Chapter 5
Answers
1
QUICK-CHECK questions
Identify, giving an example, which is more variable: your external environment
or the internal environment of your cells.
The external environment is far more variable than the internal environment. For
example, the external temperature can vary over time, with extremely hot weather on
one day and very cold conditions the next. In contrast, the internal environment of
cells, such as the tissue fluid and the plasma that bathe cells, is normally maintained
within narrow limits and so is very stable in a healthy person.
2
List the essential difference between an intra- and an extracellular fluid.
An intracellular fluid is the cytosol that is contained within the cell membrane and
surrounds the nucleus. An extracellular fluid is the tissue fluid and the plasma that is
outside cells and bathes their outer membranes.
3
Identify each of the following as intracellular or extracellular:
a
plasma
Plasma is extracellular because it exists outside cells.
b
cytosol
Cytosol is intracellular because it is contained within cells.
c
tissue fluid.
Tissue fluid is extracellular because it exists outside cells.
4
List up to three substances that can move between the internal fluid
compartments of the body.
The internal fluid compartment of the body comprises tissue fluid, blood plasma and
cytosol (see figure 5.4). Many substances can move between these internal fluid
compartments including:
 hormones that are transferred in the blood plasma and then move into the cytosol
of target cells
 glucose (and other nutrients) that can move from the plasma into the cytosol
 waste products, such as carbon dioxide, that pass from cells into the blood for
transport to an excretory organ, such as the lungs for carbon dioxide.
5
Define the term ‘homeostasis’ using words suitable for a non-biology student.
‘Homeostasis’ is the term used to describe the events that take place in the body to
counteract a change that has occurred so that the conditions within the body are kept
within a narrow range. For example, if too much of a particular hormone (a chemical) is
produced within the body, events occur that cause a reduction in the production of that
hormone.
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Chapter 5: QUICK-CHECK answers
6
List up to three variables that are subject to homeostasis.
Variables that are subject to homeostatic regulation include:
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body temperature
blood level of the hormone thyroxin
blood pressure
pH of body tissues and fluids
water content of the body.
Briefly describe how the following body systems contribute to homeostasis:
a
excretory
The excretory system removes nitrogenous waste and other ions from the body in
the urine, which the kidneys produce and excrete. This system is important in
regulating both the water content of the body and the pH of body fluids,
maintaining them within narrow limits.
b
respiratory
The lungs of the respiratory system remove carbon dioxide, a waste product of
cellular metabolism, from the body. The removal of carbon dioxide assists in
maintaining the pH of the body within narrow limits.
8
Outline an example of a negative feedback system.
In a negative feedback system, when a change in a variable in one direction is
detected, this change is counteracted by a response that produces a change in the
opposite direction. For example, the control of core body temperature involves a
negative feedback system: if the body temperature starts to increase above a certain
level, typically 37°C, cooling mechanisms are activated. Likewise, if the body
temperature starts to fall below a particular level, warming mechanisms are activated.
(Note: Cooling mechanisms in humans include sweating and increased blood flow in
surface blood vessels. Warming mechanisms include shivering and increased
production of the hormone thyroxin.)
9
Name two sensors that monitor the level of glucose in the blood.
Sensors that monitor blood glucose levels are the alpha cells and the beta cells of the
pancreas.
10
For each of the sensors you have listed in question 9, what are the effectors?
When alpha cells in the pancreas detect a fall in blood glucose level, the effector that
they produce is the hormone glucagon. Glucagon counters the decrease in blood
glucose by stimulating the conversion of stored glycogen in the liver to glucose, which
is released into the bloodstream, raising the blood glucose level.
When beta cells in the pancreas detect a rise in blood glucose, the effector they
produce is the hormone insulin. Insulin counters the increase in blood glucose by
stimulating cells to take up glucose from the blood, lowering the blood glucose level.
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Chapter 5: QUICK-CHECK answers
11
Assume that sensors had detected a fall in the variable under consideration.
a
What would you expect the response of the effectors to be?
If a fall in blood glucose was detected, the expected responses would be to
activate effectors to increase blood glucose level through:
b
 increased production of glucagon by alpha cells
 decreased production of insulin by beta cells.
Outline changes that would occur as a result of the responses.
An increase in glucagon production by alpha cells acts on the liver and stimulates
the conversion of stored glycogen to glucose, which is discharged into the
bloodstream and raises blood glucose levels.
A reduction in insulin production results in reduced absorption of glucose by cells,
which raises blood glucose levels.
The overall expected result of these two responses is a rise in the level of blood
sugar.
12
Assume that sensors had detected a rise in the variable under consideration.
a
What would you expect the response of the effectors to be?
