Coordination and regulation
Defining homeostasis
• Homeostasis is the maintenance of the internal environment in a
relatively stable state despite changes in either the external or
internal environment. A stable internal environment is important
because organisms function more efficiently when they are under
optimum conditions.
• The external environment of a cell can vary greatly. The internal
environment is generally relatively stable. Variables controlled in
living things include: nutrients, temperature, pH, water, ions (e.g.
Na+, Cl–, Ca2+), oxygen levels, carbon dioxide levels and blood
pressure.
• Organisms that can regulate their internal environment can tolerate
a wide range of environmental conditions.
• There are several key interfaces in humans between the external
and internal environments including alveoli (air), villi (food) and
nephrons (urine). The cellular structure of these interfaces presents
a large surface area for maximum exchange.
Defining homeostasis
• Homeostatic mechanisms require a system to
coordinate the organism’s response.
• Signal transduction is cell-to-cell
communication.
• In this process, a signalling cell releases
messenger molecules that move about and
interact with the specific receptors of a target
cell. This interaction is then converted into
changes within the target cell.
• Cells can respond to the messenger molecules
only if they have specific receptors that can
recognise and bind to the messenger molecules.
Defining homeostasis
• The signalling pathway is a series of proteins
working in a sequence. Each protein affects the
structure of the next one in the pathway by either
activating or inhibiting its function.
• The final protein affected is the target protein.
• The pathway may alter the activity of a metabolic
enzyme, alter a genetic expression, increase or
decrease cell mobility, activate DNA synthesis,
change ion permeability or initiate cell death.
The basic elements of signal transduction
Source: Figure 15.1 from Cell and Molecular Biology: Concepts and Experiments by Karp, 4th edition, © 2004 John Wiley
• Neurotransmitters, hormones,
pheromones and plant growth regulators
act as signalling molecules.
• The nervous system and the endocrine
system regulate internal functions and
work to maintain homeostasis.
The nervous system
– The nervous system is made up of the central nervous
system (CNS), which consists of the brain and spinal
cord, and the peripheral nervous system (PNS), which
consists of all other nerve cells that connect the CNS to
other parts of the body.
– Nerve cells form the basic structure of the nervous system.
They are also known as neurons.
– A neuron has a cell body, which contains the nucleus.
Extensions from the cell body are called axons and
dendrites. The axons carry information away from the cell
body to another neuron or tissue. Dendrites are highly
branched extensions that receive information from other
neurons and carry the information into the cell body. A
group of many axons bound together is called a nerve.
A nerve cell
Neurons
• There are three basic types of neurons: affector (sensory)
neurons, effector (motor) neurons and connecting neurons
(interneurons).
• The junction of two nerve cells is called a synapse. A nerve impulse
is transmitted across the synaptic gap by several chemical
substances known as transmitter substances, such as
acetylcholine. These chemicals are packaged in vesicles.
• Transmitter substances diffuse across the synapse and bind to
neurotransmitter receptors. These are located on ion channels
on the adjoining neuron. When the transmitter substance binds to its
specific receptors, the ion channels open and a nerve impulse is
generated. The action of the transmitter substance is regulated by
enzymes; for example, acetylcholine is deactivated by
acetylcholinesterase. The action of the transmitter substances last
for a short time only.
• The release of transmitter substances or the action of the
regulating enzymes can be affected by other chemicals, such
as poisons and pain killers. This has an effect on the
transmission of nerve impulses.
• Nerve impulses travel in one direction only.
• They are generated from changes in the distribution of
electrical charges along the axon. A non-stimulated ‘resting’
nerve cell has small differences between the electrical charge
on the inside and outside of the cell membrane.
• When stimulated, the nerve cell is ‘activated’, the permeability
of the membrane changes and positive sodium ions (Na+)
move into the cell. This change in permeability is passed on to
the next part of the membrane and the original distribution of
charges is restored. The action potential, or impulse, is
generated when a stimulus changes the resting potential to
the threshold level.
• The action potential is ‘all or nothing’; it is the same strength
regardless of the strength of the stimulus.
