2 Mammals keep their internal environment relatively constant, so providing steady and appropriate conditions within which cells can carry out their activities. This is known as homeostasis.
3 Homeostatic equilibrium requires receptors that detect changes in physiological parameters such as the temperature, water potential and pH of the blood. Effectors are the cells, tissues and organs that carry out the functions necessary to restore those parameters to their set points. Homeostatic control systems use negative feedback in which any change in a parameter stimulates actions by effectors to restore the parameter to its set point.
4 Excretion is the removal of toxic waste products of
metabolism, especially carbon dioxide and urea.
The deamination of excess amino acids in the liver
produces ammonia, which is converted into urea, the
main nitrogenous waste product. Urea is excreted in
solution in water, as urine.
5 The kidneys regulate the concentration of various
substances in the body fluids, by excreting
appropriate amounts of them. Each kidney is
made up of thousands of nephrons and their
associated blood vessels. The kidneys produce
urine by ultrafiltration and reabsorption, plus some
secretion of unwanted substances. Different regions
of a nephron have different functions, and this is
reflected in the structure of the cells that make up
their walls.
6 Blood is brought to the glomerulus in an afferent
arteriole. High hydrostatic pressure in the
glomerulus forces substances through the capillary
walls, the basement membrane and inner lining of
Bowman’s capsule. The basement membrane acts as
a filter, allowing only small molecules through. This
filtrate collects in Bowman’s capsule and then enters
the proximal convoluted tubule, where most reabsorption
occurs by diffusion and active transport;
substances are also reabsorbed in the distal convoluted tubule and collecting duct. The loop of
Henle acts as a counter-current multiplier, producing
high concentrations of sodium and chloride ions
in the tissue fluid in the medulla. This tissue has a
very low water potential. Water is reabsorbed from
fluid in the collecting duct by osmosis if the body is
dehydrated.
7 The water content of the blood is controlled by
changing the amount of water excreted in the
urine by the kidneys. This is done by regulating the
permeability of the walls of the collecting ducts to
water, and hence the volume of water reabsorbed
from the collecting ducts into the blood. The
permeability is increased by the hormone ADH,
which is secreted by the posterior pituitary gland
in response to stimulation of osmoreceptors in the
hypothalamus.
8 Neurones are cells adapted for the rapid transmission
of electrical impulses. Sensory neurones transmit
impulses from receptors to the central nervous
system (brain and spinal cord); motor neurones
transmit impulses from the central nervous system
to eff ectors; intermediate neurones transmit
impulses within the central nervous system. The
three neurones are found in series in reflex arcs
that control fast, automatic responses to stimuli. In
vertebrates, the axons of many neurones are insulated
by a myelin sheath, which speeds up transmission.
9 Neurones have a resting potential, which is a
potential difference across their membranes, with
the inside having a negative potential compared
with the outside; this potential difference is about
−65 mV. An action potential is a rapid reversal of
this potential, caused by changes in permeability of
the cell surface membrane to potassium and sodium
ions. Action potentials are always the same size.
Information about the strength of a stimulus is given
by the frequency of action potentials produced.
10 Action potentials are propagated along axons by local
circuits that depolarise regions of membrane ahead
of the action potential. This depolarisation stimulates
sodium ion voltage-gated channels to open, so that
the permeability to sodium increases and the action
potential occurs further down the axon. Axons are repolarised by the opening of potassium ion voltagegated
channels that allow potassium ions to diffuse
out of the axon. After a short refractory period when
the sodium channels cannot open, the membrane is
able to respond again. Refractory periods determine
the maximum speed of impulses.
11 Action potentials may be initiated within the brain
or at a receptor. Receptors respond to information
from the environment. Environmental changes
result in permeability changes in the membranes
of receptor cells, which in turn produce changes
in potential diff erence across the membrane. If the
potential difference is sufficiently great and above
the threshold for the receptor cell, this will trigger
an action potential in a sensory neurone. Receptors
are transducers converting the energy of stimuli into
electrical impulses.
12 A synapse is a junction between two neurones
or between a motor neurone and a muscle cell.
