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A-Level Biology > Exchange of Substances > Flashcards

Exchange of Substances Flashcards

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1
Q

how do small organisms transport substances in and out? why is this possible?

A
  • they have a large SA:volume ratio, so they can diffuse substances across surface efficiently
  • shorter diffusion distance from outside to centre of organisms so can simply exchange substances across their surface
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2
Q

why isn’t it possible for larger organisms to simply diffuse substances across their surface?

A
  • as an organism gets larger, SA:volume ratio decreases + therefore will need adaptions to help increase surface area
    larger object = smaller sa:vol
  • larger organisms typically have a higher metabolic rate, so more efficient transport of waste out of calls and reactants into cells is required
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3
Q

list some adaptions of organisms that increase surface area. what is their purpose?

A
  • villi + microvilli - more efficient absorption of digested food
  • alveoli + bronchioles - more efficient gas exchange
  • spiracles + tracheoles (insects) - more efficient gas exchange
  • gill filaments + lamellae (fish) - more efficient gas exchange
  • thin wide leaves (plants) - more efficient gas exchange
  • many capillaries - link to form capillary network - more efficient gas exchange
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4
Q

define breathing

A

movement of air into and out of the lungs

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5
Q

define respiration

A

chemical reaction to release energy in the form of ATP

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6
Q

define ventilation

A
  • correct term for breathing
  • movement of air into and out of the lungs
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7
Q

describe the structure and function of the alveoli

A
  • minute air sacs at end of bronchioles
  • consists of collagen and elastic fibres lined with epithelial cells
  • stretch as they fill with air and spring back when breathing out
  • increase surface area for more efficient gas exchange
  • alveolar epithelium one cell thick and lies in close contact with the capillary endothelium
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8
Q

outline the pathway of air from when it enters the body to when it reaches the blood

A

air ->trachea -> bronchi -> bronchioles -> alveoli -> alveolar epithelium -> capillary endothelium -> blood

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9
Q

describe the structure and function of the trachea

A
  • has tough c shaped cartilage rings that support the trachea to helps to keep it open
  • ciliated epithelium to move mucus up throat
  • goblet cells to secrete mucus
  • branches into 2 bronchi
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10
Q

describe the structure and function of the bronchioles

A
  • small tubes leading from bronchi to alveoli
  • muscular walls
  • lined with epithelial cells
  • control air flow in/out of the alveoli
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11
Q

what is an antagonistic interaction?

A

one muscle contracts and the other relaxes

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12
Q

describe how the intercostal muscles are used for ventilation

A
  • external intercostal muscles contract leading to inspiration (inhaling)
  • internal intercostal muscles contract leading to expiration (exhaling)
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13
Q

describe the process of inspiration

A
  • external intercostal muscles contract (pulling ribs upwards and outwards)
  • internal intercostal muscles relax
  • diaphragm contracts downwards from dome position
  • air pressure in lungs (pulmonary pressure) lower (in comp. to atmospheric pressure)
  • lung volume increases
  • air moves into lungs from atm pressure into lower pressure
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14
Q

describe the process of expiration

A
  • external intercostal muscles relax
  • internal intercostal muscles contract (pulling ribs upwards and outwards)
  • diaphragm relaxes, elasticity returns to dome position
  • air pressure in lungs (pulmonary pressure) higher (in comp. to atmospheric pressure)
  • lung volume decreases to increase the pressure
  • air moves out of lungs from lower pressure into atm pressure
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15
Q

describe the process of gas exchange in the alveoli

A
  • alveoli surrounded by capillaries + made up of a single layer of cells (short diffusion distance)
  • CO2 diffuses out of blood into alveoli
  • O2 diffuses into blood out of alveoli
  • once gases are in alveoli, gas exchanges between the epithelium and the blood
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16
Q

describe how the alveoli are adapted to their function

A
  • alveolar epithelium cells very thin to minimise diffusion distance
  • each alveolus is surrounded by a network of capillaries to remove exchanged gases + therefore maintains a concentration gradient resulting in faster diffusion
  • walls of alveoli contain elastin which allows them to recoil to their normal shape following inspiration
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17
Q

what is used to measure tidal volume?

A

spirometer

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18
Q

what is tidal volume?

A

the volume of air normally taken in at each breath when the body is at rest - usually around 0.5dm3

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19
Q

what is vital capacity?

A

maximum volume air a person can inhale/exhale

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20
Q

what is residual volume?

