Section 3 - Chapter 7: Mass Transport - old

Question 1

What is haemoglobin

Answer 1

• Group of chemically similar molecules found in many different organisms
• A protein molecule with a quaternary structure that has evolved to make it as efficient at loading oxygen under a set of conditions and unloading under a different set of conditions

Question 2

What are the different structures of haemoglobin

Answer 2

• Primary Structure - sequence of amino acids in the four polypepeptide chains
• Secondary Structure - Each polypeptide chain is coiled into helix
• Tertiary Structure - Each polypeptide chain is folded in a precise shape- important to carry oxygen
• Quaternary Structure - All 4 chains are linked to form a spherical molecule. Each chain is associated with a haem group - contains a ferrous group (Fe2+). This can combine with a single Oxygen molecule. Each haemoglobin can carry 4O2

Question 3

What is unloading

Answer 3

• The process by which haemoglobin releases its oxygen.
• Happens in the tissue.

Question 4

What is loading

Answer 4

• Process by which haemoglobin binds with oxygen. Happens in the lungs

Question 5

What is the role of haemoglobin

Answer 5

• To transport oxygen. To be efficient at transporting it must
  
  • Readily associate with oxygen where gas exchange takes place
  • Readily dissociate from oxygen at respiring tissue
  • It changes affinity (chemical attraction) for oxygen under different conditions - it achieves this because the shape chnages in the presence of substances (CO2). New shape binds more loosely - releases O2

Question 6

How is oxygen measured

Answer 6

• pO₂ is a measure of oxygen concentration
• The greater the concentration of dissolved oxygen the higher the partial pressure
• As pO₂ increases haemoglobin’s affinity also increases and when pO₂ decreases oxyhaemoglobin unloads its oxygen

Question 7

Why are there different haemoglobins

Answer 7

• Shape of the molecule - each species produced a haemoglobin with a slightly different amino acid sequence
• The haemoglobin of each species has different tertiary and quaternary structure hence different oixygen binding properties.
• Depending on the structure, haemoglobin range from those with high affinity and those with a low affinity.

Question 8

What is an oxygen dissociation curve

Answer 8

• When haemoglobin is exposed to different partial pressures of oxygen, it doesnt bind to oxygen evenly
• The graph of the relationship between th saturation of haemoglobin with oxygen at any partial pressure

Question 9

What is the explanation of the oxygen dissociation curve

Answer 9

1. The shape of the haemoglobin makes it difficult for the oxygen to bind to 1 of 4 subunits because they are closely united. Therefore low o2 conc. Gradient of the curve is shallow
2. However the binding of the first molecule changes the quaternary structure, causing it to change shape. Change allows other subunits to bind to a O2 molecule
3. Therefore smaller increase in partial pressure of o2 to bind to the second oxygen molecule than the first one. Positive cooperativity. Easier binding. Gradient of curve steepens
4. After binding of third molecule. It is harder for the fourth to bind. With majority of sites occupied, less likely that single o2 molecule will find an empty site. The gradient reduces and graph flattens

Question 10

What are the two different facts about dissociation curves

Answer 10

• The further to the left - greater affinity of haemoglobin for oxygen (loads oxygen readily and unloads less easily)
• The further to the right - the lower the affinity of haemoglobin for oxygen (so it loads oxygen less readily and unloads more easily)

Question 11

What does carbon dioxide do to the affinity of haemoglobin

Answer 11

• Haemoglobin has a reduced affinity for oxygen in the presence of CO2.
  
  • Respiring tissues make CO2
  • Dissolved CO2 is acidic - lowers pH
  • Causes shape to change - causes reduced affinity - increases rate of unloading
  • Dissociation curve shifts to the right - The bohr effect.

Question 12

What are the different behaviours of haemoglobin in different areas of the body.

