exchange1

Question 1

Introductory points.

Answer 1

• Internal cell environment different from outside/external.
• Exchange takes place at exchange surfaces.
• To enter/leave an organism, most substances must cross cell plasma membranes.

Question 2

Comment on mass transport and its necessity.

Answer 2

Mass transport maintains the final diffusion gradients that bring substances to and from the cell membranes of individual cells.

Helps to maintain relatively stable environment of tissue fluid.

Most cells are too far from exchange surfaces to rely on diffusion alone to supply tissue fluid/remove waste from fluid.

Large organisms need a mass transport system, as diffusion of a substance to necessary cells would take too long, even if the outer surface could supply enough of the substance.

Question 3

How does SA:V ratio affect exchange?

Answer 3

Smaller organisms have high SA:V ratios —> increase efficiency of gas exchange

Question 4

What adaptations do some organisms have for exchange (in terms of SA:V ratio)?

Answer 4

• Flattened shape so that no cell is ever far from the surface.
• Specialised exchange surfaces with large areas to increase the SA:V ratio —> lungs in animals, gills in fish etc.

Question 5

List common features of exchange surfaces.

Answer 5

• Thin so short diffusion distance.
• Selectively permeable.
• Movement of environmental medium to maintain concentration gradient.

Question 6

Why are some exchange surfaces located inside organisms?

Answer 6

Many exchange surfaces are thin - need to be located inside organisms as they’re easily damaged.

=> therefore need to have a method of moving the external environmental medium over the surface —> ventilation of lungs etc.

Question 7

What is the relationship between SA:V ratio and metabolic rate?

Answer 7

Smaller organisms —> higher SA:V ratio —> higher metabolic rate.

In endothermic and ectothermic animals, metabolic rate is inversely proportional to size.

Question 8

How do single-celled organisms exchange gases?

Answer 8

1. High SA:V ratio therefore oxygen absorbed across the body surface.
2. Cell-membrane is the only barrier - cell wall is no additional barrier.

Question 9

Structure of insects’ gas exchange system? How do insects exchange gases?

Answer 9

• Have an internal network of tracheae tubes for gas exchange, supported by strengthened rings to prevent collapse.
• Tracheae extend and divide into smaller tracheoles, which extend throughout all body tissues of the insect - oxygen brought directly to the respiring tissues —> short diffusion pathway from tracheoles to any body cell.

Question 10

How do insects exchange gases?

Answer 10

1. Oxygen used up as cells aerobically respire - [O2] at end of tracheoles is low.

—> O2 diffuses down gradient from atmosphere into tracheoles.

(opposite for CO2)

2. Contraction of muscles in insects can squeeze the tracheae enabling mass movements of air in and out.
3. Tracheoles are filled with water at their ends; soluble lactate produced by anaerobic respiration during periods of major activity, lowering wp of muscle cells.

—> water drawn in by osmosis, reducing volume of tracheoles, drawing in air

=> final diffusion pathway is in a gas phase > liquid so faster diffusion, but some water lost by evaporation.

• 1-3 all lead to faster diffusion.

Question 11

How do insects limit water loss?

Answer 11

• Thin, permeable surface with a large area conflict with the need to conserve water —> need to compromise and balance opposing needs of exchanging respiratory gases limiting water loss.
  1. Small SA:V ratio - minimises area over which water is lost.
  2. Waterproof coverings - over body surfaces - rigid outer-skeleton of chitin, covered with a waterproof cuticle.
  3. Spiracles - can be closed to reduce water loss - conflicts with the need for oxygen so occurs briefly when insect is at rest.

Question 12

Limitations of insect gas exchange method?

Answer 12

Relying mostly on diffusion for gas exchange means the diffusion pathway needs to be short —> limits size of insects.

Question 13

Suggest what causes spiracles to open.

Answer 13

Increasing [CO2].

Question 14

Fossil insects were much larger. Suggest how the composition of the atmosphere then compares to now.

Answer 14

Then, [O2] in atmosphere was higher.

Short diffusion pathway not as essential as now, as larger [O2] gradient so adequate O2 available for insects to be larger.

