3.4.1
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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.
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.
3.4.1
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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.
 Simple aquatic organisms (like Amoeba and seaweeds)
obtain their food, water and oxygen directly from the
water that surrounds them.
 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
 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.
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”?
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
Ion content
Water content
Blood glucose
Increased by shivering, lost by sweating
Increased by eating, lost by sweating + urine
Increased by drinking, lost by sweating + urine
Increased and decreased by hormones
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.
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.
 Most animals are ectotherms (cold blooded) —their body
temperature varies with the external temperature, e.g. fish,
amphibians, reptiles.
LEARNING CHECK
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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?
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.
The SKIN – Temperature control
 If our temperature rises, more blood flows close to
surface of the skin and glands release sweat.
 If our temperature drops, blood flows beneath the adipose
tissue, we stop sweating, our hairs stand up and we shiver.
LEARNING CHECK
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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?
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
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.
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.
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.
 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.
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.
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.
 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.
 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.
LEARNING CHECK
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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?
 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.
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.
Winter twig – showing lenticels
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?