Adaptations for gas exchange

Question 1

Name some excellent respiratory surfaces

Answer 1

• Gills of a fish
• alveoli in the lungs of a mammal
• tracheae of an insect
• Spongy mesophyll cells in leaves

Question 2

Explain some essential features of exchange surfaces

Answer 2

• Have a large surface area to volume ratio
• Be thin–> short diffusion pathway
• Permeable–> so respiratory gases diffuse easily
• mechanism to produce a steep diffusion gradient

Question 3

Characteristics of unicellular organisms

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

Question 4

How can unicellular organisms exchange gases

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 acid for enzymes to function

Question 5

Why might diffusion across the surface of larger organisms not be efficient enough?

Answer 5

• In larger organisms many cells are aggregated together
• These aggregations are seen in fossils of early multicellular organisms
• But they have a lower surface area to volume ratio

Question 6

How is an earthworm adapted for gas exchange?

Answer 6

• It is cylindrical and so its surface area to volume ratio is smaller than a flatworm but larger than that of a compact organism of the same volume
• 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 the blood vessels.Carrying the oxygen away from the surface maintains a diffusion gradient at the respiratory surface
• Carbon dioxide is also carried in the blood and it diffuses out across the skin, down diffusion gradient

Question 7

Why do multicellular animals, such as insects and mammals, have special features not seen in unicellular organisms?

Answer 7

• They generally have a higher metabolic rate. They need to deliver more oxygen to respiring cells and remove more carbon dioxide
• With an increase in size and specialisation of cells, tissues and organs become more interdependent
• They must actively maintain a steep concentration gradient across their respiratory surfaces by moving the environmental medium, air or water, and in larger animals, the internal medium, the blood. So they need ventilation mechanism
• Respiratory surfaces must be thin to make the diffusion pathway short, but then they area fragile and could be easily damaged. But as they are inside the organism, such as the lungs of a mammal or the gills of a fish, they are protected

Question 8

What are major problems for terrestrial organisms?

Answer 8

• 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 loose a lot of water

Question 9

Animals have evolved different methods of overcoming the conflict of needing to conserve water with the risk of water loss at the gas exchange surface

Answer 9

• Gills cannot function out of water but on land, the trachea of insects and the lungs of vertebrates do
• Lungs are internal, minimising the loss of water and heat. They allow gas exchange with air and allow animals to be very active

Question 10

Gas exchange: Amphibians

Answer 10

• Include frogs, toads and newts
• Their 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 11

Gas exchange: Reptiles

Answer 11

• Include crocodiles, lizards and snakes
• Their lungs have a more complex internal structure than those of amphibians, increasing the surface area for gas exchange

Question 12

Gas exchange: Birds

Answer 12

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

Question 13

Gas exchange in fish

Answer 13

Fish are active and need a good oxygen supply. Gas exchange takes place across a special respiratory surface, the gills have:

• A one-way current of water, 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

Question 14

Describe the two main groups of fish

Answer 14

• Cartilaginous fish: Have a skeleton of cartilage

- Bony fish: Have a skeleton of bone

Question 15

Describe the ventilation system of the cartilaginous fish

Answer 15

• They do not have a special mechanism to force water over the gills, and many 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 less concentrated, in the blood. But this diffusion can only continue until the concentrations are equal. So the blood’s oxygen concentration is limited to 50% of its possible maximum value
• Gas exchange in parallel flow does not occur across the whole gill lamella, only part of it until the oxygen concentration in the blood and water is equal

Question 16

Describe the ventilation system of the bony fish

Answer 16

• Bony fish have an internal skeleton made of bone and gills are covered with a flap called the operculum, rather than opening directly on the side of the fish, as in cartilaginous fish
• Bony fish live in both freshwater and seawater and are the most numerous of aquatic vertebrates

Question 17

Briefly describe ventilation in fish

Answer 17

• To maintain a continuous, unidirectional flow, water is forced over the gill filaments by pressure differences
• The water pressure in the 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 pump also acts as a pump

