2.2 Adaptations for Gas Exchange

Question 1

What is the respiratory surface?

Answer 1

The site of gas exchange

Question 2

For rapid diffusion of gases, a respiratory surface must …

Answer 2

Have a large enough surface area, relative to the volume of the organism so that the rate of gas exchange satisfies the organism’s needs.

Be thin so that diffusion pathways are short

Be permeable so that the respiratory gases diffuse easily

Have a mechanism to produce a steep diffusion gradient across the respiratory surface by bringing in oxygen, or removing carbon dioxide rapidly.

Question 3

How are unicellular organisms like amoeba adapted to gas exchange?

Answer 3

Single cells have a large surface area to volume ratio

The cell membrane is thin so diffusion into the cell is rapid

A single cell is thin so diffusion distances inside the cell are short

Question 4

What can single-celled organisms therefore due to having adaptations?

Answer 4

Absorb enough oxygen across the cell membrane to meet their needs for respiration

Remove carbon dioxide fast enough to prevent building up a high concentration and making the cytoplasm too acidic for enzymes to function

Question 5

Why is diffusion across larger organism surfaces not efficient enough for their gas exchange?

Answer 5

In larger organisms, many cells are aggregated together.

Larger organisms have a lower surface area to volume ratio than small organisms of the same overall shape.

Question 6

Explain flatworms and their adaptations

Answer 6

Flatworms are aquatic organisms which, being flat, have a much larger surface area than a spherical organism of the same volume. Their large surface area to volume ratio has overcome the problem of size increase because no part of the body is far from the surface and so diffusion paths are short.

Question 7

What kind of organism is an earthworm?

Answer 7

Terrestrial

Question 8

How are earthworms adapted for gas exchange?

Answer 8

It is cylindrical and so its surface area to volume ratio is smaller than a flatworm’s

Its skin is the respiratory surface which it keeps moist by secreting mucus. The need for a moist surface restricts the earthworm to the damp environment of the soil.

It has a low oxygen requirement because it is slow moving and has a low metabolic rate. Enough oxygen diffuses across its skin into the blood capillaries beneath.

Haemoglobin is present in its blood, carrying oxygen around the body in blood vessels.

Question 9

How does the earthworm maintain a concentration gradient?

Answer 9

Haemoglobin carries the oxygen away from the surface which maintains a diffusion gradient at the respiratory surface.

Carbon dioxide is also carried in the blood and it diffuses out across the skin, down a concentration gradient.

Question 10

What do most multicellular animals have and why?

Answer 10

They generally have a higher metabolic rate. They need to deliver more oxygen to respiring cells and remove more carbon dioxide.

Question 11

The major problems for terrestrial organisms :

Answer 11

Water evaporates from body surfaces, which could result in dehydration

Gas exchange surfaces must be thin and permeable with a large surface area. But water molecules are very small and pass through gas exchange surfaces, so gas exchange surfaces are always moist. They are consequently likely to lose a lot of water.

Question 12

Amphibians

Answer 12

Amphibian’s skin is moist and permeable with a well developed capillary network just below the surface. Gas exchange takes place through the skin and when the animal is active, in the lungs also.

Question 13

Reptiles

Answer 13

Reptile’s lungs have a more complex internal structure than those of amphibians, increasing the surface area for gas exchange.

Question 14

Birds

Answer 14

The lungs of birds process large volumes of oxygen because flight requires a lot of energy. Birds don’t have a diaphragm but their ribs and flight muscles ventilate their lungs more efficiently than the methods used by other vertebrates.

Question 15

What do gills have in cartilaginous fish?

Answer 15

A one way current of water, which is kept flowing by a specialised ventilation mechanism.

Many folds, providing a large surface area over which water can flow and over which gases can be exchanged.

A large surface area, maintained as the density of the water flowing through prevents the gills from collapsing on top of each other.

An extensive network of blood capillaries, with blood carrying haemoglobin allowing efficient diffusion of oxygen into the blood and carbon dioxide out.

Question 16

How do cartilaginous fish carry out gas exchange?

Answer 16

Cartilaginous fish have gills in five spaces on each side called gill pouches which open to the outside at gill slits.

They must keep swimming for ventilation to happen.

