Gas exchange

Question 1

What is the metabolic rate of an organism?

Answer 1

The amount of energy expended by that organism in a given period of time

Question 2

What is the basal metabolic rate? (1)

Answer 2

The metabolic rate of an organism when at rest

Question 3

How can metabolic rate be measured? (3)

Answer 3

Oxygen consumption
Carbon dioxide production
Heat production

Question 4

How does an organism’s surface area to volume ratio relate to their metabolic rate? (1)

Answer 4

The higher the SA:V ratio the higher the metabolic rate

Question 5

Smaller animals have a greater SA:V ratio. What does this mean?

Answer 5

They lose more heat so they have to use up more energy to maintain their body temperature

Question 6

How might large organisms adapt to compensate for its small surface area to volume ratio? (2)

Answer 6

Changes that increase the surface area (e.g. folding; body parts becoming larger - elephant’s ears; elongating shape)
Developing a specialised gas exchange surface

Question 7

What is Fick’s law?

Answer 7

rate of diffusion = (surface area x concentration gradient) / diffusion distance

Question 8

What adaptations do insects have to limit water loss? (3)

Answer 8

Waterproof covering over their body surfaces
Relatively small surface area to volume ratio to minimise the area over which water is lost
Close spiracles when dehydrated

Question 9

How does oxygen move through the insect? (3)

Answer 9

Oxygen enters the insects through spiracles and into the trachea. Spiracles close.
Oxygen diffuses through the trachea into the tracheoles down a concentration gradient of oxygen
Oxygen is delivered directly to the respiring tissues

Question 10

Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange (3)

Answer 10

Tracheoles have thin walls so short diffusion distance to cells
Highly branched so large surface area for gas exchange
Trachea provides tubes full of air so fast diffusion into insect tissues

Question 11

How do gases move in and out of the tracheal system in insects? (3)

Answer 11

Down a diffusion gradient
Mass transport; muscle contractions (abdominal pumping) squeeze the trachea moving air in and out (ventilation)
End of tracheoles are filled with water

Question 12

Explain the movement of oxygen into the gas exchange system of an insect when it is at rest (3)

Answer 12

Oxygen used in aerobic respiration
So oxygen concentration gradient established
So oxygen diffuses in

Question 13

How do insects get additional oxygen during flight? (6)

Answer 13

When an insect is at rest, water can build up in the tracheoles
During flight, the insect may partly respire anaerobically and produce some lactate (lactic acid)
This lowers the water potential of the muscle cells. As the lactate builds up, water passes via osmosis from the tracheoles into the muscle cells
This adaptation draws air into the tracheoles closer to the muscle cells and therefore, reduces the diffusion distance for oxygen when its most needed

Question 14

Explain two ways in which the structure of fish gills is adapted for efficient gas exchange (4)

Answer 14

Many filaments/lamella so there’s a large surface area
Lamella are thin for a short diffusion pathway
Each gill is made up of lots of gill filaments which create a large surface area for the exchange of gases
The lamellae have lots of blood capillaries to speed up diffusion

Question 15

A fish uses its gills to absorb oxygen from water. Explain how the gills of a fish are adapted for efficient gas exchange (6)

Answer 15

Large surface area provided by many lamella so that increases diffusion
Thin epithelium of lamella so that there is a short diffusion distance between blood and water
Water and blood flow in opposite directions so that a concentration gradient is maintained
As water always next to blood with lower concentration of oxygen
Circulation replaces blood saturated with oxygen
Ventilation replaces water (as oxygen removed)

Question 16

Explain how the counter current mechanism in fish gills ensures the maximum amount of the oxygen passes into the blood flowing through the gills (3)

Answer 16

Water and blood flow in opposite directions
Blood always passing water with a higher oxygen concentration
Diffusion/concentration gradient maintained throughout the length of the gill lamella/filament

Question 17

How are leaves adapted for gas exchange? (3)

Answer 17

Flat - large surface area to volume ratio
Many stomata - pores to allow air to move in and out of leaf
Air spaces in leaf so short distance between mesophyll cells and air

Question 18

How does CO2 diffuse into a plant for gas exchange? (3)

Answer 18

Mesophyll cells photosynthesise and this reduces the concentration of CO2 in the cells
CO2 diffuses from the air spaces into the cells
This in turn reduces the CO2 concentration in the air spaces causing CO2 to move into the air spaces from the air outside the leaf, through the stomata