A

volume of air left in lungs after the strongest inhalation

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21
Q

how do you calculate total lung capacity? what is a normal lung capacity

A
  • vital capacity + residual volume
  • 5-6 dm3 (5-6L)
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22
Q

what is pulmonary ventilation?

A

the total volume of air that is moved into the lungs during one minute (dm3min-1)

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23
Q

what is ventilation//breathing rate?

A

the number of breaths taken in during one minute - normally 12-20 in a healthy adult

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24
Q

give an equation linking tidal volume, pulmonary ventilation and ventilation rate. include units.

A

pulmonary ventilation (dm3min-1)= tidal volume (dm3) x ventilation rate (min-1)

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25
Q

how do bronchitis and asthma reduce gas exchange?

A
  • lumen narrows due to inflammation
  • less air entering and exiting alveoli = lower conc gradient = not enough oxygen delivered to alveoli
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26
Q

how does emphysema reduce gas exchange?

A
  • alveoli walls begin to break down, which reduces the surface area for gas exchange
  • not enough oxygen in and carbon dioxide out
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27
Q

how are insects adapted to prevent water loss?

A
  • small sa:volume ratio where water can evaporate from
  • waterproof exoskeleton
  • spiracles, where gases enter and water can evaporate from, can open and close as required to prevent water loss
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28
Q

describe the components of the insect tracheal system

A
  • spiracles: round valve-like openings running along the length of the abdomen. oxygen and carbon dioxide enter and leave via spiracles. trachea is attached to these openings
  • trachea: network of internal tubes, trachea tubes have rings within them to strengthen the tubes and keep them open, branch into smaller tubes deeper into the abdomen
  • tracheoles: extend throughout all the tissues in the oxygen to deliver oxygen to all respiring cells
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29
Q

list 3 ways gas is moved in the insect tracheal system

A
  • simple diffusion
  • mass transport as a result of muscle contraction
  • as a result of volume changes in the tracheoles
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30
Q

how is simple diffusion used in gas exchange in terrestrial insects?

A

gas can exchange by diffusion, as when cells respire, they use up oxygen and produce carbon dioxide, creating concentration gradient from the tracheoles to the atmosphere

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31
Q

how is mass transport used in gas exchange of terrestrial insects?

A

insects contract and relax abdominal muscles to move gases on mass

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32
Q

how is gas moved in and out of the insect tracheal system as a result of volume changes in the tracheoles?

A
  • ends of tracheoles are filled with water
  • when insect is very active/in flight muscles cells begin to anaerobically respire and lactate is produced
  • lactate lowers the water potential of muscle cells
  • water moves from tracheoles into cells by osmosis, decreasing the volume in the tracheoles
  • as a result more air from the atmosphere is drawn in
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33
Q

how are insects adapted for efficient diffusion?

A
  • large number of fine tracheoles, provides large sa
  • walls of tracheoles are thin, and there is a short distance between spiracles and tracheoles, provides short diffusion pathway
  • use of oxygen and production of carbon dioxide, sets up steep concentration gradient
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34
Q

state the equation for fick’s law

A

diffusion = (surface area x difference in concentration) / length of diffusion path

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35
Q

describe the anatomy of a fish gill

A
  • 4 layers of gills on both sides of the head
  • gills made up of stacks of gill filaments
  • each gill filament is covered in gill lamellae positioned at right angles to the filament, creating a large surface area
  • when fish open their mouth, water rushes in and over the gills, then out through a hole in the sides of their head
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36
Q

how are fish adapted for efficient gas exchange?

A
  • many gill filaments covered in lamellae provide large surface area:volume ratio
  • capillary network and very thin gill lamellae provide short diffusion distance
  • countercurrent flow mechanism maintains the concentration gradient
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37
Q

describe the countercurrent exchange principle in fish. why is it useful?

A
  • water flows over the gills in the opposite direction due to the flow of blood in the capillaries
  • ensures that equilibrium is not reached, therefore ensuring a diffusion gradient is maintained across the entire length of the lamellae
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38
Q

describe the structure of a leaf

A
  • stomata: found on underside of leaf, pore surrounded by 2 guard cells, site of gas exchange, carbon dioxide diffuses into spongy mesophyll
  • spongy mesophyll: empty space where gases diffuse in, helps to maintain concentration gradient, then diffuses into palisade mesophyll
  • palisade mesophyll: where most photosynthesis occurs
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39
Q

describe how gases are exchanged in plants

A
  • oxygen diffuses out of stomata
  • carbon dioxide diffuses in through the stomata
  • to reduce water loss by evaporation, stomata close at night when photosynthesis wouldn’t be occuring
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40
Q

describe how xerophytes are adapted to survive in environments with limited water