At cholinergic synapses, a transmitter substance,
acetylcholine, is released when action potentials
arrive. Impulses pass in one direction only, because
transmitter substances are released by exocytosis by
the presynaptic neurone to bind to receptor proteins
that are only found on the postsynaptic neurone.
13 Any one neurone within the central nervous
system is likely to have at least several hundred
synapses with other neurones, some of which will be stimulatory and some inhibitory. This allows
integration within the nervous system, resulting in
complex and variable patterns of behaviour, and in
learning and memory.
14 Hormones are chemicals that are made in endocrine
glands and transported in blood plasma to their
target cells, where they bind to specific receptors and
so affect the behaviour of the cells.
15 The concentration of glucose in the blood is
controlled by the action of insulin and glucagon,
which are secreted by the islets of Langerhans in
the pancreas and aff ect liver and muscle cells. The
use of negative feedback keeps the blood glucose
concentration near the set point.
16 Plants produce several chemicals known as plant
growth substances that are involved in the control
of growth and responses to environmental changes.
Auxin is synthesised mainly in growing tips of shoots
and roots, and appears to be involved in preventing
the growth of lateral buds when an intact and active
apical bud is present. Gibberellin is synthesised
in young leaves and in seeds. It stimulates growth
of stems and germination of seeds such as those
of wheat and barley. Abscisic acid is synthesised
by any cells in a plant that contain chloroplasts or
amyloplasts, especially in stress conditions. The
presence of large concentrations of abscisic acid in
leaves causes stomata to close.
1. End-of-chapter questions
1 Which of the following is an incorrect statement about the endocrine system?
A All hormones bind to receptors on the cell surface of their target cells.
B Endocrine glands are ductless.
C Endocrine glands secrete hormones into the blood.
D Hormones are transported in the blood plasma.
2 Glucose is small enough to be filtered from the blood in glomeruli in the kidney, but is not normally found in the urine. This is because glucose is:
A reabsorbed in distal convoluted tubules
B reabsorbed in proximal convoluted tubules
C reabsorbed along the whole length of the nephrons
D respired by cells in the kidney
3 Which of the following is responsible for saltatory conduction in myelinated neurones?
A axon membranes
B nodes of Ranvier
C Schwarm cells
D voltage-gated channel proteins
4 Which of the following correctly identifies the effects of the three plant hormones,abscisic acid (ABA), auxin and gibberellin?
5 The figure shows the changes in potential difference across the membrane of a neurone over a period of time. The membrane was stimulated at time A and time B with stimuli of different intensities.
a Stimulus B resulted in an action potential.
Describe what is occurring at C, D and E. [6]
b Suggest why stimulus A did not result in
an action potential being produced whereas stimulus B did. [2]
[Total: 8]
[Cambridge International AS and A Level Biology 9700104, Question 8, October/November 2007]
6 a Explain the meaning of the term excretion. [3]
b The figure is a photomicrograph of part of the kidney.
i Name A, B, C and D. [4]
ii Identify the region of the kidneyshown in the figure
and give a reason for your identification. [2]
iii Calculate the actual maximum width of the
structure labelled A. Show your working. [2]
7 The control of the water content of the blood is an example of homeostasis.
a Name the part of the body that monitors the water potential of the blood. [1]
In an investigation of the factors that influence urine production, a person drank one litre of water. The person's urine was collected at half-hourly intervals for four hours after drinking. The results are shown as line A on the figure. On the following day, the same person drank one litre of a dilute salt solution and the urine was collected in the same way (line B). Dilute salt solution has about the same water potential as blood plasma.
b Calculate how much urine was produced in the two hours after drinking the litre of water. [1]
c Explain why the person produced so much urine after drinking the litre of water. [4]
d Suggest why the results during the second day were so different from those on the first day. [2]
8 Mammals have internal communication systems for signalling between cells.
a Explain why animals such as mammals have internal communication systems. [4]
Organs that secrete substances through ducts are known as exocrine glands. Most of the pancreas is made up of exocrine tissue. The rest, about 2%, is endocrine tissue which secretes insulin and glucagon.
