Exchange of Substances (Topic 3)

Question 1

How does an organism’s size relate to
their surface area to volume ratio?

Answer 1

The larger the organism, the lower the
surface area to volume ratio.

Question 2

How does an organism’s surface area to
volume ratio relate to their metabolic
rate?

Answer 2

The lower the surface area to volume
ratio, the lower the metabolic rate.

Question 3

How might a large organism adapt to
compensate for its small surface area to
volume ratio?

Answer 3

Changes that increase surface area e.g.
folding; body parts become larger e.g.
elephant’s ears; elongating shape;
developing a specialised gas exchange
surface.

Question 4

Why do multicellular organisms require
specialised gas exchange surfaces?

Answer 4

Their smaller surface area to volume ratio
means the distance that needs to be crossed
is larger and substances cannot easily enter
the cells as in a single-celled organism.

Question 5

Name three features of an efficient gas
exchange surface.

Answer 5

1. Large surface area, e.g. folded membranes
   in mitochondria.
2. Thin/short distance, e.g. wall of capillaries.
3. Steep concentration gradient, maintained
   by blood supply or ventilation, e.g. alveoli.

Question 6

Why can’t insects use their bodies as an
exchange surface?

Answer 6

They have a waterproof chitin
exoskeleton and a small surface area to
volume ratio in order to conserve water.

Question 7

Name and describe the three main
features of an insect’s gas transport
system.

Answer 7

• Spiracles= holes on the body’s surface which may be
opened or closed by a valve for gas or water exchange.
• Tracheae= large tubes extending through all body
tissues, supported by rings to prevent collapse.
• Tracheoles= smaller branches dividing off the tracheae.

Question 8

Explain the process of gas exchange in
insects.

Answer 8

• Gases move in and out of the tracheae through
the spiracles.
• A diffusion gradient allows oxygen to diffuse into
the body tissue while waste CO2
diffuses out.
• Contraction of muscles in the tracheae allows
mass movement of air in and out.

Question 9

Why can’t fish use their bodies as an
exchange surface?

Answer 9

They have a waterproof, impermeable
outer membrane and a small surface
area to volume ratio.

Question 10

Name and describe the two main
features of a fish’s gas transport system.

Answer 10

Gills= located within the body, supported by arches, along
which are multiple projections of gill filaments, which are
stacked up in piles.
Lamellae= at right angles to the gill filaments, give an
increased surface area. Blood and water flow across them
in opposite directions (countercurrent exchange system).

Question 11

Explain the process of gas exchange in
fish.

Answer 11

• The fish opens its mouth to enable water to flow
in, then closes its mouth to increase pressure.
• The water passes over the lamellae, and the
oxygen diffuses into the bloodstream.
• Waste carbon dioxide diffuses into the water
and flows back out of the gills.

Question 12

How does the countercurrent exchange
system maximise oxygen absorbed by
the fish?

Answer 12

Maintains a steep concentration gradient, as
water is always next to blood of a lower
oxygen concentration. Keeps rate of diffusion
constant along whole length of gill enabling
80% of available oxygen to be absorbed.

Question 13

Name and describe three adaptations of
a leaf that allow efficient gas exchange.

Answer 13

1. Thin and flat to provide short diffusion pathway and large
   surface area to volume ratio.
2. Many minute pores in the underside of the leaf (stomata)
   allow gases to easily enter.
3. Air spaces in the mesophyll allow gases to move around
   the leaf, facilitating photosynthesis.

Question 14

How do plants limit their water loss while
still allowing gases to be exchanged?

Answer 14

Stomata regulated by guard cells which
allows them to open and close as needed.
Most stay closed to prevent water loss
while some open to let oxygen in.

Question 15

Describe the pathway taken by air as it
enters the mammalian gaseous
exchange system.

Answer 15

Nasal cavity → trachea → bronchi →
bronchioles → alveoli

Question 16

Describe the function of the nasal cavity
in the mammalian gaseous exchange
system.

Answer 16

A good blood supply warms and moistens
the air entering the lungs. Goblet cells in
the membrane secrete mucus which traps
dust and bacteria.

Question 17

Describe the trachea and its function in
the mammalian gaseous exchange
system.

Answer 17

• Wide tube supported by C-shaped cartilage to keep
the air passage open during pressure changes.
• Lined by ciliated epithelium cells which move
mucus towards the throat to be swallowed,
preventing lung infections.
• Carries air to the bronchi.

