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AS level Biology > Section 3 - Mass transport > Flashcards

Section 3 - Mass transport Flashcards

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1
Q

What are the haemoglobins?

A
  • Group of - Chemically similar molecules
  • Protein molecules
  • Quaternary structure
  • To make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions.
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2
Q

What makes up the structure of a haemoglobin molecule?

A
  • Primary structure 0 sequences of amino acids in 4 polypeptide chains
  • Secondary structure - the polypeptide chains are coiled into a helix
  • tertiary structure - folded into a precise shape - an important factor in its ability to carry oxygen.
  • Quaternary structure - 4 polypeptides linked together forming almost spherical molecule. Associated with haem group.
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3
Q

Which structure of the haemoglobin molecule is important in its ability to carry oxygen?

A

The tertiary structure as each of the polypeptide chains is folded into a precise shape.

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4
Q

What is the quarternary structure of a haemoglobin molecule?

A
  • All four polypeptides are linked together to form an almost spherical molecule.
  • Each polypeptide is associated with a haem group
  • Ferrous (FE2+) ion.
  • Each FE2+ ion can combine with a single oxygen molecule (O2)
  • Product -four O2 molecules that can be carried by a single haemoglobin molecule in humans.
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5
Q

What is loading in the context of haemoglobin molecules?

A

The process by which haemoglobin binds with oxygen.

AKA associating.

In humans this takes place in the lungs

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6
Q

What is unloading in the context of haemoglobin molecules?

A

The process by which haemoglobin releases its oxygen.

AKA dissociating.

In humans this takes place in the tissues.

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7
Q

What does it mean if haemoglobin has a high affinity for oxygen?

A

Takes up oxygen more easily but releases it less easily.

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8
Q

What does it mean if haemoglobin has a low affinity to oxygen?

A

Takes up oxygen-less easily but releases it more easily.

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9
Q

What is the role of haemoglobin?

A

To transport oxygen.

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10
Q

In order for haemoglobin molecules to be efficient at transporting oxygen, what functions must they have?

A
  • Readily associate with oxygen at the surface where gas exchange takes place
  • readily dissociate from oxygen at those tissues requiring it.
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11
Q

How is it possible for haemoglobin molecules to easily load and unload oxygen molecules at similar rates of efficiency?

A
  • It changes its affinity (chemical attraction) for oxygen under different conditions.
  • Shape changes in the presence of certain substances, such as carbon dioxide.
  • Presence of CO2 (Higher conc in respiring tissue), new shape of haemoglobin binds more loosely to oxygen.
  • As result haemoglobin releases its oxygen.
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12
Q

Explain the affinity of haemoglobin for oxygen in on a gas exchange surface

A
  • High O2 conc.
  • Low CO2 conc.
  • High affinity of haemoglobin for oxygen
  • Results in oxygen being associated.
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13
Q

Explain the affinity of haemoglobin for oxygen on respiring tissues.

A
  • Low O2 conc.
  • High CO2 conc.
  • Low affinity of haemoglobin for oxygen
  • Results in oxygen being dissociated.
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14
Q

Why do different haemoglobins have different affinities for oxygen?

A

The shape of the molecule.

  • Each species produces haemoglobin with slightly diff amino acid sequence.
  • Different tertiary and quaternary structures and so diff oxygen binding properties.
  • Molecules range from those with high affinity for oxygen to those with low.
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15
Q

What happens when haemoglobin is exposed to different partial pressures of oxygen?

A

It does not bind to oxygen evenly.

The graph of this relationship is the oxygen dissociation curve.

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16
Q

What does the oxygen dissociation curve depict?

A

The relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen.

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17
Q

Why is the gradient of the oxygen dissociation shallow initially?

A
  • Shape of haemoglobin changed as each oxygen binds as they are closely united.
  • At low oxygen concentrations, little oxygen binds to haemoglobin.
  • The gradient of the curve is shallow initially.
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18
Q

What is the second stage of the oxygen dissociation curve?

