Surface area to volume ratio and 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 3 features of gas exchange.

Answer 5

1. Large SA (folded membranes in mitochondria).
2. Thin / short distance (wall of capillaries).
3. Steep concentration gradient, maintained by blood supply or ventilated (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 SA:V 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 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 collpase.
• Tracheoles = smaller branches dividing off the tracheae.

Question 8

Explain the process of gas exchange in insects.

Answer 8

• Gases move in and out the tracheae through 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 (counter current exchange system).

Question 11

Explain the process of gas exchange in fish.

Answer 11

• The fish opens its mouth to enable water flow in, then it 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 counter current 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 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.
   - Air spaces in the mesophyll allow gases to move around the leaf, facilitating photosynthesis.

Question 14

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

Answer 14

Stomata are 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

• Supported by rings of cartilage and are lined by ciliated epithelium cells.
• Narrow.
• One for each lung.
• Allow passage of air into bronchioles.

Question 19

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

Answer 19

• Narrower than 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 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 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.