Answer 12

• At the gas exchange surface - conc of CO2 is low because it diffuses across the exchange surface and is exerted by the organism. Affinity increases - oxygen readily loaded. Reduced CO2 has shifted curve to the left
• In respiring tissue (muscle) the conc of CO2 is high. Affinity for haemoglobin is reduced. O2 is readily unloaded into muscle cells. Increased CO2 conc has shifted curve tonthe right.
• Greater Carbon dioxide concentration, the more readily haemoglobin releases O2

Question 13

What happens on the loading, transport and unloading of oxygen

Answer 13

• At the gas-exchange surface carbon dioxide is constantly being removed
• The pH is raised due to low conc of Co2
• The higher the pH changes the shape of haemoglobin enables to load oxygen readily - increases the affinity for oxygen so not released on transport
• In respiring cells - CO2 is produced. CO2 is acidic in solution so pH in blood in respiring tissue is lowered - lower affinity for oxygen
• Haemoglobin releases O2 into respiring tissue

Question 14

EXAM QUESTION: Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why?

Answer 14

1. Binding of first oxygen changes the tertiary/quaternary structure of haemoglobin
2. Creates/leads to/uncovers second/ another binding site OR uncovers another iron/ haem grouo to bind to

Question 15

Haemoglobin saturation in humans

Answer 15

• In humans, haemoglobin becomes saturated with oxygen passing through lungs. Not all haemoglobin are loaded with 4 O2
• When this haemoglobin reaches a tissue with low respiratory rate - only one of these molecules are released - blood returning (75% saturated with O2)
• Active tissue (muscle) 3 O2 molecules are unloaded

Question 16

What is the oxygen dissociation curve for a lugworm

Answer 16

• Animal that lives on the seashore
• Not very active. Most of the time covered in sea water
• There is a low concentration of oxygen - it has a high affinity for oxygen
• Dissociation curve is shifted to the left that of a human - means that haemoglobin of the lugworm is fully loaded with oxygen

Question 17

What is the oxygen dissociation curve for a hawk

Answer 17

• A hawk has a high respiratory rate (active) and high oxygen demand and lives where there is plenty of oxygen.
• Its haemoglobin must unload oxygen quickly for activity
• Low affinity for oxygen
• Curve is shifted to the right of a human

Question 18

What is the oxygen dissociation curve for a rat

Answer 18

• A rat has a higher surface area to volume ratio than a human. Lose heat more quickly - high metabolic rate to keep warm - higher oxygen demand
• Its haemoglobin needs to unload oxygen easily to meet the greater oxygen demand
• Has a lower affinity for oxygen - graph shifted to the right

Question 19

What does the s-shaped dissociation curve mean?

Answer 19

• At very low oxygen concentrations it is hard for Hb to take up oxygen initially, but as it starts to load, it continues to load very quickly
• Very efficient unloading - a small drop of oxygen results in very rapid unloading of oxygen from HB

Question 20

Why do LARGE organisms need a mass transport system?

Answer 20

• All organisms need to exchange materials with their environment
• Some exchange via their body surface (small SA to vol ratio)
• The more active an organism is and the larger it is (small SA: vol) the need for a mass transport with a pump.
• Specialist exchange surfaces are required to absorb nutrients and respiratory gases and remove excretory products

Question 21

What are the features of a transport system

Answer 21

• Sutiable medium to carry the materials - e.g blood/air liquid based cause it readily dissolves substances
• A form of mass transport system - moved over large distances
• Closed system of vessels containing transport medium - forms branching network to distribute e.g vessels, arteries
• A pump (heart)/ passive process (evaporation) or mechanism that transports medium within vessels - requires a pressure difference
• Valves - ensure one way flow
• Control the flow - suit changing needs e.g change in heart rate

Question 22

What is the circulatory system in mammals

Answer 22

• Mammals have a closed, double circulatory system in which blood is confined to vessels and passes twice through the heart for each complete circuit
• Because - blood passing into lungs (pressure reduced) - if it passes into body - circulation = slow.
• Done so substances can be delivered to the rest of the body quickly - important mammals have high body temperature (high metabolism)

Question 23

How many pumps is the heart made of and what are the names of the chambers in each pump

Answer 23

• 2 separate pumps lying side by side. Left = oxygenated blood from lungs, Right = deoxygenated blood from the body
• Each pump has 2 chambers
  
