Chapter 7 - Mass Transport

Question 1

Haemoglobin structure
Haemoglobin is a water soluble globular protein with a quaternary structure
How many polypeptide chains does haemoglobin have?
Describe structure of haemoglobin

Answer 1

Four polypeptide chains- each bonded to a haem group which contains an iron ion (Fe2+)

Question 2

What is haemoglobin bound to oxygen is called?

Answer 2

Oxyhaemoglobin

Question 2

What is positive cooperativity?

Answer 2

When one oxygen moleucle binds, the quaternary structure changes to make it easier for other oxygen molecules to bind. Sme happens for disassociation too, for oxygen to unbind.
Slightly harder for 4th oxygen molecule to bind because harder to find a binding site

Question 2

There are four polypeptide chains each binded to a haem group that contains one iron ion
How many oxygen molecules can one iron ion bind to?
Therefore how many oxygen molecules can a haemoglobin molecule carry?

Answer 2

Each polypeptide chain is bonded to a haem group that contains an ion of iron, which is able to bind to one oxygen molecule.
So one haemoglobin molecule can bind to 4 oxygen molecules
Oxygen molecules bind one at a time.

Question 3

Describe what each structure of the haemoglobin is ?

Answer 3

Primary - sequence of amino acids in the four polypeptide chains
Secondary- each of the four chains is coiled into a helix
Tertiary- polypeptide chain is folded into a precise shape which helps with its function
Quaternary - All four polypeptide chains are linked- contains haem groups and iron ions.

Question 4

What is the process in which hameglobin binds with oxygen called and where does it occur?

Answer 4

Loading/ Associating
Happens in lungs

Question 5

What is the process in which haemoglobin releases/unbinds its oxygen and where does it occur?

Answer 5

Unloading/ Disossiating
Happens in tissues

Question 6

How does positive cooperativity affect association and dissociation

Answer 6

Allows oxygen to rapidly associated and dissociate from haemoglobin

Question 7

Why do different haemoglobins have different affinities to oxygen and how does this affect association?

Answer 7

Diff haemoglobin molecules have slightly diff amino acid sequences so different shapes- shape can cause the haemoglobin to have either a high or low affinity for oxygen
Haemoglobin with a high affinity for oxygen take up more oxygen easily but release it less easily
Those with low affinity for oxygen take up oxygen less easily but release it more easily

Question 8

Partial pressure of oxygen-What is it?

Answer 8

Amount of oxygen that is present in a mixtyre of gases
measured by the pressure it contributes to the total pressure of the gas mixture
measure in kilopascals

Question 8

Oxygen always moves from high to low partial pressure.
How does partial pressure affect oxygen association and dissociation?

Answer 8

When partial pressure is high, Leaving the lungs, oxygen diffuses downits pressure gradient and binds to haemoglobin in a process called oxygen association.
In respiring tissues, the partial pressure of oxygen islow. Oxygen diffuses down its pressure gradient and unbinds from haemoglobin in a process called oxygen dissociation when pp is low
The partial pressure of oxygen is lowest in the respiring tissues and highest in the lungs. Positive cooperativity is involved in increasing the rate of both binding and unbinding between oxygen and haemoglobin.

Question 9

As the partial pressure of oxygen increases, the percentage saturation of haemoglobin with oxygen…

Answer 9

also increases

Question 10

What is the percentage saturation of haemoglobin

Answer 10

The percentage saturation of haemoglobin with oxygen is a measure of how many iron ions in the haemoglobin molecules are bound to oxygen

Question 11

What does oxygen dissociation curve show?

Answer 11

The relationship between the percentage saturation of haemoglobin and the partial pressure of oxygen
(Curve flashcard is on quizlet) - saturation on y should go from 0 to 100
partial pressure on x

Question 12

Explain the shape of the oxygen dissociation curve
Explain why goes flat, steep then flat.

Answer 12

The increase in gradient is due to positive cooperativity. When an oxygen molecule binds to haemoglobin, it makes it easier for another oxygen molecule to bind. This causes the percentage saturation of haemoglobin with oxygen to increase rapidly.
Steep gradient is due to positive copperativity
At higher partial pressures, the curve flattens - plateaus at around 98% saturation- This is because as it reaches full saturation it is harder for oxygen to find an empty binding site

Question 13

How does carbon dioxide affect the curve?

