Exchange Flashcards

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1
Q

What are the features of specialised exchange surfaces?

A

A large surface area relative to the volume of the organism which increases the rate of exchange.
Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly.
Selectively permeable to allow selected materials to cross.
Movement of the environment medium, for example, air, to maintain a diffusion gradient.
A transport system to ensure the movement of the internal medium, for example, blood, in order to maintain a diffusion gradient.

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2
Q

What is the relationship between diffusion?

A

Diffusion ∝ surface area x difference in concentration / length of diffusion pathway.

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3
Q

Why are exchange surfaces within the organism?

A

Being thin, specialised exchange surfaces are easily damaged and dehydrated.
They are therefore often located inside an organism.
Where an exchange surface is located inside the body, the organism needs to have a means of moving the external medium over the surface, e.g. a means of ventilating the lungs in a mammal.

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4
Q

Why do fish have specialised exchange surfaces?

A

Fish have a waterproof, and therefore a gas-tight, outer covering.
Being relatively large they also have a small surface area to volume ratio.
Their body surface is therefore not adequate to supply and remove their respiratory gases and so have evolved - the gills.

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5
Q

What is the structure of the gills?

A

The gills are located within the body of the fish, behind the head.
They are made up of gill filaments, stacked in a pile.
At right angles to the filaments are gill lamellae, which increase the surface area of the gills.
Water is taken through the mouth and forced over the gills and out through the opening on each side of the body.

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6
Q

What is the counter current flow?

A

The flow of water over the gill lamellae and the flow of blood within them are opposite directions.
It is important that the maximum possible gas exchange is achieved.
If the water and blood flowed in the same direction, far less gas exchange would take place.

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7
Q

What is the counter current exchange principle?

A

The blood and water flow over the gill lamellae in opposite directions.
This arrangement means that:
Blood that is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. Therefore diffusion of oxygen from the water to the blood takes place.
Blood with little oxygen in it meets water which has had most, but not all, its oxygen removed. Again, diffusion of oxygen from the water to blood takes place.

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8
Q

Why is the counter current exchange needed in fish?

A

A diffusion gradient for oxygen uptake is maintained across the entire width of the gill lamellae.
In this way, about 80% of the oxygen available in the water is absorbed into the blood of the fish.
If the flow of water and blood had been in the same direction (parallel flow), the diffusion gradient would only be maintained across part of the length of the gill lamellae and only 50% of the available oxygen would be absorbed by the blood.

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9
Q

What is ventilation?

A

To maintain diffusion of gases across the alveolar epithelium, air is constantly moved in and out of the lungs - breathing/ventilation.

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10
Q

What is inspiration?

A

When the air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the lungs.

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11
Q

What is expiration?

A

When the air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs.

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12
Q

How do the pressure changes in the lungs occur?

A

By movement of three sets of muscles:
The diaphragm, which is a sheet of muscle that separates the thorax from the abdomen.
The intercostal muscles, which lie between the ribs, two sets:
The internal intercostal muscles, whose contraction leads to expiration.
The external intercostal muscles, whose contraction leads to inspiration.

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13
Q

What is the process of inspiration?

A

The external intercostal muscles contract, while the internal intercostal muscles relax.
The ribs are pulled upwards and outwards, increasing the volume of the thorax.
The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax.
The increased volume of the thorax results in the reduction of pressure in the lungs.
Atmospheric pressure is now greater than pulmonary pressure, so air is forced into the lungs.

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14
Q

What is the process of expiration?

A

The internal intercostal muscles contract, while the external intercostal muscles relax.
The ribs are pulled downwards and inwards, decreasing the volume of the thorax.
The diaphragm muscles relax and so it is pushed up again by the contents of the abdomen that were compressed during inspiration. The volume of the thorax is further decreased.
The decreased volume of the thorax increases pressure in the lungs.
Pulmonary pressure is now greater than that of the atmosphere, so air is forced out of the lungs.