Detection of a rise in blood glucose levels above a particular level would be
expected to produce responses that would result in a reduction in blood glucose
levels. These responses would be a reduction in glucagon production by alpha
cells and an increase in insulin production by beta cells.
b
Outline changes that would occur as a result of the responses.
Reduction in glucagon production would result in glucose moving from the
bloodstream into the liver, where it would be converted to glycogen for storage.
An increase in insulin production would result in more glucose being absorbed
from the blood by body cells.
The overall expected result would be a fall in blood sugar level.
13
Explain whether all hormones need to enter the bloodstream to be able to affect
a cell.
While many hormones do need to enter the bloodstream to be transported to distant
cells that respond to the hormone, this is not the case for all hormones. Some
hormones exert their effect on the same cells within which they are produced and
other hormones simply diffuse through tissue fluid to nearby cells where they produce
a response. These latter hormones do not need to enter the bloodstream for transport.
14
Name two differences between steroid hormones and protein hormones.
Differences between steroid hormones and protein hormones include:
 solubility in water: Protein hormones are soluble in water whereas steroid hormones
are insoluble in water.
 ability to pass through cell membranes: Protein hormones are unable to pass
through cell membranes whereas steroid hormones can pass through cell
membranes.
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Chapter 5: QUICK-CHECK answers
15
Why do some hormones require a second messenger protein in order to have
their signal detected within a cell?
Because of their hydrophilic nature, protein hormones (also known as signal
molecules) are unable to pass through the plasma membranes of their target cells.
Instead, the protein hormone binds to receptor molecules on the membrane surface
and, when this occurs, the signal is detected and a second messenger inside the cell
is activated. This second messenger is necessary to transmit the original signal into
the cell.
16
a
Explain what happens in signal transduction.
Signal transduction is a sequence of events that occurs inside a cell after receptor
molecules in the cytosol are activated. This sequence of events is termed signal
transduction. The original signal is altered by interactions with a series of relay
molecules. The final response of the cell to the hormone signal occurs after this
sequence of events. (See figure 5.15.)
b
Outline one example of a response a cell may make to a hormone signal.
Examples include:
 An increase in the amount of the hormone insulin reaching a cell results in an
increase in the amount of glucose absorbed by the cell.
 A decrease in the amount of glucagon received by liver cells results in an
increase in the amount of glucose absorbed by the liver and converted into
glycogen for storage.
17
By what means do male moths detect pheromones produced by females of the
same species?
Male moths detect pheromones released by females of the same species through the
smell (olfactory) receptors on their feathery antennae.
18
Outline one way in which pheromones are used to reduce pest infestation of
crops.
Insect pheromones of a pest species can be produced artificially. These pheromones
can be placed in small traps and the ‘baited’ traps are then placed around fields or
glasshouses. Males of the pest species are then attracted to, and captured in, the
traps.
19
In what ways are plant hormones similar to animal hormones?
Similarities between plant hormones and animal hormones include:
 Both kinds of hormone are produced in relatively small amounts.
 Both kinds of hormones are signal molecules.
 Both kinds of hormone are produced in one kind of cell or a group of cells and most
commonly act on other types of cells.
 Both kinds of hormones are transported through tubular systems, such as the
circulatory system in mammals and the phloem in flowering plants.
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Chapter 5: QUICK-CHECK answers
20
In what ways are plant hormones different from animal hormones?
Differences between plant hormones and animal hormones include:
 Plant hormones are generally produced in growing regions of roots and shoots
whereas animal hormones are produced by special glands or particular groups of
cells in those glands.
 The tubular system through which animal hormones are transported includes a
pump (the heart) that facilitates rapid movement of hormones from one to another
area of a human body; pumps are not present in plant transport systems.
21
22
Match each item on the left with the appropriate response(s) on the right.
Hormone
Result of signal transduction event
auxins
apical dominance, phototropism
gibberellins
cell reproduction, cell elongation
ethylene
fruit ripening, leaf abscission
cytokinins
cell replication
abscisic acid
stomatal closure
florigen
promotes flowering
indoleacetic acid
apical dominance, phototropism
List the cascade of signal transduction events when gibberellin is activated in a
wheat seed.
The sequence of events involved in signal transduction is:
 Water acts on gibberellin in the plant embryo and gives a signal for the conversion
of gibberellin to gibberellic acid.
 Gibberellic acid diffuses from embryo into the endosperm of the wheat seed and
then into the aleurone layer that surrounds the endosperm, where it gives a signal
for the production of the enzyme amylase.
 Amylase acts on starch reserve in endosperm and signals the conversion of starch
to glucose.
 Glucose diffuses to the embryo where it is a nutrient and energy source for the
developing embryo.
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