Answer 13

The lower the conc of carbon dioxide in the blood, the further left the curve is
The greater the conc of carbon dioxide, the more readily haemoglobin releases its oxygen, curve further to the right

Question 13

At lower carbon dioxide concentrations, oxygen…?

Answer 13

Associates more readily
Dissociates less readily
Because it has a greater percentage saturation of haemoglobin
Vice versa for high conc of carbon dioxide

Question 14

What is the Bohr Effect?

Answer 14

The curve shifiting left or right as a result of a change in conc of carbon dioxide

Question 15

Carbon dioxide and pH
How does it affect acidity

Answer 15

At the gas exchange surfacel, C02 is constantly being removed
At a low conc of C02, the pH is slightly raised, less acidic
At a high conc of C02, the pH is lowered, more acidic
So curves to the right have higher acidity

Question 16

How does the C02 increase acidity

Answer 16

Carbon dioxide reacts with water in our blood to form H+ ions, more H+ ions means more acidic

Question 17

How does acidity affect the haemoglobin?
What happens to association/dissocitation when acidic and alkali?

Answer 17

Haemoglobin molecules are proteins, so changes in acidity can change the shape of the haemoglobin molecule - changes shgape of tertiary strcutre
When acidic, easier for oxygen to dissociate
When less acidic, easier for oxygen to associate

Question 18

Why is the Bohr effect beneficial for exercise

Answer 18

Respiring muscle cells release carbon dioxide, which decreases the pH of the blood near the tissues. This causes haemoglobin to change shape, allowing oxygen to dissociate more readily. As a result, cells are provided with more oxygen for aerobic respiration.

Question 19

Explain the advantage of the changes seen in the haemoglobin of people with anaemia- who have a lower conc of haemoglobin in the blood

Answer 19

At every measured partial pressure of oxygen, the percentage saturation of haemoglobin with oxygen is lower in people with anaemia. This corresponds to a shift in the oxyhaemoglobin dissociation curve to the right. It means that haemoglobin releases oxygen more readily in the tissues for respiration. This allows the body to meet its energy demands despite having less haemoglobin in the blood.

Question 20

What does higher affinity mean?
Haemoglobin has a higher affinity for oxygen
Where does this happen and what ph?

Answer 20

Associates more readily and dissociates less readily
‘haemoglobin likes oxygen’
Happens in lungs bc there is a lower conc of c02-higher pH

Question 21

What does low affinity mean? Haemoglobin has a lower affinity for oxygen Where does this happen and what pH?

Answer 21

Associates less readily and dissociates more readily Happens in tissue as they respire aerobically and produce high conc of c02- low pH

Question 22

How does lactic acid affect the affinity of haemoglobin?

Answer 22

The presence of lactic acid therefore increases the acidity of the blood. So, the oxyhaemoglobin dissociation curve shifts to the right and haemoglobin’s affinity for oxygen decreases.

Question 23

How do different species' haemoglobin differ?

Answer 23

They have different affinities - different oxygen dissociation curves, with affinites that are best adaped for them E.g llamas have a higher affinity bc they live on high altitudes so more oxygen will assoosiate at lower partial pressures

Question 24

Compare human affinity to that of a hummingbird

Answer 24

Hummingbird haemoglobin has a lower affinity for oxygen than human haemoglobin, which means it associates less readily with oxygen and dissociates more readily. This means that plenty of oxygen is released into the tissues to sustain the high rate of aerobic respiration that is required to keep the hummingbird hovering.

Question 25

Why do multicellular organisms need internal transport systems?

Answer 25

This is because, compared to single-celled organisms: Multicellular organisms are larger, so the diffusion distance across their surface would be too large. Multicellular organisms have a higher metabolic rate. Multicellular organisms need to supply nutrients and oxygen rapidly to a larger number of active cells.

Question 26

What type of circulatory system do mammals have? What does this mean?

Answer 26

Closed, double circulatory system It is a closed system - Blood is contained in blood vessels. It is a double system - Blood passes through heart twice per circuit.

Question 27

Name three features of a mammalian circulatory system?

Answer 27

1. Suitable medium - in mammals the transport medium is the blood. It is water based so substances can easily dissolve into it. 2. Means of moving the medium - animals often have a pump known as the heart to maintain pressure differences around the body. 3. Mechanism to control flow around the body - valves are used in veins to prevent any backflow.