b Describe the structure of the endocrine tissue in the pancreas. 5]
An investigation was carried out to determine the response of pancreatic cells to an increase in the glucose concentration of the blood. A person who had been told not to eat or drink anything other than water for 12 hours took a drink of a glucose solution. Blood samples were taken from the person at one hour intervals for five hours, and the concentrations of glucose, insulin and glucagon in the blood were determined. The results are shown in the figure.
c i Explain why the person was told not to eat or drink anything other than water for 12 hours before having the glucose drink. [3]
ii Use the information in the figure to describe the response of the pancreatic cells to an increase in the glucose concentration. [4]
iii Outline the role of insulin when the glucose concentration in the blood increases. [5]
9 Gibberellin is a plant growth regulator.
a Outline the role of gibberellin in the germination of seeds such as those of wheat and barley. [5]
In an investigation of the effects of gibberellin, plants of short-stemmed and long-stemmed varieties of five cultivated species were grown from seed. The young plants of each species were divided into two groups. One group of plants was sprayed with a solution of gibberellin each day. A control group was sprayed with the same volume of water. After eight weeks, the stem length of each plant was measured and means calculated for each group of plants. A statistical test was carried out to determine whether the difference between the treatments for each species was significant.
The results are shown in the figure. The p value for each species is given.
Cambridge International Examinations bears no responsibility for the example answers to questions taken from its past question papers which are contained in this publication.
1 A
2 B
3 B
4 C
Exam-style questions
1. End-of-chapter questions
1 Which of the following is an incorrect statement about the endocrine system?
A All hormones bind to receptors on the cell surface of their target cells.
B Endocrine glands are ductless.
C Endocrine glands secrete hormones into the blood.
D Hormones are transported in the blood plasma.
2 Glucose is small enough to be filtered from the blood in glomeruli in the kidney, but is not normally found in the urine. This is because glucose is:
A reabsorbed in distal convoluted tubules
B reabsorbed in proximal convoluted tubules
C reabsorbed along the whole length of the nephrons
D respired by cells in the kidney
3 Which of the following is responsible for saltatory conduction in myelinated neurones?
A axon membranes
B nodes of Ranvier
C Schwarm cells
D voltage-gated channel proteins
4 Which of the following correctly identifies the effects of the three plant hormones,abscisic acid (ABA), auxin and gibberellin?
a Stimulus B resulted in an action potential.
Describe what is occurring at C, D and E. [6]
b Suggest why stimulus A did not result in
an action potential being produced whereas stimulus B did. [2]
[Total: 8]
[Cambridge International AS and A Level Biology 9700104, Question 8, October/November 2007]
6 a Explain the meaning of the term excretion. [3]
b The figure is a photomicrograph of part of the kidney.
i Name A, B, C and D. [4]
ii Identify the region of the kidneyshown in the figure
and give a reason for your identification. [2]
iii Calculate the actual maximum width of the
structure labelled A. Show your working. [2]
[Total: 11]
7 The control of the water content of the blood is an example of homeostasis.
a Name the part of the body that monitors the water potential of the blood. [1]
In an investigation of the factors that influence urine production, a person drank one litre of water. The person's urine was collected at half-hourly intervals for four hours after drinking. The results are shown as line A on the figure. On the following day, the same person drank one litre of a dilute salt solution and the urine was collected in the same way (line B). Dilute salt solution has about the same water potential as blood plasma.
b Calculate how much urine was produced in the two hours after drinking the litre of water. [1]
c Explain why the person produced so much urine after drinking the litre of water. [4]
d Suggest why the results during the second day were so different from those on the first day. [2]
e Explain why negative feedback, and not positive feedback is involved in homeostatic mechanisms.
