Surface Area to Volume Ratio and Gas Exchange

Question 1

what do organisms need to exchange with its environment?

Answer 1

nutrients, respiratory gases (eg oxygen and carbon dioxide), heat and excretory products (eg urea)

Question 2

what happens to an organisms’ sa:v ratio if it gets larger?

Answer 2

the sa:v ratio gets smaller

Question 3

which process do single-celled organisms use for gas exchange, and how?

Answer 3

they use diffusion to exchange oxygen and carbon dioxide down their concentration gradients.
they have a large enough sa:v ratio to meet their gas exchange needs with simple diffusion across their surface

Question 4

why are multi-cellular organisms unable to exchange gases with their environment via diffusion?

Answer 4

their sa:v ratio is too big, which however is an advantage in terms of heat loss as the heat won’t leave the body quickly due to slow diffusion rate

Question 5

what adaptations do small animals have to keep them warm?

Answer 5

high rate of respiration which releases heat energy

Question 6

what is fick’s law, and therefore, what factors make a good exchange surface?

Answer 6

(surface area x concentration gradient) / diffusion distance

large surface are and concentration gradient, and short diffusion distance

Question 7

what features do insects have to reduce water loss?

Answer 7

1) waterproof covering over body surfaces- usually rigid exoskeleton covered by waterproof cuticle
2) small sa:v ratio to minimise the area over which water is lost
3) closing spiracles
4) hairs around spiracles- trap water vapour and prevent water leaving

Question 8

label this structure of an insect’s respiratory system + functions

Answer 8

1) spiracles- tiny pores which gas leaves + enters- open + close to control water loss by evaporation
2) tracheae- network of tubes supported by strengthened rings of chitin
3) tracheoles- small tubes which extend through body tissues- where gas exchange happens
4) muscle cells- every insect cell is close to a tracheole or tracheae- short diffusion pathway

Question 9

how does an oxygen concentration gradient form in an insect’s tracheal system?

Answer 9

1) cells aerobically respire using oxygen, which lowers oxygen concentration in cell
2) oxygen diffuses from a high conc in the tracheae to low conc in the cell
3) this lowers the oxygen conc in the tracheae so oxygen diffuses into the tracheae from outside the insect via the spiracles

Question 10

how does a carbon dioxide concentration gradient form in an insect’s tracheal system?

Answer 10

1) aerobic respiration produces co2, increasing the conc in the cell
2) co2 diffuses from a high conc in the cell to a low conc in the tracheae
3) co2 then moves from a high conc in the tracheae to a low conc outside the insect via spiracles

Question 11

what does the movement of insects induce?

Answer 11

it creates a mass movement of air in and out of the tracheae, speeding up gas exchange

Question 12

at rest, why is water present in the ends of tracheoles?

Answer 12

high water potential in tissue cells than the tracheoles- water moves into the tracheoles by osmosis

Question 13

why does water move into the muscle cells and how does this increase gas exchange rate?

Answer 13

1) insect is active and there is a high rate of respiration, so muscle cells produce lactic acid
2) lactic acid lower muscle cells’ water potential lower than tracheoles’
3) water moves out of tracheole into muscle cells by osmosis
4) less water in tracheole so larger surface area for gas exchange and faster diffusion of air

Question 14

describe the flow of water over the gills

Answer 14

1) mouth opens and operculum shuts
2) mouth volume increases and pressure decreases
3) water moves in down pressure gradient
4) mouth closes and operculum opens
5) mouth volume decreases and pressure increases
6) water forced out over gills down pressure gradient

Question 15

what organ in fishes allows for gases to be exchanged with water?

Answer 15

gills

Question 16

label the structure of a fish’s gas exchange system

Answer 16

Question 17

when a fish swims through water, in what direction does the water flow over the gills compared to blood flow direction, and what is this called?

Answer 17

the blood and water flow in opposite directions- called countercurrent flow

Question 18

how are gills adapted for efficient gas exchange?