Question 15

Structure of fish gas exchange system?

Answer 15

• Relatively large organisms —> small SA:V ratio.
• Waterproof, gas-tight outer covering => specialised internal gas exchange surface (gills) as body surface inadequate for gas exchange.
• Gills made up of gill filaments, with gill lamellae perpendicular to filaments => increase SA of gas exchange surface.

Question 16

How do fish exchange gases?

Answer 16

• Countercurrent exchange principle:
  1. Water flowing over gill lamellae flows in opposite direction to blood flow in gill lamellae.
  2. Blood already well loaded with O2 meets water with max [O2] (and vice versa) —> diffusion as still a gradient.
  3. => Maintains a favourable O2 concentration gradient along the entire gas-exchange surface - gill lamellae.

Question 17

What are the advantages of the counter-current exchange principle?

Answer 17

80% of O2 in water absorbed, rather than 50% that would be absorbed if blood flow in same direction as water due to the equilibrium that would be reached.

Question 18

Suggest why one-way flow of gases is advantageous for the fish.

Answer 18

Less energy is required, because flow does not have to be reversed, which is important and water is dense and difficult to move.

Question 19

Suggest difference in gills for more metabolically active fish.

Answer 19

More gill lamellae and more gill filaments => larger SA of gills.

Question 20

Comment on respiration-photosynthesis balance’s effect on plant gas exchange.

Answer 20

1. Volumes of and types of gases being exchanged depend on balance between respiration and photosynthesis - at times gases from each process can be used in the other.
2. a) Photosynthesis is taking place:

• Some CO2 from respiration, but most from external air.
• Some O2 used in respiration is from photosynthesis, but most diffuses out of the plant.

2. b) Photosynthesis is not taking place:
   - In the dark etc., O2 diffuses into leaf as it is constantly being used by cells during aerobic respiration.

Question 21

Adaptations of the leaf for gas exchange.

Answer 21

1. No living cell is far from the external air —> close to sources of O2/CO2.
2. Diffusion takes place in gas phase - faster than in water.
3. Many small stomata - no cell is far from a stoma and also therefore a short diffusion pathway.
4. Many interconnecting air-spaces that occur throughout the mesophyll so that gases can readily come into contact with mesophyll cells.
5. Large SA of mesophyll cells for rapid diffusion.

Question 22

Describe stomata and comment on their importance.

Answer 22

= Minute pores occurring mainly on leaves, especially on their underside.

1. Each stoma surrounded by a pair of guard cells - can open and close the stomatal pore, controlling the rate of gas exchange.
2. This is important as terrestrial organisms lose their water by evaporation —> must balance conflicting needs of gas exchange and water loss control by closing stomata at times when water loss would be excessive.

NB = some herbicides kill plants by shutting stomata —> respn and photo could continue for a short time by exchanging gases between them, but not indefinitely.

Question 23

Comment on conflict between water loss and gas exchange.

Answer 23

• Terrestrial organisms must limit their water loss without compromising the efficiency of their gas exchange surfaces.
• Air at exchange surfaces (within the body) is almost 100% saturated with water vapour —> less evaporation of water from the exchange surface.

Question 24

How do xerophytic plants limit water loss?

Answer 24

Can’t have a small SA:V ratio as large SA required for exchange and photosynthesis.

• Certain plants (xerophytes) have evolved a range of other adaptations to limit their water loss by transpiration.
  1. Thick cuticle - waxy cuticle - as thickness increases, less water can escape via this route.
  2. Rolling up of leaves - trap a region of still air under lower epidermis - high wp created as air becomes saturated with water - no wp gradient between inside/outside of leaf so no water loss.
  3. Hairy leaves - traps a layer of air.
  4. Stomata in pits/grooves - traps a layer of air.

Question 25

Suggest a reason why having a smaller leaf area does not reduce the rate of photosynthesis in the same way as it would for plants in warmer climates.

Answer 25

Smaller leaf area in cold climates does not reduce rate of photosynthesis as cold temperatures mean that rate of photosynthesis is slower - temperature likely limiting factor (Enzyme-controlled process), and not light..

