gas exchange

Explain one reason gas exchange is more challenging in larger species of organisms.(2)

1. (as organisms get larger) distance to centre of the body increases
   OR
   the surface area for diffusion relative to the volume/ surface area to volume ratio becomes smaller/ decreases;  
   
   Explanation:

2. which causes gas exchange/ diffusion of oxygen in/ carbon dioxide out to be too slow
   OR
   there is less area/space for diffusion to occur;

Explain how ventilation and lung structure contribute to passive gas exchange.(7)

a. air carried through trachea AND bronchi/bronchioles AND alveoli ✔ All three required in correct order.

b. alveoli increase the surface area/thin walled for gas exchange ✔

c. gas exchange carried out through type I pneumocytes ✔

d. type II pneumocytes secrete surfactant to reduce surface tension ✔

e. moist surface/surfactant allows gases to diffuse in solution ✔

f. ventilation/moving blood maintains concentration gradients of oxygen and carbon dioxide ✔

g. between air in alveoli and blood in «adjacent» capillaries
OR
oxygen diffuses from alveoli to capillaries and carbon dioxide from capillaries to alveoli ✔ OWTTE

h. external intercostal muscles/diaphragm contract during inspiration ✔

i. lowering air pressure «in lungs»/increasing thorax volume ✔

j. relaxation of external intercostal muscles/diaphragm enable «passive» expiration ✔

k. internal intercostal «and abdominal muscles» contract «to force» expiration ✔

l. expiration due to increasing air pressure «in lungs»/decreasing thorax volume ✔

Explain the process of gas exchange taking place in the alveoli (7)

a. O₂ diffuses into blood and CO₂ diffuses out from blood ✔

b. blood entering the alveoli is high in CO₂/low in O₂ ✔
OR
air in alveolus is high in O₂/low in CO₂ ✔

c. diffusion (in either direction) take place due to concentration gradients ✔

d. concentration gradients maintained by ventilation/blood flow ✔

e. large surface area created by many alveoli/spherical shape of alveoli for more efficient diffusion ✔

f. rich supply of capillaries (around alveoli) allows efficient exchange ✔

g. type I pneumocytes are thin to allow easy diffusion/short distances ✔

h. gases must dissolve in liquid lining of alveolus in order to be exchanged ✔

i. type II pneumocytes secrete surfactants to reduce surface tension/prevent lungs sticking together ✔

j. type II pneumocytes create moist conditions in alveoli ✔

Explain how gases are exchanged between the air in the alveolus and the blood in the capillaries. (3)

a. O₂ concentration in alveolar air greater than in capillary/blood «prior to gas exchange»
OR
hemoglobin in blood binds oxygen maintaining the concentration gradient ✔

b. O₂ gas dissolves in water lining the alveolus ✔

c. O₂ diffuses through wall of alveolus and capillary into blood ✔

d. CO₂ concentration in blood greater than in alveolar air «prior to gas exchange» ✔

e. CO₂ diffuses through wall of capillary and alveolus into alveolar airspace ✔

Outline the action taken by the diaphragm during inhalation. (1)

contracts/flattens/becomes less domed/increases volume of thorax;

Outline the process of inhalation. (4)

a. diaphragm contracts / moves downwards/flattens ✔

b. external intercostal muscles contract ✔

c. (muscle contraction) moves the rib cage upwards and outwards ✔

d. increases volume of the thorax / lungs ✔

e. difference in pressure/decreasing pressure causes air to flow into lungs / lungs inflate ✔

Explain how breathing is controlled by the brain. (6)

1. breathing is an automatic process/can occur without conscious intervention/is involuntary/ autonomic;

2. voluntary/conscious factors can override automatic functions (for a limited time);

3. control of the breathing comes from the respiratory centre;

4. (respiratory centre) located in the medulla of the brain;

5. exercise results in higher CO2 levels in blood;

6. breathing rate changes in response changes to blood pH/acidity/CO2 level;

7. medulla contains chemoreceptors
   OR
   chemoreptors in aortic/carotid bodies send signals to medulla;

8. respiratory centre/medulla sends nerve impulses to diaphragm/intercostal muscles;

9. stimulates (the intercostal muscles/diaphragm) to control breathing rate/depth of inspiration/contraction;

Identify one adaptation of the leaf for the absorption of light visible in this micrograph. (4)

1.  double layer of palisade cells;

2.  palisade layer on upper surface;

3.  dimensions of palisade cells maximise light absorption;

4.  thin/transparent epidermis/cuticle;