Question 28

Blood vessels and thier functions What are the 4 types of blood vessel and which way do they carry blood?

Answer 28

1. Arteries - carry blood away from the heart and into arterioles 2. Arterioles- smaller arteries that control blood flow from arteries to capillaries 3. Capillaries - tiny vessels that link arterioles and veins] 4. Veins - carry blood from capillaries back to the heart

Question 29

Diagram of all layers of artery and vein on quizlet flashcard What are the different layers ?

Answer 29

Tough outer layer, muscle layer, elastic layer, thin inner lining( smooth to reduce friction and thin to allow diffusion) and lumen

Question 30

Function of tough outer layer?

Answer 30

Resists pressure changes from both within and outside

Question 31

Function of muscle layer

Answer 31

The function of the muscle layer is to contract to control the flow of blood.

Question 32

Function of the elastic layer?

Answer 32

The function of the elastic layer is to stretch and recoil to maintain blood pressure.

Question 33

Arteries transpoprt blood rapidly, under high pressure. How are they adapted for their function

Answer 33

Thick walls to withstand high pressure - Thicker muscle layer to control volume of blood passing through them - Thicker elastic layer - important that blood pressure in arteries is kept high so blood can reach everywhere in the body - No valves because blood is under constant high pressure so tends to not flow backwards

Question 34

Arterioles carry blood under lower pressure from arteries to capillaries How does their structure compare to arteries?

Answer 34

Muscle layer is thicker - needed for contraction of the lumen which restricts blood flow and so controls movement of blood into the capillaries. Thinner elastic layer because the blood pressure is lower

Question 35

Veins transport blood slowly, under low pressure How are they adaoated for their function?

Answer 35

Muscle layer is thin compared to arteries Overall thinner walls because there is low pressure and no risk of bursting. Also allows them to be flattened easily which aids the blood flow within them Thin elastic layer- pressure to low to create a recoil action Valves - Have valves which ensure blood does not flow backwards because pressure is low, so blood flows only in one direction

Question 36

How are capillaries' structure adaped to their function

Answer 36

Capillaries are numerous and highly branched. -they provide a large surface area for exchange. -Narrow lumen which reduces the diffusion pathway Walls are only one cell thick , so short diffusiion pathway

Question 37

What are the 4 parts of blood and their function?

Answer 37

Blood consists of: Plasma - Mostly water, transports substances in solution. Red blood cells - Carry oxygen. White blood cells - Immune cells. Platelets - Involved in clotting.

Question 38

What is tissue fluid and why is it needed?

Answer 38

It is a watery liquid that contains glucose, amino acids, fatty acids, ions in solution and oxygen Tissye fluid supplies all of these substances to the tissues

Question 39

What does tissure fluid receive back from tissues and overall what is its function?

Answer 39

Receives carbon dioxide and other wasteb materials from the tissues. It is the means by which materials are exchanged between blood and cells - it bathes all the cells of the body - it is the immediate environment of cells

Question 40

What is tissue fluid formed by?

Answer 40

Formed from blood plasma

Question 41

What is composition of blood plasma controlled by?

Answer 41

Homeostatic systems

Question 42

How is tissue fluid formed? Why does the hydrostatic pressure vary like it does?

Answer 42

Blood passes through the arteries, the narrower artieroles and then the even narrower capillaries- through increasingly small vessels. This creates a high hydrostatic pressure at the arterial end of the capillary. This causes tissue fluid to be forced out of the blood plasma(at the arterial end) - only small molecules are forced out because pressure is only enough for small molecules. This leaves tissue fluid that bathes the cells

Question 43

What are the two forces that oppose the outward hydrostatic pressure?

Answer 43

The hydrostatic pressure of the tissue fluid outside of the coillaries which resists outward movement of liquids - The lower water potential of the blood due to the plasma proteins that causes water to move back into the blood within the capillaries

Question 44

What is the process of filtration under pressure called?

Answer 44

Ultrafiltration

Question 45

Describe the process of the return of tissue fluid to the circulatory system/capillaries.

