3.3.2 Gas Exchange

Question 1

How does gas exchange in single celled organisms occur?

Answer 1

+ Oxygen is absorbed by diffusion across their body surface, carbon dioxide also diffuses across their body surface
+ small (large SA:V)
+ thin membrane only surrounding the entire organism
+ single celled organisms with an additional cell wall have walls that are permeable

Question 2

What are the features of insects?

Answer 2

+ most are terrestrial
+ a problem for them is that water easily
evaporates from their body surface
+ however efficient gas exchange requires a thin,
permeable surface with a large area
+ have a small SA:V

Question 3

What is the exoskeleton?

Answer 3

+ found in insects
+ composed of a hard fibrous material called
chitin (for protection)
+ covered by a lipid rich layer (to prevent water
loss)

Question 4

What is the tracheae?

Answer 4

+internal network of tubes
+ supported by strengthened rings to prevent
them from collapsing

Question 5

What are tracheoles?

Answer 5

+ smaller divisions from the trachea, which
extend throughout the body of the insect
+ tracheoles branch directly to the cells

Question 5

What are air sacs?

Answer 5

+ temporary store of air within the insects body
(located at the end of the tracheoles)

Question 5

What are spiracles?

Answer 5

+ tiny pores on the surface of the insects body
+ contain valves

Question 6

Some smaller insects rely on simple diffusion of gases through the tracheal system. Why might this not be sufficient for larger insects?

Answer 6

+ longer diffusion distance
+ more active (require more energy)
+ therefore require more oxygen

Question 7

How does gas exchange occur in insects?

Answer 7

1. Valves in the spiracles open, to allow air to
   enter into the trachea
2. From the trachea air is passed onto the
   tracheoles ( thin walls)
3. From the tracheoles air is carried directly into t
   the cells within the body of the insect
   + tracheoles also carry air into air sacs where it is
   temporarily stored

Question 8

What are the advantages of gas exchange in insects?

Answer 8

+ short diffusion pathway for gases (from air into
the respiring cells)
+ diffusion gradient maintained ( for both oxygen
and carbon dioxide)

Question 9

What are the disadvantages of gas exchange in insects?

Answer 9

+ when spiracles are open, water can evaporate
from the insect causing it to dehydrate
+ relies on diffusion and for this to be effective
the pathway needs to be short
+ this limits the size that insects can get to

Question 10

How do insects overcome these disadvantages in gas exchange?

Answer 10

+ for most of the time the spiracles are closed to
prevent water loss, only open to allow gas e
exchange
+ spiracles can be closed to conserve water in
hot conditions
+ in the mean time insects can use air stored
within their air sacs to require
+ being small has not hindered the survival of i
insects - they are the most successful
organisms on Earth

Question 11

How do respiratory gases move in the tracheal system?

Answer 11

1. Along a diffusion gradient:
   + low concentration of oxygen towards the end of the tracheoles
   + high concentration of oxygen outside of the insects body, in the atmosphere
   + oxygen diffuses along the trachea into the
   tracheoles
   + opposite direction for carbon dioxide
2. Ventilation
   + rings of muscles within the insects body
   contract and relax (rhythmic abdominal
   movement) to allow mass movement of the air
   in and out of the trachea

Question 12

What happens when insects are rest?

Answer 12

+ water fills the end of the tracheoles

Question 13

What happens when insects are flying?

Answer 13

1. muscles cells respire anaerobically and
   produce lactic acid
2. lowers the water potential in muscle cells
3. water moves by osmosis from the tracheoles
   into the muscles cells
4. this increases the exposed surface area to air
5. as gases diffuse faster through air than water
   this enables a greater volume of oxygen to be
   supplied

Question 14

Why can a fish not obtain oxygen in the same way as a single celled organism?

Answer 14

+ fish have a waterproof outer covering, which
will not allow gases to diffuse through
+ also they have a small SA:V, which means that
their body surface will not be able to supply
then with a sufficient supply of respiratory
gases
+ it is also much harder to obtain oxygen from
the environment because of the lower oxygen
concentration in water than in air

Question 15

What are gills?