A
  • After first oxygen is bound
  • quaternary structure of the haemoglobin changes.
  • This change makes it easier for the other subunits to bind to an oxygen molecule.
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19
Q

Explain the third stage of the oxygen dissociation curve.

A
  • After first and second oxygen is bound
  • A smaller increase in the partial pressure to oxygen is needed to bind the second oxygen molecule
  • Positive cooperativity because binding of the first molecule makes binding of the second easier and so on.
  • The gradient of the curve steepens.
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20
Q

Explain the fourth and final stage of the oxygen dissociation curve

A
  • Three oxygen bound and situation changes.
  • Harder to attach last oxygen
  • Due to probability.
  • With the majority of the binding sites occupied, it is less likely that a single oxygen molecule will find an empty site to bind to,
  • The gradient of the curve reduces and the graph flattens off.
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25
Q

At the gas exchange surface, why is the concentration of carbon dioxide low?

A
  • It diffuses across the exchange surface and is excreted from the organism.
  • Affinity of haemoglobin for oxygen is increased
  • And high concentration of oxygen in the lungs
  • oxygen is readily loaded by haemoglobin.
  • This reduces CO2 concentration has shifted the oxygen dissociation curve to the left.
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26
Q

In rapidly respiring tissues why is the concentration of carbon dioxide high?

A
  • The affinity of haemoglobin for oxygen is reduced
  • low concentration of oxygen in the muscles
  • oxygen is readily unloaded from the haemoglobin into the muscle cells.
  • The increased CO2 concentration had shifted the oxygen dissociation curve to the right.
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27
Q

What does it mean if the oxygen dissociation curve is more to the left of the curve in its position on the axes?

A

The greater the affinity of haemoglobin for oxygen (It loads oxygen readily but unloads it less easily)

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28
Q

What does it mean if the oxygen dissociation curve is more to the right of the curve in its position on the axes?

A

The lower the affinity of haemoglobin for oxygen

So loads oxygen less readily but unloads it more easily.

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29
Q

How does CO2 affect haemoglobins affinity for oxygen?

A

Reduced affinity in the presence of carbon dioxide.

The greater conc the more readily the haemoglobin releases its oxygen (The Bohr Effect)

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30
Q

What does the Bohr effect explain?

A

The Bohr Effect - The greater the conc. of CO2 the more readily the haemoglobin releases its oxygen.

Why the behaviour of haemoglobin changes in different regions of the body.

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33
Q

Why does an increased concentration of CO2 cause an increase in oxygen affinity?

A

Dissolved CO2 is acidic and the low pH causes haemoglobin to change shape.

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34
Q

Explain, in detail, the process of loading, transporting and unloading of oxygen

A
  1. At exchange surface, CO2 is constantly being removed
  2. Low CO2 = raised pH
  3. the higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily
  4. this shape also increases the affinity of haemoglobin for oxygen, so it is not released while being transported in the blood of the tissues.
  5. in the tissues carbon dioxide is produced by respiring cells
  6. carbon dioxide is acidic in solution, so the pH of the blood within the tissues is lowered
  7. the lower pH changes the shape of haemoglobin into one with a lower affinity for oxygen
  8. haemoglobin releases its oxygen onto the respiring tissues.
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35
Q

How does the loading, transporting and unloading of oxygen occur in a more active tissue?

A

Higher rate of respiration ->

More CO2 tissues produce ->

Lower the pH ->

Greater the haemoglobin shape change ->

More readily oxygen is unloaded ->

more oxygen is available for respiration.

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36
Q

On average what is the overall saturation of haemoglobin at atmospheric pressure?

A

97%

As not all haemoglobin molecules are loaded with their maximum four oxygen molecules.

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37
Q
A
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38
Q

In terms of oxygen affinity, what adaptations have some animals made?

A

Species with the lower partial pressure of oxygen have evolved haemoglobin that had a higher affinity for oxygen than the haemoglobin of animals that live where the partial pressure of oxygen is higher.