Answer 18

1) gill filaments- increase sa
2) many lamellae on filaments- increase sa
3) lamellae contain many capillaries + thin epithelium - short diffusion distance between water and blood

Question 19

how does counter current flow ensure maximum amount of oxygen passes into blood flowing over gills?

Answer 19

1) water and blood flow in opposite directions which maintains the oxygen conc gradient
2) oxygen conc is always higher in water than blood
3) oxygen diffusion can happen along the whole lamellae length

Question 20

why is parallel flow less efficient for gas exchange than counter current flow?

Answer 20

there is a oxygen conc gradient from water to blood for only part of the lamellae length- only 1/2 of oxygen from water diffuses into blood- equilibrium is reached

Question 21

label this diagram of the lungs

Answer 21

Question 22

what is ventilation?

Answer 22

a sequence of breathing movements that moves gases to and from the internal gas exchange surface. during ventilation air always flows from a higher pressure to a lower pressure.

Question 23

describe inhalation and exhalation

Answer 23

1) external intercostal muscles and diaphragm contract
2) rib cage rises and diaphragm flattens
3) increase in thoracic cavity volume and decrease in pressure (below atmospheric)
4) air moves into lungs down pressure gradient

1) external intercostal muscles and diaphragm relax
2) rib cage falls + diaphragm rises
3) decrease in thoracic cavity volume and increase in pressure (above atmospheric)
4) air moves out of lungs down pressure gradient

Question 24

how is forced expiration carried out?

Answer 24

requires:
1) relaxation of external intercostal muscles and diaphragm
2) contraction of internal intercostal muscles

Question 25

how are mammalian lungs adapted for efficient gas exchange?

Answer 25

1) many alveoli- large sa
2) thin alveolar epithelium (one layer of squamous cells)- short diffusion distance
3) many capillaries surrounding alveoli- maintains large conc gradient
4) ventilation- maintains large conc gradient
5) thin capillary endothelium- short diffusion distance

Question 26

label the internal structure of a dicotyledonous leaf

Answer 26

Question 27

what is the advantage of guard cells opening and closing stomata?

Answer 27

to reduce water loss

Question 28

how are leaves adapted for gas exchange?

Answer 28

1) irregular, spongy mesophyll cells- large sa
2) mesophyll cells in contact with air spaces- short diffusion pathway
3) thin and flat- large sa:v ratio
4) spaces filled with air not water- diffusion occurs faster in gas not liquid
5) many stomata- allow air to move in an out

Question 29

describe diffusion of CO2 as a result of photosynthesis

Answer 29

1) mesophyll cells photsynthesis, using CO2, reducing CO2 conc in cells
2) CO2 diffuses from air spaces in cell down a conc gradient
3) this reduces CO2 conc in the air spaces causing CO2 to diffuse from air outside leaf into air spaces, down a conc gradient

Question 30

describe diffusion of O2 as a result of photosynthesis

Answer 30

1) mesophyll cells produce O2 in photosynthesis, increasing O2 conc in cells
2) O2 diffuses into air spaces from the cells down a conc gradient
3) this increases the O2 conc in the air spaces, causing O2 to diffuse from air spaces to outside the leaf down a conc gradient

Question 31

what are xerophytes?

Answer 31

plants that live in dry climates that have adapted to reduce water loss eg closing stomata at night

Question 32

label this diagram of a xerophytic plant leaf

Answer 32

Question 33

how are xerophytic plants adapted to reduce water loss?

Answer 33

1) reduced stomata number- reduced sa for evaporation
2) thick waxy cuticle- waterproof
3) leaves reduced to spines- small sa:v ratio
4) stomata sunk in pits
5) hairs to trap water
6) rolled leaves
4, 5 + 6- reduced water potential gradient by increasing humidity directly outside of stomata

Question 34

what is the definition of digestion?