  • The atrium is thin walled and elastic and stretches as it collects blood
  • The ventricle has a thicker muscular wall and contracts strongly to pump blood some distance either lungs or body

Question 24

Why are there 2 separate pumps in the heart

Answer 24

• Blood has to pass through tiny capillaries in the lungs to present a large surface area for the exchange of gases.
• Therefore a large drop in pressure and so blood flow to the rest of the body is slow
• Therefore mammals have a system in which blood is returned to the heart to increase pressure before it is distributed

Question 25

Where does the right ventricle pump blood to

Answer 25

• Only to lungs
• Has a thinner muscular wall than left ventricle
• Left ventricle has thicker walls enabling it to contract to create enough pressure to pump blood to the rest of the body

Question 26

What are the names of the valves that are between the atrium and ventricle

Answer 26

• The left atrioventricular (bicuspid) valve - found between left ventricle and left atrium
• The right atrioventricular (tricuspid) valve - found between right ventricle and right atria
• Prevent backflow of blood into the atria when ventricles contract

Question 27

Where do the following carry blood from and to:

1. Aorta
2. Vena cava
3. Pulmonary artery
4. Pulmonary vein
5. Renal artery
6. Renal vein

Answer 27

1. Aorta - connected to left ventricle and carries oxygenated blood from heart to body
2. Vena cava - connected to right atrium and carriesd deoxygenated blood from body to heart
3. Pulmonary artery - connected to right ventricle and carries deoxygenated blood from heart to lungs
4. Pulmonary vein - connected to left atrium and brings oxygenated blood from lungs to heart
5. Renal artery - body to kidneys
6. Renal vein - kidneys to vena cava (heart)

Question 28

What are the name of the blood vessels that supply the heart with oxygen

Answer 28

• Heart muscle is supplied with own blood vessels called the coronary arteries - branch off aorta ahortly after it leaves the heart
• Blocking of these arteries leads to myocardial infarction or heart attack - cause an area of the heart is deprived of blood and oxygen. Muscle cells in the region can’t respire and therefore die

Question 29

What are the risk factors of cardiovascular disease - increase the risk of disease. About smoking

Answer 29

• Smoking
  
  • Carbon monoxide - combines irreversibly to haemoglobin in red blood cells forms carboxyhaemoglobin. Reducing oxygen carrying capacity. Heart works harder - raised blood pressure. Also insufficient in supplying oxygen to heart.
  • Nicotine

Question 30

What are the other risk factors of cardiovascular disease

Answer 30

• High blood pressure - caused by genes, stress, diet, no exercise. As there is a higher pressure in arteries - heart must work harder and weakening of the wall (aneursym) and burst. To resist higher pressure walls are thickened and harden and reduce blood flow
• Blood cholestrol - Cholestrol is essential in membranes. Two types: High density lipoproteins - remove cholestrol from tissues and transports to liver. Protects arteries and Low density lipoproteins transports cholestrol from liver to tissues - may lead to heart disease
• Diet - high levels of salt - raise blood pressure, high saturated fat - increases low density lipoproteins and blood cholestrol comcentration

Question 31

1. Where are the semi-lunar valves found and what do they do
2. Flow map to illustrate the blood journey as it travels from the heart to the lungs back to the heart and then the body then back to the heart

Answer 31

1. Are found in aorta and pulmonary artery. Prevent backflow of blood into and ventricles. This arises when elastic walls of the vessels recoil.
2. Pulmonary arteries - lungs - Pulmonary vein - left atrium - left atrioventricular valve - left ventricle - semi-lunar valve - aorta - body - vena cava - right atrium - right atrioventricular valve - right ventricle - semi lunar valve - pulmonary artery

Question 32

How do valves work

Answer 32

• The valves open one way
• Whether they are open or closed is relative to the pressure
• If there is a higher pressure behind the valve it is forced open
• Pressure in front of the valve is higher it is forced shut