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15
Q

How does exercise affect breathing?

A

During normal quiet breathing, the recoil of elastic tissue in the lungs is the main cause of air being forced out.
Only under more strenuous conditions such as exercise do the various muscles play a major part.

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16
Q

How does gas exchange take place in single-celled organisms?

A

They are small and therefore have a large surface area : volume ratio.
Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrane.
Carbon dioxide from respiration diffuses out across their body surface similarly.
Where a living cell is surrounded by a cell wall, this is no additional barrier to the diffusion of gases.

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17
Q

How do respiratory gases move in and out of the tracheal system?

A

Along a diffusion gradient.
Mass transport
The ends of the tracheoles are filled with water.

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18
Q

How do gases move along diffusion gradient?

A

When cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles falls.
This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells.
Carbon dioxide is produced by cells during respiration.
This creates a diffusion gradient in the opposite direction.
This causes gaseous carbon dioxide to diffuse along the tracheoles and trachea from the cells to the atmosphere.
As diffusion in air is much more rapid than in water, respiratory gases are exchanged quicker by this method.

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19
Q

How do gases move by mass transport?

A

The contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out.
This further speeds up the exchange of respiratory gases.

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20
Q

How do gases move - the ends of the tracheoles are filled with water?

A

During periods of major activity, the muscle cells around the tracheoles respire and carry out some anaerobic respiration.
This produces lactate, which is soluble and lowers the water potential of the muscle cells.
Water therefore moves into the cells from the tracheoles by osmosis.
The water in the ends of the tracheoles decreases in volume and in doing so draws air further into them.
This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid.
This increases the rate at which air is moved in the tracheoles but leads to greater water evaportation.

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21
Q

What is the structure of the gas exchange system in insects?

A

For gas exchange, insects have evolved an internal network of tubes called trachea.
The tracheae are supported by strengthened rings to prevent them from collapsing.
The tracheae divide into smaller dead-end tubes called tracheoles, that extend throughout all the body tissues of the insect.
In this way, atmospheric air, with the oxygen it contains, is brought directly to the respiring tissues, as there is a short diffusion pathway from a tracheole to any body cell.

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22
Q

What are spiracles?

A

Gases enter and leave tracheae through tiny spores - spiracles, on the body surface.
They may be opened and closed by a valve.
When they are open, water vapour can evaporate from the insect.
For much of the time insects keep their spiracles closed to prevent this water loss.
Periodically they open the spiracles to allow gas exchange.

23
Q

What are the limitations of the tracheal system?

A

While it is efficient, it relies mostly on diffusion to exchange gases between the environment and the cells.
For diffusion to be effective, the diffusion pathway needs to be short which is why insects are of a small size.
As a result, the length of the diffusion pathway limits the size that insects can attain.

24
Q

How is plant gas exchange different to animals?

A

Some plant cells carry out photosynthesis.
During photosynthesis, plant cells take in carbon dioxide and produce oxygen.
At times the gases produced in one process can be used for the other.
This reduces gas exchange with the external air.
Overall, this means that the volumes and types of gases that are being exchanged by a plant leaf change.

25
Q

How does the balance between the rates of photosynthesis and respiration change during photosynthesis?

A

When photosynthesis is taking place, although some carbon dioxide comes from respiration of cells, most of it is obtained from the external air.
In the same way, some oxygen from photosynthesis is used in respiration by most of it diffuses out of the plant.

26
Q

How does the balance between the rates of photosynthesis and respiration change without photosynthesis?

A

When photosynthesis is not occurring, for example, in the dark, oxygen diffuses into the leaf because it is constantly being used by the cells during respiration.
In the same way, carbon dioxide produced during respiration diffuses out.

27
Q

How is gas exchange in plants similar to insects?

A

No living cell is far from the external air, and therefore a source of oxygen and carbon dioxide.
Diffusion takes place in the gas phase, which makes it more rapid than if it were in water.

