3 Organisms exchange substances: 7 Mass Transport

Question 1

What are haemoglobins?

Answer 1

A group of chemically similar molecules found in many different organisms.
Haemoglobin is a protein with a quaternary structure.

Question 2

What is the role of haemoglobin and red blood cells in the transport of oxygen? When does it associate and dissociate with oxygen?

Answer 2

Haemoglobin has a high affinity for oxygen in the lungs so oxygen associates.
It has a low affinity for oxygen near respiring tissues, so oxygen dissociates.

Oxygen loads onto haemoglobin when there’s a high pO2 (high partial pressure of O2) - alveoli have high pO2.
Oxygen unloads when there’s a lower pO2 - tissues respire so there’s a low pO2.

Question 3

How do oxyhaemoglobin dissociation curves show how affinity for oxygen varies?

Answer 3

1. The haemoglobin shape makes it difficult for the first oxygen to bind to a site. Therefore, at low pO2, little oxygen binds so the gradient is shallow.
2. The binding of the first O2 changes the quaternary structure of the haemoglobin, making it easier to bind a second O2. The binding of the first O2 induces the other subunits to an O2 molecule.
3. Positive cooperativity - binding of the first O2 makes it easier to bind others. It takes a smaller increase in pO2. Gradient steepens.
4. After the third O2 binds, the binding of the fourth is harder as it’s less likely for an O2 to find an empty site. Gradient reduces and curve flattens off.

Question 4

What are the effects of carbon dioxide concentration on the dissociation of oxyhaemoglobin?

Answer 4

The Bohr Effect:

• the greater the concentration of CO2, the more readily haemoglobin releases its oxygen.

At gas-exchange surface:
- CO2 concentration is low
- affinity for oxygen increases so oxygen is readily loaded
- oxygen dissociation curve shifts left

At respiring tissues:
- CO2 concentration is high
- affinity for oxygen decreases so oxygen is readily unloaded
- oxygen dissociation curve shifts right

Dissolved CO2 is acidic and the low pH causes haemoglobin to change shape to either a high affinity or low affinity one.

Question 5

How are animals adapted to their environment in relation to haemoglobin?

Answer 5

Different animals possess different types of haemoglobin with different oxygen transport properties.
Organisms that live in low O2 environments have higher affinity for oxygen haemoglobin - dissociation curve is to the left of a human’s.
Organisms that are very active (so have a high oxygen demand) have lower affinity for oxygen haemoglobin - curve is to the right.

Question 6

What is the general pattern of blood circulation in a mammal?

Answer 6

Deoxygenated blood:
Right ventricle -> pulmonary artery -> lungs

Oxygenated blood:
Lungs -> pulmonary vein -> left atrium -> left ventricle -> aorta -> arteries -> renal artery -> kidneys

Deoxygenated blood:
Kidneys -> renal vein -> veins -> vena cava -> right atrium - > right ventricle

Question 7

What is the gross structure of the human heart?

Question 8

What are the stages of the cardiac cycle?

Answer 8

1. Diastole
2. Atrial systole
3. Ventricular systole

Question 9

What happens in diastole?

Answer 9

• ventricles and atria are relaxed
• the semi-lunar valves close due to higher pressure in the pulmonary artery and aorta (prevents backflow into ventricles)
• atria fill due to higher pressure in vena cava and pulmonary vein
  
  • atria pressure increases
• ventricle pressure is lower so atrioventricular valves open and blood flows into the ventricles

Question 10

What happens in atrial systole?

Answer 10

• ventricles are relaxed
• atria contract, decreasing chamber volume and increasing its pressure
• blood is pushed into the ventricles
• ventricular pressure and chamber volume increases

Question 11

What happens in ventricular systole?

Answer 11

• atria are relaxed
• ventricles contract, decreasing chamber volume and increasing its pressure
• ventricle pressure is higher than in the atria so atrioventricular valves shut to prevent backflow
• ventricle pressure is higher than in the aorta and pulmonary artery
• semi-lunar valves open and blood is forced out into the arteries.