Answer 45

At the venous/venule end of the capillaries the loss of fluid, combined with the remaining plasma proteins, reduces the water potential and hydrostatic pressure. The loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside them. At the venous end of capillary network, the hydrostatic pressure is lower than the tissue fluid outside it The water potential is lower in capillaries than in tissue fluid due to fluid loss. This allows tissue fluid to return to the blood as a result of osmosis and external hydrostatic pressure.

Question 46

Not all tissue fluid can return to the capillaries, the rest is carried back to the blood by the lymphatic system 1.What is the lymphatic system? 2. What are the contents of the lymphatic system called? 3. Lymph are not moved by the pumping of the heart What are the two ways they are moved?

Answer 46

1. A system of vessels that begin in the tissues and gradually merge into larger vessels and form a network around the body These larger vessels drain the tissue fluid back into the bloodstream via two ducts that join veins close to the heart 2. Lymph 3. Contraction of body musckes - squeeze the lymph vessels, fluid moves away from tissues in the direction of the heart Also hydrostatic pressure of the tissue fluid that has left the capillaries

Question 47

Strcuture of the heart - labelled diagram on quizlet- atrium on top, ventricle on bottom Between each atrium and ventricle are atrioventricular valves that prevent that backflow of blood back into the atria. Another name for: 1 Right atroventricular valve 2. Left atrioventricular valve Semi lunar valves are those found in the pulmonary artery on the right and aorta on the left

Answer 47

1. Tricuspid valve 2. Bicuspid valve

Question 48

Order of oxygenated blood flow Pulmonary vein and aorta transport it - found on left of heart

Answer 48

Blood becomes oxygenated in the lungs. It moves from the lungs to the heart via the pulmonary vein. into left atrium,biscuspid valve, left ventricle, semi lunar valve It moves from the heart to the rest of the body via the aorta.

Question 49

Pulmonary artery and vena cava transport dexoygenated blood from body - heart - to lungs What side found on?

Answer 49

Right

Question 50

Which side of the heart contains which type of blood?

Answer 50

The left side of the heart contains oxygenated blood, and the right side contains deoxygenated blood. This forms two separate pumping mechanisms.

Question 51

What are the four blood vessels in the heart, their function on what chamber they are connected to?

Answer 51

These blood vessels are: The pulmonary vein - This moves oxygenated blood into the left atrium from the lungs. The aorta - This moves oxygenated blood from the left ventricle to the body. The vena cava - This moves deoxygenated blood into the right atrium from the body. The pulmonary artery - This moves deoxygenated blood from the right ventricle to the lungs.

Question 52

How does structure of atrium vary to ventricle? A is always linked to V Atrium receive blood from veins Ventricles- pumps bloods into arteries

Answer 52

The ventricles have thicker walls with more muscle than the atria. This is because: The atria only need enough pressure to pump blood a short distance into the ventricles.- have thin walled and elastic as it stretches as it collects blood The ventricles need a lot of pressure to pump blood a long distance out of the heart to other organs - has to contract strongly tp pump blood

Question 53

What is a double circulatory system?

Answer 53

Blood passes through the heart twice for each cycle of the body

Question 54

Which ventricle has a thicker more muscular wall and why?

Answer 54

The left ventricle has a thicker wall with more muscle than the wall of the right ventricle. This is because: The right ventricle only needs enough pressure to pump deoxygenated blood a short distance to the lungs. The left ventricle needs a lot of pressure to pump oxygenated blood to the other more distant organs of the body.

Question 55

Order of blood flow

Answer 55

On a quizlet flashcard- diagram

Question 56

What is the role of the coronary artery? What happens if it gets blocked? Another name for a heart attack?

Answer 56

supply the heart with blood If the coronary arteries get blocked then less blood can flow through them. As a result, less oxygen reaches cells in muscle tissue so less respiration 3. Myocardial infarction

Question 57

The cardiac cycle - two phases?

Answer 57

Contraction(systole)- atrial and ventricular systole and relaxation(diastole)

Question 57

1st stage Diastole - relaxation What happens during diastole? Why do A.V valves open?