28
Q

What is the structure of a plant leaf?

A

There is a short, fast diffusion pathway.
In addition, the air spaces inside a leaf have a very large surface area compared with the volume of living tissue.
There is no specific transport system for gases, which simply move in and through the plant by diffusion.
Most gaseous exchange occurs in the leaves.

29
Q

How are the leaves adapted for rapid diffusion?

A

Many small pores, called stomata, and so no cell is far from a stoma and therefore the diffusion pathway is short.
Numerous interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come in contact with mesophyll cells.
Large surface area of mesophyll cells for rapid diffusion.

30
Q

What are stomata?

A

Minute pores that occur mainly on the leaves, especially the underside.
Each stoma is surrounded by a pair of special guard cells.
These cells can open and close the stomatal pore.
In this way they can control the rate of gaseous exchange.

31
Q

Why is it important for guard cells to control the stoma?

A

This is important because terrestrial organisms lose water by evaporation.
Plants have evolved to balance the conflicting needs of gas exchange and control of water loss.
They do this by closing stomata at times when water loss could be excessive.

32
Q

Why must water loss be limited in insects?

A

Most insects are terrestrial (live on land), so water easily evaporates from the surface of their bodies and they can become dehydrated.
They have evolved adaptations to conserve water.
However, efficient gas exchange requires a thin, permeable surface with a large area.
These features conflict with the need to conserve water.

33
Q

How have insects evolved to reduce water loss?

A

Small surface area to volume ratio to minimise the area over which water is lost.
Waterproof coverings over their body surfaces. In insects this covering is a rigid outer skeleton of chitin that is covered with a waterproof covering.
Spiracles can be closed to reduce water loss. This conflicts with the need for oxygen and so occurs largely at rest.

34
Q

How do plants limit water loss?

A

Plants cannot have a small surface area to volume ratio because they photosynthesise, which requires a large leaf surface area for the capture of light and gas exchange.
To reduce water loss, terrestrial plants have a waterproof covering over parts of the leaves and the ability to close stomata when necessary.

35
Q

What are xerophytes?

A

Certain plants with a restricted water supply have evolved a range of other adaptations to limit water loss through transpiration.
Xerophytes are plants that are adapted to living in areas where water is in short supply.
Without these adaptations these plants would become desiccated and die.

36
Q

How are plant leaves modified to reduce water loss through evaporation?

A

Thick cuticle
Rolling up of leaves
Hairy leaves
Stomata in pits or grooves
A reduced surface area to volume ratio of the leaves.

37
Q

How does a thick cuticle reduce water loss through the leaves?

A

Although the waxy cuticle on leaves forms a waterproof barrier, up to 10% of water loss can still occur by this route.
The thicker the cuticle, the less water can escape by this means.
For example, holly.

38
Q

How does rolling up of leaves reduce water loss?

A

Most leaves have their stomata largely, confined to their lower epidermis.
The rolling of leaves in a way that protects the lower epidermis from the outside helps to trap a region of still air within the rolled leaf.
This region becomes saturated with water vapour and so has a very high water potential.
There is no water potential gradient between the inside and outside of the leaf and therefore no water loss.
For example, marram grass rolls its leaves.

39
Q

How do hairy leaves reduce water loss?

A

A thick layer of hairs on leaves, especially on the lower epidermis, traps still, moist air next to the leaf surface.
The water potential gradient between the inside and the outside of the leaves is reduced and therefore less water is lost by evaporation.
For example, a heather plant.

40
Q

How do stomata in pits or grooves reduce water loss?

A

These trap still, moist air next to the leaf and reduce the water potential gradient.
For example, pine trees.

41
Q

How does a reduced surface area to volume ratio of the leaves reduce water loss?

A

The smaller the surface area : volume ratio, the slower the rate of diffusion.
By having leaves that are small and roughly circular in cross-section, as in pine needles, rather than leaves that are broad and flat, the rate of water loss can be considerably reduced.
This reduction in surface area is balanced against the need for a sufficient area for photosynthesis to meet the requirements of the plant.