Question 33

What are the different stages of the cardiac cycle

Answer 33

• Diastole
• Atrial Systole
• Ventricular systole

Question 34

What happens in the diastole stage in the cardiac cycle

Answer 34

• Blood returns to the atria of the heart through the pulmonary vein (lungs) or vena cava (body).
• As atria fill, pressure rises. When the pressure exceeds that in the ventricles, av valves open and allows blood to pass into ventricles.
• Ventricle and atria relax. Ventricle relaxing causes recoil and reduces pressure.
• Therefore pressure is lower than in aorta and pulmonary artery. So semi lunar valves close

Question 35

What happens in the atrial systole stage in the cardiac cycle

Answer 35

• The contraction of atrial walls along with recoil of relaxed ventricle walls forces remaining blood into ventricles from atria.
• Throughout this stage the muscle of the ventricle walls remains relaxed
• Pressure in ventricles increase

Question 36

What is the ventricular systole stage of the cardiac cycle

Answer 36

• After a short delay to allow ventricles to fill with blood, their walls contract simultaneously.
• This increases blood pressure within them
• Forces atrioventricular valves to close and preventing backflow into atria
• Atria relax and semilunar valves forced open

Question 37

What are pocket valves

Answer 37

• In veins that occur throughout the venous system.
• This ensures when veins are squeezed, blood flows back towards the heart than away

Question 38

What type of circulatory system do mammals have and why is it important

Answer 38

• Mammals have a closed circulatory system, blood is confined to vessels - allows to maintain pressure and be regulated

Question 39

What is cardiac output and what is the equation

Answer 39

• Cardiac output is the volume of blood pumped by one ventricle (heart) in one minute. Usually measured in dm3 min-1 and depends on 2 factors
  
  • Heart rate - rate at which heart beats per minute
  • Stroke volume - volume of blood pumped during each heartbeat (dm3 or cm3)
• Cardiac output = stroke volume x heart rate

Question 40

What are the pressure and volume changes and associated valve movements

Answer 40

Question 41

What are the different types of blood vessel

Answer 41

• Arteries - carry blood away from the heart and into arterioles
• Arterioles - are smaller arteries that control blood flow from arteries into capillaries
• Capillaries - are tiny vessels that link arterioles to veins
• Veins - carry blood from capillaries to the heart

Question 42

What are the basic layered structure of the arteries, arterioles and veins

Answer 42

• Tough fibrous outer layer - that resists pressure changes from within and outside
• Muscle layer - that can contract and control blood flow
• Elastic layer - That helps maintain blood pressure by streching and springing (recoiling)
• Thin inner lining (endothelium) - that is smooth to reduce friction and thin to allow diffusion
• Lumen - central cavity of the blood vessel , blood flows

Question 43

What is the artery structure related to its function

Answer 43

The function of the arteries is to transport blood rapidly from heart to the tissues

• The muscle layer is thick compared to veins - smaller arteries can be constricted/dilated to control blood flow
• The elastic layer is thicker than veins - Important that blood pressure is kept high if is to reach extremeties of body. Streching and recoiling helps maintain pressure
• The overall thickness of walls - resists bursting in pressure
• No valves - blood is constantly in high pressure due to heart pumping blood and tends not to flow back

Question 44

What is the arteriole structure related to function

Answer 44

Arterioles carry blood under lower pressure than arteries from arteries to capillaries. They control flow of blood between them.

• The muscle layer is relatively thicker than in arteries - The contraction of the muscle layer allows constriction. This restricts flow and controls movement into capillaries
• The elastic layer is relatively thinner than in arteries - because blood pressure is lower

Question 45

What is the vein structure related to its function

Answer 45

Vein transport blood slowly under low pressure from the capillaries in tissues to the heart

• The muscle layer is relatively thin compared to arteries - veins carry blood away from tissues and therefore constriction/dilation cant control blood flow to tissues
• The elastic layer is relatively thin - lower pressure of blood within veins wont cause them to burst/ pressure is too low to recoil
• The overall thickness of the wall is small - no need. Pressure = too low, allows them to be flattened easily allowing flow of blood within them
• There are valves - ensures blood doesnt flow backwards

