1. Homeostasis

2. Glossary
Negative feedback – a stimulus changes some condition and it triggers a response that reverses the change.
An example of negative feedback is your home’s thermostat
Positive feedback - the original stimulus initiates a chain of events that intensify change from an original condition. Positive feedback usually moves away from homeostasis.
An example of positive feedback is childbirth.

3. What is Homeostasis? What do animals need to keep constant?
Body cells work best if they have the correct
Temperature
Water levels
pH level
Sugar levels
Your body has mechanisms to keep the cells in a constant environment.

4. What is Homeostasis?
The maintenance of a constant internal environment in the body is called Homeostasis

5. Controlling body temperature
All mammals maintain a constant body temperature.
Human beings have a body temperature of about 37ºC.
E.g. If your body is in a hot environment your body temperature is 37ºC
If your body is in a cold environment your body temperature is still 37ºC

6. Controlling body temperature
Animals with a large surface area compared to their volume will lose heat faster than animals with a small surface area.
Volume = _______
Surface area = ______
Volume : Surface area ratio = ___________
Volume = _______
Surface area = ______
Volume : Surface area ratio = ___________

7. Controlling body temperature
Volume : Surface area ratio = 1:6
Volume : Surface area ratio = 1:5
For every 1 unit of heat made, heat is lost out of 6 sides
For every 1 unit of heat made, heat is lost out of 5 sides

8. Controlling body temperature
Volume : Surface area ratio = 1:6
Volume : Surface area ratio = 1:5
The bigger the Volume : Surface Area ratio is, the faster heat will be lost.

9. Penguins huddling to keep warm

10. What mechanisms are there to cool the body down?
Sweating
When your body is hot, sweat glands are stimulated to release sweat.
The liquid sweat turns into a gas (it evaporates)
To do this, it needs heat.
It gets that heat from your skin.
As your skin loses heat, it cools down.

11. Sweating
The skin

12. What mechanisms are there to cool the body down?
Vasodilation
Your blood carries most of the heat energy around your body.
There are capillaries underneath your skin that can be filled with blood if you get too hot.
This brings the blood closer to the surface of the skin so more heat can be lost.
This is why you look red when you are hot!

13. This means more heat is lost from the surface of the skin
If the temperature rises, the blood vessel dilates (gets bigger).

14. What mechanisms are there to warm the body up?
Vasoconstriction
This is the opposite of vasodilation
The capillaries underneath your skin get constricted (shut off).
This takes the blood away from the surface of the skin so less heat can be lost.

15. This means less heat is lost from the surface of the skin
If the temperature falls, the blood vessel constricts (gets shut off).

16. What mechanisms are there to warm the body up?
Piloerection
This is when the hairs on your skin “stand up” .
It is sometimes called “goose bumps” or “chicken skin”!
The hairs trap a layer of air next to the skin which is then warmed by the body heat
The air becomes an insulating layer.

18. Homeostasis in Plants What do you think plants need to keep constant?
Water
Sunlight
CO2
Control of water levels in the plant is important to the survival of the plant.
This is achieved by a number of methods:
Waxy cuticle on leaves
Storage of water
Opening and closing of stomates

19. Function of Stomata Stoma
Guard Cells
What process involves using CO2 and H2O releasing O2 as a waste product?
Photosynthesis
What is the plant using this process to make?
Carbohydrates-glucose
If the plant needs water for photosynthesis, why is water coming out of the stoma?
What goes out? O2
H2O
CO2
What goes in?
Stoma Open
Stoma Closed
Stoma

20. Function of Guard Cells
These stomata (leaf openings) allow water and O2 out of the leaf.
Why would the plant close stomata with guard cells?
Prevent excess water loss through transpiration. (conserve water)
So what is the point of having stomata?
Allow gas exchange for photosynthesis and control water loss
Stoma Open
Stoma Closed
Guard cells open by inflating with extra water. They do this by pumping K+ ions into the cell, which causes water to rush in via osmosis to diffuse the high ion concentration.

21. Phototropism Bending shoots
Diagram of a shoot bending towards the light.
Note how auxin released only on the dark side of the shoot is able to cause cell elongation on that side. It is the uneven cell growth which causes the shoot to bend - towards the light in this case.
Stem bends to optimize the exposure to the light and keep the amount of photosynthesis constant.

22. Homeostasis in Plants
The opening and closing of the stomata maintains water balance in the plant and thus maintains homeostasis.
Stems bend towards sunlight to maintain the amount of photosynthesis.

23. NEGATIVE FEEDBACK
Temperature regulation, and water and sunlight regulation in plants are all examples of negative feedback mechanisms.
Processes in which an initial change will bring about an additional change in the opposite direction.

24. POSITIVE FEEDBACK
A positive feedback is a process in which an initial change will bring about an additional change in the same direction.
An example of a simple positive feedback in everyday life is the growth of an interest-earning savings account. As interest is accrued the principal will begin to grow (assuming money is not withdrawn). As the principal grows, even more interest will be accrued, quickening the rate of principal growth.
Ice has a higher albedo (or reflectivity) than vegetation, soil, or water. As ice expands, more solar radiation is reflected to space, less is absorbed by the surface, and temperatures decrease. Cooler temperatures lead to more ice growth, more reflection of solar radiation back to space, and even cooler temperatures - a positive feedback.