Session 2. Area of Study 2 – Detecting and Responding
Coordination and regulation
Signal transduction
Organisms need to be able to detect and respond to changes in the external and internal
environment. That means they need cells or tissues which can detect changes in the
external and internal environment, transfer information about those changes
(transduction) to an organ or cells that can respond in the appropriate way to this
change. In higher organisms, like ourselves, these messages may pass through a
central processing unit such as the central nervous system.
There are five principles to communication:
1. a signal is produced (by production of a signalling molecule in a cell)
2. the signal is detected (by a receptor protein on the target cell)
3. the signal is transferred until it reaches its target – transduction (through the cell)
4. a response is made to the signal (the cell responds to the signal)
5. the signal is switched off after the response is made (the signalling cell switches
of production of the signalling molecule).
These principles can be applied to external signals and to internal signals of the
organism and the cells.
External Stimuli
To detect external stimuli organisms have a collection of receptors collectively known as
exteroreceptors:
chemoreceptors – found in the nose (smell) and mouth (taste)
mechanoreceptors – found in the ear (sound and balance) and skin (touch and
pressure)
photoreceptors – found in the eye (light)
thermoreceptors – found in the skin (temperature change)
pain receptors – free nerve endings in the skin
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The organisms needs to receive these stimuli, analyse what they mean for the organism
and respond in an appropriate manner to keep the organism safe.
Internal Stimuli
Organisms also need to respond to changes within the internal environment of the
organism. Specialised cells called interoceptors measure changes in the internal
environment – such as temperature, water content, carbon dioxide levels, and blood
sugar content.
Responding to signals from the external and internal environments of organisms and
cells is important:

For the major developmental processes including growth and reproduction.

To maintain a relatively stable internal environment within a small range – this is
called homeostasis.

To survive challenges from the external environment.
Intra and Inter cellular Communication
Cells communicate within the cytosol and with neighbouring cells. Within the cell various
proteins are produced which relay messages to target proteins to act in a certain way.
For example, the production of the hormone insulin is the result of proteins within the
cells causing the gene for insulin to be unwound, transcribed and then translated. That
hormone is then packaged by the Golgi Apparatus and taken to the cell membrane in a
vesicle and secreted into the blood stream by the cell. It then communicates to other
cells to take up glucose.
1. Signal
molecule
2. Receptor
protein
3.
Intracellular
signal proteins
4. Target
proteins
5. Response is switched off by enzyme breakdown of one of the signaling molecules.
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Principles of homeostasis
It is important that the concentration of materials like glucose, water, carbon dioxide, and
oxygen are kept within certain limited ranges in the blood plasma, interstitial fluids and
intracellular fluids for the normal function of the cell. The organism must respond to
changes in these levels to maintain homeostasis.
Homeostasis in organisms relies on the stimulus-response model. This is a simple
model which basically says a stimulus is received by a receptor. The receptor sends the
information to a central processing unit which analyses the information and sends a
message to an effector which displays the response.
Stimulus
Receptor
Central Processing Unit
Response
Effector
Touching a hot-iron is the classic example of what is called the negative feedback
stimulus-response model. The effect is the opposite of the stimulus. That is, I touch hot
object – my hand flings back away from the source of the pain.
Positive feedback is when a change of variable occurs in a system, the system
responds. In the case of positive feedback the response of the system is to change that
variable even more in the same direction. For a simple example, imagine an ecosystem
with only one species and an unlimited amount of food. The population will grow at a
rate proportional to the current population, which leads to positive feedback. This has a
de-stabilizing effect, so left unchecked, does not result in homeostasis. In some cases (if
not controlled by negative feedback), a positive feedback loop can run out of control, and
can result in the collapse of the system.
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Nervous System
The nervous system is made of two parts – the central nervous system (CNS) which is
the brain and spinal cord and the peripheral nervous system (PNS) which is the nerve
cells throughout the rest of the body. These are only found in animals – the higher order
the animal the more complex the nervous system.
The nervous system allows for an organism to respond quickly to a stimulus, although
the effect is generally short lived. This quick response is based on electrical messages
being sent along specialised cells called neurones. There are many types of neurons
including:
Sensory neurons – transmit messages from receptor organs to the CNS
Motor neurons – transmit impulses from the CNS to the effector cells
Connector neurons – relay messages between connected neurons.
Each neuron contains a cell body (nucleus and many other important organelles),
dendrites (slender, branched projections that conduct electrical messages from other
cells to the cell body), and an axon (extension of the neuron that transmit the nerve
impulse away from the cell body) which is covered with an insulating layer of fat known
as the myelin sheath. The myelin sheath is made up of Schwann cells. The insulation
stops the electrical impulse being lost to the surrounding tissue and fluids and keeps it
moving very quickly.
