OCR A2 UNIT F214 COMMUNICATION
Specification:
1. Outline the need for communication systems within multicellular
organisms, with reference to the need to respond to changes in the
internal and external environment and to co-ordinate the activities of
different organs
2. State that cells need to communicate with each other by a process
called cell signalling
3. State that neuronal and hormonal systems are examples of cell
signalling
4. Define the terms negative feedback, positive feedback and
homeostasis
5. Explain the principles of homeostasis in terms of receptors, effectors
and negative feedback
6. Describe the physiological and behavioural responses that maintain a
constant core body temperature in ectotherms and endotherms, with
reference to peripheral temperature receptors, the hypothalamus and
effectors in skin and muscles
Why Multicellular Organisms need Communication Systems

The majority of animals and plants are multicellular with different
organs and tissues carrying out specific functions

To ensure that these organisms are efficient, it is vital that the activities
of different cells, tissues and organs are co-ordinated

Cell signalling and interactions between cells, tissues and organs was
referenced in unit F211

In this section, the emphasis is on cells detecting changes in the
external or internal environment (stimuli) so that a response can be
brought about

In multicellular animals, the endocrine system (hormone production)
and the nervous system are responsible for co-ordinating the
activities of different cells, tissues and organs

In multicellular plants there is no nervous system but plant cells do
produce chemicals with hormone type effects called plant growth
regulators (this is covered in F215)
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
Co-ordination between cells, tissues and organs depends upon cell
signalling. This is the communication between cells so that they work
together to coordinate the activities of different organs to bring about a
response

In multicellular animals, coordination brought about by the nervous
system and the endocrine system works by cell signalling
Definitions:
TERM
Homeostasis
DEFINITION
Maintenance of a constant internal
environment of an organism within narrow
limits, despite greater fluctuations in the
external environment
This refers to the air, water or soil
environment that living organisms are
exposed to. A change in this environment
may cause a living organism stress eg
temperature changes
In multicellular animals, the blood and
tissue fluid bathe cells and are the
immediate environment around cells
A mechanism to ensure that any change
from the optimum condition in the internal
environment is detected, corrected and
returned to the optimum, constant/steady
state condition
This mechanism is less common. It occurs
when a change in the optimum condition is
detected and moved further away from the
constant/steady state condition
A change in the internal or external
environment that causes a response
Receptors may be cells or molecules within
plasma membranes that detect a change
from the internal optimum steady state
condition (detect a stimulus)
A change in behaviour or physiology as a
result of the environmental change
(stimulus)
Effectors are cells, tissues or organs that
respond to cell signalling and bring about a
change to maintain the optimum condition
(this response would involve the negative
feedback mechanism)
External environment
Internal environment
Negative feedback
Positive feedback
Stimulus
Receptor
Response
Effector
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Features of an Efficient Communication System in Multicellular Animals

Cover the whole organism

Allow cell to cell communication

Allow specific communication

Allow rapid communication

Result in both short term and long term responses
Some Physiological Factors that must be controlled in Mammals
Physiological
Factor
Reason for
Control
Organs
detecting
Changes
Effector
Organs
Core body
temperature
Blood plasma
glucose
concentration
Blood plasma
water potential
Blood pressure
Blood plasma CO2
and urea
concentrations
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Co-ordinating
Systems
Feedback
Mechanism
Principles of Negative Feedback
This feedback mechanism involves the following cell signalling processes:
Stimulus  Receptor  Communication Pathway  Effector  Response
(nervous or hormonal systems)
Principles of Positive Feedback
Positive feedback may be beneficial or harmful. Some examples are detailed
below
Beneficial Positive Feedback Mechanisms - Effect of Oxytocin in
Childbirth and Breast Feeding

Oxytocin is a hormone secreted from the posterior pituitary gland

Oxytocin causes contractions of the uterus muscles that start labour, at
the end of pregnancy

Although oxytocin causes the uterine muscle contractions, these
muscle contractions (response) stimulate the release of more oxytocin
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and this increased hormone release stimulates more uterine
contractions

Oxytocin also stimulates the release of milk from the nipples during
breast feeding. The stimulus for this hormone release is the baby
sucking the nipples. The more the baby sucks (stimulus) the more
oxytocin is released and the more milk is ejected from the nipples
Harmful Positive Feedback Examples
1. Too Low Body Temperature

Changes in body temperature change enzyme activity

If body temperature is too low, enzyme activity will fall. The exergonic
reactions (respiration) that release thermal energy will be slower and
less heat will be released. The body will cool still further and enzyme
reactions will be far too slow to support life
2. Too High Body Temperature

If body temperature is too high, enzyme activity increases. The
exergonic reactions in respiration that release thermal energy will be
faster and more heat will be released. The body will heat up still further
3. Breathing in Air containing a High Concentration of CO2

Breathing in air containing a high concentration of CO2 increases the
blood plasma CO2 concentration. Chemoreceptors detect the high
plasma CO2 concentration. The response is to breathe more rapidly to
remove the CO2 at a faster rate. In turn, more CO2 enters the blood
causing the breathing to be even more rapid
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Importance of Body Temperature Control
Changes in body temperature affect the structure and activity of globular
proteins including enzymes
Comparison of Endotherms and Ectotherms – two groups of animals
with different body temperature control mechanisms
Comparative Feature
Endotherms
Ectotherms
Types of animal
Mammals and birds
All animals except
mammals and birds
Are physiological
methods used?
Yes – many
mechanisms are used
Less likely
Is change of metabolic
rate a major part of
temperature control?
Are behavioural
methods used?
Yes
No
Yes
A constant body
temperature is
maintained within
narrow limits
Yes
Body temperature is
maintained above
external temperature
Yes
Yes – these are the
main mechanisms for
controlling body
temperature.
Ectotherms aim to
maximise heat
exchange directly with
their environment
No – body temperature
fluctuates with changes
in external
environmental
temperatures
No
Temperature Regulation (Thermoregulation) in Endotherms

