(7) Mass Transport

Question 1

Explain the Sigmoid shape of the oxygen dissociation curve?

Answer 1

First O₂ binding is difficult → polypeptide subunits are closely united, so haem groups are less accessible.

First O₂ binding causes a quaternary structure change → haem groups become more exposed, making it easier for more O₂ to bind (positive cooperativity).

Second & third O₂ bind more easily due to this structural shift.

Fourth O₂ binding is harder → fewer available binding sites, so probability of O₂ finding the last site is lower.

Question 2

Explain the Bohr Effect:

Answer 2

Increased respiration = Increased partial pressure of CO2 = decreased blood pH = Oxyhaemoglobin disassociation curve to shift right. So Haemoglobin has lower affinity for oxygen. So it releases oxygen more readily to respiring tissues.

Question 3

Explain Formation of Tissue Fluid.

Answer 3

Arteriole end of capillary:
High hydrostatic pressure from contractions of the heart forces water, oxygen, amino acids etc out of capillaries into surrounding tissues.

Large molecules eg: (plasma) proteins and red blood cells stay in capillary. (Too large)

This is called ultrafiltration.

Venule end of capillary:
(plasma) proteins remaining reduces water potential in blood.

So water moves back into blood by osmosis.
(AND)
Some excess tissue fluid drains into the lymphatic system

Question 4

Describe how blood is circulated around the body.

Answer 4

Pulmonary Circulation:
Deoxygenated blood from the body → right atrium → right ventricle → pulmonary artery → lungs (blood becomes oxygenated).

Oxygenated blood from the lungs → pulmonary veins → left atrium.

Systemic Circulation:
Oxygenated blood from the left ventricle → aorta → to the body tissues.

Deoxygenated blood from body tissues → returns to the right atrium via the superior and inferior vena cava.

Question 5

Define Systole and Diastole.

Answer 5

Systole: Contraction of atria / ventricles

Diastole: Relaxing / Non contraction of atria /ventricles.

Question 6

Describe the cardiac cycle:

Answer 6

Blood flows into atria through vena cava and pulmonary vein.
Pressure in atria hence rises.

Atrioventricular valves open due to pressure being greater in atria than ventricles. Atrial systole (contraction occurs)

Pressure in ventricles now rises. Atrioventricular valves close due to a higher pressure in ventricles than atria.
(Prevents backflow of blood)

Semi-lunar valves open in pulmonary artery and aorta. Blood pumped from ventricles out of heart.
Ventricles then diastole (relax)

Semi lunar valves shut when pressure in ventricles is less than aorta and pulmonary artery.

Question 7

Describe Translocation in phloem cells.
(Mass flow hypothesis)

Answer 7

(At source) sucrose is actively (transported) into the phloem/sieve
element/tube;
2. By companion/transfer cells;
3. Lowers water potential in
phloem/sieve element/tube and
water enters by osmosis;
4. (Produces) high (hydrostatic)
pressure;
5. Mass flow/transport towards
sink/roots/storage tissue;
6. At sink/roots sugars are
removed/unloaded;

Question 8

Describe Cohesion Tension Theory.

Answer 8

Water transpires (evaporates out of stomata) from leaves.

This reduces water potential in mesophyll cells creating water potential gradient.

So water is drawn out of xylem by osmosis.

This creates tension (negative pressure) in the xylem

(Cohesive forces between water molecules caused by hydrogen bonding) So water pulled up xylem as a column.