Answer 34

the hydrolysis of large, insoluble biological molecules into small, soluble molecules

Question 35

label this diagram of the digestive system

Answer 35

Question 36

what is the function of the salivary glands?

Answer 36

secretes amylase-containing saliva, that hydrolyses starch into maltose

Question 37

what is the function of the oesophagus?

Answer 37

carries food from mouth to stomach by peristalsis, thick muscle walls

Question 38

what is the function of the stomach?

Answer 38

where food is mixed with acidic gastric juice killing microorganisms, also contains proteases which hydrolyse proteins into amino acids

Question 39

what is the function of the pancreas?

Answer 39

gland which secretes pancreatic juice containing amylase, exo and endopeptidases and lipases

Question 40

what is the function of the small intestine?

Answer 40

wall folded into villi made of epithelial cells which have microvilli to increase sa for rate of absorption. membranes also contain disaccharidases which hydrolyse disaccharides into monosaccharides.

Question 41

what is the function of the large intestine?

Answer 41

absorbs water from food turning the remains into faeces

Question 42

what is the function of the rectum?

Answer 42

stores faeces before periodical release via anus

Question 43

write a diagram showing how starch is hydrolysed

Answer 43

starch + water –> maltose + water –> glucose
I I
amylase maltase

Question 44

where are salivary amylase produced and where do they function?

Answer 44

produced in salivary glands, used in mouth

Question 45

where are endopeptidases produced and where do they function?

Answer 45

produced in the stomach/ pancreas, used in stomach/ lumen of ileum

Question 46

where are pancreatic amylase, lipase, exo and endopeptidases produced and where do they function?

Answer 46

produced in pancreas, used in lumen of ileum

Question 47

where are disaccharidases and dipeptidases produced and where do they function?

Answer 47

produced in epithelial cells of small intestine, embedded in membrane of epithelial cells of ileum

Question 48

how do salivary amylase hydrolyse starch into glucose?

Answer 48

salivary amylase hydrolyses the glycosidic bonds of starch using water, producing maltose. maltose is then hydrolysed with the enzyme maltase, which is embedded in the ileum’s epithelial cells membranes to form glucose.

Question 49

why is it useful to have membrane-bound enzymes?

Answer 49

1) enzymes don’t get removed in faeces
2) monosaccharides and amino acids are close to transport proteins in cell membrane for facilitated diffusion into epithelial cell

Question 50

what are lipids hydrolysed into and by what enzymes?

Answer 50

they are hydrolysed into monoglycerides and 2 fatty acids by lipase, which hydrolyse the ester bonds

Question 51

where are lipase enzymes produced and where are they used?

Answer 51

produced in the pancreas, used in the small intestine

Question 52

where are bile salts produced and where are they stored?

Answer 52

produced in the liver, stored in the gall bladder

Question 53

what do bile salts do?

Answer 53

emulsify lipids into smaller droplets, called micelles, increasing surface area for faster hydrolysis of lipids

Question 54

describe lipid absorption

Answer 54

1) monoglycerides and fatty acids form a micelle with the bile salts
2) micelles make the fatty acids and monoglycerides more soluble in water, allowing them to be transported
3) micelles carry fatty acids and monoglycerides to epithelial cell membrane
4) monoglycerides and fatty acids diffuse through the phospholipid bilayer
5) monoglycerides and fatty acids reform triglycerides in the smooth ER and are encased in a vesicle
6) the golgi apparatus modifies the triglycerides by combining them with proteins forming chylomicrons
7) chylomicrons are packaged into vesicles for secretion
8) chylomicrons then enter the lymph vessels

Question 55

where do endopeptidases hydrolyse polypeptides and what do they form?

Answer 55

the hydrolyse peptide bonds in the middle of the chain forming shorter polypeptides

Question 56

where exopeptidases hydrolyse polypeptides and what do they form?

Answer 56

they hydrolyse peptide bonds at the end of the chain releasing single amino acids

Question 57

why is the combined action of exo and endopeptidases more efficient than on their own?