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39
Q

What are the two types of phases in the cardiac cycle?

A

Contraction - systole

Relaxation - diastole.

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40
Q

Why is systole explained in two stages?

A

Contraction occurs separately in the ventricles and the atria and is therefore described in two stages.

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41
Q

Explain relaxation of the heart

A

Diastole

  1. As atria fill the pressure rises.
  2. When pressure exceeds that in the ventricles, the atrioventricular valves open allowing the blood to pass into the ventricles.
  3. This passage is aided by gravity
  4. Muscular walls of both atria and ventricles are relaxed at this stage.
  5. Relaxation causes them to recoil and reduces the pressure within the ventricle.
  6. Causes pressure to be lower than that in the aorta and pulmonary artery so semi-lunar valves in the aorta and pulmonary artery close, accompanied by the ‘dud’ sound of the heart beat.
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42
Q

Explain contraction of the atria

A

Atrial systole

Contraction of the atrial wallas, along with the recoil of the relaxed ventricle walls, forces the remaining blood in the ventricles from the atria.

Throughout this stage, the muscle of the ventricle walls remains relaxed.

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43
Q

Explain contraction of the ventricles

A

Ventricular systole

  1. After a short delay to allow the ventricles to fill with blood, their walls contract simultaneously.
  2. This increases the blood pressure within them, forcing shut the atrioventricular valves and preventing backflow of blood in the atria.
  3. The ‘lub’ sound of these valves closing is the characteristic of the heartbeat.
  4. With the atrioventricular valves closed, the pressure in the ventricles rises further,
  5. When exceeded that of the aorta and pulmonary artery, blood forced from the ventricles into vessels.
  6. Venticles contract forcefully creating high pressure.
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46
Q

How are valves in the cardiovascular system designed?

A

So they open whenever the difference in blood pressure either side of them favours the movement of blood in the required direction.

When the pressure difference is reversed the valves close.

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47
Q

Describe atrioventicular valves

A

Between the left atrium and ventricle and the right atrium and ventricle.

Prevent backflow of blood when contraction of the ventricles mean that ventricular pressure exceeds atrial pressure.

Closure of these valves ensures that when the ventricles contract blood within them moves to the aorta and pulmonary artery rather than back to the atria.

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48
Q

Describe semi-lunar valves

A

In the aorta and pulmonary artery.

Prevent backflow of blood into the ventricles when the pressure in these vessels exceeds that in the ventricles.

This arises when the elastic walls of the vessels recoil increase ng the pressure within them and when the ventricle walls relax reducing the pressure within the ventricles.

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49
Q

What is a closed circulatory system?

A

Blood is confined to vessels.

Allows the pressure within them to be maintained and regulated.

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50
Q

Describe pocket valves

A

In veins

Occur throughout the venous system

Ensure that when the veins are squeezed blood flows back towards the heart rather than away from it.

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51
Q

What is the structure of a valve?

A

A number of flaps of though, but flexible, fibrous tissue, which are cusp-shaped, in other words like deep bowls.

When pressure is greater on the convex side of the cusps they move apart to let blood pass between.

When pressure is greater on the concave side blood collects within the ‘bowl’ of the cusps. Pishes them together to form a tight fit that prevents the passage of blood.

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53
Q

Define cardiac output

A

The volume of blood pumped by one ventricle of the heart in one minute.

Measured in dm^3min^-1

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54
Q

What two factors does cardiac output depend on?

A

The heart rate

stroke volume

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55
Q

What is the cardiac output equation?

A

Cardiac output = heart rate x stroke volume

56
Q

What are the different types of blood vessel?

A
  1. Arteries
  2. Arterioles
  3. Capillaries
    1. Veins
57
Q

What are arteries?

A

Blood vessels that carry blood away from the heart and into arterioles

58
Q

What are arterioles?