Question 26

Points about human gas exchange system.

Answer 26

• O2 needed for ATP production and CO2 removal necessary as its build-up could be harmful to the body.
• Lungs located within body because air is not enough to support and protect these delicate structures and because body as a whole would lose a lot of water and dry out.

Question 27

Describe the structure of the human gas exchange system.

Answer 27

Lungs —> trachea -> bronchi -> bronchioles -> alveoli.

Question 28

Describe lung structure.

Answer 28

• Lobed structures.

Question 29

Describe trachea structure.

Answer 29

• Flexible airway - cartilage rings prevent the trachea collapsing as the air pressure inside falls when breathing in.
• Tracheal walls made up of muscle, lined with ciliated epithelium and goblet cells.

Question 30

Describe bronchi structure.

Answer 30

• 2 divisions of trachea (one per lung) —> similar structure to trachea.
• Also produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus towards the throat.
• Amount of cartilage supporting them is reduced as bronchi get smaller.

Question 31

Describe bronchiole structure.

Answer 31

• Branching subdivisions of bronchi —> walls of muscle lined with epithelial cells —> can constrict to control air flow in/out of alveoli.

Question 32

Describe alveoli structure.

Answer 32

• Minute air-sacs at end of bronchioles.
• Some collagen and elastic fibres (elastin).
• Allow to stretch when breathing in and recoil during expiration in order to expel CO2-rich air.
• Alveoli are lined with epithelium, thin (one cell thick).

NB=> the alveolar membrane is the gas-exchange surface.

Question 33

Essential features of the alveolar epithelium as a gas exchange surface.

Answer 33

1. Made up of epithelial cells - very thin walls —> short diffusion distance —> diffusion is more rapid.
2. Surrounded by a dense network of/many pulmonary capillaries (one cell thick) —> constant flow of blood taking O2 away from the area of diffusion —> speeding up diffusion as favourable [O2] and [CO2] gradients are maintained.
3. Many alveoli in lungs —> large total SA —> faster diffusion.
4. Mucus lining the alveoli —> helps gases to diffuse —> faster diffusion.

Question 34

Why does rapid gas diffusion take place in humans?

Answer 34

1. RBCs slowed as they pass through pulmonary capillaries —> more time for diffusion.
2. Distance between alveolar air and RBCs reduced —> RBCs flattened against the capillary walls.
3. Capillary endothelium and alveolar epithelium walls are thin (one cell thick) —> short diffusion distance.
4. Alveoli and pulmonary capillaries have a very large total SA.
5. Constant ventilation by breathing movements and constant circulation of blood by the heart —> ensure a steep [O2] / [CO2] gradient for gas exchange.

Question 35

Define Tidal Volume. Shown as what on graph?

Answer 35

Volume of air in each breath. Small repeated change.

Question 36

Define Ventilation Rate.

Answer 36

Number of breaths/minute.

Question 37

Define FEV1.

Answer 37

Forced Expiratory Volume - max volume of air that can be breathed out in one second.

Question 38

Define FVC. Shown as what on graph?

Answer 38

Forced Vital Capacity - max volume of air that can be forcefully expired after a deep breath in. Large increase in volume then decrease below typical volume level range.

Question 39

How does TB arise? Effect? Symptoms?

Answer 39

1. Infection with TB bacteria —> immune cells build a wall around bacteria in lungs —> forms small, hard lumps called tubercles.
2. Infected tissue in tubercles dies and gas exchange surface is damaged, decreasing tidal volume.
3. Fibrosis also caused, further reducing tidal volume.
4. Reduced TV —> less O2 breathed in per breath so VR increases to compensate.

Common Symptoms:

1. Persistent cough.
2. Coughing up blood.
3. Mucus.
4. Chest pains.
5. Shortness of breath and fatigue.

Question 40

How does Fibrosis arise? Effect? Symptoms?