Question 46

What is the structure of capillary related to its function

Answer 46

Is to exchange metabolic materials (oxygen, carbon dioxide, glucose) between blood and cells of the body. Blood flow is slower to allow exchange

• Their walls consist mostly of lining - extremely thin, so diffusion distance is short - rapid diffusion
• Numerous and highly branched - large surface area
• Narrow diameter - so permeate tissues, no cell is far from a capillary and short diffusion pathway
• Lumen is narrow - red blood cells are squeezed flat against capillary wall. Reduces diffusion distance
• Spaces between the lining (endothelial) cells - allow white blood cells to escape in order to deal with infections

Question 47

What is tissue fluid

Answer 47

• A watery liquid that contains glucose, amino acids, fatty acids, ions and oxygen. (small molecules
• Tissue fluid supplies all of these substances to the tissues and in return recieves carbon dioxide and waste materials
• Subsatances move in and out of capillaries into tissue fluid by pressure filtration
• It is formed from blood plasma, and the composition of blood plasma is controlled by homeostatic systems.
• As a result tissue fluid provides a constant environment for cells it surrounds.

Question 48

Exam Q) How is tissue fluid formed

Answer 48

• Hydrostatic pressure of blood high at arterial end
• Fluid/water/soluble molecules pass out
• Proteins/ large molecules remain
• This lowers the WP and water moves back into the venous end of the capillary by osmosis
• Lymph system collects any excess tissue fluid which returns to blood/circulatory system and returns tissue fluid to vein

Question 49

Return of tissue fluid to the circulatory system

Answer 49

Once tissue fluid has exchanged metabolic materials. It returns to the circulatory system. Most TF returns to the blood plasma via capillaries:

• The loss of TF from the capillaries reduces HP inside them
• As a result, by the time blood reaches venous end, HP is lower than that of the tissue fluid outside it
• Therefore tissue fluid is forced back into capillaries by the higher HP outside them
• Plasma has lost water and still contains proteins - lower WP than TF
• Water leaves the tissue by osmosis down a water potential gradient

Question 50

What happens to the tissue fluid that can’t be returned to the capillaries

Answer 50

• The remainder is carried back via the lymphatic system.
• This is a system of vessels that begin in the tissues.
• Initially, resemble capillaries (except have dead ends) but merge into a larger vessel that forms a network throughout the body
• These larger vessels drain their contents back into the bloodstream via 2 ducts that join veins close to the heart

Question 51

The contents of the lymphatic system (lymph) are not moved by the pumping of the heart. Instead they are moved by:

Answer 51

• 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

Question 52

In plants what is water transported in

Answer 52

• Through hollow, thick-walled tubes called xylem vessels
• Transports water and mineral ions up the plant
• The main force that pulls water through the xylem vessel is evaporation (transpiration) from leaves
• Energy is supplied by the sun (passive)

Question 53

Movement of water out through the stomata

Answer 53

• The humididty of the atmosphere is less that the air spaces next to stomata
• There is a water potential gradient from air spaces to the air in atmosphere
• When stomata are open, water diffuse out of air spaces into surrounding air
• Water lost is replaced by water evaporating from the cell walls of surrounding mesophyll cells

Question 54

Movement of water across the cells of a leaf

Answer 54

• Water is lost from mesophyll cells by evaporation from their cell walls
• This is replaced by water reaching the mesophyll cells from the xylem via cell walls or cytoplasm
  
  • Mesophyll cells lose water to the air spaces due to evaporation
  • The cells have a lower WP and so water enters by osmosis from neighbouring cells
  • The loss of water from neighbouring cells lowers WP
• In this way, WP gradient is established that pulls water up xylem, across leaf mesophyll and into atmosphere

Question 55

Movement of water up the stem in the xylem

Answer 55

• The main factor that is responsible for the movement of water up the xylem from the roots to leaves is cohesion tension
  
