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 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. elongating body parts (elephant’s ears)
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 travelled by gas is larger, & substances cannot easily enter the cells as in a single-celled organism.

Question 5

Name 3 features of an efficient gas exchange system, and give an example for each.

Answer 5

1) Large surface area (e.g. folded membranes in mitochondria)
2) Thin / short diffusion pathway (e.g. walls 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 & a small surface area to volume ratio in order to conserve water.

Question 7

Name the 3 main features of an insect’s gas transport system.

Answer 7

Spiracles
Tracheae
Tracheoles

Question 8

What are spiracles in an insect’s gas transport system?

Answer 8

Holes on the insect body’s surface which may be opened or closed by a valve for gas or water exchange.

Question 9

What are tracheae in an insect’s gas transport system?

Answer 9

Large tubes extending through all body tissues, supported by rings to prevent collapse.

Question 10

What are tracheoles in an insect’s gas transport system?

Answer 10

Smaller branches off the tracheae which have thin, permeable walls, and go into individual cells.

Question 11

Explain the process of gas exchange in insects.

Answer 11

Air moves into microscopic pipes called the tracheae, through pores on the surface called spiracles.
A diffusion gradient allows oxygen to diffuse into the smaller tracheoles which deliver oxygen directly into respiring cells. (Insect’s circulatory system doesn’t transport oxygen).
Carbon dioxide from cells moves down its own concentration gradient towards the spiracles to be released into the atmosphere.
Insects use rhythmic abdominal movements (contraction of muscles) to move air in & out.

Question 12

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

Answer 12

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

Question 13

Name the 2 main features of a fish’s gas exchange system.

Answer 13

Gills
Lamellae

Question 14

What are gills in a fish’s gas exchange system?

Answer 14

Located within the body
Supported by arches
Made of lots of thin plates called gill filaments, which are stacked parallel, and give a large surface area.

Question 15

What are lamellae in a fish’s gas exchange system.

Answer 15

At right angles to the gill filaments, giving an increased surface area
Have lots of blood capillaries and a thin surface layer of cells to speed up diffusion, between the water and the blood.

Question 16

What is the counter-current system in fish’s gas exchange?

Answer 16

In gills, blood flows through lamellae in one direction, & water flows over them in opposite direction.
This means that water with a relatively high oxygen concentration always flows next to blood with a lower oxygen concentration.
So, a steep concentration gradient is maintained over the entire length of the gill.

Question 17

Explain the process of gas exchange in fish.

Answer 17

Fish opens mouth to enable water to flow in, then closes mouth to increase pressure.
Water passes over lamellae, & oxygen diffuses into bloodstream.
Waste carbon dioxide diffuses into water & flows back out of gills.

Question 18

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

Answer 18

Maintains a steep concentration gradient across entire length of gill, 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 19

Name & describe 3 adaptations of a leaf that allow for efficient gas exchange.

Answer 19

1) Thin & flat = to provide short diffusion pathway & 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 20

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

Answer 20

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

Question 21

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

Answer 21

Nasal cavity
Trachea
Bronchi
Bronchioles
Alveoli

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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

Question 24

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

Answer 24

Like the trachea, they are supported by rings of cartilage and are lined by ciliated epithelium cells.
However, they are narrower and there are 2 of them, 1 for each lung.
Allow passage of air into the bronchioles.

Question 25

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

Answer 25

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

Question 26

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

Answer 26

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 27

Explain the process of inspiration

Answer 27

• External intercostal muscles contract (while internal relax), pulling the ribs up & out.
• Diaphragm contacts & 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 28

Explain the process of expiration.

Answer 28

• External intercostal muscles relax (internal stay released, unless forced respiration = contract), bringing the ribs down & in.
• Diaphragm relaxes and domes upwards.
• Volume of thorax decreases.
• Air pressure inside lungs is therefore higher than pressure outside, so air move out to rebalance.

Question 29

What is tidal volume?

Answer 29

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

Question 30

What is breathing rate?

Answer 30

The number of breaths we take per minute.

Question 31

How do you calculate pulmonary ventilation rate?

Answer 31

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