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At the point where axon endings and dendrite endings meet there is a gap known as the
synapse. For a message to cross this gap requires the electrical message in the axon to
stimulate vesicles which contain a neurotransmitter which is a chemical substance.
The neurotransmitter leaves the axon and is received by the dendrite and stimulates an
electrical impulse to travel down the dendrite. There are many types of neurotransmitters
and each appears to have a specific job. Research into this area is ongoing and new
transmitters are being discovered all the time.
The electrical impulse is passed down neurons by a special relationship between
potassium ions (K+) and sodium ions (Na+). Electrons pass along between the two
positive ions, passing the electrical charge with them.
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Endocrine Systems in Animals
Both plants and animals use chemical
messengers, called hormones, to respond to
changes within the internal environment. The
response time to a stimulus is slow, but generally
long lasting. Hormones are generally only
produced in small amounts as they tend to work
on a specific target cell. Hormones are generally
either steroids (lipid soluble derived from
cholesterols, such as oestrogen and testosterone)
or polypeptide based hormones including
adrenaline, insulin and glucagon. The endocrine
system comprises a series of glands spread
around the body (see figure) including the gonads,
pancreas, adrenal glands, thymus gland, thyroid
gland, pituitary and hypothalamus. The hormones
released play a critical role in Homeostasis,
regulating water and blood glucose levels, foetal
development and menstruation in females.
Plant Hormones
As for animals, plants regulate growth, development, and reproduction using hormones.
However, unlike animal hormones, plant hormones are involved in more than one type of
response.
Auxins – are produced in the growing tips of plants and their major action is to control
the enlargement and elongation of cells, particularly in the tips of stems (in response to
light) and in the tips of roots (in response to gravity). They are also involved in apical
dominance (which means that the central tip grows at the expense of other tips), and
stem enlargement.
Cytokinins – these hormones act on shoots, roots and fruits. This hormone is
associated with cell reproduction in new shoot growth and growth of young fruits.
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Gibberellins – promote plant growth, by stimulating both cell elongation and cell
reproduction in the tissues of plant stems and leaves – gibberellins also initiate seed
germination and bud development.
Abscisic Acid – controls the opening and closing of stomata and the cell death
associated with leaf, fruit and limb drop.
Ethylene – stimulates the abscission zone in ageing leaves and stimulates ripening of
mature fruit. May also be responsible for controlling the flowering of plants.
External Factors
Light – plants grow toward light. Auxins move to the dark side of stems causing
elongation on the dark side of the stem and the stem therefore bends toward the light
source. Phototropism
Gravity – in the roots auxins are also responsible for growth toward the centre of the
earth. Geotropism
Contact – auxins are also implicated in the manner in which climbing plants respond to
contact and twine themselves around other objects. Thigmotropism
Time – the length of day causes different plants to respond in different ways. Some will
flower due to a short day, others require a long day. Seeds also germinate in response
to the amount of light. The light heats the soil and as the warmth increases the seeds
begin to germinate.
Homework
1. Write down three questions that you can’t work out the answer for yourself and
ask your teacher.
2. Select 20 glossary words for this section of the course and create a mix and
match activity. Share this with a study partner.
3. Create 20 short answer questions related to this section of the course. Share
these with a study partner.
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VCAA 2008
Question 10
In a multicellular organism, the term ‘internal environment’ refers to the
A. cytoplasm.
B. cell organelles.
C. nuclear regions.
D. extracellular fluid.
Question 11
The following diagram shows a synapse between two neurons.
The arrows X, Y and Z point respectively to a
A. pre-synaptic terminal, a Golgi body and acetylcholine.
B. pre-synaptic terminal, a vesicle and a neurotransmitter.
C. post-synaptic terminal, a vacuole and a neurotransmitter.
D. post-synaptic terminal, a mitochondrion and acetylcholine.
Question 22
Consider the following list of biological molecules and their abbreviations.
hormones. . . . . . . . . . . . . . . ho
neurotransmitters. . . . . . . . . nt
gibberellins . . . . . . . . . . . . . gb
signalling molecules . . . . . . sm
The Venn diagram that best represents the relationship between hormones, neurotransmitters,
gibberellins and signalling molecules is
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Question 23
In mammals the parathyroid gland secretes parathyroid hormone (PTH). PTH is involved in
regulating the concentration of calcium in blood plasma. Parathyroid hormone increases the
amount of calcium in plasma by causing calcium to move from bone to the plasma, and by
assisting the uptake of calcium from the alimentary canal. PTH also stimulates the kidney to
activate vitamin D. The concentration of calcium in plasma acts directly, in negative feedback, to
regulate the output of parathyroid hormone.