A

Smaller arteries that control blood flow from arteries and capillaries.

59
Q

What are capillaries?

A

Tiny blood vessels that link arterioles to veins.

60
Q

What are veins?

A

Carry blood from capillaries back to the heart.

61
Q

What structures do arteries, arterioles and veins all share?

A
  • Tough fibrous outer layer - resists pressure changes from both within and outside
  • muscle layer - contract and so control the flow of blood
  • elastic layer - helps to maintain blood pressure by stretching and springing back (recoiling)
  • thin inner lining (endothelium) - smooth to reduce friction and thin to allow diffusion
  • lumen - not actually a layer but the central cavity of the blood vessel through which the blood flows.
63
Q

What is the structure of a capillary?

A

Lumen

Lining layer

64
Q

What is the function of arteries?

A

To transport blood rapidly under high pressure from the heart to the tissues.

65
Q

How are arteries adapted from their function?

A
  • Muscle layer is thick compared to veins - constricted and dilated in order to control the volume of blood passing through them
  • Thick elastic layer - maintain high pressure to reach extremities
  • The great overall thickness of the wall - resists the vessel bursting under pressure
  • no valves - except in arteries leaving the heart as constant high pressure due to heart pumping blood into the arteries tends to be no backflow.
66
Q

What is the function of arterioles?

A

Carry blood, under lower pressure than arteries, from arteries to capillaries.

Also, control the flow of blood between the two.

67
Q

What adaptations have arterioles made?

A
  • Muscular layer relatively thicker than in arteries - contraction of this muscle layer allows constrictions of the lumen of the arteriole. Restricts the flow of blood and so controls its movement into the capillaries that supply the tissues with blood.
  • Elastic layer relatively thinner than arteries - blood pressure is lower.
68
Q

What is the function of veins?

A

Transport blood slowly, under low pressure from the capillaries in tissues to the heart.

69
Q

What adaptions have veins made?

A
  • Thin muscle layer - carry blood away from tissues and constriction and dilation cannot control the flow of blood to tissues
  • Thin elastic layer - low pressure of blood not prone to bursting and pressure too low to recreate recoil action
  • Small overall thickness of the wall - no risk of bursting. Also allows them to flatten easily aiding flow of blood within them.
  • Valves throughout - ensure blood doesn’t flow backwards.
70
Q

What is the function of capillaries?

A

To exchange metabolic materials such as oxygen, carbon dioxide and glucose between the blood and the cells of the body. Flow of blood is much smaller allowing more time for the exchange of materials.

71
Q

What are the adaptations of capillaries?

A
  • Wall mostly lining layer - extremely thin so diffusion pathway small.
  • Numerous and highly branched - providing a large surface area for exchange
  • Narrow diameter - permeate tissues
  • lumen is narrow - red blood cells squeezed flat against the side of a capillary. Brings them even closer to the cells they supply oxygen.
  • Spaces between the lining cells allowing white blood cells to escape in order to deal with infections within tissues.
72
Q

Why does tissue fluid exist?

A

Capillaries can not serve severy cell directly.

SO final journey of metabolic materials is made in a liquid solution that bathes the tissues.

73
Q

What is tissue fluid?

A

a watery liquid containing glucose, amino acids, fatty acids, ions in solution and oxygen.

Supplies all these substances to the tissues.

Receives carbon dioxide and other waste materials from the tissues.

74
Q

How is blood plasma formed?

A

From blood plasma and the composition of blood plasma is controlled by various homeostatic systems.

As a result, it provided cells with a mostly constant

75
Q

What is hydrostatic pressure?

A

Pumping by the heart creates a pressure at the arterial end of the capillaries.

76
Q

What does the hydrostatic pressure cause?

A

Tissue fluid to move out of the blood plasma.

77
Q

What opposes the outward hydrostatic pressure?