Question 19

How does O2 diffuse out of a plant for gas exchange? (3)

Answer 19

Mesophyll cells produce O2 as a result of photosynthesis
O2 diffuses into the air spaces from the cells
This increases the concentration of O2 in the air spaces, causing O2 to move from the air spaces to outside the leaf via the stomata

Question 20

Name one adaptation of the upper epidermis and explain its function (2)

Answer 20

Waxy cuticle layers
Makes it waterproof and prevents gas exchange (water vapour)

Question 21

Where does gas exchange occur in plants and briefly explain how (4)

Answer 21

At the stomata
Allows diffusion of gases in and out of the leaf
Is controlled by the opening/closing of the guard cells
CO2 diffuses in through the stomata into the spongy mesophyll layer then the palisade mesophyll layer (where photosynthesis occurs)

Question 22

Describe an adaptation of the spongy mesophyll layer for gas exchange (2)

Answer 22

Lots of air spaces
Provides short diffusion distance between gas (CO2) and mesophyll cells

Question 23

What adaptions do plants have to reduce water loss? (4)

Answer 23

At night, the guard cells close the stomata to prevent water loss
Air spaces saturated with water vapour from the xylem and water diffuses out of the stomata as it evaporates
Upper and lower surfaces have a waxy cuticle
Most stomata are found on the lower surface

Question 24

What are xerophytes? (1)

Answer 24

Plants that are adapted to living in areas where water is in short supply

Question 25

How are xerophytes adapted to reduce water loss? (6)

Answer 25

Stomata sunken in pits so reduced concentration gradient
Reduced number of stomata so less surface area for water loss
Hair to trap water vapour so reduced concentration gradient
Rolled leaves so reduced concentration gradient
Thick waxy cuticles so increased diffusion distance
Leaves reduced to spines reduces surface area for water loss

Question 26

Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf (4)

Answer 26

Carbon dioxide enters via stomata
Stomata opened by guard cells
Diffuses through air spaces
Down diffusion gradient

Question 27

How are alveoli adapted for gas exchange? (4)

Answer 27

The structure of the alveoli gives it an extremely large surface area
Dense capillary network to maintain efficient diffusion rates
Rich blood supply which circulates to maintain a large concentration gradient between the gases in the blood and in the alveoli
One cell thick squamous epithelium so thin diffusion distance

Question 28

How are lungs adapted for gas exchange? (3)

Answer 28

Alveoli have a Large Surface Area for diffusion

Have a Large Concentration Gradient
Maintained by a rich blood supply

Alveoli have a one cell thick squamous epithelium so thin diffusion distance

Question 29

Describe the pathway of oxygen from the atmosphere to the blood (4)

Answer 29

Oxygen moves through the trachea, bronchi, bronchioles and finally reaches the alveoli
Oxygen then passes by diffusion through the alveolar epithelium
Then through the endothelial cell of the capillary
And then combines with haemoglobin on red blood cell

Question 30

What is inhalation? (5)

Answer 30

External intercostal muscles contract pulling rib cage up and out
Diaphragm contracts and pulls down
Thoracic cavity volume increases
Pressure in lungs lower than atmospheric pressure
Air moves into lungs down a pressure gradient.

Question 31

What is exhalation? (5)

Answer 31

Internal Intercostal (rib) muscles contract
Diaphragm relaxes and moves up
Thoracic cavity volume decreases
Pressure in lungs greater than atmospheric pressure
Air moves out of lungs down a pressure gradient.

Question 32

Describe how ventilation helps to maintain an oxygen concentration gradient in the alveoli and lung capillaries (2)

Answer 32

Brings in air containing a higher oxygen concentration
Removes air with a lower oxygen concentration

Question 33

Give one other way that helps to maintain the difference in oxygen concentration

Answer 33

Circulation of blood

Question 34

Explain the role of the diaphragm in breathing out (3)

Answer 34

Diaphragm moves up
Reduces volume of thorax
Pressure in thorax higher than outside

Question 35

Why do multicellular organisms require specialised gas exchange surfaces? (2)

Answer 35

Smaller SA:V ratio means that they need to overcome long diffusion pathway
For faster rate of diffusion

Question 36

What is tidal volume? (1)

Answer 36

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