Download Chapter 5: Homeostasis and regulatory mechanisms Key questions

Chapter 5: Homeostasis and regulatory mechanisms
Key questions
1
2
a
The external environment of an organism is the environment in which the organism lives. Its internal
environment is the watery medium that surrounds its cells.
b
The intracellular environment is the fluid contained within the cells. The extracellular environment is the
fluid surrounding the cells.
a
A relatively stable internal environment is important to the normal functioning of organisms because cell
processes operate under optimal conditions that are usually within a narrow range.
b
Internal factors that are regulated: pH, temperature, concentration of salts and mineral ions, concentration
of nutrients, concentration of waste materials.
3
Unicellular organisms do not have body systems to regulate internal conditions; their internal and external
environments are similar. Unicellular organisms must be able to tolerate the environmental conditions in
which they are immersed, or else they die. Multicellular organisms have an internal environment that is quite
different from their external environment; they have mechanisms that allow them to regulate aspects of their
internal environment despite external conditions.
4
Multicellular organisms have specialised structures that allow them to regulate aspects of their internal
environment. These structures may be specialised tissues or systems. For example, the root hair cells of plant
roots are specialised for taking up the mineral ions required by the plant; excretory tissue in animals removes
wastes. (A range of relevant responses acceptable.)
5
Homeostasis is the process by which the internal conditions of an organism are maintained in a relatively
stable state.
6
a
Osmoconformers are aquatic animals whose body fluids are similar in concentration to the external
environment. Osmoregulators regulate the concentration of their body fluids, which can be quite different
from that of the external environment.
b
Osmoconformers expend less energy than osmoregulators, but because the concentration of their body
fluids varies in line with the concentration of the surrounding water, their internal environment is not
always optimal for the various processes and reactions, resulting in considerable stress. Osmoregulators
expend a great deal of energy in ensuring constant internal conditions. While this is a cost, the benefit is
that conditions are always optimal for cell processes.
7
The circulatory system maintains homeostasis by maintaining temperature and levels of nutrients, oxygen,
carbon dioxide, nitrogenous waste and hormones.
8
Birds and mammals expend considerable energy in maintaining a constant internal temperature. However the
benefit is that internal conditions are always optimal for cell processes. Enzymes for example, operate most
efficiently in a narrow optimal temperature range; a constant internal temperature means enzyme efficiency
regardless of external temperature conditions.
9
a
i Stimulus: a change in an internal or external factor that provokes a response. Example: an increase in
blood CO2 levels is a stimulus that provokes the response of increased lung ventilation.
ii Receptor: specialised structure that detects change in an internal or external factor. Example: the
respiratory centre in the brain detects a change in blood CO2 levels.
iii Effector: specialised structure that responds to a stimulus. Example: when blood CO2 levels increase,
the ventilation rate of the lungs (effector) increases.
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iv Response: the way in which an organism responds to a stimulus—this may be physiological or
behavioural. Example: when blood CO2 levels increase, the response is the increased breathing or
ventilation rate of the lungs.
b
10 a
The threshold is the lowest intensity of a stimulus to which a receptor will respond. Once the threshold is
reached, the stimulus triggers the receptor and a response occurs. Fluctuations in stimuli that don’t reach
the threshold do not trigger any response.
In a stimulus–response model, a stimulus is detected by a receptor, which generates a signal. The signal
is transmitted through a sensory neuron directly, or via a connector neuron, to a motor neuron, which
controls an effector to produce a response.
In a negative feedback mechanism, a stimulus is detected by a receptor, which generates a signal. The
signal is transmitted through a sensory neuron to connector neurons to the brain, or other parts of the
nervous system, so that a response occurs, which reduces the original stimulus.
b
11 a
b
Negative feedback mechanisms act to restore the balance or normal setting. For instance, an increase in
internal temperature causes responses, such as sweating and dilation of peripheral blood vessels, to
increase heat loss, thereby reducing the internal temperature.
A feature of the internal environment that is tightly controlled is temperature. This is controlled by a
misalignment detector.
A feature of the internal environment that varies widely is blood glucose level.
12 A proportional control system is one in which the size of the response is proportional to the size of the
stimulus. For example, breathing rate is proportional to the level of carbon dioxide in the blood. Although a
neuron either responds or does not respond, the intensity of a response can be varied by the number of cells
that respond or the rate of production of action potentials.
13 Disturbance detectors alert the body to changes in external or internal environmental factors so that the body
can respond in ways that ensure there is no change to internal body set points. For example, temperature
disturbance detectors in the skin detect changes to environmental temperature and the body responds in ways
that ensure there is no change to core body temperature. Misalignment detectors detect changes to the body’s
set point for a particular factor. For example, when core body temperature increases or decreases from its set
point, misalignment detectors register that the factor is out of line.
14 a
b
The two communication systems of multicellular organisms are the hormonal and nervous systems.
