Biology Topic 3

Question 1

Describe the relationship between the size of an organism and its SA:V ratio.

Answer 1

As the size of an organism increases, its SA:V ratio increases.

Question 2

What are some features of an efficient exchange surface?

Answer 2

• Large SA, e.g. folded membranes.
• Thin - shorter diffusion pathway.
• A good blood supply in order to maintain a steep diffusion gradient.

Question 3

What is the relationship between
surface area to volume ratio and metabolic rate?

Answer 3

To minimize heat loss from the body, animals with a smaller surface area do not have a high metabolic rate. However, animals with a higher surface area have a high metabolic rate to produce more heat to maintain body temperature.

Question 4

How does air enter the body of an insect?

Answer 4

It enters the body through spiracles, leading to a system of air-filled tubes called tracheoles.

Question 5

What holds the tracheae open?

Answer 5

Rigid rings made of chitin.

Question 6

What do the tracheae branch into? What takes place here?

Answer 6

They branch into smaller tracheoles that penetrate between cells and into the muscle fibres, where gas exchange takes place.

Question 7

How do larger insects ventilate the tracheal system?

Answer 7

They open and close the spiracles and contract the abdominal muscles which squeeze the tracheal system, causing air to be pumped through the system.

Question 8

How does gas exchange occur in single-celled organisms?

Answer 8

As their whole cell-surface membrane is in touch with the environment, they need no special adaptations.

Question 9

How do fish obtain their oxygen from the water they swim in?

Answer 9

Water passes over their gills, which are adapted to absorb oxygen from the water.

Question 10

What do thin filaments have over their surface?

Answer 10

Rows of thin lamellae that vertically stick up from the filaments.

Question 11

What does the surface of the lamellae consist of?

Answer 11

A single layer of thin, flattened cells. Underneath this layer is a network of capillaries.

Question 12

Why is the gas exchange system of a fish named the ‘counter-current system?’

Answer 12

The blood vessels in the lamellae are arranged so that water and blood flow in opposite directions.

Question 13

What is an advantage of a counter-current system?

Answer 13

It maintains a concentration gradient all the way along the capillary.

Question 14

In dicotyledonous plants, where does gas exchange take place?

Answer 14

Inside of the leaves.

Question 15

What is the gas exchange surface in a leaf?

Answer 15

The surface of the mesophyll cells.

Question 16

Why are mesophyll cells considered a suitable gas exchange surface?

Answer 16

• Most of their surface is exposed to the air spaces in the leaf.
• The palisade mesophyll cells have small gaps between them, making sure that most of the surface is exposed to the air.

Question 17

How does air enter the leaf?

Answer 17

Through the stomata.

Question 18

Why does CO2 diffuse into the mesophyll cells?

Answer 18

The air is higher in CO2 than the cytoplasm of the cells.

Question 19

Why does oxygen diffuse out of the mesophyll cells?

Answer 19

The mesophyll cells are higher in oxygen than the air spaces, therefore oxygen diffuses out of the cells into the air spaces.

Question 20

How does oxygen diffuse out of the leaf?

Answer 20

Oxygen diffuses out of the leaf via. the stomata.

Question 21

How do insects reduce the chances of drying out?

Answer 21

• They have an outside skeleton that is rigid and coated with a waxy substance, making it waterproof.
• Their spiracles can be closed using valves.
• Hairs surrounding the spiracles trap humid air, reduce the WP gradient between the tracheae and the outside air.

Question 22

How are leaves adapted to reduce water loss?

Answer 22

• They have a waterproof cuticle on the upper and lower surfaces of the leaf.
• The stomata can be closed if water loss is too high.

Question 23

How does an example of a xerophyte (a cactus) reduce water loss?

Answer 23

• Stomata in grooves to reduce WP gradient between the environment and the cells.
• Leaves reduced to spines to reduce SA:V ratio.
• Shallow roots spread over a wide area to obtain as much water as possible from a shower of rain.
• Thick waxy cuticle to keep water loss at a minimum.
• Swollen stem with chloroplasts for photosynthesis and thickened for water storage and to reduce SA:V ratio.
• Roots penetrating deeply to water table.

Question 24

Adaptations of another xerophyte (marram grass) to reduce water loss.

Answer 24

• Thick waxy cuticle to reduce water loss to a minimum.
• Photosynthetic tissue.
• Exposed external surface has no stomata, reducing water loss.
• Hairs surround stomata to trap moist air.
• Hinge cells roll leaf up when conditions are dry. When rolled up, moist air is trapped inside - reducing the WP gradient between the cells and environment.

Question 25

How does air travel through the human gas exchange system?

Answer 25

Air is breathed into the trachea, which is held open by rings of cartilage. The trachea branches into two bronchi, the bronchi branch into bronchioles that lead to clusters of alveoli.

Question 26

What features ensure that there is a short diffusion pathway for oxygen to pass through from the alveoli to the red blood cells?

Answer 26

• The alveolar walls are one-cell thick and made of thin, flattened cells.
• The capillary walls are made of a single layer of thin, flat cells.
• The capillaries are narrow, so red blood cells are squeezed through them one by one.
  This means that most of their surface is pressed against the capillary walls.

Question 27

What does the ventilation of the lungs maintain?

Answer 27

It maintains a steep concentration gradient for efficient gas exchange.