3.1 Surface Area to Volume Ratio + 3.2 Gas Exchange

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 smaller the surface area to volume

ratio, the higher 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 and enables 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.