From this information it would be expected that
A. increased production of PTH results in reduction of vitamin D activation.
B. reduced production of PTH results in increased calcium in the faeces.
C. sustained overproduction of PTH results in strengthened bones.
D. high levels of blood calcium stimulate release of PTH.
Question 24
The following diagram outlines the events that occur as a result of a particular signalling
molecule
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Signal transduction is represented by stage
A. 1
B. 2
C. 3
D. 4
Question 4
Living organisms cannot survive without the presence of enzymes.
a. Explain why enzymes are necessary in living organisms.
1 mark
b. Describe the ‘active site’ of an enzyme and explain its role.
2 marks
The blood pressure of an individual can change significantly to ensure an appropriate supply of
water, nutrients and oxygen to cells and to remove wastes that may be harmful. However, people
who have long-term high blood pressure develop characteristics that can be life threatening.
Scientists have decided that they may be able to treat patients suffering high blood pressure by
designing and developing a drug to lower high blood pressure. This technique is called ‘rational
drug design’.
c. Write a short paragraph to explain the phrase rational drug design.
2 marks
It is known that Angiotensin-converting enzyme (ACE) acts on the polypeptide Angiotensin I to
produce Angiotensin II, a powerful blood-pressure raising agent.
People with long-term high blood pressure have raised blood levels of ACE.
The following diagram represents the active site of a molecule of ACE.
d. i. Which drug is likely to be the most effective in preventing excessive high blood pressure?
ii. Give the reasons for your choice in part i.
iii. Explain the process by which this drug would contribute to lowering blood pressure in a
person.
1 + 1 + 2 = 4 marks
Total 9 marks
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Question 7
The diagram below shows a young plant growing with a sufficient supply of nutrients in normal
light.
a. What type of growth response is being shown by the plant?
1 mark
Some VCE students were asked to perform an experiment to test the effect of lack of sunlight on
the growth of
a sample of seeds of the same plant.
b. i. Name one controlled (fixed) variable that students should keep constant.
ii. Name the one experimental (independent) variable that students should change.
1 + 1 = 2 marks
c. Which part of the diagram represents an interneuron?
1 mark
d. What is the general name given to the type of nerve pathway shown in the diagram?
1 mark
The myelin sheath along structure Q can be damaged by disease.
e. Describe how such damage would affect the person involved.
1 mark
Total 6 marks
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Question 8
a. Name a homeostatic system you have studied this year. Draw a labelled diagram outlining how
the system operates.
3 marks
Copper is an essential trace element for most organisms. Over time, organisms have developed
complex homeostatic mechanisms to regulate the uptake, distribution and removal of copper.
These mechanisms, coordinated by the nucleus, involve the metal transcription factor (MTF-1).
In the fruit fl y, Drosophila, MTF-1 acts as an activator under both high and low copper
concentrations. Under high copper concentrations, MTF-1 activates metallothioneins that prevent
additional copper entering the cell.
Under low copper concentrations within a cell, MTF-1 activates the copper importer Ctr1B which
enables more copper to enter the cell from the surroundings.
b. What is the stimulus for homeostasis of copper?
1 mark
c. Suggest why the level of copper in the cells of an organism is controlled.
1 mark
Total 5 marks
VCAA 2007
Question 3
Recent research has demonstrated that members of some families are unable to feel
pain. They respond normally to touch, temperature and pressure and have no signs of
nerve disease.
From this information it is reasonable to conclude that in the affected people
A. neurons in the skin are unable to respond to external stimuli.
B. ion channels are blocked in one group of specialised neurons.
C. neurons are unable to produce neurotransmitters.
D. brain cells fail to respond to electrical signals.
Question 16
Homeostatic systems comprise components such as sensors, effectors and variables.
In such systems, the component being kept relatively constant is
A. the variable.
B. input to the sensor.
C. input to the effectors.
D. output from the effectors.
Question 5
Networks of neurons interact to support the complex functioning of an organism. Some
signals from neurons activate processes. Other signals inhibit processes. Activation
signals are indicated by a + sign, inhibitory signals by a - sign. A single activation signal
is cancelled out by a single inhibition signal if they are given at the same time to the
same neuron. Several neuron networks, each associated with a muscle fibre, were
investigated. If neurons J, K, L and M were activated at the same time, you would expect
neuron N to activate the muscle fibre in network
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Question 15
When fats and proteins are present in the duodenum, events occur that result in
digestive enzymes being released into the duodenum from the pancreas. In addition, the
gall bladder contracts and releases bile into the duodenum.
It is reasonable to conclude that
A. signalling molecules carry messages from the duodenum to both the gall bladder and
the pancreas.
B. signalling molecules would diffuse through tissue fluid that fills the spaces between
the different organs.
C. an increase in digestive enzymes from the pancreas results in an increase of fats and
proteins in the duodenum.
