CHAPTER 11 - Gas Exchange Review questions
ANSWERS
1 It is necessary for organisms to exchange oxygen and carbon dioxide with their external environment to
provide the necessary requirements for cellular respiration and photosynthesis.
During cellular respiration oxygen is required for the conversion of glucose into carbon dioxide and
water. Energy is released as a result of this conversion. The carbon dioxide produced by this process will
cause the tissue to become more acidic if it is not removed. Increased acidity would damage the structure of
many of the important molecules thus affecting their function.
In photosynthesis, carbon dioxide is required from the external environment. Carbon dioxide and water
are converted into glucose and oxygen. The oxygen is released into the external environment.
2 Diffusion is the process by which gases move across an exchange surface. The direction and rate of
diffusion are dependent upon the concentration gradient of the substance being transferred across the
exchange surface. Molecules will move to a region of lower concentration and they will move more quickly if
there is large concentration gradient.
3 Algae do not have any stomata for gaseous exchange and carbon dioxide diffuses across their exterior
surface into the photosynthetic cells.
4
Cellular respiration
Photosynthesis
Takes place in all organisms
Only takes place in plants
Takes place all the time
Only happens in sunlight or under artificial lights
Produces energy
Requires energy
5 During the day two processes occur i.e. photosynthesis and respiration. Because the rate of photosynthesis
is usually faster than the rate of respiration, carbon dioxide produced in respiration is not sufficient to supply
enough carbon dioxide for photosynthesis and it diffuses into the leaf. At night only one process i.e.
respiration and this requires oxygen.
6 Take a potted plant and paint a section of the underside of a leaf with Vaseline or a substance
impermeable to gases. Leave the plant in bright sunlight for a couple of hours. Remove the leaf and test it for
starch. The area that was painted should be negative.
7a Leaves have stomata whose opening and closing is controlled by guard cells. Stems have lenticels which
are a group of loosely packed cells through which air passes freely to the cells beneath. Just behind the
growing points of roots are root hairs. Oxygen readily diffuses into the film of moisture surrounding the root
hairs, and then into the roots themselves.
7b air  stomata  air spaces  dissolves in moisture surrounding the cell membrane  passes through
the membrane
8a They have chloroplasts which undergo photosynthesis and produce glucose which is used in respiration to
make energy for movement i.e. opening and closing. The glucose produced by photosynthesis causes water
to move into the guard cells by osmosis. The cells become turgid and the stomata open. They also have a
thickened inner cell wall with cellulose fibres that prevent the cells expanding in width. Therefore they
expand lengthways and this buckles the guard cells and opens the stoma.
8b If the water supply is abundant the guard cells will be full of water (turgid) and the stomata will be open.
If water is lost the guard cells lose their turgor and the stomata close. In bright sunlight photosynthesis will
take place in the guard cells. As the concentration of glucose in the cells increases water will move in,
increasing the turgor and the stomata will open. If the concentration of carbon dioxide is low inside the leaf
the stomata will open so that photosynthesis can continue.
8c Land plants need to conserve water. The rays of the sun are very drying and they strike the tops of the
leaves. By not having any stomata in the upper surface water loss is minimised. Aquatic plants need to float
on top of the water without coming water-logged and having stomata only on the top of the leaf allows free
exchange of gases between the leaf and the atmosphere.
8d The soil has poor drainage and becomes saturated with water. This reduces the amount of oxygen
available to the root cells. The roots cannot take up oxygen which is needed for respiration and the plant
could die.
9 There could be special structures in their stems above the level of the water which allow exchange of
gases. They also could have large intercellular spaces in their stems to allow rapid diffusion.
10 Comparison of lungs and fish gills:
Feature
Large surface area
Thin and moist
Good ventilation
Good blood supply
Protection/support
Features
Large surface area
Thin and moist
Good ventilation
Good blood supply
Protection/support
Lungs
Greater gas exchange per unit of time
To allow gases to pass through easily. Gas can only pass across an exchange surface if it is in
solution, thus the surface must be moist. It is difficult for gases to pass through thick barriers
To provide an adequate supply of the gas being transferred or the gas will become scarce
and the rate of exchange will drop
Inadequate blood flow means oxygen accumulates and so diffusion slows down
Respiratory membranes are delicate and as such require protection from physical damage
and, in terrestrial animals, protection from water loss
Fish gills
Fish gills are composed of a series of gill arches each of which has many finger-like gill
filaments. Each of these has a row of closely packed, flat, leaf-like lamellae across which gas
exchange takes place. The lamellae provide a large surface area for gas exchange
Lamellae are thin and moist
Gills are ventilated with oxygen by the movement of water over the gills
Lamellae contain many blood vessels
The gills are covered by the operculum, which protects the fragile gills
11a Gills are not suitable for breathing air because they are external respiratory surfaces. Being external, gills
would be prone to water loss due to evaporation of water from the respiratory surface. Also, when gills are
out of water the filaments clump together and the surface area is reduced.