Answer 57

endopeptidases hydrolyse peptide bonds in the middle of the polypeptide which creates more ends and more surface area for hydrolysis by exopeptidases

Question 58

what do dipeptidases hydrolyse?

Answer 58

didpeptides into amino acids

Question 59

where does absorption occur?

Answer 59

the small intestine

Question 60

how are the cells of the small intestine adapted for absorption of nutrients?

Answer 60

1) microvilli increase surface area for diffusion
2) more carrier and channel proteins in membrane for facilitated diffusion / active transport
3) many mitochondria to produce more ATP for active transport via aerobic respiration
4) epithelial lining is one cell thick providing a short diffusion pathway

Question 61

how is the small intestine adapted for the absorption of nutrients?

Answer 61

the constant blood flow creates a conc gradient between the inside of the cell and blood

Question 62

describe monosaccharide and amino acid absorption

Answer 62

1) sodium ions are actively transported into the blood from the epithelial cells, using energy from atp hydrolysis and a carrier protein
2) this creates a sodium ion conc gradient from the lumen of the small intestine into the epithelial cells
3) the monosaccharide/amino acid and sodium ion are co-transported into the epithelial cell from the lumen via a carrier protein down the sodium ion conc gradient
4) the monosaccharide/ amino acid is now transported into the blood from the epithelial cell via facilitated diffusion using a carrier protein down a conc gradient

Question 63

what happens to glucose absorption if atp production is inhibited?

Answer 63

active transport can’t occur if there’s no atp. if the sodium ions aren’t actively transported into the blood from the cell, sodium ion conc will rise in the cell so the conc gradient between the lumen and cell isn’t maintained. co-transport will stop as glucose can’t enter without sodium ions, and glucose absorption would stop.

Question 64

how does having haemoglobin aid many large organisms?

Answer 64

many organisms use the circulatory system to carry blood round their bodies to carry oxygen to their tissues while also removing waste CO2.
however, oxygen solubility is low in solutions so a more efficient transport method is needed, rather than dissolving oxygen in the blood

Question 65

describe the structure of haemoglobin?

Answer 65

a protein with a quaternary structure, composed of 4 polypeptide chains. each polypeptide has a haem group containing Fe+ which binds to one oxygen molecule. so, each haemoglobin molecule can bind to 4 O2 molecules

Question 66

write a word equation to show how haemoglobin and oxygen combine

Answer 66

oxygen + haemoglobin –> oxyhaemoglobin
<–

Question 67

what is partial pressure of oxygen?

Answer 67

a measure of the concentration of oxygen present in tissues

Question 68

what is affinity?

Answer 68

how well oxygen is bound to haemoglobin

Question 69

what is percentage saturation?

Answer 69

the amount of oxygen combined with the haemoglobin

oxygenated haemoglobin / max. saturation

Question 70

describe the loading of oxygen onto haemoglobin, including where it occurs and the conditions including affinity, partial pressure and saturation

Answer 70

at the lungs. high partial pressure of oxygen, haemoglobin has a high affinity for oxygen, haemoglobin becomes saturated with oxygen

Question 71

describe the unloading of oxygen onto haemoglobin, including where it occurs and the conditions including affinity, partial pressure and saturation

Answer 71

at the respiring tissues. low partial pressure of oxygen, haemoglobin has a low affinity for oxygen, haemoglobin becomes less saturated with oxygen

Question 72

sketch an oxygen dissociation curve

Answer 72

Question 73

why is an oxygen dissociation curve always sigmoid (s-shaped) ?

Answer 73

binding of the first molecule of oxygen to haemoglobin changes the tertiary shape and structure of haemglobin. this uncovers another haem group for oxygen for bind to. so oxygen molecules will bind more readily to the haemglobin.

Question 74

what is the effect of increased respiration on oxygen dissociation?