[Total: 13]
8 Mammals have internal communication systems for signalling between cells.
a Explain why animals such as mammals have internal communication systems. [4]
Organs that secrete substances through ducts are known as exocrine glands. Most of the pancreas is made up of exocrine tissue. The rest, about 2%, is endocrine tissue which secretes insulin and glucagon.
b Describe the structure of the endocrine tissue in the pancreas. 5]
An investigation was carried out to determine the response of pancreatic cells to an increase in the glucose concentration of the blood. A person who had been told not to eat or drink anything other than water for 12 hours took a drink of a glucose solution. Blood samples were taken from the person at one hour intervals for five hours, and the concentrations of glucose, insulin and glucagon in the blood were determined. The results are shown in the figure.
c i Explain why the person was told not to eat or drink anything other than water for 12 hours before having the glucose drink. [3]
ii Use the information in the figure to describe the response of the pancreatic cells to an increase in the glucose concentration. [4]
iii Outline the role of insulin when the glucose concentration in the blood increases. [5]
[Total: 21]
9 Gibberellin is a plant growth regulator.
a Outline the role of gibberellin in the germination of seeds such as those of wheat and barley. [5]
In an investigation of the effects of gibberellin, plants of short-stemmed and long-stemmed varieties of five cultivated species were grown from seed. The young plants of each species were divided into two groups. One group of plants was sprayed with a solution of gibberellin each day. A control group was sprayed with the same volume of water. After eight weeks, the stem length of each plant was measured and means calculated for each group of plants. A statistical test was carried out to determine whether the difference between the treatments for each species was significant.
The results are shown in the figure. The p value for each species is given.
b Using the information in the figure, describe the effect of adding gibberellin solution to the two varieties of the five species. [5]
c Explain why the short-stemmed variety of pea showed a more significant growth in height when treated with gibberellin than the long-stemmed variety. [3]
d Suggest the advantages of cultivating crops of short-stemmed varieties of peas and beans rather than long-stemmed varieties. [3]
[Total: 16]
10 Abscisic acid (ABA) is a weak acid. Its structure can be represented as ABA-H. It dissociates into positively charged H+ ions (protons) and negatively charged ABA- ions as shown:
The following observations have been made by scientists:
light stimulates proton (H+ ion) uptake into the grana of chloroplasts;
ABA-H can diffuse into and out of chloroplasts, but ABA- cannot.
This information is summarised in the figure.
a Using all the information provided, predict what happens to the pH in the stroma in the light.[1]
b i When light shines on the chloroplast, dissociation of ABA-H is stimulated. Explain why this happens. [2]
ii Explain the effect that this will have on diffusion of ABA-H into or out of the chloroplast. [2]
When the mesophyll cells of leaves become dehydrated, some of the ABA stored in the chloroplasts is released into the transpiration stream in the apoplast,
c ABA travels in the apoplast pathway to the guard cells. Explain why this is an advantage when the leaf is dehydrated. [2]
[Total: 7]
11 a Explain how a nerve impulse is transmitted along a motor neurone. [9]
b Describe how an impulse crosses a synapse. [6]
[Total: 15]
12 a Describe a reflex arc and explain why such reflex arcs are important. [7]
b Describe the structure of a myelin sheath and explain its role in the speed of transmission of a nerve impulse. [8]
[Total: 15]
[Cambridge International AS and A Level Biology Paper 41, Question 10, October/November2009]
13 a Compare the roles of the endocrine and nervous systems in control and coordination in animals. [8]
b Describe the part played by auxins in apical dominance in a plant shoot. [7]
[Total:15]
[Cambridge International AS and A Level Biology 9700/04, Question 10, May/June 2008]
2. End-of-chapter answers
Cambridge International Examinations bears no responsibility for the example answers to questions taken from its past question papers which are contained in this publication.