A
  • curled leaves to trap water vapour to increase local humidity
  • hairs to trap water vapour to increase local humidity
  • sunken stomata to trap water vapour and increase local humidity
  • thicker cuticle to reduce evaporation
  • longer root network to reach more water
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41
Q

outline the digestion of carbohydrates

A
  1. amylase produced by pancreas + salivary glands - hydrolyses polysaccharides into maltose by hydrolysing glycosidic bonds
  2. digestion begins in the mouth, continues in the duodenum (1st part of small intestine) and is completed in the ileum
  3. sucrase and lactase (membrane bound enzymes) hydrolyse sucrose and lactose into monosaccharides
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42
Q

which enzymes are involved in the digestion of proteins? what is their role?

A
  • endopeptidases: hydrolyse peptide bonds between amino acids in the middle of a polymer chain
  • exopeptodases hydrolyse peptide bonds between amino acids at the end of a polymer chain
  • membrane bound dipeptidases: hydrolyse peptide bonds between two amino acids
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43
Q

outline the locations in the body at which protein digestion takes place

A
  1. starts in stomach
  2. continues in duodenum
  3. is fully digested in the ileum
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44
Q

what substances are produced to digest lipids? where are they produced and how are they involved in the digestion?

A
  • lipids digested by lipase and the action of bile salts
  • lipase produced in pancreas + can hydrolyse the ester bond in triglycerides to form monoglycerides and fatty acids
  • bile salts are produced in the liver and can emuslify lipids to form micelles, which increase the surface area for lipase to act on
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45
Q

what are the 2 stages involved in the digestion of lipids?

A
  • physical (emulsification + micelle formation)
  • chemical (lipase)
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46
Q

what are micelles? how are they involved in digestion?

A
  • water soluble vesicles formed of the fatty acids, glycerol, monoglycerides and bile salts
  • they deliver the fatty acids, glycerol + monoglycerides to the epithelial cells of the ileum for absorption
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47
Q

how are mammals adapted for absorption?

A
  • products of digestion are absorbed across the cells lining the ileum
  • ileum wall covered in villi, which have tin walls surrounded by a network of capillaries
  • epithelial cells have microvilli
  • maximises absorption by increasing the surface are, decreasing the diffusion distance and maintaing a concentration gradient
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48
Q

why are active transport and cotransport required in the absorption of monosaccharides and amino acids?

A
  • to absorb gluose and aminoacids from the lumen to the gut, there must be a higher concentration in the lumen than in the epithelial cell for facilitated diffusion to take place
  • however usually higher conc in epithelial cells, so active transport and co transport are required
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49
Q

describe the cotransport of glucose and sodium ions in the ileum

A

1) sodium ions are actively transported out of the epithelial cell into the blood (reduces the sodium ion concentration in the epithelial cell)
2) sodium ions can then diffuse from the lumen down their concentration gradient into the epithelial cell
3) the proteins the sodium ions diffuse through is a cotransporter protein (2 different molecules attach before either of them are transported to the other side), so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient
4) glucose then moves by facilitated diffusion from epithelial cell to blood

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50
Q

describe how micelles enter the epithelial cells in the absorption of lipids

A
  • simply diffuse through cell surface membrane into ileum epithelial cells as non-polar
  • once in cell, modified back into triglycerides inside of ER + golgi body
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51
Q

outline the absorbtion of lipids

A
  • fatty acids + monoglycerides from lipid digestion leave micelles and enter the epithelial cells
  • fatty acids link to form triglycerides
  • fatty globules combine with proteins to form chylomicrons inside golgi apparatus
  • chylomicrons are extruded from the epithelial cell and enter a lacteal (lymph capillary)
  • lymph in the lacteal transports chylomicrons away from intestsine
52
Q

describe the structure of haemoglobin

A
  • quaternary structure protein
  • 2 alpha and 2 beta chains, each with a haem group containing an iron ion
53
Q

what does “affinity of haemoglobin for oxygen” mean?

A

the ability of haemoglobin to attract/bind oxygen

54
Q

what does “saturation of haemoglobin with oxygen” mean?

A

when haemoglobin is holding the max amount of oxygen it can bind

55
Q

what is the loading/association of haemoglobin?