Question 12

What is the structure of arteries related to their function?

Answer 12

Thick muscle tissue
- arteries can be constricted to control the volume of blood passing through and the flow of it

Thick elastic tissue
- stretching and recoil helps maintain high pressure and evens out pressure surges

Thick wall
- resists high pressure

Smooth endothelium
- reduces friction

No valves
- blood is under constant high pressure so tends not to flow backwards

Question 13

What is the structure of the arterioles related to their function?

Answer 13

Thick muscle layer
- contraction constricts the lumen, restricts the flow of blood

Thin elastic layer
- blood pressure is lower

Question 14

What is the structure of the veins related to their function?

Answer 14

Thin muscle layer
- their contraction can’t control the flow of blood to tissue since they are carrying blood away from tissues
- instead, the contraction of the tissues helps blood flow

Thin elastic layer
- blood pressure is low

Thinner wall
- pressure is low so no need for thick wall
- allows them to be flattened easily, aiding blood flow

Valves
- ensures blood doesn’t flow backwards

Question 15

What is the structure of the capillaries related to their function?

Answer 15

Thin walls
- short diffusion distance for substance exchange

Capillary beds (networks of capillaries in tissues)
- large surface area for exchange

Found near cells in exchange tissues
- short diffusion pathway

Narrow lumen
- red blood cells are flattened against the walls, reducing diffusion distance

Spaces between endothelial cells
- allows white blood cells to escape to deal with infections

Question 16

How is tissue fluid formed?

Answer 16

Ultrafiltration
1. At the arterial end, the hydrostatic pressure inside the capillaries is greater than in the tissue fluid.
2. Difference in hydrostatic pressure forces water out of the capillaries, forming tissue fluid.
- water potential is higher outside which would cause water to move back into the capillaries but pressure difference is greater.

Question 17

How does tissue fluid return to the circulatory system?

Answer 17

1. As water left and plasma proteins remained, the hydrostatic pressure and water potential in the capillaries at the venule end is much lower.
2. Water re-enters the capillaries via osmosis down the water potential gradient.
3. Excess tissue fluid is drained by the lymphatic system.

Question 18

What is the equation for cardiac output?

Answer 18

cardiac output = stroke volume x heart rate

Question 19

What are the most common risk factors for cardiovascular disease?

Answer 19

High blood cholesterol and poor diet.
Cigarette smoking.
High blood pressure.

Question 20

Why is high blood cholesterol and a poor diet a risk factor for cardiovascular disease?

Answer 20

Cholesterol forms atheromas which can lead to increased blood pressure and clots.
Could block flow of blood to coronary arteries which could cause a myocardial infarction.

Question 21

Why is cigarette smoking a risk factor for cardiovascular disease?

Answer 21

Nicotine increases risk of high blood pressure.
CO combines with haemoglobin which reduces the amount of oxygen going to the heart which can lead to a heart attack.

Question 22

Why is high blood pressure a risk factor for cardiovascular disease?

Answer 22

High blood pressure increases the risk of damage to artery walls, increasing the risk of atheroma formation.
Blood clots can form, blocking flow of blood which can lead to a myocardial infarction.

Question 23

What is xylem?

Answer 23

The tissue that transports water in the stem and leaves of plants.

Question 24

What is the cohesion-tension theory of water transport in the xylem?

Answer 24

1. Water evaporates from the leaves (transpiration).
2. This creates tension which pulls more water up.
3. Cohesion between water molecules means a column of water moves up the xylem.

Question 25

What is the structure of xylem?

Answer 25

Long tubes formed from dead cells with no end cell walls.

Question 26

What is transpiration?

Answer 26

Evaporation of water from a plant’s surface.

Question 27

What is the process of using a potometer to estimate transpiration rate?

