HOMEOSTASIS
Multi cellular organisms function best, and at optimum metabolic efficiency, if the internal
environment for their cells is maintained at a constant level. Multi cellular organisms
regulate their internal environment in order to remain healthy. They monitor all the
activities of their cells, their requirements and the wastes they produce, using feedback
systems. The organism, as a whole, coordinates the surroundings of its cells to ensure that
the composition of the internal body fluid remains within the required limits. The internal
environment for cells can never be controlled completely but the coordinating systems
work to maintain a balance within certain limits. This is known as homeostasis.
Link to syllabus outcomes:

explain why the maintenance of a constant internal environment is important for optimal metabolic
efficiency

describe homeostasis as the process by which organisms maintain a relatively stable internal
environment

explain that homeostasis consists of two stages:
- detecting changes from the stable state
- counteracting changes from the stable state

gather, process and analyse information from secondary sources and use available evidence to
develop a model of a feedback mechanism
Activities & Questions
 What is optimal metabolic efficiency?
 Why is it so important for multi cellular organisms to maintain a constant internal
environment for optimal metabolic efficiency? HINT: Think about the function of
enzymes and the conditions they need to work.
 Define homeostasis
In order for organisms to survive, it is essential for them to be able to pick up information
and react accordingly. Any information that causes a response is called a stimulus.
Environments contain many stimuli (plural of stimulus) and organisms have special
receptors that detect them. Some receptors detect a variety of stimuli.
 Construct a table that shows at least 5 different stimuli and the types of receptors
that would detect these stimuli. For example, light is a stimulus and the receptor that
detects light is known as a photoreceptor.
 Find or create your own diagram to illustrate the stimulus-response pathway. Write a
short explanation in your own words.
 What are effectors?
 In order to get an effector to respond, a receptor needs to send a message via a
“control centre”. What is the control centre in multi cellular organisms?
When there are changes to an organism’s stable (normal) state, the response is
homeostatic. This means that actions occur which counteract the change and bring the
organism back to its stable state.
If receptors in the skin detect heat, they relay information via the nerves to the
hypothalamus, which also contains receptors sensitive to the heat of passing blood. This
triggers the sympathetic nervous system to dilate skin capillaries and activate sweat glands.
When receptors in the skin detect a low temperature, a negative feedback mechanism is
activated to stop the original action. If skin temperature is still low, the hypothalamus may
activate thyroid hormones to increase metabolic rate, activate the sympathetic nervous
system to shut down skin capillaries and sweat glands and activate food metabolism in the
liver to produce heat. In this way, the body can maintain a stable body temperature.
 Find the definition of the underlined words in the passage above
 What is a positive feedback mechanism?
You need to conduct some research so that you can create your own feedback model.
There should be two loops or cycles in the model that you create. This is because there are
two stages in homeostasis: detecting a change from the stable stage and counteracting the
change to return the body to a stable state.
SAMPLE EXAM QUESTIONS
Q. In organisms, the maintenance of a constant internal environment is
(A) necessary because organisms must have a constant body temperature.
(B) necessary because enzyme activity is highest at specific temperatures.
(C) unnecessary because organisms are found in environments with a broad range of
temperatures.
(D) unnecessary because the nervous system detects and responds to changes in
ambient temperature.
Q. Amylase is an enzyme that catalyses the breakdown of starch. As the temperature
rises, the activity of this enzyme will
(A)increase until the enzyme begins to denature.
(B)decrease until the substrate begins to denature.
(C)increase until the optimum pH level is reached.
(D)decrease until the substrate reaches a maximum concentration.
Q. An experiment was conducted to determine the effects on body function of increasing
ambient temperature. Participants (100 individuals) were asked to lie down and remain as
still as possible during the entire experiment. The following observations were made:
•
The starting temperature of the room was 22°C.
•
The mean body temperature of all participants at the start of the experiment was
37.1°C.
•
As the room temperature increased, sweating and heart rate also increased.
•
As the room temperature returned to 22°C, sweating and heart rate returned to
normal.
In a control group of 100 participants who were lying down in a room where the ambient
temperature was maintained at 22°C, no changes in sweating and heart rate were
observed.
a) Identify the independent and dependent variables in this experiment
b) Identify the body system that monitors and responds to changes in external temperature
Q. Use an example to outline the role of the nervous system in maintaining homeostasis .
Q. Draw a labelled diagram to show how a specific feedback mechanism plays an
essential role in homeostasis.