Transmission of a nerve impulse along an axon. The action potential, or
impulse, is a sequence of depolarisation and repolarisation.
•The movement of a nerve impulse is very quick.
•It is quickest along axons that are covered with a
myelin sheath.
•Damage to the structure of the myelin sheath can
affect the transmission of nerve impulses.
An electron micrograph of the
myelin sheath
Source: © Dr Enrico Mugnaini/Visuals Unlimited
The endocrine system
• The endocrine system is the internal body system that
deals with chemical communication. It is made up of
hormones, the ductless glands that secrete hormones,
and the target cells that respond to hormones.
• Hormones are chemical messengers produced by
endocrine glands; for example, oestrogen is produced
by the ovary, testosterone by the testes, thyroxine by the
thyroid gland, and insulin by the pancreas.
• Hormones are secreted by the secretory cells that
synthesise them into the fluid around the cell; they then
pass into the bloodstream and are transported around
the body by the circulatory system.
• Hormones are inactivated by liver cells.
• Hormones regulate metabolism, growth, development
and sexual reproduction.
The endocrine system
• Hormones are soluble either in lipids or in water. Lipid-soluble
hormones include the steroid hormones and the thyroid
hormones. Water-soluble hormones include protein hormones
(such as insulin), smaller peptide hormones (such as
antidiuretic hormone) and hormones synthesised from amino
acids (such as epinephrine).
• Hormones act only on cells that have receptors for them.
Lipid-soluble hormones move through the plasma membrane
by diffusing through the lipid bilayer. They bind to and activate
receptors inside the cell, which alters the function of the cell.
The receptors for water-soluble hormones are proteins on the
surface of the target cells. The signalling pathway of watersoluble hormones within the cell involves enzyme action.
• Target cells can respond differently to the same hormone; for
example, insulin stimulates the synthesis of glycogen from
glucose in a liver cell, but the synthesis of lipids in adipose
cells.
Pheromones
– Pheromones are external signalling molecules that send
messages from the cells of one individual to the cells of another
individual of the same species. These organic molecules are
made in one individual but act in another individual. Pheromones
are used to mark territory, for attracting the opposite sex and for
defence.
– Insects and mammals secrete pheromones. Examples of
species that release pheromones include ants, bees, moths,
rats, mice, dogs and cats. In humans, pheromones are thought
to trigger hormonal changes.
– Pheromones can be used commercially for pest control. For
example, in a technique called mating disruption, artificial
pheromones are released from many sites within a crop to
confuse male insects and make it difficult them to locate females;
this reduces mating and the numbers of offspring produced.
Plant growth regulators
• Directional plant growth in response to an
external environmental stimuli is called a
tropism; for example, phototropism is
growth in response to light.
• Growth towards a stimulus is called a
positive tropism. Growth away from a
stimulus is called a negative tropism.
Auxin is a plant hormone that causes the
elongation of cells. It is involved in
phototropism.
• Auxin is a plant hormone that causes the
elongation of cells. It is involved in
phototropism.
• Plants rely on hormones for
communication. Plant hormones act on a
range of cell types and they can produce
different effects in different parts of a plant.
There are five groups of plant hormones
– Auxins, such as indoleacetic acid (IAA), enlarge and
elongate cells. Auxins act to trigger phototropism and
geotropism. IAA causes apical dominance, which is
the inhibition of lateral growth.
– Cytokinins promote cell reproduction.
– Gibberellins promote plant growth in stems and
leaves by increasing cell elongation and cell
reproduction. They also initiate seed development,
bud formation and the formation of juvenile leaves.
– Abscisic acid inhibits growth and reduces water loss
by closing the stomata. It causes fruit to fall from a
plant and is present in dormant buds.
– Ethylene stimulates the ripening process and causes
petals and flowers to fall from plants.
Models for representing homeostasis
• The sequence of events that occurs in response to a
change in the external or internal environment can be
represented by a stimulus–response model.
• The change is detected as a stimulus by a receptor.
• The stimulus is sent as a signal by an affector
(sensory) neuron to a connecting neuron
(interneuron) located in the central nervous system.