Blood travels through the gill capillaries in the same direction as the water travels, described as parallel flow. Oxygen diffuses from where it is more concentrated in the water, to where it is less concentrated in the blood. But this diffusion can only continue until the concentrations are equal.

Question 17

Why do cartilaginous fish have a less efficient ventilation system than bony fish?

Answer 17

Their ventilation system is less efficient than bony fish because they do not have a special mechanism to force water over the gills.

Question 18

Explain parallel flow in cartilaginous fish

Answer 18

Gas exchange in parallel flow does not occur continuously across the whole gill lamella, it occurs only until the oxygen concentration in the blood and water is equal.

With increased distance along the gill lamella, the concentration of oxygen in the blood goes up and the concentration of oxygen in the water goes down until they are equal.

Question 19

What do bony fish have?

Answer 19

Bony fish have an internal skeleton made of bone and the gills are covered with a flap called the operculum.

Question 20

How do bony fish maintain a continuous unidirectional flow?

Answer 20

To maintain a continuous, unidirectional flow, water is forced over the gill filaments by pressure differences. The water pressure in the mouth cavity is higher than in the opercular cavity. The operculum acts as both a valve, letting water out and as a pump, moving water past the gill filaments. The mouth also acts as a pump.

Question 21

How do bony fish take in water?

Answer 21

The mouth opens

The operculum closes

The floor of the mouth is lowered

The volume inside the mouth cavity increases

The pressure inside the mouth cavity decreases

Water flows in, as the external pressure is higher than the pressure inside the mouth.

Question 22

How do bony fish force water out over the gills?

Answer 22

The mouth closes

The operculum opens

The floor of the mouth is raised

The volume inside the mouth cavity decreases

The pressure inside the mouth cavity increases

Water flows out over the gills because the pressure in the mouth cavity is higher than in the opercular cavity and outside.

Question 23

What are bony fish gills like?

Answer 23

Each gill is supported by a gill arch made of bone.

Along each gill arch are many thin projections called gill filaments.

On the gill filaments are the gas exchange surfaces, the gill lamellae, sometimes called gill plates. These are held apart by water flowing between them and they provide a large surface area for gas exchange.

Question 24

What happens to bony fish when their gills are out of the water?

Answer 24

Out of water they stick together and the gills collapse. Much less area is exposed and so not enough gas exchange can take place. This is why fish die if out of the water.

Question 25

Explain countercurrent flow

Answer 25

Water moves from the mouth cavity, to the opercular cavity and into the gill pouches, where it flows between the gill lamellae. The blood in the gill capillaries flows in the opposite direction to the water flowing over the gill surface known as countercurrent flow.

Question 26

With countercurrent flow, at every point along the gill lamellae

Answer 26

At every point along the gill lamellae, the water has a higher concentration than the blood, so oxygen diffuses into the blood along the whole length of the gill lamellae.

Question 27

Is parallel flow or countercurrent flow more efficient?

Answer 27

Countercurrent flow is a more efficient system than the parallel flow of the cartilaginous fish. The gills of a bony fish remove about 80% of the oxygen from the water. This high level of extraction is important to fish as water contains much less oxygen than air.

Question 28

With increased distance along the gill lamella...

Answer 28

With increased distance along the gill lamella, the concentration of oxygen in the blood goes up and that of the water goes down, until the concentration in the blood is very high and the concentration in water is very low.

Question 29

Explain carbon dioxide exchange in bony fish

Answer 29

As in cartilaginous fish, carbon dioxide diffuses from the blood to the water. In bony fish, however, because there is a countercurrent system, carbon dioxide diffuses out of the blood along the whole length of the gill lamellae.

Question 30

Where are the lungs enclosed in?

Answer 30

The thorax

Question 31

What surrounds each lung and line the thorax?

Answer 31

Pleural membranes

Question 32

What is between the pleural membranes?

Answer 32

The pleural cavity containing a few cm3 of pleural fluid.

Question 33

What does the pleural fluid do?

Answer 33

The fluid is a lubricant, preventing friction between the lungs and inner wall of the thorax when they move during ventilation.

Question 34

Describe the structure of the thorax

Answer 34

At the base of the thorax is a dome-shaped sheet of muscle, the diaphragm, separating the thorax from the abdomen. The ribs surround the thorax. The intercostal muscles are between the ribs. The trachea is a flexible airway bringing air to the lungs. The 2 bronchi are the branches of the trachea. The lungs consist of a branching network of tubes called bronchioles which arise from the bronchi. At the ends of the bronchioles are air sacs called alveoli.

