3.4 - Mass Transport

Question 1

Describe the structure of haemoglobin.

Answer 1

Globular, water soluble. Consists of four polypeptide chains, each carrying a haem group (quaternary structure).

Question 2

Describe the role of haemoglobin.

Answer 2

Present in red blood cells. Oxygen molecules bind in the haem groups and are carried around the bogy to where they are needed in respiring tissues.

Question 3

Name the three factors affecting oxygen-haemoglobin binding.

Answer 3

1. Partial pressure/concentration of oxygen
2. Partial pressure/concentration of CO2
3. Saturation of haemoglobin with oxygen.

Question 4

How does saturation of haemoglobin with oxygen affect oxygen-haemoglobin binding?

Answer 4

It s hard for the first oxygen molecule to bind. Once it does, it changes its shape to make it easier for the second and third molecules o bind, known as positive cooperativity. It is then slightly harder for the fourth oxygen molecule to bind because there is a low chance of finding a binding site.

Question 4

How does partial pressure of oxygen affect oxygen-haemoglobin binding?

Answer 4

As partial pressure of oxygen increases, the affinity of haemoglobin for oxygen also increases, so oxygen binds tightly to haemoglobin. When partial pressure is low, oxygen is released from haemoglobin.

Question 5

Explain why oxygen binds to haemoglobin in the lungs.

Answer 5

• Partial pressure of oxygen is high.
• Low concentration of CO2 in the lungs, so affinity is high.
• Positive cooperativity, after the first oxygen molecule
  binds, binding of subsequent molecules is easier.

Question 6

Explain why oxygen is released from haemoglobin in the respiring tissues.

Answer 6

• Partial pressure of oxygen is low.
• High concentration of CO2 in respiring tissues, so affinity
  decreases.

Question 7

What do oxyhaemoglobin dissociation curves show?

Answer 7

Saturation of haemoglobin with oxygen (in %), plotted against partial pressure of oxygen (in kPA). Curves further to the left show the haemoglobin has a higher affinity for oxygen.

Question 8

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

Answer 8

Curve shifts to the right because haemoglobin’s affinity for oxygen has decreased.

Question 9

Name 3 common features of a mammalian circulatory system.

Answer 9

1. Suitable medium for transport, water-based to allow
   substances to dissolve.
2. Means of moving the medium and maintaining pressure
   throughout the body, such as the heart.
3. Means of controlling flow so it remains unidirectional, such as
   valves.

Question 10

Relate the chambers in the heart to their function>

Answer 10

Atria: thin-walled and elastic, so they can stretch when filled with blood.
Ventricles: thick muscular walls pump blood under high pressure. The left ventricle is thicker than the right because it has to pump blood all the way around the body.

Question 11

Relate the structure of the blood vessels to their function.

Answer 11

• Arteries have thick walls to handle blood under high pressure
  without tearing, and are muscular and elastic to control blood flow.
• Veins have thin walls due to low pressure, therefore requiring valves
  to ensure blood doesn’t flow backwards. Have less muscular and
  elastic tissue as they don’t have to control blood flow.

Question 12

Why are two pumps needed (left and right) instead of one?

Answer 12

To maintain blood pressure around the whole body. When blood passes through the capillaries of the lungs, the pressure drops sharply and therefore would not be flowing strongly enough to continue around the whole body. Therefore it is returned to the heart to increase the pressure.

Question 13

Describe what happens during cardiac diastole.

Answer 13

The heart is relaxed. Blood enters the atria, increasing the pressure and pushing open the atrioventricular valves. This allows blood to flow into the ventricles. Pressure in the heart is lower than in the arteries, so semi-lunar valves remain closed.

Question 14

Describe what happens during atrial systole.

Answer 14

The atria contract, pushing any remaining blood into the ventricles.

Question 15

Describe what happens during ventricular systole.

Answer 15

The ventricles contract. The pressure increases, closing the atrioventricular valves to prevent backflow, and opening the semiluna valves. Blood flows into the ateries.

Question 16

Name the nodes involved in heatt contraction and where they are situated.

