Homeostasis

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.

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.

What is Homeostasis?

The maintenance of a constant internal environment in the

body is called Homeostasis

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

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 = ___________

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

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.

Penguins huddling to keep warm

What mechanisms are there to cool the body down?

1. 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.

Sweating

The skin

2. 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!

What mechanisms are there to cool the body down?

If the temperature rises, the blood vessel dilates (gets bigger).

This means more heat is lost from the surface of the skin

What mechanisms are there to warm the body up?

1. 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.

If the temperature falls, the blood vessel constricts (gets shut off).

This means less heat is lost from the surface of the skin

What mechanisms are there to warm the body up?

2. 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.

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

Stoma Open Stoma Closed

Guard Cells

Stoma

Function of Stomata

CO2

O2 H2O

What goes in?

What goes out?

•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?

Stoma Open Stoma Closed

Guard CellsFunction of Guard Cells

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

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.

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.

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.

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.