# Gas Exchange

## Exam-Style Questions

1. a) (i) Give one similarity between the way in which oxygen from the atmosphere reaches a muscle in an insect and the way it reaches a mesophyll cell in a leaf.

(ii) Give one difference in the way in which carbon dioxide is removed from a muscle in an insect and the way in which it is removed from a muscle in a fish.

(2 marks)

The diagram shows the way in which water flows over the gills of a fish.

The graph below shows the changes in pressure in the buccal cavity and in the opercular cavity during a ventilation cycle.

b) Use the graph to calculate the rate of ventilation in cycles per second.

(1 mark)

c) For most of this ventilation cycle, water will be flowing in one direction over the gills explain the evidence from the graph that supports this.

(2 marks)

d) Explain how the fish increases pressure in the buccal cavity.

(2 marks)

(Marks available: 7)

Answer outline and marking scheme for question: 1

Give yourself marks for mentioning any of the points below:

a) (i) Diffuses (directly to cells concerned).

(ii) Transported in blood of a fish/lost through gills in fish/through tracheae/spiracle in insect.

(2 marks)

b) 100 (cycles per minute)

(1 mark)

c) Water will flow from high pressure to low pressure; pressure in buccal cavity is higher than in opercular cavity.

(2 marks)

d) Muscles surrounding buccal cavity contract; mouth shuts; floor of buccal cavity rises/volume decreases.

(2 marks)

(Marks available: 7)

2. The diagram shows a chamber set up for an investigation into the movement of woodlice in response to humidity.

Eleven chambers were set up, each with a different relative humidity obtained by using different concentrations of a solution in the base of the chamber. A woodlouse was placed in each chamber.

The rate of movement was recorded. This was repeated ten times for each of the chambers using different woodlice each time, and the means were plotted on a graph.

a) Explain how the response shown increases the chance of survival of woodlice in natural conditions.

(2 marks)

b) (i) Suggest why woodlice were kept in a dry environment for a short time before the investigation was carried out.

(ii) suggest why different woodlice were used each time.

(2 marks)

c) (i) name the type of behaviour observed in this investigation.

(2 marks)

(Marks available: 6)

Answer outline and marking scheme for question: 2

Give yourself marks for mentioning any of the points below:

a) Move faster in environment where they are more likely to desiccate/dry/slower in environments where no dehydration occurs; increases chance of finding suitable environment/remaining in a favourable environment.

(2 marks)

b) (i) To make them more active (at the beginning of the experiment) animals all in same state of hydration at beginning.

(ii) Natural variation in response/large sample enable the typical response to be found.

(2 marks)

c) (i) kinesis

(ii) (Rate of movement) related to intensity of a stimulus.

(2 marks)

(Marks available: 6)

3. The drawing shows a 24-hour cycle for the opening and closing of stomata from the same plant.

a) Explain how this opening and closing of stomata is adventageous to the plant.

(2 marks)

b) The diagram shows the potassium (K+) ion concentrations in the cells around an open and closed stoma in Commelina. The concentrations are in arbitrary units.

(i) Explain how the movement of K+ ions accounts for the opening of the stoma.

(ii) Explain how K+ ions are moved against a concentration gradient.

(5 marks)

(Marks available: 7)

Answer outline and marking scheme for question: 3

Give yourself marks for mentioning any of the points below:

a)

Open during the day to allow entry of carbon dioxide;

Closed at night/midday to reduce transpiration/evaporation/water loss.

b)

(i) K+ ions move into guard cells;

Water potential of guard cells becomes more negative;

Water enters;

How uptake of water causes stoma to open;

(ii) Energy/respiration/ATP/active transport;

Intrinsic proteins/carriers/channels.

(5 marks)

(Marks available: 7)

## Gas Exchange in Plants

Plants obtain the gases they need through their leaves. They require oxygen for respiration and carbon dioxide for photosynthesis.

The gases diffuse into the intercellular spaces of the leaf through pores, which are normally on the underside of the leaf - stomata. From these spaces they will diffuse into the cells that require them.

Stomatal opening and closing depends on changes in the turgor of the guard cells. When water flows into the guard cells by osmosis, their turgor increases and they expand. Due to the relatively inelastic inner wall, the guard cells bend and draw away from each other, so the pore opens. If the guard cells loose water the opposite happens and the pore closes. The guard cells lower their water potential to draw in water from the surrounding epidermal cells, by actively accumulating potassium ions. This requires energy in the form of ATP which, is supplied by the chloroplasts in the guard cells.

Respiration occurs throughout the day and night, providing the plant with a supply of energy. Photosynthesis can only occur during sunlight hours so it stops at night. A product of respiration is carbon dioxide.

This can be used directly by the plant in photosynthesis.