A
  1. Hydrostatic pressure of the tissue fluid outside the capillaries which resists outward movement of liquid
  2. lower water potential of the blood due to the plasma proteins, that causes water to move back into the blood within the capillaries.
78
Q

How does the return of tissue fluid to blood plasma directly via the capillaries occur?

A
  1. Loss of tissue fluid from capillaries reduces the hydrostatic pressure inside them
  2. As a result, by the time the blood has reached the venous end of the capillary network its hydrostatic pressure is usually lower than that of the tissue fluid outside it
  3. Therefore, tissue fluid is forced back into the capillaries by the higher hydrostatic pressure outside them
  4. in addition, the plasma has lost water and still contains proteins. It, therefore, has a lower water potential than the tissue fluid
  5. As a result, water leaves the tissue by osmosis down a water potential gradient.
79
Q

Describe the process of ultrafiltration

A
  • The combined effect of all forces creates an overall pressure that pushes tissue fluid out of the capillaries at the arterial end.
  • This pressure is only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood because these are too large to cross the membranes.
  • This type of filtration under pressure is called ultrafiltration
80
Q

What moves the contents of the lymphatic system?

A

Can not be pumped by the heart.

  • Hydrostatic pressure - of the tissue fluid that has left the capillaries
  • Contraction of body muscles that squeeze the lymph vessels - valves in the lymph vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart.
82
Q

Where does the remainder of tissue fluid go that can’t return to the capillaries?

A

Carried back via the lymphatic system.

This is the system of vessels that begin in the tissues.

Initially, they resemble capillaries but they gradually merge into larger vessels that form a network throughout the body.

These larger vessels drain their contents back into the bloodstream via two ducts that join veins close to the heart.

83
Q

What are xylem vessels?

A

Thick-walled tubes that transport the vast majority of water in plants.

Hollow.

84
Q

What is Transpiration?

A

The main force that pulls water through the xylem vessels in the stem of a plant. It requires the evaporation of water from leaves.

The energy for this is supplied by the sun and the process is therefore passive.

85
Q

The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.

What does this mean?

A

There is a water potential gradient from the air spaces through the stomata to the air.

Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air.

Water loss by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.

Changing the shape of the stomatal pores means the plants can control their rate of transpiration.

86
Q

How is water lost from the mesophyll cells?

A

By evaporation from their cell walls to the air spaces of the leaf.

This is replaced by water reaching the mesophyll cells from the xylem either via cell walls or via the cytoplasm.

87
Q

How does water movement occur on the cytoplasmic route?

A
  • Mesophyll cells lose water to the air spaces by evaporation due to heat supplied by the sun
  • These cells now have a lower water potential and so water enters by osmosis from neighbouring cells
  • the loss of water from these neighbouring cells lowers their water potential
  • they in turn take in water from their neighbours by osmosis.
88
Q

What is the main factor that is responsible for the movement of water up the xylem?

A

Cohesion-tension.

89
Q

How does the movement of water up the stem occur?

Cohesion-tension theory

A
  • water evaporates from mesophyll cells due to heat from the sun leading to transpiration
  • Water molecules form hydrogen bonds between one another and hence tend to stick together. This is known as Cohesion.
  • Water forms a continuous, unbroken column across the mesophyll cells and down the xylem.
  • As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion.
  • A column of water is, therefore, pulled up the xylem as a result of transpiration. This is called the transpiration pull.
  • Transpiration pull puts the xylem under tension, that is, there is a negative pressure within the xylem, hence the name cohesion-tension theory.
90
Q

What is cohesion in terms of water transpiration?

A

Water molecules form hydrogen bonds between one another and hence tend to stick together.

91
Q

What is transpiration pull?

A

A column of water that is pulled up the xylem as a result of transpiration

92
Q

How strong is the force of the transpiration pull?

A

Easily raise water up to 100m or more of the tallest trees.

93
Q

What are three pieces of evidence that support the cohesion-tension theory?