Answer 74

1) cells aerobically respire more quickly
2) respiration uses more of the O2 surrounding the tissue
3) this reduces the partail pressure to a level lower than normal
4) the haemoglobin will dissociate more of the O2 (less saturated) and more oxygen will be released from the haemoglobin to the respiring cells

Question 75

sketch another line on an oxygen dissociation curve displaying the bohr effect

Answer 75

Question 76

explain the bohr effect

Answer 76

1) in the presence of CO2 the oxygen dissociation shifts right, because the affinity of haemoglobin for oxygen is reduced
2) so at the partial pressures of oxygen found at the tissues, haemoglobin is less saturated
3) oxygen unloads more readily to be used for aerobic respiration at the tissues
4) this delays the onset of anaerobic respiration at the tissues so less lactic acid is produced
5) because CO2 lowers the pH of blood which alters the haemoglobin tertiary structure

Question 77

what is metabolism?

Answer 77

rate of cellular reactions

Question 78

name the 3 basic types of haemoglobin

Answer 78

1) found in adult humans and other species that live on land at sea level
2) in species that live where the environmental pO2 is low (high altidue, lake bottoms, etc). as there isn’t a lot of O2, normal HG won’t full saturate at the gas exchange surface, so this form of HG shows the dissociation curve shifted left, so it fully saturates. human foetuses also similar.
3) in species that have a high metabolic rate. curve shited right and much steeper, this means the HG will unload its oxygen much faster to the tissues

Question 79

describe what happens to haemoglobin in terms of affinity and partial pressure when the oxygen dissociation curve is shifted left

Answer 79

HG has a higher affinity for oxygen. at the pO2 found at the gas exchange surface, HG is more saturated with O2. so HG will load oxygen more readily and can be transported to tissues for aerobic respiration

Question 80

describe what happens to haemoglobin in terms of affinity and partial pressure when the oxygen dissociation curve is shifted right

Answer 80

HG has a lower affinity for oxygen. at the pO2 found in tissues, HG is less saturated with oxygen. so Hg unloads O2 more readily at tissues for faster aeobic respiration

Question 81

why must foetal haemoglobin have a curve to the left of adult haemoglobin?

Answer 81

so it has a higher affinity than its mother’s HG, so oxygen loads onto foetal haemglobin from mother’s haemoglobin to be used for aerobic respiration

Question 82

label the structure of the heart

Answer 82

Question 83

why do mammals have a double circulatory system?

Answer 83

blood passes through the heart, is pumped to lungs and returned to heart. then blood passes a second time, is pumped around the body and returns to heart.

Question 84

why does the left ventricle have a thicker wall of muscle than the right?

Answer 84

the left ventricle contracts more forcefully in order to generate a higher blood pressure as it must transport blood around the whole body.

Question 85

what is mass flow?

Answer 85

the bulk movement of liquids and gases due to pressure difference

Question 86

why are closed systems more efficient than open systems in terms of mass flow?

Answer 86

it’s easier to generate and maintain a pressure gradient

Question 87

how is a pressure gradient generated in the heart?

Answer 87

blood moves down the pressure gradient from arteries to veins to capillaries

Question 88

describe blood flow in the vena cava, pulmonary artery, pulmonary vein and aorta

Answer 88

vena cava- carries deoxygenated blood from body to heart’s right atrium
pulmonary artery- carries deoxygenated blood from heart to lungs
pulmonary vein- carries carries oxygenated blood from lungs to heart
aorta- carries blood from the oxygenated heart to the organs

Question 89

what are the atrio-ventricular valves?

Answer 89

open when atria pressure > ventricle pressure
close when ventricle pressure > atria pressure

Question 90

what are the semi-lunar valves?

Answer 90

open when ventricle pressure > artery pressure
close when artery pressure > ventricle pressure

Question 91

what is the cardiac cycle?

Answer 91

the sequence of events that lead to the emptying and filling of the heart eg. the events of one heartbeat

Question 92

how do changes in blood pressure come about?