Answer 16

• Sinoatrial node (SAN) = wall of right atrium.
• Atrioventricular node (AVN) = in between the two atria.

Question 17

What does myogenic mean?

Answer 17

The heart’s contraction is initiated from within the muscle itself, rather than by nerve impulses.

Question 18

Explain how the heart contracts.

Answer 18

• SAN initiates and spreads impulse across the atria, so they contract.
• AVN receives, delays and then conveys the impulse down the bundle of His.
• Impulse travels into the Purkinje fibres which branch across the ventricles, so they contract from the bottom up.

Question 19

Why does the impulse need to be delayed?

Answer 19

If the impulse spread straight from the atria into the ventricles, there would not be enough time for all the blood to pass through and for the valves to close.

Question 20

How is the structure of capillaried suited to their function?

Answer 20

• Walls are only one cell thick; short diffusion pathway.
• Very anrrow, so can permeate tissues and red blood cells can lie flat against the want effectively delivering oxygen to tissues.
• Numerous and highly branched, providing a large surface area.

Question 21

What is tissue fluid?

Answer 21

A watery substance containing glucose, amino acids, oxygen, and other nutrients. It supplies these to the cells, while also removing any waste materials.

Question 22

How is tissue fluid formed?

Answer 22

As blood is pumped through increasingly small vessels, this created hydrostatic pressure which forces fluid out of the capillaries. It bathes the cells, and then resturns to the capillaries when the hydrostatic pressure is low enough.

Question 23

How is water transported in plants?

Answer 23

Through xylem vessels; long, continuous columns that also provide structural support to the stem.

Question 25

Explain the cohesion-tension theory.

Answer 25

Water molecules form hydrogen bonds with each other, causing them to ‘stick’ together (cohesion). The surface tension of the water also created this sticking effect. Therefore as water is lost through transpiration, more can be drawn up the stem.

Question 26

What are the three components of phloem vessels?

Answer 26

• Seive tube elements = form a tube to transport sucrose in the dissolved form of sap.
• Companion cells = involved in ATP production for active loading of sucrose into sieve tubes.
• Plasmodesmata = gaps between cell walls where the cytoplasm links, allowing substances to flow.

Question 27

Name the process whereby organic materials are transported around the plant.

Answer 27

Translocation.

Question 28

How does sucrose in the leaf move into the phloem?

Answer 28

Sucrose enters companion cells of the phloem vessels by active loading, which uses ATP and a diffusion gradient of hydrogen ions. Sucrose then diffuses from companion cells into the sieve tube elements through the plasmodesmata.

Question 29

How do phloem vessels transport sucrose around the plant?

Answer 29

As sucrose moves into the tube of elements, water potential inside the phloem is reduced. This causes water to enter via osmosis from the xylem and increases hydrostatic pressure. Water moved along the sieve tube towards areas of lower hydrostatic pressure. Sucrose diffuses into surrounding cells where it is needed.

Question 30

Give evidence for the mass flow hypothesis of translocation.

Answer 30

• Sap is released when a stem is cut, therefore there must be pressure in the phloem.
• There is a higher sucrose concentration in the leaves than the roots.
• Increasing sucrose levels in the leaves results in increased sucroses in the phloem.

Question 31

Give evidence against the mass flow hypothesis of translocation.

Answer 31

• The structure of sieve tubes seems to hinder mass flow.
• Not all soluted move at the same speed, as they would in mass flow.
• Sucrose is delivered at the same rate throughout the plant, rather than to areas with the lowest sucrose concentration first.

Question 32

How can ringing experiments be used to investigate transport in plants?

Answer 32

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissure above the missing ring swells due to accumulation of sucrose as the tissue below begins to die. Therefore sucrose must be transported in the phloem.

Question 33

How can tracing experiments be used to investigate transport in plants?

Answer 33

Plants are grown in the presence of radioactive CO2, which will be incorporated into the plant’s sugars. Using autoradiography, we can see that the areas exposed to radiation correspond to where the phloem is.