D. an increase in bile from the gall bladder reduces the break down of fats and proteins.
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Question 21
Some plant cells have connections through their cell walls through which they can
communicate. The following diagram includes a section of
It is reasonable to conclude that
A. such cells lack a nucleus.
B. structure X is made of cellulose.
C. the cytosols of the two cells are isolated from each other.
D. material is able to pass from cell to cell without having to travel through a plasma
membrane.
Question 22
Signal molecules that pass from one cell to another in plants include
A. gibberellins that inhibit seed germination.
B. ethylene that gives the instruction for fruits to ripen.
C. abscisic acid that gives the instruction for leaves to grow.
D. auxin that gives the instruction for stems to stop growing.
Question 24
The olfactory part of a brain is responsible for an animal’s ability to smell. The brain of a
black flying fox has a relatively large olfactory lobe, indicating that these flying foxes rely
heavily on smell to find their food. Signals between neurons in the olfactory lobe would
occur by means of
A. enzymes.
B. hormones.
C. neurohormones.
D. neurotransmitters.
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Question 25
The following graph compares the hearing of humans with the hearing of two flying
foxes.
The threshold is a measure of sensitivity where the higher the value, the louder is the
volume of sound required to hear a particular frequency. Assume that the human, the
grey-headed flying fox and the little red flying fox were exposed to a range of sounds.
Although the sounds were at different frequencies, they were all at the same volume.
From the data provided on the graph, one can conclude that
A. it is more difficult for a human to hear the sound if it has a frequency of 1 kHz than if it
has a frequency of 10 kHz.
B. a grey-headed flying fox can hear the sound at 10 kHz better than can the little red
flying fox.
C. a grey-headed flying fox hears all sounds better than the little red flying fox whatever
the frequency of the sound.
D. at frequencies below 1 kHz, humans have more difficulty hearing the sound than both
flying foxes.
Question 1
a. What is the general name for the chemicals that act as sex attractants in many
insects?
1 mark
A hormone was produced in one cell, entered the blood stream and travelled to two
groups of cells adjacent to each other. One group of cells responded to the hormone but
the neighbouring group did not.
b. What is the most likely reason for this difference in response by cells to the same
hormone?
1 mark
Consider one hormone you have studied this year that is transported through the blood
to one or more types of cells.
c. i. Name the hormone.
ii. Name the tissue or gland that produces the hormone.
iii. Explain what the term signal transduction refers to.
iv. What is the outcome as a result of signal transduction in the cells that were targets
for the hormone you chose in part c.i. of this question?
1 + 1 + 1 + 1 = 4 marks
Total 6 marks
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Question 9
Researchers measured the internal body temperatures of a number of perentie
lizards,Varanus giganteus, on a day in March 1983. The following graph shows the
average body and air temperature for that day.
a. i. On this day, at what time did the lizards reach their highest internal body
temperature?
ii. Does the answer you gave in part i. coincide with the highest air temperature for the
day? Explain.
1 + 1 = 2 marks
b. Is a lizard likely to detect skin temperature by the nervous system or the hormonal
system? Explain.
1 mark
Total 3 marks
VCAA 2006
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Question 10
Consider two cells (X and Y) in a reflex arc in the nervous system. Cell X is transmitting a signal
to cell Y. Signal transduction commences
A. in the cytosol of cell X.
B. along the axon of cell X.
C. in the myelin sheath of cell X.
D. at the synapse between cells X and Y.
Question 21
Pheromones are
A. hormones found only in plants.
B. used to repel pest insects from crops.
C. effective only over very short distances.
D. chemicals that often act as sex attractants.
Question 5
Examine the following table.
Summary carbon dioxide and oxygen levels in air and water
The respiratory gases, oxygen and carbon dioxide, are under homeostatic control. The gas in
shorter supply is normally regulated because it is the limiting factor. For instance, aquatic
organisms, like fish, regulate levels of oxygen. Terrestrial organisms, like mammals, regulate
levels of carbon dioxide. A build-up of carbon dioxide in the blood can cause the pH of the blood
to become acidic. Mammals are more susceptible to this build-up of carbon dioxide than fish.
Receptors in the brain and arteries detect such changes in carbon dioxide and pH and stimulate
ongoing breathing.
a. Why are mammals more susceptible to a build-up of carbon dioxide than fish? 1 mark
______________________________________________________________________________
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b. Consider a mammal that experiences an increase in its blood pH. Name one effector and
suggest what its response would be? 2 marks
______________________________________________________________________________
______________________________________________________________________________
Hyperventilation is the practice of breathing deeply and rapidly to remove carbon dioxide from
the lungs. This results in a significant lowering of the amount of carbon dioxide in the blood. It is
extremely dangerous and can cause a person to become unconscious.
c. Explain why underwater pearl divers, who use no special diving apparatus, would
hyperventilate. 1 mark
______________________________________________________________________________
d. Why does hyperventilating cause a person to become unconscious? 1 mark
______________________________________________________________________________
Total 5 marks
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