A

the binding of oxygen to haemoglobin

56
Q

what is the unloading/dissociation of haemoglobin?

A

when oxygen detaches or unbinds from haemoglobin

57
Q

is oxygen loaded in areas with high or low partial pressure? give an example of one of these areas.

A
  • high partial pressure
  • alveoli
58
Q

is oxygen unloaded in areas with high or low partial pressure? give an example of one of these areas.

A
  • low partial pressure
  • repiring tissues
59
Q

describe the oxyhaemoglobin dissociation curve and what it shows

A
  • sigma curve
  • oxygen partial pressure on x axis, % saturation on y axis
  • at lower partial pressure, there is lower affinity, showing the unloading of O2 at sites where it’s needed
  • almost 100% saturation of O2 at higher partial pressure, showing haemoglobin has a high affinity for oxygen in areas of high partial pressure, as will be loading lots of oxygen which will be transported by red bood cells in blood to be unloaded in areas where respiration is occuring (low partial pressure areas)
60
Q

why does cooperative binding happen?

A
  • haemoglobin changes shape when the first oxygen binds
  • makes it easier for further oxygens to bind
61
Q

what is the Bohr effect?

A
  • when high CO2 concentration causes the oxyhaemoglobin curve to shift to the right
  • the affinity for oxygen decreases because the acidic (CO2 forms weak carbonic acid which lowers the pH) CO2 changes the shape of the haemoglobin slightly
62
Q

when would an oxyhaemoglobin curve shift left? give an example

A
  • when there is a low partial pressure of CO2 (e.g. in alveoli) curve shift left
  • there is increased affinity and therefore more oxygen is loaded
63
Q

when would an oxyhaemoglobin curve shift right? give an example

A
  • when there is a high partial pressure of CO2 (e.g. in respiring tissues), curve shifts right
  • there is a decreased affinity and therefore more oxygen is unloaded
64
Q

describe how fetal haemoglobin differs from other types of haemoglobin and how this affects the oxyhaemoglobin curve. why is this an advantage?

A
  • curve shifts left
  • at the same partial pressure of oxygen, fetal haemoglobin has a higher affinity and so is more saturated in oxygen
  • advantage as fetus unable to inhale/exhale so only source of oxygen is from mother’s haemoglobin in the blood supply
65
Q

how would the oxyhaemoglobin curve change for an animal that lives at a high altitude?

A
  • curve shifts left
  • there is a lower partial pressure of oxygen so they have a higher affinity for oxygen
66
Q

how would the oxyhaemoglobon curve change for animals with faster metabolism?

A
  • curve shifts right
  • they have a higher partial pressure for oxygen as they need more for respiration to provide energy for contracting muscles
67
Q

how would the oxyhaemoglobin curve change for animals that live underground

A
  • curve would shift left
  • there is a lower partial pressure for oxygen, so there is a higher affinity for oxygen
  • allows them to get oxygen they need from the soil despite the limited amount
68
Q

mammals have a closed double circulatory system. what does this mean? why is it needed?

A
  • closed = blood remains within the blood vessels
  • double circulatory system = the blood passes through the heart twice in each circuit, one circuit delivers blood to lungs and other delivers blood to rest of the body
  • mammals need a double circulatory system to manage the pressure of blood flow
69
Q

does blood flow through the lungs at a higher or lower pressure? why?

A
  • blood flows through lungs at a lower pressure
  • prevents damage to capillaries in the alveoli
  • reduces the speed at which the blood flows, enabling more time for gas exchange
70
Q

list the key blood vessels in the heart, lungs and kidneys, as well as any others

A
  • coronary arteries
  • heart: vena cava, aorta, pulmonary artery, pulmonary vein
  • lungs: pulmonary artery, pulmonary vein
  • kidneys: renal artery, renal vein
71
Q

describe the properties of the cardiac muscle

A
  • thick muscular layer in walls of heart
  • myogenic: can contract and relax without nervous or hormonal stimulation
  • never fatigues, as long as there is a supply of oxygen
72
Q

what are the coronary arteries? what happens if they become blocked?