Question 18

Describe the bronchi and their function in
the mammalian gaseous exchange
system.

Answer 18

• Like the trachea they are supported by rings of
cartilage and are lined by ciliated epithelium cells.
• However they are narrower and there are two of
them, one for each lung.
• Allow passage of air into the bronchioles.

Question 19

Describe the bronchioles and their
function in the mammalian gaseous
exchange system.

Answer 19

• Narrower than the bronchi.
• Do not need to be kept open by cartilage, therefore
mostly have only muscle and elastic fibres so that
they can contract and relax easily during ventilation.
• Allow passage of air into the alveoli.

Question 20

Describe the alveoli and their function in
the mammalian gaseous exchange
system.

Answer 20

• Mini air sacs, lined with epithelium cells, site of
gas exchange.
• Walls only one cell thick, covered with a
network of capillaries, 300 million in each lung,
all of which facilitates gas diffusion.

Question 21

Explain the process of inspiration and
the changes that occur throughout the
thorax.

Answer 21

• External intercostal muscles contract (while internal
relax), pulling the ribs up and out.
• Diaphragm contracts and flattens.
• Volume of the thorax increases.
• Air pressure outside the lungs is therefore higher than
the air pressure inside, so air moves in to rebalance.

Question 22

Explain the process of expiration and the
changes that occur throughout the
thorax.

Answer 22

• External intercostal muscles relax (while internal
contract), bringing the ribs down and in.
• Diaphragm relaxes and domes upwards.
• Volume of the thorax decreases.
• Air pressure inside the lungs is therefore higher than the
air pressure outside, so air moves out to rebalance.

Question 23

What is tidal volume?

Answer 23

The volume of air we breathe in and out
during each breath at rest.

Question 24

What is breathing rate?

Answer 24

The number of breaths we take per minute.

Question 25

How do you calculate pulmonary
ventilation rate?

Answer 25

Tidal volume x breathing rate. These can
be measured using a spirometer, a device
which records volume changes onto a
graph as a person breathes.

Question 26

Define digestion.

Answer 26

The hydrolysis of large, insoluble
molecules into smaller molecules that
can be absorbed across cell
membranes.

Question 27

Which enzymes are involved in
carbohydrate digestion? Where are they
found?

Answer 27

• Amylase in mouth
• Maltase, sucrase, lactase in
membrane of small intestine

Question 28

What are the substrates and products of
the carbohydrate digestive enzymes?

Answer 28

• Amylase → starch into smaller polysaccharides
• Maltase → maltose into 2 x glucose
• Sucrase → sucrose into glucose and fructose
• Lactase → lactose into glucose and galactose

Question 29

Where are lipids digested?

Answer 29

The small intestine.

Question 30

What needs to happen before lipids can
be digested?

Answer 30

They must be emulsified by bile salts
produced by the liver. This breaks down
large fat molecules into smaller, soluble
molecules called micelles, increasing
surface area.

Question 31

How are lipids digested?

Answer 31

Lipase hydrolyses the ester bond
between the monoglycerides and fatty
acids.

Question 32

Which enzymes are involved in protein
digestion? What are their roles?

Answer 32

• Endopeptidases= break between specific
amino acids in the middle of a polypeptide.
• Exopeptidases= break between specific amino
acids at the end of a polypeptide.
• Dipeptidases= break dipeptides into amino
acids.

Question 33

How are certain molecules absorbed into
the ileum despite a negative
concentration gradient?

Answer 33

Through co-transport.

Question 34

Which molecules require co-transport?

Answer 34

Amino acids and monosaccharides.

Question 35

Explain how sodium ions are involved in
co-transport.

Answer 35

Sodium ions (Na+
) are actively transported
out of the cell into the lumen, creating a
diffusion gradient. Nutrients are then taken
up into the cells along with Na+
ions.

Question 36

Why do fatty acids and monoglycerides
not require co-transport?

Answer 36

The molecules are nonpolar, meaning
they can easily diffuse across the
membrane of the epithelial cells.

Question 37

Describe the structure of haemoglobin.

Answer 37

Globular, water soluble. Consists of four
polypeptide chains, each carrying a
haem group (quaternary structure).

Question 38

Describe the role of haemoglobin.