Answer 15

+ located behind the head of a fish
+ water enters through the mouth and is forced over these before leaving through the sides

Question 16

What are gill filaments?

Answer 16

+ made from gill filaments (thin plates)
+these are stacked up in a pile

Question 17

What are gill lamellae?

Answer 17

+ at right angle to gill filaments

Question 18

How are the gills involved in diffusion?

Answer 18

+ as the water passes through the gills, gas
exchange occurs in the lamellae
+ in the lamellae, the distance between the water
and the blood cells is approx 5µm
+ the distance between two lamellae is approx
50µm (short diffusion pathway)
+ gills provide a large SA
+ gills have an extensive network of blood
capillaries to maintain a concentration gradient

Question 19

What is the counter current system?

Answer 19

+ flow of blood and water over the gill lamellae
occur in opposite directions
+ this is done to maintain a concentration
gradient of oxygen and carbon dioxide
throughout the gill lamellae

Question 20

What are the stages of parallel flow?

Answer 20

1. At the start: high concentration gradient
   present so oxygen from the water diffuses into
   the blood
2. Towards the end: concentration gradient no
   longer available. Only approx 50% of oxygen
   available has been observed. Problem: a
   sufficient amount of oxygen is not being taken
   out of the water and into the blood - because
   diffusion isn’t happening across the whole of
   the lamellae

Question 21

What are the stages of counter flow?

Answer 21

1. At the start: blood already loaded with oxygen meets water with an even higher concentration of oxygen and diffusion can take place
2. Towards the end:
   + blood with slightly less/no oxygen meets
   water that has had some oxygen removed
   already
   + BUT still has enough oxygen left for diffusion
   to take place
   + therefore, at all points through the gill
   lamellae diffusion of oxygen into the blood
   occurs

Question 22

How does ventilation happen in fish?

Answer 22

1. When the fish open their mouth they lower the
   floor of the buccal cavity
2. This causes the volume inside the buccal cavity
   to increase, which causes a decrease in
   pressure within the cavity
3. The pressure is higher outisde the mouth of
   the fish and so water flows into the buccal
   cavity
4. This fish then raises the floor if the buccal
   cavity to close its mouth, increasing the
   pressure within the buccal cavity
5. Water flows from the buccal cavity (high
   pressure) into the gill cavity (low pressure)
6. As water enters pressure begins to build up in
   the gill cavity and causes the operculum to be
   forced open and water to exit the fish

Question 23

Compare and contrast the methods by which insects and bony fish exchange oxygen and
carbon dioxide with the environment

Answer 23

Insects

Spiracles - location and function

Trachea - structure (chitin rings) and function

Tracheoles - thin walls & end at body cells, large number

Ability of some insects to compress air sacs with abdominal pumping

Removal of water from tracheoles when insect is active

Fish

Filaments – location, function, many of them

Lamellae – many of them, thin walls

Blood supply – many capillaries in lamellae

Counter-current system

Similarities

Short distance for diffusion

Large surface area

Possibility of ventilation

Simple diffusion of gases (or dissolved gases) into/out of cells

Differences

Blood supply in fish – due to small SA:vol ratio

Water flows through fish (in one direction), air moved in and out of insec

Question 24

Describe two adaptions of the structure of alveoli for efficient gas exchange

Answer 24

+ large surface area
+ thin walls

Question 25

People who are fire-breathers for many years often find they cannot breathe out properly. Explain why.

Answer 25

+ fibrosis reduces elasticity of
alveoli
+ due to scar tissue

Question 26

Destruction of phagocytes causes the lungs to be more susceptible to infections. Explain why.

Answer 26

+ phagocytes destroy/engulf
pathogens
+ lung diseases are caused by
pathogens

Question 27

The reduced elasticity of the lungs causes breathing difficulty. Explain how.

Answer 27

+ lungs can no longer expand to their maximum capacity
+ breathing out particularly affected

Question 28

explain how the elastic tissue in the wall helps to even out the pressure of blood flowing through the artery

Answer 28

+ stretches as a result of high pressure
+ then recoils