Question 18

The ventilation mechanism operates as follows: To take in water

Answer 18

a) The mouth opens
b) The operculum closes
c) The floor of the mouth is lowered
d) The volume inside the mouth cavity increases
e) The pressure inside the mouth cavity decreases
f) Water flows in, as the external pressure is higher than the pressure in the mouth cavity is higher than in the opercular cavity and outside

Question 19

The ventilation mechanism operates as follows: To force water out over the gills the precesses are reversed

Answer 19

a) The mouth closes
b) The operculum opens
c) The floor of the mouth is raised
d) The volume inside the mouth cavity decreases
e) The pressure inside the mouth cavity increases
f) Water flows out over the gills because the pressure in the mouth cavity is higher than in the opercular cavity and outside

Question 20

Bony fish have 4 pairs of gills:

Answer 20

• Each gill is supported by a gill arch, sometimes called a gill bar, 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 help apart by water flowing between them and they provide a large surface area for gas exchange
• Out of water they sick together and the gills collapse
• Much less area is exposed and so not enough can take place
• This is why fisher die if out of water for more than a very short time

Question 21

Counter-current flow

Answer 21

• 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

Question 22

Counter-current flow across gill lamella

Answer 22

• The water always has a higher oxygen concentration than the blood, so oxygen diffuses into the blood along the whole length of the gill lamellae
• This 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 23

Carbon dioxide exchange

Answer 23

• As in cartilaginous fish, carbon dioxide diffuses from the blood to the water
• In bony fish, however, because there is a counter-current system, carbon dioxide diffuses out of the blood along the whole length of the gill lamellae
• This is, like oxygen uptake, more efficient than the carbon dioxide loss from the gills of cartilaginous fish

Question 24

Gills provide

Answer 24

• A specialised respiratory surface, rather than using the whole body surface
• A large surface extended by the gill filaments and gill lamellae
• An extensive network of blood capillaries, with blood carrying haemoglobin, allowing efficient diffusion of oxygen into the blood and carbon dioxide out

Question 25

Structure of the human breathing system

Answer 25

• The lungs are enclosed in an airtight compartment, the thorax
• Pleural membranes line in the thorax and cover each lung. The fluid between the membranes prevents friction between the lungs and chest cavity as the lungs move
• 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 intercoastal muscles are between the ribs
• The trachea is a flexible airway, bringing air into the lungs
• The two 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 end of the bronchioles are sacs called alveoli

Question 26

Ventilation of the lungs

Answer 26

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

Question 27

Inspiration (inhalation)

Answer 27

Breathing in is an active process because muscle contraction requires energy

a) The external intercostal muscle contract
b) The ribs are pulled upwards and outwards
c) At the same time, the diaphragm muscles contract, so it flattens
d) Both actions increase the thorax volume
e) This reduces the pressure in the lungs
f) Atmospheric air pressure is now greater than the pressure in the lungs, so air is forced into the lungs

Question 28

Expiration (exhalation)

Answer 28

Breathing out is a mainly passive process and is, in part, the opposite of inspiration

a) The external intercostal muscles relax
b) The ribs move downwards and inwards
c) At the same time, the diaphragm muscles relax, so it domes upwards
d) Both actions decrease the thorax volume
e) This increases the pressure in the lungs
f) Air pressure in the lungs is now greater than atmospheric pressure so air is forced out of the lungs

Question 29

Describe the structures of the lungs

Answer 29

• Lung tissue is elastic and, like a stretched elastic band, lungs recoil and regain their original shape when not being actively expanded
• The recoil plays a major part in pushing air out of the lungs
• Surrounding each lung and lining the thorax are pleural membranes, between which is a cavity containing pleural fluid
• This fluid acts as a lubricant, allowing friction free movement against the inner wall of the thorax during ventilation

Question 30

Describe the structure of the alveoli

Answer 30

• the inside surfaces of the alveoli are coated with a surfactant, which can be thought of as an anti-sticking mixture
• It is made of moist secretions, containing phospholipid and protein, and has a low surface tension, preventing the alveoli collapse during exhalation, when the air pressure inside them is low
• It also allows gases to dissolve, before they diffuse in or out

Question 31

Gas exchange in the alveolus

Answer 31

The gas exchange surfaces are the alveoli. They are very efficient at gas exchange

• They provide a large surface area relative to the volume of the body
• Gasses 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 alveoli and maintains diffusion gradients, as carbon dioxide is rapidly bought to the alveoli and oxygen is rapidly carried away
• The capillary walls are also one cell thick, contributing to the short diffusion pathway for gases

Question 32

What happens after deoxygenated blood enters the capillaries surrounding the alveoli?