1 A
2 B
3 B
4 C
Exam-style questions
5 a C: depolarisation/the inside of the membrane
becomes more positive/less negative;
sodium ions/Na+
, flow in;
D: repolarisation/inside of the membrane
becomes more negative/less positive;
potassium ions/K+
, flow out;
E: hyperpolarisation/refractory period;
more negative than resting potential; [6]
6 a excretion: removal from the body;
of waste products of metabolism;
carbon dioxide/nitrogenous waste/urea/uric acid/
any other example;
substances in excess of requirements;
water/salts/sodium ions/potassium ions/any
other example; [max. 3]
b i A: distal convoluted tubule;
B: Bowman’s capsule;
C: glomerulus/capillary;
D: proximal convoluted tubule; [4]
ii cortex; glomeruli/convoluted tubules, are only
found in the cortex; [2]
7 a hypothalamus; [1]
b 1555 cm3
(or any answer within the range 1150 to
1160 cm3
or equivalent in dm3
); [1]
c any four from:
water was absorbed into the blood;
water increases the water potential of the plasma;
any effect of an increase in water potential of
the plasma on, cells/tissues, e.g. water enters
cells by osmosis/cells will swell/decreases
efficiency of reactions inside cells/cells may burst; osmoreceptors detect increase in water potential;
do not, secrete/release, ADH; collecting ducts remain impermeable to water; excess water lost
in urine; until water potential returns to normal/
set point; [max. 4]
d (after absorption of dilute salt solution) no change
in water potential of blood plasma; water and
salt is not lost in the urine, so must remain in
the body; giving an increase in volume, of blood
or body fl uids; body tolerates changes in blood
volume, but not its water potential; [max. 2]
e homeostasis is maintenance of constant internal
conditions;
negative feedback: a deviation from the set point;
is detected by a receptor; a control centre instructs
eff ector to carry out an action;
to reverse the
change/return factor to set point;
positive feedback: any (small) deviation in a factor
leads to an increase in the change (not a reversal);
e.g. opening of voltage-gated sodium ion channels
in rising phase of action potential; [max. 5]
[Total: 13]
8 a animals are multicellular/complex organisms;
cells are long distances apart;
coordination;
of cells/tissues/organs, so that they work
together;
regulation of internal environment/
refer to homeostasis;
response to, changes in
the environment/external stimuli;
any
example; [max. 4]
b islets of Langerhans;
small groups of cells;
scattered among the exocrine tissue;
blood spaces/
capillaries, in between the cells;
α cells;
β cells;
cells full of vesicles containing (molecules of),
hormones/protein;
cells with rough endoplasmic
reticulum; [max. 5]
c i glucose concentration may already be high;
if person had eaten within 12 hours;
effect of
sudden increase would not be seen/so there
was a sudden increase;
may already be a high
concentration of insulin; [max. 3]
ii β cells secrete insulin; concentration of
insulin increases over first hour after taking
the glucose solution;
insulin concentration
increases from 60 to 300 pmol dm−3
;
α
cells do not secrete glucagon;
glucagon
concentration, remains constant/decreases;
from 42 to 36 pmol dm−3
; [max. 4]
iii insulin: stimulates, liver/muscle, cells;
increase in uptake of glucose from the blood;
stimulates enzymes; to increase conversion of
glucose to glycogen;
brings about a decrease
in the blood glucose concentration; [5]
[Total: 21]
9 a gibberellin secreted by embryo;
stimulates cells in
the aleurone layer;
stimulates protein synthesis;
to make amylase;
to break down starch in the
endosperm;
mobilises glucose;
for respiration;
to
provide energy for germination; [max. 5]
b gibberellin increases the mean length of the stem
in the short-stemmed varieties of all species;
figures for any one species;
increase is significant
in all but one species;
gibberellin increases the
mean length of the stem in the long-stemmed
varieties of four of the species; not tomato;
increases are not signifi cant; [max. 5]
c short-stemmed plants do not make (much)
gibberellin;
(because) they do not have the
allele for the enzyme that makes gibberellin;
long-stemmed variety has dominant allele for
gibberellin synthesis;
gibberellin supplied each
day promotes growth of stems; [max. 3]