Answer 57

Both atria and ventricles are relaxed. Blood returns to atria of heart through pulmonary vein(from the lungs) and the vena cava (from the body) As the atria fill, the pressure rises so A.V valves open so blood does not flow backwards. When the pressure of the blood in the atria is greater than the ventricles, theatrioventricularvalves open. S.L valves are closed

Question 58

2nd stage- Atrial systole What happens during atrial systole - to volume and pressure? Both atrioventricular valves are open, semi-lunar are closer

Answer 58

The atria contract, ventricles remain relaxed As the atria contract, this causes the volume of the atria to increase, and therefore causes the pressure of the atria to increase, which pushes blood from the atria to the ventricles

Question 59

3rd stage- ventricular systole What happens? Which valves are open/closed? When do semi lunar valves open?

Answer 59

Ventricles fill with blood When pressure in ventricles>than that in atria, the atrioventricular valves close to prevent the backflow of blood into the atria Ventricles contract, which decreases volume and increases pressure in the ventricles, allows blood to be pumped up into aorta and pulmonary artery When the pressure in the ventricles is greater than the pressure in the blood vessels, the semi-lunar valves open.

Question 60

What happens when pressure in the blood vessels is greater than the pressure in the ventricles?

Answer 60

Semi-lunar valves close and atrioventricular valve open- marks the start of diastole

Question 61

Describe how pressure changes bring about the opening and closing of valves during the cardiac cycle.

Answer 61

When the pressure of the blood in the atria is greater than in the ventricles, the atrioventricular valves open. This allows blood to start entering the ventricles. In atrial systole, the atria contract. This causes the volume of the atria to decrease and the pressure in the atria to increase. This pushes blood out of the atria and into the ventricles. In ventricular systole, the ventricles fill with blood. When the pressure in the ventricles is greater than the pressure in the atria, the atrioventricular valves close. Next, the ventricles contract. When this happens, the volume in the ventricles decreases and the pressure of the blood increases. When the pressure of the ventricles is greater than the pressure in the blood vessels, the semi-lunar valves open So, blood is pushed out of the heart. Finally, when the pressure in the blood vessels is greater than the pressure in the ventricles, the semi-lunar valves close.

Question 62

What is cardiac output? measured in what unit?

Answer 62

The volume of blood pumped by the heart in one minute dm^3/min stroke volume x heart rate= cardiac output

Question 63

What is stroke volume? What is heart rate?

Answer 63

1. Volume of blood pumped in one heart beat - average 75cm^3 2. the rate at which the heart beats - average -70 bpm

Question 64

What is transpiration? Where does gas exchange in plants take place?

Answer 64

The evaporation of water from leaves - out of stomata down a water potential gradient The stomata

Question 65

What does xylem transport and in what direction?

Answer 65

Xylem tissue transports water and mineral ions around plants. Wter enters roots by osmosis at root hair cells and then oasses into the root cortex into the xylem where it is transported UP the plant.

Question 66

Structure of xylem cells and adaptations

Answer 66

Xylem vessels have no end walls, made from dead cells joined end to end Adaptations of xylem vessels: They are elongated, hollow tubes without end walls. They lack organelles. Their walls are thickened with lignin for support. They have non-lignified pits that allow movement of water and ions into and out of vessels.

Question 67

How are root hair cells efficient for gas exchange of water and minerals?

Answer 67

- large surface area - thin surface layer - soil has a much higher water potential than root cells

Question 68

What are the 4 factors affecting rate of transpiration and how does it work?

Answer 68

1. Light - more light means more stomata are open - stomata open for maximum CO2 absorption for photosynthesis, increasing the transpiration rate. 2.Temperature - increasing temp increases the kinetic energy of water molecules, so evaporation of water is faster -> faster rate of transpiration 3. Humidity - the lower humidity, faster rate of transpiration - Low humidity increases the water potential gradient between the leaf and atmosphere, increasing the transpiration rate. 4. More wind=faster rate of transpiration - higher wind speeds aid blowing away of the water molecules from the stomata which increases the water potential gradient between the leaf and atmosphere, increasing the transpiration rate.

Question 69

What are the 2 roots by which water enters the roots and how do they work?

Answer 69

1. Apoplast pathway - through the cell wall - as water is drawn in it pulls more water behind it, creates a tension that draws water along the cell wall of the root cortex, goes as far as the endodermis, cell walls have many water filled spaces so tgere is little resistance to this pull of water along the cell walls 2. Symplast pathway - through the cytoplasm

Question 70

Why does water move out of the stomata?