Answer 40

1. Formation of scar tissue in lungs —> result of infection/exposure to substances like asbestos/dust.
2. Scar tissue is thicker and less elastic than normal lung tissue.
3. Lungs are less able to expand and can’t hold as much air —> TV and FVC reduced.
4. Reduction of gas exchange efficiency —> diffusion is slower across a thicker, scarred membrane.
5. Faster VR to compensate.

Common Symptoms:

1. Shortness of breath.
2. Dry cough.
3. Chest pain.
4. Fatigue and weakness.

Question 41

How does Asthma arise? Effect? Symptoms?

Answer 41

1. Inflamed and irritated airways —> due to (usually) an allergic reaction to substances like pollen and dust.
2. During an asthma attack, smooth muscle lining the bronchioles contracts, producing a large amount of mucus.
3. Causing constriction of the airways —> air flow in/out of lungs is severely reduced —> less O2 in alveoli and blood —> FEV1 severely reduced.

Common Symptoms:
1. Wheezing.
2. A tight chest.
3. Shortness of breath.
All come on very suddenly during an asthma attack.

• Can be relieved by drugs in inhalers which relax bronchiole muscle, opening up the airways.

Question 42

How does Emphysema arise? Effect? Symptoms?

Answer 42

1. Caused by smoking/long-term exposure to air pollution —> foreign particles in air/smoke become trapped in the alveoli.
2. Causes inflammation, attracting phagocytes to the area which produces an enzyme that breaks down elastin protein in alveoli walls.
3. Elastin is elastic —> helps alveoli to return to their normal shape after inhaling/exhaling air (recoil).
4. Leads to destruction of the alveoli walls —> reducing SA of alveoli, decreasing efficiency of gas exchange.
5. VR increases to compensate, FVC reduced.

Common Symptoms:

1. Shortness of breath.
2. Wheezing.

Question 43

What do all aforementioned lung diseases do?

Answer 43

All reduce the rate of gas exchange —> less O2 can diffuse into bloodstream.

Body cells receive less O2, so rate of aerobic respiration decreases.

Less energy released so sufferers often feel tired and weak.

Question 44

Explain how Emphysema could reduce FVC (2).

Answer 44

1. Elastin protein in alveoli walls broken down by immune response, caused by inflammation from trapped particles/air.
2. Alveoli can’t recoil to expel air as well.

Question 45

List lung disease risk factors.

Answer 45

NB = Lung diseases here referring to COPD.

1. Smoking.
2. Air pollution.
3. Genetic makeup - some more likely to get it - some die early while others never get it.
4. Infections - people with a high incidence of other infections also show a higher incidence of COPD.
5. Occupation - people with harmful chemicals that can be inhaled have an increased rate of lung disease.

Question 46

Suggest 2 reasons why humans need to absorb large volumes of oxygen from the lungs.

Answer 46

1. Humans have a large volume of cells.

2. High metabolic rate.

Question 47

Explain how cells lining the trachea and bronchi protect the alveoli from damage.

Answer 47

• They produce mucus, trapping particles of dirt and bacteria in inhaled air —> cilia on these cells move this debris up the trachea —> swallowed into stomach.

=> dirt/bacteria could cause damage/infection in the alveoli.

Question 48

Why is ventilation necessary?

Answer 48

We must constantly ventilate the lungs to maintain diffusion of gases across the alveolar epithelium.

Question 49

Describe the 3 sets of muscles involved in ventilation.

Answer 49

3 sets of muscles bring about pressure changes in the lungs,

1. Diaphragm - Sheet of muscle separating the thorax from the abdomen.
2. Internal intercostal muscles - between ribs, contraction —> expiration.
3. External intercostal muscles - between ribs, contraction —> inspiration.

Question 50

Describe process of Inspiration.

Answer 50

• An active process, requiring energy.
  1. External intercostal muscles contract - while the internal intercostal muscles relax.
  2. Ribs pulled upwards and outwards —> increasing volume of thorax.
  3. Diaphragm muscles contract —> flattens —> increasing volume of thorax.
  4. Increased volume of thorax —> reduced pressure in lungs.
  5. Atmospheric pressure > pulmonary pressure —> air is forced into the lungs.