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 longer e.g. elephant’s ears
• Elongating shape
• Developing a specialised gas exchange system

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 enter easily into the cells as in a single celled organism

Question 5

Name 3 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 5

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

Answer 5

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

Question 6

Name and describe 3 main features of an insect’s gas transport system

Answer 6

• 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 7

Explain the process of gas exchange in insects

Answer 7

• 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 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 9

Explain the process of gas exchange in fish

Answer 9

• 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
• Water carbon dioxide diffuses into the water and flows back out of the gills

Question 10

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

Answer 10

• 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 11

Name and describe 3 adaptations of a leaf that allows efficient gas exchange

Answer 11

1. Thin and flat to provide short diffusion pathway and large surface area to volume ratio
2. Many tiny pores on 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 12

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

Answer 12

• 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 13

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

Answer 13

Nasal cavity –> trachea –> bronchi –> bronchioles –> alveoli

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

• 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 for passage of air into the bronchioles

Question 17

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

Answer 17

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

Question 18

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

Answer 18

• 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 19

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

Answer 19

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

Question 20

Explain the process of expiration and the changes that occur during the thorax

Answer 20

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

Question 21

What is tidal volume?

Answer 21

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

Question 22

What is breathing rate?

Answer 22

The number of breaths we take per minute

Question 23

How do you calculate pulmonary ventilation rate

Answer 23

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