Hormones are chemical substances that act on specific target cells. Nervous communication involves
conduction of electrical signals along neurons and chemical transmission to effector cells.
Hormonal and nervous systems transmit signals by specific chemical substances (hormones and
neurotransmitters, respectively) that act on or within target cells.
15 Signalling molecules are involved in communication between cells within the body, e.g. hormones are
signalling molecules because they communicate between cells.
16 Hormones are chemical substances that act on specific target cells, are released in response to specific stimuli,
usually travel through an organism’s internal transport system, are effective in low concentrations and have a
slower effect than a nervous response.
17 Hormonal communication is specific because only a specific group of cells detects a stimulus and produces a
particular hormone, and only specific cells are able to respond to the hormone.
18 The function of the digestive system is to use physical and chemical methods to break down the complex
molecules of food into molecules that are small enough to pass through the membranes of the cells that line
the wall of the gut.
19 Stimulus—presence of acidic food in the gut
Receptor—mucosal lining of the duodenum secretes secretin
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Effector—pancreas
Response—release of digestive enzymes
20 Carbohydrates and fats can be stored in the body. Vitamins and amino acids cannot be stored in the body.
21 a
The regulatory roles of the liver include:
regulating the level of blood glucose
storing and metabolising carbohydrates
recycling lactic acid produced in anaerobic respiration
synthesising and breaking down proteins
forming urea from nitrogenous wastes
synthesising, storing and using fats
storing iron and vitamins
forming bile salts
breaking down poisons and hormones
generating heat from metabolism
removing bacteria and worn out red blood cells.
b
Blood from the intestine is carried to the liver by the hepatic portal vein. It regulates the level of nutrients
in the blood so that blood being pumped to the body does not have dangerously high nutrient levels.
22 Insulin converts glucose to glycogen.
Glucagon converts glycogen to glucose.
Glycogen is the storage product of glucose.
The pancreas produces insulin and glucagon.
The adrenal medulla produces adrenaline, which is involved in raising blood glucose by acting on fats,
skeletal muscles and the liver.
Fat can be converted to glucose.
Skeletal muscle stores glycogen.
Fat tissue stores fats.
23 During exercise, the level of activity of skeletal and cardiac muscles increases, the heart rate and breathing
rate increase, and the depth of lung ventilation increases.
24 When blood glucose concentration falls during exercise:
a
the level of insulin decreases
b
the level of glucagon increases.
25 Changes in the circulatory system during exercise include:
increase in heart rate
increase in blood pressure
increase in the volume of blood pumped by the heart
redirection of blood flow away from the internal organs and towards the skin.
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26 a
b
Oxygen debt is the amount of oxygen required to provide the energy needed to convert lactic acid to
glucose.
An oxygen debt occurs when anaerobic respiration takes place and lactic acid is formed. The lactic acid
is subsequently converted back to glucose and respired aerobically.
27 Regular strenuous exercise causes the following changes:
a
level of haemoglobin increases
b
resting heart rate decreases
c
size of muscles increases
d
bone strength increases.
28 Metabolic changes occur during pregnancy because the fetus requires energy for development, maintenance
and transport. These changes include:
increase in metabolic rate of about 15%
increased energy expenditure due to the need to carry extra weight
lung ventilation increases due to the need to maintain more aerobic respiration
cardiac output rises to about 30% above normal.
29 Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. If fetal haemoglobin had the same
affinity for oxygen as adult haemoglobin, the fetal blood would not be able to take up enough oxygen from the
mother’s blood for its needs. Higher oxygen affinity allows this to occur.
30 Stomata regulate the movement of water, carbon dioxide and oxygen into and out of leaves.
31 Structural features of guard cells, and how each contributes to stomatal opening and closing, include:
pairs of guard cells are joined at their ends—when they swell, they buckle and increase the size of the
stomata
cell walls are thickest on the side nearest the pore—this causes the cells to swell lengthwise only
bands of inelastic fibres run around each cell—these cause the cells to shrink when turgor pressure is
reduced.
32 Environmental factors that affect the opening and closing of stomata include water availability, carbon dioxide
concentration, light intensity and temperature.
33 Potassium pumps in cell membrane are activated ! potassium ions are pumped into guard cells ! water
moves into guard cells by osmosis ! guard cells swell
Chapter review questions
1
a
The concentration of the fluid in the external environment of the cells of Hyas araneus is the same as that
of the external environment of its body.
b
i The solute concentration of the blood of Carcinus maenas is approx 400 mmol/L.
ii The crab takes up salts across its gills by the process of active transport.
c
i Hyla araneus is an osmoconformer; Carcinus maenas is an osmoregulator.
ii Carcinus maenas uses energy in active transport that Hyas araneus does not.
2
Activity
Stimulus
Receptor
Means of
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Effector
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Response