  • Water evaporates from mesophyll cells due to transpiration
  • Water molecules form hydrogen bonds between one another and stick together - cohesion
  • Water forms a continuous, unbroken column across the mesophyll cells and down the xylem
  • As water evaporates, more molecules are drawn up due to cohesion
  • A column of water is therefore pulled up xylem due to transpiration - transpiration pull
  • Transpiration pull puts the xylem under tension, there is a negative pressure. Cohesion tension theory

Question 56

What is the supporting evidence for the cohesion tension theory

Answer 56

• Hug a tree - Change in diameter of tree trunk according to the rate of transpiration. When transpiration rate is high there is more tension. This pulls xylem vessels inwards and causes the tree to shrink. At night transpiration is low - less tension - diameter of the trunk increases
• Break in xylem vessel - if a xylem vessel is broken and air enters it, the tree can no longer draw up water. Continous column is broken and water molecules can no longer stick.
• When a xylem vessel is broken, water doesn’t leak out as would happen if it were under pressure. Instead air is drawn in - under tension

Question 57

What is transpiration pull

Answer 57

• Transpiration pull is a passive process and therefore does not require metabolic energy to take place
• The xylem vessels through which water passes are dead and can’t actively ,move the water.
• Xylem vessels have no end walls - xylem forms a series of continous, unbroken tubes which is essential for cohesion tension theory
• Energy is in the form of heat that evaporates from the leaves and comes from teh sun.

Question 58

A plant is sprayed with herbicide - why does this have no effect on the water flow initially

Answer 58

• Water flow doesnt require energy (passive)
• Xylem is non-living
• Water flow would continue through killed cells for a while

Question 59

What are the problems and benefits of transpiration

Answer 59

• Water loss - less than 1% of the water moved in the transpiration stream is used by the plant
• Benefits
  
  • Cools plant
  • Mineral ions, sugars, hormones are moved with it (without transport would be slow)

Question 60

What are the factors affecting transpiration rate

Answer 60

• Increase light - Stomata open - water moves from leaf to atmosphere - Increase transpiration rate
• Increase temperature - Kinetic energy increases - Water molecule move faster - Increases evaporation rate from the leaf surface - Increase transpiration rate
• Increase humidity (more water in air) - Less steep WP gradient between leaf and surrounding atmosphere - Reduced transpiration rate
• Increase air movement - Water naturally accumulates around stomata - Air movement disperses this humid layer - Gradient between leaf and air is higher - Transpiration rate increases

Question 61

What can the rate of water loss in a plant be measured by and what is the experiment

Answer 61

A potometer

• A leafy shoot is cut under water - prevents air from entering the xylem. Cut at a slant to increase surface area
• Assemble potometer under water and insert the shoot using a rubber tube so no air enter in
• Remove the apparatus from the water and all joints are selaed wityh waterproof jelly. Make sure its water and air tight
• An air bubble is introduced into capillary tube
• Distance moved by air bubble in a given time is measured and mean calculated
• The experiment can be repeated to compare rates of water uptake under different conditions

Question 62

What is the process by which organic molecules are transported called

Answer 62

• Translocation
• The tissue that transports biological molecules is called phloem.
  
  • Phloem is made of sieve tube elements, long thin structures arranged end to end. Their end walls are perforated to form sieve plates
  • Associated with sieve tube elements are companion cells

Question 63

What is the name of the site where photosynthesis occurs to produce sugars and what is the name of the site where sugars are used or stored called

Answer 63

• Sources
• Sinks
• Translocation can occur in either direction
• Organic molecules to be transported include sucrose and amino acids or inorganic substances like magnesium ions
• Precise mechanism of translocation is unclear but current theory is mass flow theory.
• Solutes can’t be moving by diffusion as it would be too slow

Question 64

What are the three phases of the mass flow theory

Answer 64

1. Transfer of sucrose into sieve tube elements from photosynthsising tissue
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from the sieve tube elements into storage or sink cells

Question 65

What is the first phase of mass flow theory and what happens

Answer 65

1. Transfer of sucrose into sieve elements from photosynthesising tissue

• Sucrose is made by photosynthesis
• This diffuses down a concentration gradient by facilitated diffusion from the photosynthsising cells to comapanion cells
• Hydrogen ions are actively transported from companion cells into the spaces within cell walls using ATP
• The hydrogen ions diffuse down the concentration gradient through carrier proteins into the sieve tube elements
• Sucrose molecules are transported along with the hydrogen ions in co-transport.