Question 35

How do mammals ventilate their lungs?

Answer 35

Mammals ventilate their lungs by negative pressure breathing meaning, that for air to enter the lungs, the pressure inside the lungs must be below atmospheric pressure.

Question 36

Describe the steps of inspiration

Answer 36

The external intercostal muscles contract The ribs are pulled upwards and outwards At the same time, the diaphragm muscles contract, so the diaphragm flattens The outer pleural membrane is attached to the thoracic cavity wall so it is pulled up and out with the ribs and the lower part is pulled down with the diaphragm. The inner membrane follows and so the lungs expand, increasing the volume inside the alveoli. This reduces the pressure in the lungs. Atmospheric air pressure is now greater than the pressure in the lungs so air is forced into the lungs.

Question 37

What type of process is inspiration?

Answer 37

Breathing in is an active process because muscle contraction requires energy

Question 38

Describe the steps of expiration

Answer 38

The external intercostal muscles relax The ribs move downwards and inwards At the same time, the diaphragm muscles relax so the diaphragm domes upwards The pleural membranes move down and in with the ribs and the lower parts move up with the diaphragm, The elastic properties of the lungs allow their volume to decrease, decreasing the volume inside the alveoli. This increases the pressure in the lungs. Air pressure in the lungs is now greater than atmospheric pressure so air is forced out of the lungs.

Question 39

What type of process is expiration?

Answer 39

Mainly passive

Question 40

What is a property of lung tissue and why is it important?

Answer 40

Lung tissue is elastic and can stretch and recoil to their original shape. This recoil plays an important role in pushing air out of the lungs.

Question 41

What do the inside surfaces of the alveoli contain?

Answer 41

The inside surfaces of the alveoli are coated with a surfactant which is made of moist secretions containing phospholipid and protein and has a low surface tension, preventing the alveoli collapsing during exhalation when the air pressure inside them is low. It also allows gases to dissolve before they diffuse in or out and prevents the alveoli from sticking together.

Question 42

Adaptations of gas exchange in the alveoli

Answer 42

They provide a large surface area relative to the volume of the body Gases dissolve in the surfactant moisture lining the alveoli The alveoli have walls made of squamous epithelium, only one cell thick so the diffusion pathway for gases is short An extensive capillary network surrounds the alveoli and maintains diffusion gradients as oxygen is rapidly bought the the alveoli and carbon dioxide is rapidly carried away. The capillary walls are also only one cell thick, contributing to the short diffusion pathway for gases.

Question 43

Describe gas exchange in the alveolus

Answer 43

Deoxygenated blood enters the capillaries surrounding the alveoli. Oxygen diffuses out of the air in the alveoli into the red blood cells and in the capillary. Carbon dioxide diffuses out of the plasma in the capillary into the air in the alveoli from where it is exhaled.

Question 44

What obstacles do most adult insects face and how do they overcome this?

Answer 44

Most adult insects are terrestrial and many live in arid habitats, so as with all terrestrial organisms, water evaporates from their body surface and they risk dehydration. Many insects reduce water loss with a waterproof layer covering the body surface. The insect exoskeleton is rigid and comprises a thin waxy layer of chitin and protein.

Question 45

How does gas exchange occur in insects?

Answer 45

Occurs through paired holes called spiracles running along the side of the body. The spiracles lead into a system of branched, chitin lined air tubes called tracheae which branches into smaller tubes called tracheoles. The spiracles can open and close so gas exchange can take place and water loss can be reduced.

Question 46

What do the hairs covering spiracles in some insects do?

Answer 46

The hairs covering spiracles in some insects contribute to water loss prevention and they prevent solid particles getting in.

Question 47

What happens when insects are relaxing?

Answer 47

When they are resting, insects rely on diffusion through the spiracles, tracheae and tracheoles to take in oxygen and to remove carbon dioxide.

Question 48

What happens when insects are active?

Answer 48

During periods of activity, such as flight, movements of the abdomen ventilate the tracheae. The ends of the tracheoles are fluid-filled and extend into muscle fibres. This interface between tracheoles and muscle fibres is where gas exchange takes place. Oxygen dissolves in the fluid and diffuses directly into the muscle cells so no respiratory pigment or blood circulation is needed. Carbon dioxide diffuses out by the reverse process.