d less energy used for growth of stem; (therefore)
more energy in, peas/beans/seeds; plants do not
need (as much) support; less likely to be damaged
by wind; less plant material to harvest/less wastage
at harvest; [max. 3]
[Total: 16]
10 a increases; [1]
b i concentration of protons in the stroma
decreases (as enter grana); shifts equilibrium
to the right; [2]
ii increases diff usion into the chloroplast;
as concentration of ABA–H decreases; so
maintaining a concentration gradient into the
chloroplast; [max. 2]
c ABA stimulates closure of stomata; less water
vapour is lost; [2]
[Total: 7]
11 a resting potential;
anything in the range −60 to
−70 mV;
sodium–potassium pump uses ATP to pump Na+
out and K+
in;
many anions inside the neurone; resting potential
is due to leakage of K+
out;
action potential is
depolarisation of membrane;
up to +40 mV;
opening of voltage-gated sodium ion channels/
sodium ions flow in;
closing of voltage-gated sodium ion channels;
voltage-gated potassium ion channels open;
K+
flow out; resting potential restored;
local circuits depolarise next part of, membrane/
axon;
refractory period ensures impulse does not travel
backwards;
saltatory conduction in myelinated neurones;
action potential only at nodes of Ranvier;
[max. 9]
b action potential arrives at presynaptic membrane;
voltage-gated calcium ion channels open;
calcium ions enter to stimulate vesicles to move
to membrane;
vesicles fuse with membrane/
exocytosis, to release (named) neurotransmitter;
(named) neurotransmitter diff uses across
(synaptic) cleft;
binds with receptor on postsynaptic membrane;
stimulates opening of sodium ion channel
proteins;
sodium ions flow in through postsynaptic
membrane/depolarisation of postsynaptic
membrane; [max. 6]
[Total: 15]
12 a reflex arc: to max 5 – these points may be on a
diagram:
strong stimulus in receptor/AW;
action potential/impulses, along sensory neurone;
dorsal root of spinal nerve;
into spinal cord;
synapse with intermediate neurone;
(then) motor neurone;
action potential/impulses, to eff ector;
action potential/impulses, to brain;
response; e.g. knee jerk;
other points which may also be given on a
diagram:
fast/immediate;
stops/limits, damage/danger;
automatic/no conscious thought;
innate/stereotyped/instinctive; [max. 7]
b myelin sheath: Schwann cells;
wrap around axon;
sheath mainly lipid;
(sheath) insulates axon (membrane);
Na+
/K+
, cannot pass through sheath/can only pass
through membrane at nodes;
depolarisation (of axon membrane) cannot occur
where there is sheath/only at nodes of Ranvier;
local circuits between nodes;
action potentials ‘jump’ between nodes;
saltatory conduction;
increases speed/reduces time, of impulse
transmission;
up to 100 m s−1
;
speed in non-myelinated neurones about
0.5 m s−1
; [max. 8]
[Total: 15]
13 a endocrine system: uses hormones;
which are chemical messengers/chemicals that
transfer messages;
secreted/released, into blood by ductless glands;
influence target/organs/cells;
which have receptors on cell membranes;
an example of named hormone and eff ect;
e.g. insulin, stimulates decrease in blood glucose
concentration
nervous system: use impulses/action potentials;
not electrical, signals/current
along neurones/nerve fi bres;
not nerves
synapse (at target cell)/neuromuscular junction;
named neurone;
e.g. receptor/sensory/motor/
eff ector/intermediate/relay
differences between the two systems: endocrine
has slow effect/nervous is fast;
endocrine has long-lasting eff ect/nervous has
short-term eff ect;
endocrine has widespread eff ect/nervous has very
localised effect;
any other detail;
e.g. extra detail of synapse,
such as neurotransmitters [max. 8]
b auxins: indole 3-acetic acid (IAA/plant growth
regulator);
synthesised in, growing tips/apical
buds/meristems;
moves by diff usion;
from cell to cell;
also, by mass flow/in phloem;
stimulates cell
elongation;
not cell enlargement inhibits, side/lateral, buds/
growth
accept inhibits branching plant grows, upwards/
taller;
accept stem elongates interaction between IAA
and other plant growth regulators;
AVP; e.g. role of abscisic acid (ABA) and lateral
bud inhibition
AVP; e.g. cytokinins antagonistic to IAA/
gibberellins enhance IAA [max. 7]
[Total: 15]
really helpful website
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