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 to the haem groups and are carried around the body to where they are needed in respiring tissues

Question 3

Name 3 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

• 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

• 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
• Known as the ‘Bohr effect’

Question 6

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

Answer 6

• Hard for first oxygen molecule to bind
• Once it does, it changes the shape to make it easier for the second and third molecules to bind (positive cooperativity)
• Slightly harder for the 4th 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, affinity is high
• 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

• Partial pressure of oxygen is low

- High concentration of carbon dioxide in respiring tissues, so affinity decreases

Question 9

What do oxyhemoglobin 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

Name 3 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

State the names of chambers, vessels and valves present in the human heart (right)

Answer 12

Right (deoxygenated blood):

• Vena cava (deoxygenated blood in)
• Right atrium
• Tricuspid valve
• Right ventricle
• Pulmonary artery (blood out to lungs)

Question 13

State the names of chambers, vessels and valves present in the human heart (left)

Answer 13

Left (oxygenated blood):

• Pulmonary vein (oxygenated blood from lungs)
• Left atrium
• Semilunar valves
• Bicuspid valve
• Left ventricle
• Aorta (oxygenated blood all the way around the body)

Question 14

Relate the structure of the chambers to their function

Answer 14

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

Question 15

Relate the structure of the vessels to their function

Answer 15

• Arteries: thick walls to handle high pressure without tearing, muscular and elastic to control blood flow
• Veins: thin walls to lower pressure, requires valves to ensure blood doesn’t flow backwards, less muscular and elastic tissue as they don’t have to control blood flow

Question 16

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

Answer 16

• Maintain blood pressure around the whole body
• Blood passing through narrow capillaries of the lungs, pressure drops sharply and therefore would not be flowing strongly enough to continue around the whole body
• Returned to heart to increase the pressure

Question 17

Describe what happens during cardiac diastole

Answer 17

• Heart is relaxed
• Blood enters the atria, increasing the pressure and pushing open the atrioventricular valves
• Blood flows into ventricles
• Pressure in the heart is lower than in the arteries, so semilunar valves remain closed

Question 18

Describe what happens during atrial systole

Answer 18

The atria contract, pushing any remaining blood into the ventricles

Question 19

Describe what happens during ventricular systole

Answer 19

• Ventricles contract
• Pressure increases, closing the atrioventricular valves to prevent backflow and opening the semilunar valves
• Blood flows into the arteries

Question 20

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

Answer 20

• Sinoatrial node (SAN) = wall of right atrium

- Atrioventricular node (AVN) = in between the two atria

Question 21

What does myogenic mean?

Answer 21

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

Question 22

Cardiac conduction system (SAABPV)

Answer 22

Sally Always Aims Balls Past Vicky

• Sinoatrial node
• Atrial systole
• Bundle of His
• Purkinje fibres
• Ventricular systole

Question 23

Explain how the heart contracts

Answer 23

• 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 24

Why does the impulse need to be delayed?

Answer 24

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 25

How is the structure of capillaries suited to their function

Answer 25

- 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 26

What is tissue fluid?

Answer 26

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

Question 27

How is tissue fluid formed?

Answer 27

- 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 28

How is water transported in plants?

Answer 28

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

Question 29

Explain the cohesion-tension theory

Answer 29

- 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 - As water is lost through transpiration, more can be drawn up the stem

Question 30

What are the 3 components of phloem vessels?

Answer 30

- 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 31

Name the process whereby organic materials are transported around the plant

Answer 31

Translocation

Question 32

How does sucrose in the leaf move into the phloem?

Answer 32

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

Question 33

How do phloem vessels transport sucrose around the plant?

Answer 33

- As sucrose moves into the tube elements, water potential inside the phloem is reduced - 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 34

Give evidence for the mass flow hypothesis of translocation

Answer 34

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

Question 35

Give evidence against the mass flow hypothesis of translocation

Answer 35

- 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 36

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

Answer 36

- Bark and phloem or a tree are removed in a ring, leaving behind the xylem - Tissues above missing ring swells due to accumulation. of sucrose as the tissue below begins to die - Sucrose must be transported in the phloem

Question 37

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

Answer 37

- 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