42
Q

Why is carbon dioxide removed?

A

All aerobic organisms require a constant supply of oxygen to release energy in the form of ATP during respiration.
The carbon dioxide produced during this process needs to be removed as its build up could be harmful to the body.

43
Q

Why is the volume of oxygen that needs to be absorbed and the volume of carbon dioxide that must be removed large in mammals?

A

They are relatively large organisms with a large volume of living cells.
They maintain a high body temperature which is related to them having high metabolic and respiratory rates.
Mammals therefore have evolved specialised surfaces, lungs, to ensure efficient gas exchange between the air and their blood.

44
Q

Why are the lungs located inside the body?

A

Air is not dense enough to support and protect these delicate structures.
The body as a whole would otherwise lose a great deal of water and dry out.

45
Q

What is the ribcage?

A

The lungs are supported and protected by this bony box.
The ribs can be moved by the muscles between them.
The lungs are ventilated by a tidal stream of air, thereby ensuring that the air within them is constantly replenished.

46
Q

What are the lungs?

A

A pair of lobed structures made up of a series of highly branched tubules, bronchioles, which end in tiny air sacs called alveoli.

47
Q

What is the trachea?

A

A flexible airway that is supported by rings of cartilage.
This prevents the trachea collapsing as the air pressure inside falls when breathing in.
The tracheal walls are made up of muscle, lined with ciliated epithelium and goblet cells.

48
Q

What are the bronchi?

A

Two divisions of the trachea, each leading to one lung.
They are similar in structure to the trachea and, like the trachea, produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus towards the throat.
The larger bronchi are supported by cartilage, although the amount is reduced as the bronchi get smaller.

49
Q

What are the bronchioles?

A

A series of branching subdivisions of the bronchi.
Their walls are made up of muscle lined with epithelial cells.
This muscle allows them to constrict so that they can control the flow of air in and out of the alveoli.

50
Q

What are the alveoli?

A

Minute air sacs, with a diameter of 100um-300um, at the end of the bronchioles.
Between them there are some collagen and elastic fibres.
The alveoli are lined with epithelium.
The elastic fibres allow the alveoli to stretch as they fill with air.
They then spring back during breathing out in order to expel the carbon dioxide-rich air.

51
Q

Why are alveoli located inside an organism?

A

Being thin, they are easily damaged, so are located here for protection.
Where an exchange surface, such as the lungs, is located inside the body, the organism has some means of moving the external medium over the surface, for example a means of ventilating the lungs in a mammal.
This is because diffusion alone is not fast enough to maintain adequate transfer of oxygen and carbon dioxide along the trachea, bronchi and bronchioles.

52
Q

What is the role of the alveoli in gas exchange?

A

There are about 300 million alveoli per lung.
Their total surface area is 70m^2.
Each alveolus is lined with epithelial cells only 0.05um - 0.3um thick.
Around each alveolus is a network of pulmonary capillaries, so narrow, 7 - 10 um thick, that red blood cells are flattened against the thin capillary walls to squeeze through.
These capillaries have walls that are only a single layer of cells thick (0.04-0.2um).

53
Q

Why is diffusion between the alveoli and the blood very rapid?

A

Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion.
The distance between the alveolar air and red blood cells is reduced as the RBCs are flattened against capillary walls.
The walls of both alveoli and capillary are very thin and therefore diffusion pathway is very short.
Alveoli and pulmonary capillaries have a very large total surface area.

54
Q

Why is diffusion between the alveoli and the blood very rapid - concentration gradient?

A

Breathing movements constantly ventilate the lungs, and the action of the heart constantly circulates blood around the alveoli. These ensure a steep concentration gradient of the gases to be exchanged is maintained.
Blood flow through the pulmonary capillaries maintains a concentration gradient.