Answer 70

Atmosphere is less humid than the air spaces next to the stomata - so there is a water potential gradient and water moleucles diffuse out into the surround air . Water lost by diffusion is replaced by water evaporating from the cell walls of the surrounding mesophyll cells

Question 71

What is the theory responsible for movement of water up the xylem

Answer 71

Cohesion-tension theory

Question 72

What is cohesion?

Answer 72

Water molecules form hydrogen bonds between one another and this makes water 'sticky'. Hydrogen bonding causes water molecules to stick together and move as one continuous column.

Question 73

What is adhesion and what does it do?

Answer 73

Adhesion - Hydrogen bonding between polar water molecules and non-polar cellulose in xylem vessel walls pulls water upwards through the xylem.

Question 74

What is transpiration pul?

Answer 74

A column of water is pulled up the xylem as a result of transpirtation - this is the transpirstion oull Transpiration pull - Evaporation of water at leaves creates the transpiration pull, and this tension is transmitted down the whole water column due to cohesion.

Question 75

Describe the cohesion-tension theory step by step

Answer 75

As water is lost from the leaves it is replaced by water from the xylem. Loss of water, lowers water potential in mesophyll cells As water is removed from the xylem, more water molecules are 'pulled up' to replace it - transpiration pull - which puts the xylem under pressure

Question 76

Potometer - fully filled with water so there are no air bubbles.- Make more one certain questions on potometer What does it measure? How is rate of water uptake measured?

Answer 76

Measures rate of water uptake Measuring the distance an air bubble moves in a given time

Question 77

Steo by step using a potometer

Answer 77

Cut the shoot underwater at slant to increase the surface area for water uptake. Assemble the potometer with the shoot submerged in water. Keep the capillary tube end of the potometer submerged throughout the experiment. Check that the apparatus is airtight. Dry the leaves, and give the shoot time to acclimatise. Shut the tap, form an air bubble and record its position. Measure the distance the air bubble moves and the time taken. Change one variable at a time and keep everything else constant.

Question 78

What are the limitations of using a potometer for measuring transpiration?

Answer 78

- Plant roots are removed so the calculated rate doesnt account for the water uptake in the roots which a real plant has - Assumes that all of the plants water will be transpired, however in reality it is also used for processes like photosynthesis and keeping the plant turgid

Question 79

How does diamater of a tree trunk support the cohesion-tension theory?

Answer 79

Change of diamter of tree trunks according to the rate of transpiration During the night, diameter of trunk increases - little to no transpiration so there is less tension in xylem so diamter increases. During day, high transpiration, there is more tensions which causes trunk to shrink in diameter

Question 80

2 more pieces of evidence that support the cohesion tension theory?

Answer 80

- If a xylem vessel is broken and air enters, the tree can no longer draw up water because the continous colum of water is broken so water molcules can no longer stick together - - if a xylem vessel is broken, water does not leak out, which would happen if under pressure Instead air is drawn in as it is under tension

Question 81

What does the phloem transport? In which direction

Answer 81

Organic substances e.g sugars Both directions, UP and DOWN

Question 82

What is translocation?

Answer 82

The transport of substances from a source to a sink, taking place in the phloem. Process by which organic molcules are transported from one part of a plant to another

Question 83

What is a source and what is a sink?

Answer 83

Source - where solute is made or a stored e.g leaves Sink - where a solute is required due to low conc of that solute like the roots

Question 84

What do plants commonly transport carbohydrates as? Overall order of transport from source to sink

Answer 84

Sucrose Overall order Source to the companion cell into sieve tube elements, down sieve plates, down sieve tube elements into companion cell, into sink

Question 85

Describe the structure of the phloem

Answer 85

It is made up of sieve tube elements which are arranged end to end to form long tubes. They do not have a nucleus. Their end walls are perforated to form sieve plates which have holes to allow sucrose to pass through. Sieve tube elements are associated with companion cells. Companion cells contain nucleus

Question 86

What are companion cells responsible for?

Answer 86

Transporting sucrose from the source to the sieve tube elements and transporting sucrose from the sieve tube elements to the sink

Question 87

Describe translocation of sucrose in the phloem

Answer 87

Sucrose starts in a source cell, moves into a companion cell and moves into a sieve tube element. Sieve tube elements are responsible for transporting the sucrose solution throughout the plant. They contain sieve plates, which have holes the sucrose solution can travel through. There, the sucrose solution moves up or down through the sieve tube elements. When it gets to where it is needed, the sucrose solution moves into a second companion cell and then into the desired sink cell.