Question 66

What is the second phase of mass flow theory and what happens

Answer 66

Mass flow of sucrose through sieve tube elements

• Mass flow is the bulk movement of a substance. Mass flow of sucrose through sieve tube elements takes place as follows:

1. The sucrose produced is actively transported into sieve tubes
2. This causes sieve tubes to have a lower water potential
3. As the xylem has a much higher water potential water moves into sieve tubes by osmosis creating a high hydrostatic pressure at the source end.
4. Respiring cells (sink) have low sucrose content and sucrose is actively transported into them from the sieve tubes lowering their WP
5. Water then also moves in by osmosis
6. The HP of the sieve tubes is lowered - results in high HP at source and low at sink
7. There is therefore a mass flow of sucrose solution down the hydrostatic gradient in the sieve tubes.

Question 67

What is the third phase of mass flow theory and what happens

Answer 67

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

1. The sucrose is actively transported by companion cells out of the sieve tube elements into the sink cells
   * Mass flow is passive. Occurs due to active transport of sugars

Question 68

What is evidence supporting mass flow hypothesis

Answer 68

• There is a pressure within sieve tubes, as shown by sap being released when they are cut
• The conc of sucrose is higher in leaves (source) than roots (sink)
• Downward flow in the phloem occurs in daylight
• Incr in sucrose levels in the leaf are followed by similar incr in sucrose levels in phloem
• Metabolic poisons/ lack of oxygen inhibits translocation
• Companion cells possess many mitochondria (readily produce ATP)

Question 69

What is the evidence questioning the mass flow theory

Answer 69

• The function of the sieve plates is unclear, they would hinder mass flow (may have structural function)
• Not all solutes have the same speed - they should do if they move by mass flow
• Sucrose is delivered at the same rate to all regions, rather than those with the lowest sucrose concentration

Question 70

What is the ringing experiement that supports the mass flow theory

Answer 70

• If a ring of bark (includes phloem and xylem) is removed from a woody stem, a buldge forms above the ring. The fluid has a higher concentration of sugars than the fluid from below the ring.
• This is because sugars can’t move past the area where the bark has been removed. Interruption of sugars below and death of tissues in the region
• This is evidence that there can be a downward flow of sugars. Phloem is used not xylem.

Question 71

How does the tracer experiement support the mass flow theory

Answer 71

• Radioactive isotopes are useful for tracking movement of substances in plants. (C14)
• If the plant is grown in an atmosphere containg 14CO2. The isotope will be incorporated into sugars produced
• These sugars can be traced as they move within the plant using autoradiography.
• Involves taking thin cross sections of the plant stem and using x-ray. Blackened regions = correspond to where phloem tissue in the stem. Phloem alone is respinsible for translocation

Question 72

More evidence of translocation of organic molecules in the phloem

Answer 72

1. When phloem is cut, a solution of organic molecules flow out
2. Plants provided with radioactive carbon have radioactively labelled carbon in phloem
3. Pressure in the phloem can be investigated using aphids (they pierce the phloem, then their bodies leave mouthparts behind that allow sap to flow out) Their sap flows quicker nearer the leaves then down the stem (evidence of a pressure gradient).
4. If a metabolic inhibitor is used. Translocation stops - evidence that active transport is involved.

Question 73

EXAM QUESTION: Suggest and explain one other way in which sieve cells are adapted for mass transport

Answer 73

• No nucleus
• No/few organelles/ little cytoplasm/ more room
• Thick strong walls resist pressure

Question 74

EXAM QUESTION: Suggest and explain one way in which companion cells are adapted for the transport of sugars between cells

Answer 74

• Companion cells have mitochondria that can produce ATP and provide energy for Active Transport. Release energy.
• Ribosomes produce proteins linked to tranport