name some things in our body that we have to keep steady
What is tissue fluid?
What is meant by “internal environment”?

8. Homeostasis Homeostasis means “controlling internal conditions”:
Waste products need to be removed how ?
CO2
Produced by respiration, removed via lungs
Urea
Produced by liver breaking down amino acids, removed by kidneys and transferred to bladder
Internal conditions need controlling --- how ?
Temperature
Increased by shivering, lost by sweating
Ion content
Increased by eating, lost by sweating + urine
Water content
Increased by drinking, lost by sweating + urine
Blood glucose
Increased and decreased by hormones

10. 3.4.2 Necessity for HOMEOSTASIS
Homeostasis is necessary if an organism is to be independent of its surroundings, and if its metabolism is to function efficiently.
Most homeostatic mechanisms work by negative feedback, i.e. if there is a change away from the normal optimum value, action is automatically taken to reverse this change.

12. Homeostasis – Temperature Regulation
Temperature influences the rate of enzyme-controlled reactions that sustain life.
Mammals and birds are endotherms (warm blooded): they have a fairly constant body temperature.
They can operate in low temperature environments, as they can keep the rate of enzyme activity high.

13. Most animals are ectotherms (cold blooded) —their body temperature varies with the external temperature, e.g. fish, amphibians, reptiles.

14. LEARNING CHECK Why is homeostasis necessary? What is metabolism?
What is negative feedback?
Can you explain how an oven thermostat works?
What is an endotherm?
What is an ectotherm?

15. Homeostasis --Body Temperature
Control of body temperature is an example of how homeostasis works in humans.
Our normal core body temperature is maintained at 37°C, the heat being mainly produced from the liver during its metabolism.
Muscles, skin and blood all play a role in controlling body temperature.

16. The SKIN – Temperature control

17. If our temperature rises, more blood flows close to surface of the skin and glands release sweat.

18. If our temperature drops, blood flows beneath the adipose tissue, we stop sweating, our hairs stand up and we shiver.

20. LEARNING CHECK What is our normal body temperature?
What happens whern we get too hot ?
What happens when we get too cold?
What part of the brain detects changes in our temperature?

21. Controlling body temperature
Body temperature is controlled by the thermo-regulatory centre in the ________. It is kept at 370C as this is the best temperature for __________ to work in. If the body becomes too hot then blood vessels _________ and sweat glands release ________. If the body is too ______ then blood vessels constrict and muscles start to __________.
Words – sweat, enzymes, cold, dilate, shiver, brain

22. Control of body water levels is an example of how homeostasis works in humans.
The importance of water to living organisms
Metabolism— it is a substrate (ingredient) in many reactions
Medium for chemical reactions
Movement of materials in and out of cells.
Transport system
Lubricant
Support
Temperature control
Dispersal of reproductive cells and seeds.

24. Homeostasis – Water Levels in the Body
Being a land animal, we have a continuous need to conserve water.
Water must be taken in daily and its loss must be carefully regulated.
Water is taken in as food and drink, and is also formed inside the cells during some reactions, especially respiration.

25. Water is lost from the body through a number of ways:
Lungs – some water gets evaporated as we exhale from our warm, damp lungs.
Skin – by evaporation from cells and through sweat.
Intestines – in the faeces (undigested food).
Kidneys – in dissolving the poisons and wastes we wish to excrete from the body.

26. We have no control over the amount of water lost each day from the lungs, skin or intestines.
So the kidneys are the water control (osmoregulatory) organs of the body – conserving or eliminating water as the body requires.

27. LEARNING CHECK List 4 ways water is important to living organisms.
List the ways we gain water.
List the ways we can lose water.
Explain the term osmoregulation.

28. Homeostasis -- Plants
Plants absorb water through their roots, but lose water from their leaves by evaporation (transpiration).
Over 90% of the water that enters a leaf goes right on through it and evaporates into the surrounding air.
The evaporation of water from leaves [and stems] is called transpiration.

29. The loss of water is reduced by a waxy cuticle and by the closing of the stomata [pores] on the lower surface of the leaf.

30. These pores stay open during the day so that CO2 can enter, for photosynthesis. But, water can escape at this time too. Stomata can stay closed at night, when there is no light for photosynthesis. This allows plants to conserve water.

31. LEARNING CHECK Plants absorb water from where?
Plants can lose water from where?
What is evaporation?
What is transpiration?
How do plants try and reduce water loss?
What are stomata?

32. Leaves are often adapted to try and reduce this loss of water—they have thick cuticles; thin,spiny leaves; etc.
Since plants do not move, their oxygen can be supplied by simple diffusion from the soil or air.

33. What about trees and woody plants ?
How do gases [oxygen and carbon dioxide] get through the waterproof, corky surface of bark?
They do so through lenticels, which are areas where the packing of bark cells is loosened up a bit.
Lenticels often look like tiny raised blisters on a branch or twig.

34. Winter twig – showing lenticels

35. LEARNING CHECK What does the term “adapted” mean
How are plants adapted to living in dry, hot climates?
What is diffusion?
What are lenticels?
How are lenticels different from stomata?