Mass transport

Question 1

Describe the structure of haemoglobin

Answer 1

Globular, water soluble. Consists of four polypeptide chains, each carrying a haem group

Question 2

Describe the role of haemoglobin

Answer 2

Present 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

Name three factors affecting oxygen-haemoglobin binding

Answer 3

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

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

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

Answer 5

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

Question 6

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

Answer 6

It is hard for the first oxygen molecule to bind. 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 finding a binding site

Question 7

Explain why oxygen binds to haemoglobin in the lungs

Answer 7

Partial pressure of oxygen is high

Low concentration of carbon dioxide in the lungs so affinity is high

Positive cooperativity

Question 8

Explain why oxygen is released from haemoglobin in respiring tissues

Answer 8

Partial pressure of oxygen is low

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, plotted against partial pressure of oxygen.

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

Name 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

Relate the structure of the chambers to their function

Answer 12

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 13

Relate the structure of the vessels to their function

Answer 13

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 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 14

Why are two pumps needed instead of one?

Answer 14

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 15

Describe what happens during cardiac diastole

Answer 15

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 16

Describe what happens during atrial systole

Answer 16

The atria contract, pushing any remaining blood into the ventricles

Question 17

Describe what happens during ventricular systole

Answer 17

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

Question 18

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

Answer 18

Sinoatrial node (SAN) = wall of right atrium

Atrioventricular node (AVN) = in between the two atria

Question 19

What does myogenic mean?

Answer 19

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

Question 20

Explain how the heart contracts

Answer 20

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 21

Why does the impulse need to be delayed?

Answer 21

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 22

How is the structure of capillaries suited to their function?

Answer 22

Walls are only one cell thick; 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 23

What is tissue fluid?

Answer 23

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

Question 24

How is tissue fluid formed?

Answer 24

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 25

How is water transported in plants?

Answer 25

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

Question 26

Explain the cohesion-tension theory

Answer 26

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

Question 27

What are the three components of phloem vessels?

Answer 27

Sieve 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 28

Name the process whereby organic materials are transported around the plant

Answer 28

Translocation

Question 29

How does sucrose in the leaf move into the phloem?

Answer 29

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 sieve tube elements through the plasmodesmata

Question 30

How do phloem vessels transport sucrose around the plant?

Answer 30

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 31

Give evidence for the mass flow hypothesis of translocation

Answer 31

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 32

Give evidence against the mass flow hypothesis of translocation

Answer 32

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 33

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

Answer 33

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 34

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

Answer 34

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