Question 88

Mass flow hypothesis What is the first phase, what happens and what processes are involved. What transport method is used and how does it work? hint hydorgen ions

Answer 88

Sucrose needs to be transported from companion cells to sieve tube elements. Hydrogen ions are required. Firstly, sucrose diffuses from source cell into companion cell by facilitated diffusion. Hydrogen ions are actively transported from the cytoplasm to the cell wall using ATP. - This is increases conc gradient between the cell wall and cytoplasm. Hydorgen ions then diffuse into the sieve tube elements through carrier proteins, as well as sucrose molecules with is This is co-transport - the carrier protein transports two substances at once - sucrose and hydrogen ions. Each hydrogen ion brings one sucrose molecule JUST KNOW IT USES ACTIVE TRANSPORT SO REQUIRES ATP FOR ENERGY. SUCROSE IS TRANSPORTED INTO SIEVE TUBES BY ACTIVE TRANSPORT AND SUCROSE IS ACTIVELY TRANSPORTED BY COMPANION CELLS OUT OF SIEVE TUBES INTO SINK CELLS

Question 89

What is hydrostatic pressure?

Answer 89

Pressure exerted by fluids

Question 90

Phase 2 - Mass flow of sucrose through sieve tube elements. Explain fully the process, how water potential and hydrostatic pressure works. Explain how source and sink end differ

Answer 90

As sucrose enters the sieve tubes, this lowers their water potential - has more solutes in it. Because the xylem has a much higher water potential, water moves into the sieve tube element by osmosis which creates a high hydrostatic pressure inside the sieve tube elements. So there is a high hydrostatic pressure by the source At the sink end, the sucrose moves from sieve tube element into the companion cell and then the sink cell. The movement of sucrose from the sieve tube element into the companion cell increases the water potential in the sieve tube element. This means water leaves the sieve tubes by osmosis into the xylem or companion cells, which lowers the hydrostatic pressure in the sieve tubes by the sink. So between the source and the sink end there is a hydrostatic pressure gradient, which pushes solutes along the sieve tube towards the sink - from the area of high to low hydrostatic pressure. There is a MASS FLOW of sucrose solution

Question 91

Explain experiment that shows hydrostatic pressure- mass flow hypothesis

Answer 91

In the model, we have two containers with partially permeable membranes: A with a high concentration of sucrose, and Bwith a lower concentration. Both containers are surrounded by water. This means that container A has a lower waterpotential than the surrounding water. As a result, water moves into container A by osmosis. Then, the sucrose solution is forced to move down the hydrostatic pressure gradient into container B .This, in turn, forces water out of container B .

Question 92

Ringing experiments Explain this and what was observed

Answer 92

Proves translocation occurs in the phloem A section of the outer layers of a tree are removed - the bark and the phloem After a period of time, there is swelling above the ring of missing tissue. Some tissues below the ring, like the roots die but those above the ring stay alive

Question 93

Ringing experiment What do these observations suggest?

Answer 93

The swelling shows sugars of the phloem accumulating above the ring. Because phloem is cut, sugars cannot reach the roots so they died, phloem needed Everything above stays alive which shows they still have access to products of photosynthesis, there is phloem above the ring

Question 94

Tracer experiments Explain what is carried out and what is observed?

Answer 94

Provide evidence that translocation occurs in the phloem A tracer (a radioactive isotope) e.g radioactive carbon is absorbed by the plants and can be identified using scanners - can track the movement of the tracer and it shows it moves through phloem. The radioactive isotopes can be incorporated into the sugars which then travel through phloem and can be traced. - showed it travels in phloem

Question 95

What are some other pieces of evidence supporting the mass flow hypothesis and how does it show it?

Answer 95

- When phloem is cut, sap(solution of organic molecules) flows out - shows phloem is under pressure - As rate of respiration increased, so did translocation - because more ATP produced which is needed for active transport of sucrose molecules into phloem -Companion cells have lots of mitochondria - site of aerobic respiration so more ATP produced

Question 96

Evidence against mass flow hypothesis