However, during the day, photosynthesis can be going 10 or even 20 times faster than respiration (depending on light intensity), so the stomata must stay open so that the plant has enough carbon dioxide, most of which diffuses in from the external atmosphere.

## Gas Exchange in Insects

Insects, being larger and having a hard, chitinous and therefore impermeable exoskeleton, have a more specialised gas exchange system.

Insects have no transport system so gases need to be transported directly to the respiring tissues.

There are tiny holes called spiracles along the side of the insect.

The spiracles are openings of small tubes running into the insect's body, the larger ones being called tracheae and the smaller ones being called tracheoles.

The ends of these tubes, which are in contact with individual cells, contain a small amount of fluid in which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the surface area of air in contact with the cells. Gases diffuse in through the spiracles and down the tracheae and tracheoles.

Ventilation movements of the body during exercise may help this diffusion.

The spiracles can be closed by valves and may be surrounded by tiny hairs. These help keep humidity around the opening, ensure there is a lower concentration gradient of water vapour, and so less is lost from the insect by evaporation.

## Gas Exchange in Fish

Fish use gills for gas exchange.

Gills have numerous folds that give them a very large surface area.

The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills.

Fish also have an efficient transport system within the lamellae which maintains the concentration gradient across the lamellae.

The arrangement of water flowing past the gills in the opposite direction to the blood (called countercurrent flow) means that they can extract oxygen at 3 times the rate a human can.

#### Concurrent flow

To understand countercurrent flow, it is easiest to start by looking at concurrent flow where water and blood flow over and through the lamellae in the same direction.

When the blood first comes close to the water, the water is fully saturated with oxygen and the blood has very little.

There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the blood.

As you move along the lamella, the water is slightly less saturated and blood slightly more but the water still has more oxygen in it so it diffuses from water to blood.

This continues until the water and the blood have reached equal saturation.

After this the blood can pick up no more oxygen from the water because there is no more concentration gradient. The maximum saturation of the water is 100% so the maximum saturation of the blood is 50%.

#### Countercurrent flow

As the blood flows in the opposite direction to the water, it always flows next to water that has given up less of its oxygen.

This way, the blood is absorbing more and more oxygen as it moves along. Even as the blood reaches the end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the beginning of the lamella and is 90 or 100% saturated.

Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae, there is still a concentration gradient and it can continue to absorb oxygen from the water.

## Gas Exchange in Humans

The gas exchange surface of a mammal is the alveolus.

There are numerous alveoli - air sacs, supplied with gases via a system of tubes (trachea, splitting into two bronchi - one for each lung - and numerous bronchioles) connected to the outside by the mouth and nose.

These alveoli provide a massive surface area through which gases can diffuse. These gases diffuse a very short distance between the alveolus and the blood because the lining of the lung and the capillary are both only one cell thick.

The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood. Ventilation movements also maintain the concentration gradients because air is regularly moving in and out of the lungs.

This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from the respiratory centre in the medulla of the brain.

Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving information about the state of the lungs.

#### Process of inspiration (breathing in)

1. external intercostal muscles contract

2. ribs and sternum move up and out

3. width of thorax increases front to back and side to side

4. diaphragm contracts

5. diaphragm moves down, flattening

6. depth of thorax increases top to bottom so the...

• volume of thorax increases.
• pressure between the pleural surfaces decreases.
• lungs expand to fill thoracic cavity.
• air pressure in alveoli is less than atmospheric pressure.
• air is forced in by the higher external atmospheric pressure.

As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in.

#### Process of expiration (breathing out)

1. External intercostal muscles relax

2. ribs and sternum move down and in

3. width of thorax decreases front to back and side to side

4. diaphragm relaxes

5. diaphragm moves up

6. depth of thorax decreases top to bottom. So the ...

• volume of thorax decreases.
• pressure between the pleural surfaces increases.
• lung tissue recoils from sides of thoracic cavity
• air pressure in alveoli is more than atmospheric pressure.
• air is forced out.

As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the expiratory part of the centre) stops so breathing in can start again.

#### Chemoreceptors

There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in carbon dioxide levels in the blood.

If the level is too high (the pH would drop, enzyme action would be affected with serious results), impulses are sent from these cells to the inspiratory part of the centre so that breathing rate increases.

This means that carbon dioxide is got out of the body as quickly as possible and more oxygen comes in.

## General Principles for Efficient Gas Exchange

Different organisms have different mechanisms for obtaining the gases they require.

Diffusion is required to supply all organisms with oxygen.

The efficiency of diffusion is increased if there is:

1. A large surface area over which exchange can take place.
2. A concentration gradient without which nothing will diffuse.
3. A thin surface across which gases diffuse.

There are other factors involved, e.g. where appropriate, a good blood supply to carry the oxygen away to the tissues and to maintain a steep concentration gradient.