Answer 32

• Oxygen diffused out of the air in the alveoli into the red blood cells in the capillary
• Carbon dioxide diffuses out of the plasma in the capillary into the air in the alveoli, from wheee it is exhaled

Question 33

Why are adult insects at risk of dehydration?

Answer 33

Water evaporates from their body surface

Question 34

What does efficient gas exchange require?

Answer 34

• thin
• permeable
• large surface area

Question 35

What do many terrestrial organisms do to reduce water loss?

Give an example

Answer 35

• They have a waterproof layer covering the body surface

An example is the insect exoskeleton which is rigid and comprises a thin waxy layer over a thicker layer of chitin and protein

Question 36

In insects, gas exchange occurs through paired holes, what are they called?

Answer 36

Spiracles

-they run along the side of the body

Question 37

The spiracle lead into a system of branched, chitin-lined air-tubes, what are they called?

Answer 37

Tracheae

- They branch into smaller tubes called tracheoles

Question 38

How does gas exchange take place in insects?

Answer 38

• The spiracles can open and close so gas exchange can take place and water loss can be reduced
• The hairs covering spiracles in some insects contribute to water loss prevention and they prevent solid particles getting in

Question 39

How does gas exchange occur when insects are resting?

How does it occur during periods of activity?

Answer 39

• when they are resting, insects rely on diffusion through the spiracles, tracheae and tracheoles to take in oxygen and to remove carbon dioxide
• During periods of activity, such as flight, movements of the abdomen ventilate the trachea
• The ends of the tracheoles are fluid-filled and are close to 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 40

Gas exchange in plants

Answer 40

• Plants, like animals, need to generate energy constantly so they respire all the time
• 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 41

Gas exchange in plants: At night

Answer 41

-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
~ At night, photosynthesis does not happen so no oxygen is produced, so the gas released is carbon dioxide
~ It is the next exchange of carbon dioxide and oxygen in relation to respiration and photosynthesis that matters

Question 42

Gas exchange in plants: During the day

Answer 42

• 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

Question 43

Explain gas exchange in plants

Answer 43

• Gases diffuse through the stomata down a concentration gradient
• Once inside the leaf, the gases in the sub-stomatal air chambers diffuse through the intercellular spaces between the spongy mesophyll cells and into cells
• The direction of diffusion depends on the concentration of gases in the atmosphere and the reactions in the plant cell

Question 44

Stomata

Answer 44

• Stomata are small pores on the above-ground parts of plants and occur mostly on the lower surfaces of leaves
• Each pore is bounded by two guard cells
• 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 tissue of the leaf

Question 45

The mechanism of opening and closing: During the day

Answer 45

• If the water enters the guard cells, they become turgid and swell, and the pore opens
• If water leaves the guard cells, they become flaccid and the pore closes

Question 46

The plant may become turgid or flaccid, what processes are these changes thought to be due to?

Answer 46

• The chloroplasts in the guard cells photosynthesise, producing ATP
• This ATP provides energy for active transport of potassium ions (K+) into the guard cells from the surrounding epidermal cells
• Stored starch is converted to malate
• 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 wall 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 stomata

Question 47

At night, the reverse process occurs and the pore closes

Answer 47

• Plants lose water by evaporation through their stomata in a process called transpiration
• Plants wilt if they lose too much water
• Sunlight on the upper surface of the leaf would increase evaporation, so confirming stomata to the lower surface minimises the water loss
• The waxy cuticle on the upper surface also reduces water loss

Question 48

Gas exchange and water loss both happen through stomata, and plants must balance the conflicting needs of gas exchange and control of water loss.
So stomata close:

Answer 48

• 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