• An effector (motor) neuron then carries the impulse to
a muscle cell or gland.
• These effector cells bring about a response, either
directly or by releasing hormones.
A stimulus–response model
Negative feedback
 Negative feedback is a control system that reduces the
original stimulus.
 In negative feedback systems, the stimulus is detected
by a receptor. The signal is compared with the ‘set point’.
If a variable is not within safe limits, a message is sent to
effector cells by the nervous system and/or the hormonal
system.
 The effector cells then bring about a physiological
and/or behavioural response that reduces the original
stimulus. The events are continued until the level of the
variable returns to safe limits. The cycle is repeated if the
set point is not achieved.
Negative feedback
Glucose control
• The hormones insulin and glucagon regulate
the level of glucose in the blood. They are
antagonistic hormones as their effects oppose
each other.
• These regulatory hormones are secreted by the
pancreas. This gland is located under the
stomach. Alpha cells secrete glucagon and
beta cells secrete insulin. These cells are
located in the Islets of Langerhans, which are
clumps of tissue that make up the pancreas.
Alpha cells and beta cells are located in the
Islets of Langerhans.
Source: Figure 18.18, chapter 18c page 615 of Principles of Anatomy and Physiology, © John Wiley
Glucose control
• When blood glucose levels rise, such as after a meal, insulin is released
by beta cells in the pancreas and production of glucagon is inhibited. This
increases the uptake of glucose by target cells. Glucose is used for
respiration. In liver cells and muscle cells, excess glucose is converted to
glycogen and stored. In fat tissue cells, excess glucose is stored as fat.
Insulin also inhibits the conversion of glycogen into glucose. As a result of
these events, insulin lowers the blood glucose level.
• When blood glucose levels fall, such as after exercise, glucagon is
released by alpha cells in the pancreas and production of insulin is inhibited.
Glucagon stimulates the liver cells to convert stored glycogen into glucose
and to synthesise glucose from non-carbohydrate compounds. The glucose
enters the bloodstream and increases the blood glucose level.
Examples of negative feedback mechanisms for glucose control:
Increase in blood glucose levels (such as after a meal)
Decrease in blood glucose levels (such as after exercise)
Diabetes is a condition characterised by higher than normal blood glucose
levels and the excretion of glucose in the urine. Juvenile-onset diabetes is the
result of insufficient supply of insulin from the pancreas. This condition can
be managed by regular injections of insulin and a controlled diet. Matureonset diabetes is the result of body cells not responding the insulin. This
condition can be regulated by a controlled diet.
Osmoregulation
Osmoregulation is the homeostatic control of the levels of water and
mineral salts in the blood. Without osmoregulation, cells would either
gain or lose too much water by osmosis.
Water inputs include drinking, food and respiration. Water outputs
include urine, faeces, sweat, evaporation from the lungs during
breathing, mucus, semen and vomitus.
In osmoregulation, the kidneys regulate the amount of water to
ensure that the needs of the organism are met. In humans, water is
reabsorbed in the kidneys through the loop of Henle. The
concentration of solutes in hypertonic urine is higher than other
body fluids. The concentration of solutes in hypotonic urine is lower
than other body fluids.
Dehydration (too little water in the blood)
Waterlogging (too much water in the blood)
Thermoregulation
The balanced core body temperature of a human is 37 °C.
The receptor for thermoregulation is the hypothalamus. The cells that act as the
receptors are called thermoreceptors.
Both the hormonal and nervous systems are involved in thermoregulation. The
primary hormone involved is thyroxine (from the thyroid gland). Thyroxine regulates
metabolic changes associated with thermoregulation.
Animals have been classified by the source of their body heat:
ectotherms: The source of body heat is largely from the environment. The
body temperature of these animals varies with the temperature of the external
environment. They are said to be poikilothermic.
endotherms: The source of body heat is largely from metabolic activity. These
animals have fairly constant body temperatures. They are said to be
homeothermic.
Decrease in body temperature
Increase in body temperature
The following diagram shows how glucose is
controlled in animals.
The following diagram shows how body
temperature is controlled in animals.
The following diagram shows how water balance is
controlled in animals.