Mass Transport

Question 1

Describe the structure of haemoglobin

Answer 1

Consists of 4 polypeptide chains each carrying a haem group
Quaternary structure

Question 2

Describe the role of haemoglobin

Answer 2

Oxygen binds to haem groups
Carried round body to respiring tissues
Present in red blood cells

Question 3

Name 3 factors affecting oxygen-haemoglobin binding

Answer 3

1. Partial pressure of oxygen
2. Partial pressure of carbon dioxide
3. Saturation of Hb with oxygen

Question 4

How do each of the 3 factors affect oxygen-haemoglobin binding?

Answer 4

1. ppO2 increases as affinity of Hb for oxygen increases
   Oxygen binds tightly to Hb and is released when pp is low
2. ppCO2 increases as affinty of Hb for oxygen decreases
   Conditions become acidic which changes shape of Hb (Bohr effect)
3. Hard for first oxygen to bind
   Hb changes shape so it’s easier for second and third to bind
   Harder again for fourth to bind as there’s a low chance of finding a binding site

Question 5

How does carbon dioxide affect the position of the oxyhaemoglobin dissociation curve?

Answer 5

Curve shifts right
Hb’s affinity for oxygen has decreased

Question 6

Name 3 common features of the mammalian circulatory system

Answer 6

1. Suitable medium for transport
2. Maintains pressure throughout the body
3. Unidirectional flow / valves

Question 7

What are the names of the valves + vessels entering / leaving the heart on chronological order?

Answer 7

Vena cava
Tricuspid valve (AV)
Semilunar valve
Pulmonary artery
Pulmonary vein
Bicuspid valve (AV)
Semilunar valve
Aorta

Question 8

Relate the structure of the vessels to their function

Answer 8

Arteries = thick walls to handle high pressures without tearing + elastic, muscular walls to control blood flow
Veins = thin walls due to lower pressure + valves to prevent backflow of blood

Question 9

What happens during cardiac diastole?

Answer 9

Heart is relaxed
Blood enters atria, increasing pressure
Pushes open AV valves
Blood flows into ventricles
Pressure in heart < arteries
Semilunar valves remain closed

Question 10

What happens during atrial and ventricular systole?

Answer 10

Atrial = atria contract, pushing blood into ventricles
Ventricular = ventricles contract, increasing pressure
AV valves close to prevent backflow
Semilunar valves open as blood flows into arteries

Question 11

What does myogenic mean?

Answer 11

Heart’s contraction is initiated from within the muscle itself

Question 12

Explain how the heart contracts

Answer 12

SAN spreads impulse across both atria, causing them to contract
AVN conveys this signal down the bundle of His
Impulse travels into Purkinje fibres that contract from the bottom up

Question 13

How is the structure of the capillaries suited to their function?

Answer 13

One cell thick = short diffusion pathway
Highly branched = large SA
Permeable membrane = effectively delivering oxygen to tissues

Question 14

How is tissue fluid formed?

Answer 14

Blood is pumped through increasingly small vessels
Creates hydrostatic pressure, forcing fluid out the capillaries
Bathes cells before returning to capillaries when the hydrostatic pressure is low enough

Question 15

How is water transported in plants?

Answer 15

Through xylem tubes
Provide structural support to the stem

Question 16

Explain the cohesion-tension theory

Answer 16

Water molecules stick together by their H bonds
Surface tension of water also creates a sticking effect
Therefore water is lost through transpiration
More can be drawn up the stem

Question 17

How are organic substances transported around the plant?

Answer 17

Translocation via the phloem tube

Question 18

Explain the mass flow hypothesis?

Answer 18

Sucrose enters the companion cells, using ATP and H+ ions
Diffuses from companion cells into sieve tube elements (via plasmodesmata)
Water potential inside phloem is reduced
Water enters from xylem via osmosis, increasing hydrostatic pressure
Moves along sieve tube to areas of lower hydrostatic pressure
Sucrose diffuses into surrounding cells

Question 19

Give evidence for the mass flow hypothesis

Answer 19

+ sap released when stem is cut / must be pressure in phloem
+ higher sucrose concn in leaves than roots
+ increasing sucrose levels in leaves = increased sucrose in phloem

Question 20

Give evidence against the mass flow hypothesis

Answer 20

• structure of sieve tubes hinder mass flow
• solutes move at different speeds
• sucrose delivered at same rate throughout plant / not to areas with lower sucrose concn first

Question 21

How can ringing experiments investigate transport in plants?

Answer 21

Bark and phloem removed in a ring around the tree
Tissues above missing ring swell due to accumulation of sucrose as tissue below dies
Therefore sucrose must be transported in phloem

Question 22

How can tracing experiments investigate transport in plants?

Answer 22

Plants are grown in the presence of radioactive carbon dioxide (incorporated into plant’s sugars)
Areas exposed to radiation correspond to where the phloem is