Answer 27

1. Cut a shoot underwater to prevent air from entering the xylem (at a slant to increase the surface area for water uptake).
2. Assemble the potometer under water, making sure there are no air bubbles,
3. Insert the shoot underwater.
4. Remove the potometer from the water and make sure it’s water and air tight.
5. Dry the leaves and introduce an air bubble into the capillary tube.
6. Record the distance moved by the bubble in a given time.
7. Once the air bubble nears the end of the tube, open the reservoir tap and push the syringe until it returns to the start.

Question 28

What is phloem?

Answer 28

The tissue that transports organic substances in plants.

Question 29

What is the structure of phloem?

Answer 29

Made of sieve tube elements (no nucleus, few organelles) and companion cells (provides energy needed for active transport of solutes)

Question 30

What is the mass flow hypothesis for translocation?

Answer 30

1. At the source, solutes are actively transported from companion cells into the sieve tubes of the phloem.
2. The water potential inside the sieve tubes decreases, so water enters the tubes by osmosis from the xylem and companion cells.
3. This creates a high pressure inside the sieve tubes at the source.
4. At the sink, solutes are used up (removed from the phloem).
5. The water potential inside the sieve tubes increases, so water leaves the tubes by osmosis.
6. This lowers the pressure inside the sieve tubes.
7. A pressure gradient is formed from the source to the sink.
8. This gradient causes mass movement of solutes towards the sink.
9. At the sink, the solutes will be used or stored.

Question 31

What is evidence for and against mass flow theory?

Answer 31

For:
- sap is released when sieve tubes are cut, showing there’s pressure within
- concentration of solute is higher at sources than sinks
- downward flow in the phloem occurs in daylight, but not when shaded or at night
- increases in solute levels in the leaf are followed by increases in solute levels in the phloem
- metabolic poisons/ lack of oxygen inhibits translocation
- companion cells have many mitochondria which produce ATP

Against:
- function of the sieve tubes is unclear
- end walls should hinder mass flow
- could have a structural function, preventing tubes bursting under pressure
- not all solutes move at the same speed
- solute is delivered at the same rate to all regions, rather than going more quickly to low solute concentration areas

Question 32

How are ringing experiments used in investigating transport in plants? What conclusions can be drawn from the results?

Answer 32

1. A section of the protective bark and phloem is removed around the circumference of the stem.
2. After a period of time, swelling occurs above the missing tissue.
3. The accumulated liquid is rich in sugars and dissolved organic substances.
4. Some tissues below the ring die.

Conclusions:
- sugars in the phloem accumulated above the ring, leading to swelling
- phloem is responsible for translocating sugars since the xylem was untouched
- the interrupted flow of sugars below the ring caused the death of tissues.

Question 33

How are tracer experiments used in investigation transport in plants? What conclusions can be drawn from the results?

Answer 33

Radioactive isotopes are used to trace the movement of substances in plants.

e.g.
1. 14C makes radioactive 14CO2.
2. A plant grown in 14CO2 will incorporate 14C into the sugar produced during photosynthesis.
3. These radioactive sugars can then be traced using autoradiography.
4. Thin cross-sections of the stem are taken and placed on X-ray film.
5. Radiation causes the film to blacken.
6. The blackened regions correspond to where phloem tissue is.

Conclusions:
- as other tissues don’t blacken the film, they don’t carry sugars so phloem is the only one responsible for translocation.

Question 34

What is evidence that translocation occurs in phloem?

Answer 34

• when phloem is cut, a solution of organic molecules flows out.
• plants grown in radioactive CO2 have radioactive C in their phloem after a while.
• aphids penetrate phloem with their mouthparts to extract the contents of the sieve tube elements
  
  • sap flows out quicker nearer the leaves than the stems, showing a pressure gradient and a downward flow.
• removing a ring of phloem leads to the accumulation of sugars above the ring and their decrease below.