3.4 Mass Transport

Question 1

Describe the structure of haemoglobin

Answer 1

Globular and water soluble. Made up of four polypeptide chains, each carrying a haem group. It has a quaternary structure.

Question 2

Describe the role of haemoglobin

Answer 2

Haemoglobin can be found in red blood cells. Oxygen molecules bind to the haem groups and are carried around the body to where they are needed in respiring tissues

Question 3

Three factors that affect oxygen-haemoglobin binding

Answer 3

Partial pressure/concentration of oxygen

Partial pressure/concentration of carbon dioxide

Saturation of haemoglobin with oxygen

Question 4

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

Answer 4

When the partial pressure of oxygen increases, the affinity of haemoglobin for oxygen also increases. This means that oxygen binds tightly to haemoglobin however when partial pressure is low, oxygen is released from haemoglobin.

Question 5

How does partial pressure of carbon dioxide affect oxygen-haemoglobin binding? What is Bohr effect?

Answer 5

When partial pressure of carbon dioxide increases, the conditions become acidic causing haemoglobin to change shape. The affinity of haemoglobin for oxygen therefore decreases, so oxygen is released from haemoglobin. This is known as Bohr effect

Question 6

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

Answer 6

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

Question 7

Explain why oxygen binds to haemoglobin in the lungs

Answer 7

In the lungs the partial pressure of oxygen is high and the is a low concentration of carbon dioxide in the lungs, so affinity is high and positive cooperativity (after the first oxygen molecule binds, binding of subsequent molecules is easier)

Question 8

Explain why oxygen is released from haemoglobin in respiring tissues

Answer 8

In respiring tissues the partial pressure of oxygen is low and the is a high concentration of carbon dioxide in respiring tissues, so affinity decreases

Question 9

What do oxyhaemoglobin dissociation curves show?

Answer 9

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 10

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

Answer 10

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

Question 11

What are some common features of a mammalian circulatory system

Answer 11

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 12

Draw a diagram off the human heart including names of chambers, vessels and valves.

Answer 12

Slide 26 on PMT 3.4 mass transport

Question 13

Relate the structure of the chambers to their function

Answer 13

Atria: thin-walled and elastic, so they can stretch when filled with blood
Ventricles: thick muscular walls that 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 14

Relate the structure of the vessels to their function

Answer 14

Arteries have thick walls to handle high pressure without tearing, and are muscular and elastic to control blood flow.

Veins have thin walls due to lower pressure, therefore have valves to prevent backflow. Have less muscular and elastic tissue as they don’t have to control blood flow.

Question 15

Why are two pumps needed in the heart instead of one?

Answer 15

To maintain blood pressure around the whole body. When blood passes through the narrow 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 16

Describe what happens during cardiac diastole

Answer 16

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 semilunar valves remain closed.

Question 17

Describe what happens during atrial systole.

Answer 17

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

Question 18

Describe what happens during ventricular systole

Answer 18

The ventricles contract. The pressure increases, closing the atrioventricular valves to prevent back flow,and opening the semilunar valves. Blood flows into the arteries.

Question 19

Name the nodes involved in the heart contraction and where they are situated

Answer 19

Sinoatrial node (SAN) = wall of right atrium

Atrioventricular node (AVN)= in between the two atria

Question 20

What does myogenic mean?

Answer 20

The hearts contraction is initiated from within the muscle itself, rather than by nerve impulses

Question 21

Explain how the heart contracts

Answer 21

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

Question 22

Why does the impulse need to be delayed?

Answer 22

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 23

How is the structure of capillaries suited to their function?

Answer 23

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

Question 24

What is tissue fluid?

Answer 24

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

Question 25

How is tissue fluid formed?

Answer 25

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

Question 26

How is water transported in plants?

Answer 26

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

Question 27

Explain the cohesion-tension theory

Answer 27

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

Question 28

What are the three components of phloem vessels?

Answer 28

Sieve tube elements= form a tube to transport sucrose in the dissolved form of sap.

Companion cells = are 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 29

Name the process whereby organic materials are transported around the plant

Answer 29

Translocation

The process on which organic material are transport around a plant

Question 30

How does sucrose in the leaf move into the phloem?

Answer 30

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 31

How do phloem vessels transport sucrose around the plant?

Answer 31

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

Question 32

Give evidence for the mass flow hypothesis of translocation

Answer 32

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 root

Increasing sucrose levels in the leaves results in increased sucrose in the phloem

Question 33

Give evidence against the mass flow hypothesis of translocation

Answer 33

The structure of sieve tubes seems to hinder mass flow

Not all solutes 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 34

How can dining experiments be used it investigate transport in plants

Answer 34

The bark and phloem of a tree are removed in a ring, leaving behind the xylem. Eventually the tissues 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 35

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

Answer 35

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