Answer 92

muscle/ ventricle contraction

Question 93

what are the phases of the cardiac cycle?

Answer 93

diastole (muscle relaxed)
atrial systole (atrial muscle contracts)
ventricular systole (ventricular muscle contracts)

Question 94

describe points a,b,c,d

Answer 94

A- pressure in ventricle rises above that in atria – AV valve closes
B- pressure in ventricle rises above that in aorta – SL valve opens
C- pressure in ventricle falls below that in aorta – SL valve closes
D- pressure in ventricle falls below that in atria – AV valve open

Question 95

what is the equation for cardiac output?

Answer 95

cardiac output= heart rate x stroke volume

Question 96

label the structure of the artery

Answer 96

Question 97

label the structure of the vein

Answer 97

+ valves

Question 98

label the structure of the capillary

Answer 98

Question 99

describe the structure of an arteriole

Answer 99

Smaller vessels than arteries and connect artery to the capillaries.
o As the vessel diameter is smaller than an artery, there is greater friction between the blood and the vessel wall. This causes a fall in blood pressure.
o Structure as for an artery but there are two major differences:-
▪ The elastic layer is thinner. As the blood pressure is lower, there is less need for the elasticity required to allow the pulse of blood to pass.
▪ The muscle layer is thicker. The muscle in the arterioles can be contracted to constrict the vessel (vasoconstriction). This reduces flow into the organ. Alternatively the muscle can be relaxed which causes the vessel to dilate (vasodilation). This allows more blood into the organ.

Question 100

how does blood flow change around the body?

Answer 100

In the arteries flow is fast and pressure is high and fluctuating (pulsar).
This reflects the events in the heart during the cardiac cycle.
In the capillaries, friction causes the pressure to fall and flow changes from pulsar to smooth and speed of flow decreases.
In the veins, pressure is low and flow is slow and non-pulsar

Question 101

how are capillaries adapted to their function?

Answer 101

Very thin walls (only 1 cell thick)- increases rate of diffusion
Numerous and branched- increases overall surface area for diffusion
Narrow diameter- ensures RBC is in contact with wall
(increases effective surface area between RBC and capillary wall and reduces distance for diffusion)

Wall spaces- there are gaps between the cells of the endothelial cells which allow rapid formation of tissue fluid and white blood cells to pass into tissue spaces.

Question 102

how is tissue fluid formed?

Answer 102

1. The blood has a high hydrostatic pressure, at the arteriole end of the capillary
2. This forces water and other small molecules out of the capillaries
3. This is called tissue fluid – exchange of gases, nutrients e.g. glucose and waste products e.g. urea occurs between the tissue fluid and the cells
4. Large plasma proteins remain in the blood as they are too big to leave the capillary
5. This lowers the water potential inside the capillary, at the venule end of the capillary
6. Water moves back into the capillary by osmosis
7. Excess tissue fluid is absorbed by lymph vessels

Question 103

how is the xylem adapted to its function?

Answer 103

long cells / tubes with no end walls;
allows water to move in a continuous column;
no cytoplasm / no organelles / named organelle;
to impede / obstruct flow / allows easier water flow; . thickening / lignin;
. support / withstand tension / waterproof / keeps water in cells;
. pits in walls;
. allow lateral movement / get round blocked vessels;

Question 104

describe water movement through the plant

Answer 104

1. Water evaporates from the mesophyll cells and diffuses out of the open stomata down a water potential gradient. This is known as TRANSPIRATION.
2. This lowers the water potential of the mesophyll cells.
3. So water is drawn out from the xylem (which has a higher water potential) into the cells via osmosis.
4. Water is pulled up the xylem by a negative pressure called tension;
5. via hydrogen bonds between water molecules called COHESION
6. Forming a continuous column of water
7. Water molecules are also attracted to the walls of the xylem – called ADHESION

Question 105

how is tree trunk diameter affected by rate of transpiration?