Answer 38

Present in red blood cells. Oxygen
molecules bind to the haem groups and
are carried around the body to where
they are needed in respiring tissues.

Question 39

Name three factors affecting
oxygen-haemoglobin binding.

Answer 39

1. Partial pressure/concentration of oxygen.
2. Partial pressure/concentration of carbon
   dioxide.
3. Saturation of haemoglobin with oxygen.

Question 40

How does partial pressure of oxygen
affect oxygen-haemoglobin binding?

Answer 40

As partial pressure of oxygen increases, the
affinity of haemoglobin for oxygen also
increases, so oxygen binds tightly to
haemoglobin. When partial pressure is low,
oxygen is released from haemoglobin.

Question 41

How does partial pressure of carbon
dioxide affect oxygen-haemoglobin
binding?

Answer 41

As partial pressure of carbon dioxide increases, the
conditions become acidic causing haemoglobin to
change shape. The affinity of haemoglobin for
oxygen therefore decreases, so oxygen is released
from haemoglobin. This is known as the Bohr effect.

Question 42

How does saturation of haemoglobin
with oxygen affect oxygen-haemoglobin
binding?

Answer 42

It is hard for the first oxygen molecule to bind. Once
it does, it changes the shape to make it easier for
the second and third molecules to bind, known as
positive cooperativity. It is then slightly harder for
the fourth oxygen molecule to bind because there is
a low chance of finding a binding site.

Question 43

Explain why oxygen binds to
haemoglobin in the lungs.

Answer 43

• Partial pressure of oxygen is high.
• Low concentration of carbon dioxide in the lungs,
so affinity is high.
• Positive cooperativity (after the first oxygen
molecule binds, binding of subsequent molecules
is easier).

Question 44

Explain why oxygen is released from
haemoglobin in respiring tissues.

Answer 44

• Partial pressure of oxygen is low
• High concentration of carbon dioxide
in respiring tissues, so affinity
decreases.

Question 45

What do oxyhaemoglobin dissociation
curves show?

Answer 45

Saturation of haemoglobin with oxygen
(in %), plotted against partial pressure of
oxygen (in kPa). Curves further to the left
show the haemoglobin has a higher
affinity for oxygen.

Question 46

How does carbon dioxide affect the
position of an oxyhaemoglobin
dissociation curve?

Answer 46

Curve shifts to the right because
haemoglobin’s affinity for oxygen has
decreased.

Question 47

Name some common features of a
mammalian circulatory system.

Answer 47

1. Suitable medium for transport, water-based to
   allow substances to dissolve.
2. Means of moving the medium and maintaining
   pressure throughout the body, such as the heart.
3. Means of controlling flow so it remains
   unidirectional, such as valves.

Question 48

Relate the structure of the chambers of the heart to
their function. (Draw it out too if you have spare paper)

Answer 48

• Atria: thin-walled and elastic, so they can
stretch when filled with blood
• Ventricles: thick muscular walls pump blood
under high pressure. The left ventricle is
thicker than the right because it has to pump
blood all the way around the body.

Question 49

Relate the structure of the vessels to
their function.

Answer 49

• Arteries have thick walls to handle high pressure
without tearing, and are muscular and elastic to
control blood flow.
• Veins have thin walls due to lower pressure,
therefore requiring valves to ensure blood doesn’t
flow backwards. Have less muscular and elastic
tissue as they don’t have to control blood flow.

Question 50

Why are two pumps (left and right)
needed instead of one? (In the heart)

Answer 50

To maintain blood pressure around the whole body.
When blood passes through the narrow capillaries of
the lungs, the pressure drops sharply and therefore
would not be flowing strongly enough to continue
around the whole body. Therefore it is returned to the
heart to increase the pressure.

Question 51

Describe what happens during cardiac
diastole.

Answer 51

The heart is relaxed. Blood enters the atria,
increasing the pressure and pushing open the
atrioventricular valves. This allows blood to
flow into the ventricles. Pressure in the heart
is lower than in the arteries, so semilunar
valves remain closed.

Question 52

Describe what happens during atrial
systole.

Answer 52

The atria contract, pushing any
remaining blood into the ventricles.

Question 53

Describe what happens during
ventricular systole.

Answer 53

The ventricles contract. The pressure
increases, closing the atrioventricular
valves to prevent backflow, and opening
the semilunar valves. Blood flows into
the arteries.