11b Lungs rely on the ventilating medium being able to exit the body by the same route it entered. Water is a
heavy medium compared with air and a great deal of energy would be required to move water across the
respiratory surface in two directions. Also, water contains less oxygen than air, and it would not be possible
to maintain an effective exchange gradient for oxygen in the lungs.
12 They have highly efficient mechanisms of moving water over their respiratory surfaces i.e. they move
large volumes of water with little dissolved oxygen. Most fish actively ventilate their gills by means of a
double-action pump of the mouth and opercular cavities, crayfish use the rhythmic beating of specialised
appendages and some fish such as sharks and tuna use ram ventilation.
The respiratory surfaces have a large surface area exposed to the respiratory medium and a countercurrent flow system between the blood and respiratory medium ensures maximum oxygen uptake.
13a Water-ventilating animals use dissolved oxygen levels and air-ventilating animals use dissolved carbon
dioxide levels.
13b For water ventilators carbon dioxide removal is easy but oxygen is the crucial factor because it is in short
supply. For air ventilators, oxygen is abundant in air but carbon dioxide is only slowly removed by diffusion
and tends to accumulate in body fluids lowering the pH to dangerous levels.
14
Frog
Insect
Advantages
and
disadvantag
es
Air is drawn into the mouth through nostrils
and then is pushed into the lungs by raising and
lowering the floor of the mouth
One pair of lungs
Oxygen moves into the circulatory system by
which it is distributed to the body cells
The skin is also used as a respiratory surface. It
will only function if it is kept moist
Oxygen reaches the cells as the circulatory
system carries it there
Exchange surface can more easily dry out,
especially the skin
Rate of breathing can be adjusted according to
the needs of the organism for oxygen
Air moves by diffusion through the spiracles and trachea
Spiracles and trachea in every segment. Spiracles can
close to reduce moisture loss
Oxygen is exchanged directly with the cells where it will
be used
Oxygen reaches the cells rapidly
Exchange surface is deep within the body therefore
there is little chance of it drying out
Rate of gas exchange depends on the diffusion gradient.
To some extent this will be modified by the level of
activity of the organism. The cells that are using oxygen
at the fastest rate will gain oxygen at the fastest rate
15 The functions of various parts of the respiratory system are as follows:
• Nasal passages filter, moisten and warm air.
• The trachea, bronchi and bronchioles make up the passage along which air travels.
• The trachea and bronchi are lined with ciliated cells and secrete mucus. Particles of dust or bacteria are
trapped by the mucus and swept by the cilia back up to the pharynx and swallowed.
• Alveoli are the site of gas exchange.
16a During inhalation:
• the diaphragm contracts downwards
• the ribs are raised outwards by contraction of the intercostal muscles
• the chest cavity increases in volume.
During exhalation:
• the diaphragm relaxes and moves up
• the intercostal muscles relax and the ribs lower
• the chest cavity decreases in volume.
16b Raising the ribs and lowering the diaphragm during inhalation increases the volume of the chest cavity
and draws air into the lungs. Relaxing these muscles during exhalation reduces the volume of the chest cavity
and forces air out of the lungs.
17a Respiratory pigments make it possible to carry much more oxygen than could be carried by the fluid
(plasma) alone. They increase the oxygen-carrying capacity of the blood.
17b Respiratory pigments provide energy savings for an animal by increasing the oxygen-carrying capacity of
the blood. An increased oxygen-carrying capacity results in a reduction in the amount of energy that must be
spent pumping the blood. It also means that the animal can have a much smaller volume of blood, and that it
can be pumped more slowly.
18 Breathing carbon monoxide can be fatal because it binds irreversibly to the same position on
haemoglobin as oxygen, thus preventing haemoglobin molecules from carrying oxygen. Death can occur if
the oxygen-carrying capacity of the blood is reduced below tolerable limits.
19a About 7% of the carbon dioxide is transported in the plasma. About 23% of carbon dioxide transported in
blood combines with haemoglobin molecules forming carbaminohaemoglobin. The carbon dioxide binds to a
different site on the haemoglobin molecule to the site where oxygen binds.
About 70% of carbon dioxide transported in the blood exists as hydrogen carbonate ions and is
transported in plasma.
19b The amount of carbon dioxide dissolved in blood plasma must be kept low because when it combines
with water in the plasma carbon dioxide forms carbonic acid which results in a decrease in the pH of the
plasma. Low pH can affect the structure of proteins (e.g. enzymes) thus affecting their function.
20a Asthma: The space through which air can travel in the airways (bronchioles) is reduced.
Emphysema:The alveoli break down reducing the surface area across which gases can be exchanged.
Pneumonia: The alveoli become filled with fluid.
Anaemia: There is less haemoglobin in the blood.
20b Asthma: The amount of air that can be exchanged between the lungs and the atmosphere is limited.
Emphysema: Gas exchange may be reduced to one quarter of normal.
Pneumonia: The surface area of the alveoli in contact with the air is reduced and gas exchange becomes less
efficient.
Anaemia: The smaller amount of haemoglobin reduces the ability of the blood to carry oxygen to the cells.