H6Gas exchange – summary of mark schemes
H.6.2
Explain the oxygen dissociation curves of adult hemoglobin, fetal hemoglobin and myoglobin.
Mark Schemes
A. initial uptake of one oxygen molecule by hemoglobin facilitates the further uptake of oxygen molecules / hemoglobin
has an increasing affinity for oxygen / and vice versa;
B. shows how the saturation of hemoglobin with oxygen varies with partial pressure of oxygen / dissociation curve for
(oxy)hemoglobin is S / sigmoid-shaped;
C. low partial pressure of oxygen corresponds to the situation in the tissue;
D. when partial pressure of oxygen is low, oxygen released;
E. high partial pressure of oxygen corresponds to the situation in the lungs;
F. when partial pressure of oxygen is high, oxygen taken up by hemoglobin;
G. Bohr effect occurs when there is lower pH / increased carbon dioxide / increased lactic acid;
H. shifts the curve to the right;
I. oxygen more readily releases to (respiring) tissue;
H.6.3
Describe how carbon dioxide is carried by the blood, including the action of carbonic anhydrase, the chloride shift and buffering
by plasma proteins.
Mark Schemes
A. carbon dioxide is carried in three forms in the blood;
B. carbon dioxide can be dissolved in the blood / plasma;
C. carried as dissociated carbonic acid / H2CO3 / H+H2 CO3–;
D. carried as carbaminohemoglobin / bound to hemoglobin;
E. carbonic anhydrase found in red blood cells / erythrocytes;
F. carbonic anhydrase speeds up production of hydrogen carbonate / bicarbonate / H CO3–;
G. chloride shift / movement of chloride ions into red blood cell / erythrocyte occurs to balance movement of hydrogen
carbonate / bicarbonate / H CO3– ion movement out;
H.6.4
Explain the role of the Bohr shift in the supply of oxygen to respiring tissues.
Mark Schemes
A. oxygen dissociation curves describe saturation of hemoglobin by oxygen;
B. over a narrow range of partial pressure of oxygen;
C. which typifies oxygen pressures surrounding cells under normal metabolism;
D. increased metabolism results in greater release of CO2 into blood;
E. CO2 lowers pH of blood;
F. increased acidity shifts oxygen dissociation curve to right;
G. results in release of oxygen from hemoglobin;
H. at the same partial pressure of oxygen;
I. ensuring that respiring tissues have enough oxygen;
J. when their need for oxygen is greatest;
K. also, saturation of hemoglobin occurs at higher partial pressures of oxygen;
L. so that it can release oxygen at higher partial pressures;
H.6.5
Explain how and why ventilation rate varies with exercise.
Mark Schemes
A. exercise increases metabolism;
B. oxygen is becoming limited;
C. CO2 concentration builds up in blood;
D. lactic acid builds up in blood;
E. lowers blood pH;
F. chemosensors detect lowered pH;
G. sensors in carotid artery / aorta;
H. send impulses to breathing centre / brain stem / (Pons) medulla oblongata;
I. impulse sent to diaphragm;
J. impulse sent to intercostal muscles;
K. increases rate of breathing / contraction / relaxation of muscles;
L. involuntary control;