Question 54

Name the nodes involved in heart
contraction and where they are situated.

Answer 54

• Sinoatrial node (SAN)= wall of right
atrium.
• Atrioventricular node (AVN)= in
between the two atria.

Question 55

What does myogenic mean?

Answer 55

The heart’s contraction is initiated from
within the muscle itself, rather than by
nerve impulses.

Question 56

Explain how the heart contracts.

Answer 56

• SAN initiates and spreads impulse across the
atria, so they contract.
• AVN receives, delays, and then conveys the
impulse down the bundle of His.
• Impulse travels into the Purkinje fibres which
branch across the ventricles, so they contract
from the bottom up.

Question 57

Why does the impulse need to be
delayed?

Answer 57

If the impulse spread straight from the
atria into the ventricles, there would not
be enough time for all the blood to pass
through and for the valves to close.

Question 58

How is the structure of capillaries suited
to their function?

Answer 58

• Walls are only one cell thick; short diffusion pathway.
• Very narrow, so can permeate tissues and red blood
cells can lie flat against the wall, effectively delivering
oxygen to tissues.
• Numerous and highly branched, providing a large
surface area.

Question 59

What is tissue fluid?

Answer 59

A watery substance containing glucose,
amino acids, oxygen, and other
nutrients. It supplies these to the cells,
while also removing any waste materials.

Question 60

How is tissue fluid formed?

Answer 60

As blood is pumped through increasingly
small vessels, this creates hydrostatic
pressure which forces fluid out of the
capillaries. It bathes the cells, and then
returns to the capillaries when the hydrostatic
pressure is low enough.

Question 61

How is water transported in plants?

Answer 61

Through xylem vessels; long, continuous
columns that also provide structural
support to the stem.

Question 62

Explain the cohesion-tension theory.

Answer 62

Water molecules form hydrogen bonds with
each other, causing them to ‘stick’ together
(cohesion). The surface tension of the water
also creates this sticking effect. Therefore as
water is lost through transpiration, more can be
drawn up the stem.

Question 63

What are the three components of
phloem vessels?

Answer 63

• Sieve tube elements= form a tube to transport
sucrose in the dissolved form of sap.
• Companion cells= involved in ATP production for
active loading of sucrose into sieve tubes.
• Plasmodesmata= gaps between cell walls where
the cytoplasm links, allowing substances to flow.

Question 64

Name the process whereby organic
materials are transported around the
plant.

Answer 64

Translocation

Question 65

How does sucrose in the leaf move into
the phloem?

Answer 65

Sucrose enters companion cells of the phloem
vessels by active loading, which uses ATP and
a diffusion gradient of hydrogen ions. Sucrose
then diffuses from companion cells into the
sieve tube elements through the
plasmodesmata.

Question 66

How do phloem vessels transport
sucrose around the plant?

Answer 66

As sucrose moves into the tube elements, water potential
inside the phloem is reduced. This causes water to enter
via osmosis from the xylem and increases hydrostatic
pressure. Water moves along the sieve tube towards
areas of lower hydrostatic pressure. Sucrose diffuses into
surrounding cells where it is needed.

Question 67

Give evidence for the mass flow
hypothesis of translocation.

Answer 67

• Sap is released when a stem is cut, therefore there
must be pressure in the phloem.
• There is a higher sucrose concentration in the
leaves than the roots.
• Increasing sucrose levels in the leaves results in
increased sucroses in the phloem.

Question 68

Give evidence against the mass flow
hypothesis of translocation.

Answer 68

• The structure of sieve tubes seems to hinder mass flow.
• Not all solutes move at the same speed, as they would in
mass flow.
• Sucrose is delivered at the same rate throughout the
plant, rather than to areas with the lowest sucrose
concentration first.

Question 69

How can ringing experiments be used to
investigate transport in plants?

Answer 69

The bark and phloem of a tree are removed in a ring,
leaving behind the xylem. Eventually the tissues
above the missing ring swells due to accumulation of
sucrose as the tissue below begins to die. Therefore
sucrose must be transported in the phloem.

Question 70

How can tracing experiments be used to
investigate transport in plants?

Answer 70

Plants are grown in the presence of radioactive
CO2
, which will be incorporated into the plant’s
sugars. Using autoradiography, we can see
that the areas exposed to radiation correspond
to where the phloem is.