Answer 105

On a hot day, during rapid transpiration, the diameter of a tree trunk will reduce slightly due to the adhesion with the walls of the xylem and negative pressure (tension) making the xylem vessels slightly narrower

Question 106

what factors affect rate of transpiration?

Answer 106

1) light intensity: more stomata open in light for p.synthesis to allow co2 through, more water evaporates out increasing rate.
2) temp: increases rate of evaporation and diffusion of of water through stomata bc molecules have a higher kinetic energy.
3) humidity- DECREASES transpiration- reduces water potential gradient between inside and outside of leaf at stomata as more water molecules in air, less evaporates out.
4) air movement- Air movement over a leaf moves the water vapour away from the stomatal pores.This increases the water potential gradient between the inside and the outside of the leaf. So the greater the rate of transpiration.

Question 107

how to measure rate of transpiration

Answer 107

1. leafy shoot is cut underwater to prevent air entering the xylem which would prevent water flow. The shoot is placed in a rubber tube.
2. The potometer is filled completely
   with water making sure there are no air bubbles
3. The potometer is removed from under the water and all joints are sealed with waterproof jelly to prevent water leaking out which would produce an unreliable result.
4. An air bubble is introduced into the capillary tube
5. As transpiration occurs, water moves through the capillary tube and into the plant, and the bubble of air moves with it
6. The distance moved over a period of time is recorded and the mean is calculated of a number of repeats.
7. The volume of water lost over a period of time can be calculated by knowing the radius (r) of the tube and the distance the bubble has moved in mm (l). (Volume of a cylinder = πr2l)

Question 108

assumptions made for method of measuring transpiration

Answer 108

volume of water taken up by the plant is the same as the volume of water lost by transpiration as some water entering the plant is used in photosynthesis, used for turgor pressure and some is produced by respiration.

Question 109

structure and adaptations of phloem

Answer 109

1) sieve tubes- living cells that form the tube for transporting solutes. They have no nucleus and few organelles. Sieve tubes are connected to each other through sieve plates
2) companion cells- many mitochondria to make ATP through aerobic respiration for the active transport of solutes.

Question 110

what actual places would be the source and sink for sucrose?

Answer 110

source: leaves where it’s produced
sinks: organs and the meristems (areas of growth) in the roots stems and leaves.

Question 111

describe mass flow hypothesis

Answer 111

1) at the source, solutes (e.g. sucrose from photosynthesis) are actively transported from companion cells into the sieve tubes of the phloem at the source. This lowers water potential in sieve tubes.
2) Water enters the sieve tubes by osmosis from xylem. This creates a high hydrostatic pressure inside the sieve tubes at the source.
3) at the sink, sucrose is removed from the phloem to be used in respiration or for storage, increasing the water potential inside the sieve tubes so water leaves the tubes by osmosis.
4) This lowers the hydrostatic pressure inside the sieve tubes.
5) result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink by MASS FLOW. The higher concentration of sucrose at the source the higher rate of translocation

Question 112

Mass flow hypothesis evidence

Answer 112

Supporting
1) If a ring of bark (which includes the phloem but not the xylem) is removed from a woody stem, a bulge forms above the ring. The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring, as the sugars can’t move past the area where the bark has been removed, evidence that there can be a downward flow of sugars.
2) Aphids pierce the phloem, bodies are removed and mouth parts are left, so sap flows out, quicker nearer the leaves than further down the stem – this is evidence that there is a pressure gradient.
3) a metabolic inhibitor (which stops ATP production) is put into the phloem then translocation stops – this is evidence that active transport is involved.
Against:
1) Sugar travels to many different sinks, not just to the one with the lowest water potential as
the model suggests.
2) sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 113

features of tracheoles that make it good for diffusion

Answer 113

1) wall 1 cell thick so rapid diffusion
2) tracheoles enter muscle fibres- direct diffusion to cells.
3) highly branched- short diffusion pathway