A
  1. Change in the diameter of tree trunks according to the rate of transpiration.
  2. If a xylem vessel is broken and air enters it, the tree can no longer draw up water.
  3. When a xylem vessel is broken, water does not leak out as would be the case if it were under pressure.
94
Q

How does the change in the diameter of tree trunks according to the rate of transpiration provide evidence in support for the cohesion-tension theory?

A

During the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem.

This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter.

At night when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases.

95
Q

If a xylem vessel is broken and eir enters it, the tree can no longer draw up water.

How does this provide evidence in support for the cohesion-tension theory?

A

As the continuous column of water is broken and so the water molecules can no longer stick together.

96
Q

When a xylem vessel is broken water does not leak out as would be the case if it were under pressure.

How does this provide evidence in support of the conhesion-tension theory?

A

Instead air is drawn in which is consistent with it being under tension.

97
Q

Describe the structure of xylem vessels

A

They are dead cells with no end walls. Meaning they form a series of continuous, unbroken tubes from root to leaves which is essential to the cohesion-tension theory of water flow up a stem.

The energy needed for transpiration is in the form of heat that evaporates water from the leaves and it ultimately comes from the sun.

98
Q

What is translocation?

A

The process by which organic molecules and some mineral ions are transported from one part of a plant to another.

IN flowering plants the tissue that transports biological molecules is the phloem.

99
Q

What is the phloem?

A

In flowering plants, it is the tissue that transports biological molecules.

Made up of sieve tube elements, long thin structures arranged end to end.

Their end walls are perforated to form sieve plates.

Associated with the sieve tube elements are cells called companion cells.

101
Q

Where is sugar - from photosynthesis - transported from?

A

Sources

102
Q

Where is the sugar transported to in order to be used directly or stored for future us?

A

Sinks

103
Q

Give evidence that translocation of molecules can be in either direction

A

As sinks can be anywhere in a plant - sometimes above and sometimes below the source.

104
Q

What substances can be transferred in the phloem?

A

Organic molecules including sucrose and amino acids.

Also inorganic ions such as potassium, chloride, phosphate and magnesium ions.

105
Q

What is the believed theory of translocation?

A

mass flow theory

106
Q

What are the three phases of the mass flow theory?

A
  1. Transfer of sucrose into sieve elements from photosynthesising tissue
  2. Mass flow of sucrose through sieve tube elements
  3. Transfer of sucrose from the sieve tube elements into storage or other sink cells
107
Q

Explain the 1st phase of translocation

A

Transfer of sucrose into sieve elements from photosynthesizing tissue

  • sucrose is manufactured from the products of photosynthesis in cells with chloroplasts
  • The sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into companion cells
  • Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
  • These hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements
  • Sucrose molecules are transported along with the hydrogen ions in a process known as co-transport. The protein carriers are therefore also known as co-transport proteins.
108
Q

What is mass flow?

A

The bulk movement of a substance through a given channel or area in a specified times.

109
Q

Explain the 2nd phase of translocation

A

Mass flow of sucrose through sieve tube elements.

  • Sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes as described above.
  • This causes the sieve tubes to have a lower (more -tive) water potential
  • As the xylem has a much higher (less -tive) water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them.
  • At the sinks, sucrose either used in respiration or converted to starch
  • cells have low sucrose content and sucrose is actively transported into them from the sieve tubes lowering their water potential
  • Due to lower water potential, water also moves into these respiring cells, from the sieve tubes by osmosis.
  • The hydrostatic pressure of sieve tubes in this region is therefore lowered
  • As a result of water entering the sieve tube elements at the source and leaving at the sink, there is high hydrostatic pressure at the source and low one at the sink
  • Therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes.
110
Q

Give examples of evidence supporting the mass flow hypothesis of translocation

A
111
Q

Give examples of evidence that questions the mass flow hypothesis

A
112
Q

Explain the third stage of translocation

A

Transfer of sucrose from the sieve tube elements into storage or other sink cells

  • The sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells.
113
Q

What type of process is mass flow?

A

Passive process

A result of the active transport of sugars.