Mammals and birds are endotherms that maintain a constant body
temperature within narrow limits

Endotherms control their body temperature by both physiological and
behavioural methods. Unlike ectotherms, they can alter their metabolic
rate as a major part of their temperature control

Endotherms balance heat input and heat output – as shown on page 7
6
Summary of Radiation, Conduction and Convection as Mechanisms of
Heat Transfer
7
Physiological Mechanisms to Control Body Temperature of Endotherms
Body
Structure
Involved
Sweat glands
in skin
Response if Core* Body
Temperature is too High


Hairs on skin
Arterioles
leading to
capillaries in
skin
Liver cells
Skeletal
muscles
Lungs, mouth
and nose


Secrete more sweat
onto skin
Water in sweat
evaporates using heat
from blood to supply
latent heat of
vaporisation
Erector muscles relax to
flatten hairs against skin
to reduce the insulating
air layer (important in
hairy mammals)
More heat loss from skin
by radiation and
convection
Vasodilation of arterioles
allows more blood to
flow into skin capillaries
More heat radiated from
skin
Response if Core* Body
Temperature is too Low



Less sweat is secreted
onto skin
Less evaporation of water
Less loss of latent heat

Erector muscles contract
to raise hairs to trap a
layer of insulating air
close to the skin
(important in hairy
mammals)

 Reduces heat loss from
the skin

 Vasoconstriction of
arterioles allows less
blood to flow into skin
capillaries

 Less heat is radiated from
skin
 Rate of metabolism is
 Rate of metabolism is
reduced
increased (stimulated by
adrenaline and thyroxine)
 Less heat generated
from exergonic reactions
 More heat generated from
such as respiration
exergonic reactions such
as respiration and
transferred to the blood
No spontaneous contractions
Spontaneous contractions
(shivering) generates heat as
muscle cells respire more
Panting increases evaporation No panting therefore less loss of
of water from lungs, tongue
latent heat
and other moist surfaces, using
latent heat of vaporisation
Core body temperature is the temperature in body organs and tissues.
The temperature control centre is in the hypothalamus and this
monitors core body temperature
8
9
Behavioural Mechanisms to Control Body Temperature of Endotherms
Behaviour if Too Hot
Behaviour if Too Cold
Move to shade or hide in a burrow
Move into a warmer place– bask in
the sun
Orientate body to decrease surface
area exposed to sun
Orientate body to increase surface
area exposed to sun and heat gain
from radiation
In shade, spread out limbs to
increase surface area for heat loss by
conduction and radiation
In extreme cold, reduce body surface
area by rolling into a ball to reduce
heat losses by radiation and
conduction. Humans can wrap their
arms around them and huddle up
Remain inactive to reduce internal
heat generation
Move about to generate heat in
muscles
Humans can wear fewer clothes
Humans can put on more clothes to
increase insulation of body surface
Humans eat cool salads and drink
iced drinks
Humans can eat hot food or drink hot
drinks
Advantages and Disadvantages of Endothermy
Advantages of Endothermy
Disadvantages of Endothermy
Maintains a constant body
temperature despite fluctuating
external temperatures
A significant part of energy intake is
needed to keep the body warm in
colder climates
Activity possible regardless of
external temperatures
Animal must consume more food to
supply energy needs
Enables mammals and birds to
inhabit colder parts of the planet
Less of the food consumed is used
for growth
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Control of Temperature Regulation

The thermoregulatory control centre is in the hypothalamus, at the
base of the brain

Temperature receptors in the hypothalamus monitor blood
temperature as blood flows through the organ and detect changes in
core body temperature (for humans this should be 37oC)

The hypothalamus also receives impulses from peripheral
temperature receptors in the skin (see skin diagram page 9). This
indicates to the hypothalamus that the external environment is very
cold or very hot and is a warning that core body temperature may
change shortly

The hypothalamus sends nerve impulses along motor neurones to the
muscles and liver to control internal heat input. Nerve impulses are
also sent to the skin to control heat output
Temperature Regulation in Ectotherms

What types of animals are ectotherms? .............................................
……………………………………………………………………………….

Ectotherms obtain most of their body heat from the external
environment. They do not generate large amounts of heat inside the
body

The body temperature of an ectotherm fluctuates with the external
temperature.
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
Ectotherms mainly use ……………………. mechanisms to control their
body temperature

Temperature regulation relies upon increasing the exchange of heat
with their environment
Advantages of being an Ectotherm
Use less food in respiration
Disadvantages of being an
Ectotherm
Less active in cooler temperatures.
Need to warm up in the morning to be
active
Need to find less food. Can survive
for long periods without eating
At greater risk of predation when less
active
More energy from food used for
growth
Will need energy stores to survive in the
winter without eating
Adaptation
How it regulates
temperature
Example
Expose body to sun/
lie on a warm
surface to warm up
More heat is absorbed by
radiation/ conduction
Orientate body to
sun to warm up
Exposes larger surface area Locust, Lizard
for increased heat
absorption
Orientate body away
from sun to cool
down
Less surface area exposed
and less heat absorbed
Locust
Hide in a burrow to
cool down
Reduces heat absorption by
keeping out of the sun
Lizard
Alter body shape by
contracting or
expanding its rib
cage
Exposes more or less
surface area to the sun to
increase or decrease heat
absorption
Horned lizard
Increased breathing
movements
Evaporates more water.
More heat loss through
latent heat of evaporation
Locust
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Snake