The process of diffusion will be faster in warm conditions as molecules have more energy and therefore move faster. So temperature will also affect the efficiency of the diffusion.

#### Fick's Law

Fick's law is used to measure the rate of diffusion.

Fick's Law states that:

(The symbol a means 'proportional to')

The larger the area and difference in concentration and the thinner the surface, the quicker the rate.

This means that water, a small molecule at a higher concentration inside the body than outside, will continually diffuse out of the animal and evaporate away. This makes the diffusion surface moist.

#### Unicellular organisms

Unicellular Organisms do not have specialised gas exchange surfaces. Instead gases diffuse in through the cell membrane.

The smaller something is, the smaller the surface area is but, more importantly, the bigger the surface area is compared to its volume.

In other words, unicellular organisms have a large surface area to volume ratio. They are therefore efficient when it comes to exchanging gases through their membrane.

Also, all parts of the organism are supplied with oxygen because the diffusion path is short.

#### Multicellular organisms

Multicellular Organisms are bigger than Unicellular organisms. This makes efficient diffusion of gases more difficult.

However, if they are small, or large but very thin (like the flatworms, Platyhelminths), the outer surface of the body is sufficient as an exchange surface because the surface area to volume ratio is still high.

Larger organisms need specialised exchange surfaces e.g. gills or lungs.

Land-living organisms usually have internal gas exchange surfaces to prevent too much water being lost from the body.

## S-Cool Revision Summary

#### General Principles for Efficient Gas Exchange

Different organisms have different mechanisms for obtaining the gases they require.

Diffusion is required to supply all organisms with oxygen.

The efficiency of diffusion is increased if there is:

1. A large surface area over which exchange can take place.
2. A concentration gradient without which nothing will diffuse.
3. A thin surface across which gases diffuse.

#### Fick's Law

Fick's law is used to measure the rate of diffusion.

The larger the area and difference in concentration and the thinner the surface, the quicker the rate.

#### Unicellular organisms

Unicellular Organisms do not have specialised gas exchange surfaces. Instead gases diffuse in through the cell membrane.

The smaller something is, the smaller the surface area is but, more importantly, the bigger the surface area is compared to its volume.

#### Multicellular organisms

Multicellular Organisms are bigger than Unicellular organisms. This makes efficient diffusion of gases more difficult.

However, if they are small, or large but very thin (like the flatworms, Platyhelminths), the outer surface of the body is sufficient as an exchange surface because the surface area to volume ratio is still high.

#### Gas Exchange in Plants

Plants obtain the gases they need through their leaves. They require oxygen for respiration and carbon dioxide for photosynthesis.

The gases diffuse into the intercellular spaces of the leaf through pores, which are normally on the underside of the leaf - stomata. From these spaces they will diffuse into the cells that require them.

#### Gas Exchange in Insects

Insects have no transport system so gases need to be transported directly to the respiring tissues.

There are tiny holes called spiracles along the side of the insect.

The spiracles are openings of small tubes running into the insect's body, the larger ones being called tracheae and the smaller ones being called tracheoles.

The ends of these tubes, which are in contact with individual cells, contain a small amount of fluid in which the gases are dissolved. The fluid is drawn into the muscle tissue during exercise. This increases the surface area of air in contact with the cells. Gases diffuse in through the spiracles and down the tracheae and tracheoles.

Ventilation movements of the body during exercise may help this diffusion.

The spiracles can be closed by valves and may be surrounded by tiny hairs. These help keep humidity around the opening, ensure there is a lower concentration gradient of water vapour, and so less is lost from the insect by evaporation.

#### Gas Exchange in Fish

Fish use gills for gas exchange. Gills have numerous folds that give them a very large surface area.

The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills.

#### Gas Exchage in Humans

The gas exchange surface of a mammal is the alveolus.

There are numerous alveoli - air sacs, supplied with gases via a system of tubes (trachea, splitting into two bronchi - one for each lung - and numerous bronchioles) connected to the outside by the mouth and nose.

These alveoli provide a massive surface area through which gases can diffuse. These gases diffuse a very short distance between the alveolus and the blood because the lining of the lung and the capillary are both only one cell thick.

The blood supply is extensive, which means that oxygen is carried away to the cells as soon as it has diffused into the blood. Ventilation movements also maintain the concentration gradients because air is regularly moving in and out of the lungs.

This breathing in (inspiration) and breathing out (expiration) is controlled via nervous impulses from the respiratory centre in the medulla of the brain.

Both the intercostal muscles (in between the ribs) and the diaphragm receive impulses from the respiratory centre. Stretch receptors in the lungs send impulses to the respiratory centre in the brain giving information about the state of the lungs.

#### Chemoreceptors

There are also chemoreceptors in the medulla and certain blood vessels that are sensitive to changes in carbon dioxide levels in the blood.