Therefore the process as a whole is active which is why it is affected by for example temperatures and metabolic poisons.

114
Q

What is the structure of woody stems?

A

An outer protective layer of bark on the inside of which is a layer of phloem that entends all around the stem.

Inside the phloem layer is xylem.

115
Q

What is the ringing experiment?

A
  • Section of the outer layers (protective layer and phloem) is removed around the complete circumference of a woody stem while it is still attached to the rest of the plant.
  • After a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell.
  • Samples of liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances.
  • Some non-photosynthetic tissues in the region below the ring are found to wither and die, while those above the ring continue to grow.
118
Q

What do the observations about the ringing experiment suggest that the removing of phloem are the stem leads to?

A
  • Sugars of the phloem accumulating above the ring leading to swelling in this region
  • The interruption of flow if sugars to the region below the ring and the death of tissues in this region.
119
Q

What conclusions were drawn from the ring experiment?T

A

That the phloem rather than xylem is the tissue responsible for translocating sugars in plants.

As the ring of tissue removed had not extended into the xylem its continuity had not been broken.

If it were the tissue responsible for translocating sugars you would not have expected sugars to accumulate above the ring nor tissues below it to die.

120
Q

Explain the tracer experiment for proving transport in plants.

A

Radioactive isotopes are useful for tracing the movement of substances in plants.

These radioactive sugars can then be traced as they move within the plant using autoradiography.

Eg. 14C can be used to radioactively label 14CO2. If a plant is then grown in an atmosphere containing 14CO2 the 14C isotope will be incorporated into the sugars produced during photosynthesis. Taking cross-sections of the plant stem and placing on a piece of X-ray film. Will blacken when exposed to radiation produced by 14C in sugars. These areas found to correspond to where phloem tissue is in the time. As other tissues do not blacken it follows that they do not carry sugars and that phloem alone is responsible for their translocation.

121
Q

Give evidence that translocation of organic molecules occurs in the phloem.

A
  • When phloem is cut a solution of organic molecules flow out
  • plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in phloem after a short time.
  • Aphids are a type of insect that feed on plants. They have needle-like mouthparts which penetrate the phloem. They can therefore be used to extract the contents of the sieve tubes. These contents show daily variations in the sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content of the phloem.
  • The removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars abover the ring of their disappearance from below it.
122
Q

Define - positive cooperativity

A
  • Hemoglobin
  • Difficult to attach first Oxygen molecule
  • Once the first oxygen binds the shape changed making the other binding sites of oxygen more exposed
  • Conformational change
123
Q

What is the equation for water potential?

A

water potential = pressure potential + solute potential

124
Q

How is tissue fluid produced?

A
  • in capillaries
  • pumping of blood created hydrostatic pressure forcing fluid out of capillaries at the arteriole end
  • plasma proteins do not move out
  • outward movement is opposed by the pressure of the fluid already in the tissue fluid and by the low water potential of the blood due to the plasma proteins in the capillary
  • once fluid enters tissue fluid, oxygen, glucose ect passes into cell
125
Q

Most of the tissue fluid enters the venule end of the capillaries

Why?

A
  • The loss of fluid decreases the hydrostatic pressure of the capillaries
  • The water potential of the blood in the venule end is low due to plasma proteins
  • Therefore water moves back into the venule end by osmosis
  • Dissolved waste also enters the venule end (CO2 )
126
Q

What is the lignin in Xylem cells?

A
  • Supporting tissue
  • Forming of cell walls
  • Stops the cells from collapsing
127
Q

How does the photometer measure the amount of respiration?

A
  1. Fill potometer with water making sure there are no air bubbles
  2. leafy shoots fitted into potometer underwater
  3. Air bubble introduced into a capillary tube
  4. Distance moved by the air bubble in a given time indicates the rate of transpiration
  5. Water from the reservoir can be used to push the bubble back to its starting position.

AS level Biology (11 decks)

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