Question 49

What happens during the day in plants?

Answer 49

During the day, plant cells containing chloroplasts can carry out photosynthesis. So during the day, plants both respire and photosynthesise. Some of the carbon dioxide they need for photosynthesis is provided by their respiration but most diffuses into the leaves from the atmosphere. Some of the oxygen they produce by photosynthesis is used in respiration but most diffuses out of the leaves.

Question 50

What happens during the night in plants?

Answer 50

At night, plants respire only and so they need oxygen from the atmosphere. Some oxygen enters the stem and roots by diffusion but most gas exchange takes place at the leaves.

Question 51

What happens to the rate of photosynthesis and rate of respiration in plants at day?

Answer 51

During the day, the rate of photosynthesis is faster than the rate of respiration. More oxygen is produced in photosynthesis than is used in respiration so overall the gas released is oxygen. At night, photosynthesis does not happen so no oxygen is produced. So the gas released is carbon dioxide.

Question 52

What happens to the rate of photosynthesis and rate of respiration in plants at night?

Answer 52

At night, photosynthesis does not happen so no oxygen is produced. So the gas released is carbon dioxide.

Question 53

What are the features of the leaf for gas exchange?

Answer 53

Large Surface area : Room for many stomata Thin : Diffusion pathway for gases entering and leaving is short Air spaces in the spongy mesophyll : Allow oxygen and carbon dioxide to diffuse between the stomata and the cells Stomatal pores : Gas exchange in and out of leaf

Question 54

What are the features of the leaf for photosynthesis?

Answer 54

Large Surface area : able to capture as much light as possible Thin : Light penetrates through leaf Cuticle and epidermis are transparent : Light penetrates to the mesophyll Palisade cells are elongated : Can accommodate a large number of chloroplasts Palisade cells are packed with chloroplasts : Capture as much light as possible Chloroplasts rotate and move within mesophyll cells : They move into the best positions for maximum absorption of light Air spaces in the spongy mesophyll : allow carbon dioxide to diffuse to the photosynthesising leaf.

Question 55

What are stomata?

Answer 55

Stomata are small pores on the above-ground parts of plants and occur mostly on the lower surfaces of leaves.

Question 56

What is each stomata pore bounded by?

Answer 56

Each pore is bounded by 2 guard cells

Question 57

Why are guard cells unusual?

Answer 57

Guard cells are unusual because they are the only epidermal cells with chloroplasts and they have unevenly thickened walls, with the inner wall, next to the pore in many species, being thicker than the outer wall. The width of the stoma can change and so stomata control the exchange of gases between the atmosphere and the internal tissues of the leaf.

Question 58

What is the mechanism of opening and closing of stomata during the day?

Answer 58

If water enters the guard cells they become turgid and swell and the pore opens

Question 59

What happens during the day for the stoma and guard cells?

Answer 59

In the light, chloroplasts in the guard cells photosynthesise, producing ATP This ATP provides energy for the active transport of potassium ions K+ into the guard cells from the surrounding epidermal cells. Stored starch is converted to malate ions. The K+ and malate ions lower the water potential in the guard cells making it more negative and consequently water enters by osmosis. The cell walls of guard cells are thinner in some places than others. Guard cells expand as they absorb water but less so in the areas where the cell wall is thick. These areas are opposite each other on the two guard cells and as the guard cells stretch a pore appears between these areas with less stretching. This is the stoma.

Question 60

What is the mechanism of opening and closing of stomata during the night?

Answer 60

If water leaves the guard cells they become flaccid and the pore closes.

Question 61

How do plants lose water?

Answer 61

Plants lose water by evaporation through their stomata in a process called transpiration.

Question 62

What happens to plants if they lose too much water?

Answer 62

Plants wilt if they lose too much water.

Question 63

How do plants minimise water loss?

Answer 63

On leaves held horizontally, sunlight on the upper surface would increase evaporation so confining stomata to the lower surface minimises the water loss. The waxy cuticle on the upper surface also reduces water loss.

Question 64

When do stomata close?

Answer 64

At night, to prevent water loss when there is insufficient light for photosynthesis In very bright light, as this generally is accompanied by intense heat which would increase evaporation If there is excessive water loss.