A
  • surround the heart and supply the cardiac muscle with oxygenated blood
  • branch off from the aorta
  • if they become blocked cardiac muscle won’t recieve oxygen + therefore will not be able to respire and the cells will die
  • results in myocardial infarction (heart attack)
73
Q

describe the properties of the atria

A
  • thinner muscular walls
  • do not need to contract as hard as not pumping blood very far (only to ventricles)
  • elastic walls to stretch when blood enters
74
Q

describe the properties of the ventricles

A
  • thicker muscular walls to enable bigger contraction (creates a higher blood pressure to enable blood to flow longer distances - to lungs and rest of body)
75
Q

describe the role and properties of the right ventricle

A
  • pumps blood to lungs
  • needs to be at a low pressure to prevent damage to capillaries in the lungs and so blood flows slowly to allow time for gas exchange
  • therefore has a thinner muscular wall in comparison to the left ventricle
76
Q

describe the role and properties of the left ventricle

A
  • pumps blood to the body
  • needs to be at a higher pressure to ensure blood reaches all the cells in the body
  • therefore has much thicker muscular wall in comparison to the right ventricle to enable larger contraction of the muscle to create a higher pressure
77
Q

is the vena cava a vein or artery? what does it do?

A
  • vein
  • carries deoxygenated blood from the body into the right atrium
78
Q

what is the role of the pulmonary vein?

A

carries oxygenated blood from the lungs to the lungs

79
Q

what is the role of the pulmonary artery?

A

carries deoxygenated blood from the right ventricle to the lungs to become oxygenated

80
Q

what is the role of the aorta?

A

carries oxygenated blood from the left ventricle to the rest of the body

81
Q

where are the semilunar valves found?

A

aorta and pulmonary artery

82
Q

where are the atrioventricular valves found? what are their alternative names?

A
  • found between atria and ventricles
  • bicuspid = left side
  • tricuspid = right side
83
Q

how do valves work? why are they needed?

A
  • open when pressure is higher behind the valve
  • close when pressure is higher in front of the valve
  • prevents backflow of blood
84
Q

what is the role of the septum?

A
  • separates oxygenated blood and deoxygenated blood
  • maintains high concentration of oxygen in oxygenated blood to maintain concentration gradient for diffusion
85
Q

what are the 3 stages of the cardiac cycle?

A
  • diastole
  • atrial systole
  • ventricular systole
86
Q

what happens during diastole?

A
  • atria and ventricular muscles are relaxed
  • blood enters atria via vena cava + pulmonary vein
  • blood flowing into the atria increases the pressure within the atria
87
Q

what happens during atrial systole?

A
  • atria muscular walls contract, further increasing the pressure
  • this causes the atrioventricular valves to open and blood to flow into the ventricles
  • ventricular muscular walls are relaxed
88
Q

what happens during ventricular systole?

A
  • after a short delay the ventricle muscular walls contract, increasing the pressure beyond that of the atria
  • causes atrioventricular valves to close and the semi-lunar valves to open
  • blood pushed out of the ventricles into the arteries (pulmonary arteries + aorta)
89
Q

give an equation that links stroke volume, cardiac output and heart rate

A

cardiac output = heart rate (min-1) x stroke volume (dm3)

90
Q

define stroke volume

A

volume of blood that leaves the heart each beat

91
Q

describe the structure of arteries

A

muscle layer: thicker than veins so that constriction and dilation can occur to control the volume of blood
elastic layer: thicker than veins to help maintain blood pressure, walls can stretch + recoil in response to the heart beat
wall thickness: thicker wall than veins to help prevent the vessels bursting due to the high pressure
valves: no

92
Q

describe the structure of veins

A

muscle layer: relatively thin so can’t control blood flow
elastic layer: relatively thin as pressure much lower
wall thickness: thin as pressure much lower, so less risk of bursting, thinness = easily flattened, which helps flow of blood to heart
valves: yes

93
Q

describe the structure of capillaries

A

muscle layer: none
elastic layer: none
wall thickness: 1 cell thick, consists of only a lining layer, which provides a short diffusion distance
valves: no

94
Q

describe the structure of arterioles

A

muscle layer: thicker than arteries to help restrict blood flow into the capillaries, as high pressure could damage capillaries
elastic layer: thinner than arteries as pressure is lower
wall thickness: thinner as pressure is lower
valves: no

95
Q

what is tissue fluid?

A

fluid containing water, glucose, amino acids, fatty acids, ions, and oxygen which bathes the tissues

96
Q

how is tissue fluid formed?

A
  • capillaries have small gaps in the walls so that liquid + small molecules can be forced out
  • as blood enters the capillaries from arterioles, the smaller diameter results in high hydrostatic pressure water, glucose, amino acids, fatty acids ions, and oxygen are forced out (ultrafiltration) + therefore form the tissue fluid
97
Q

which substances are forced out of the capillary in the formation of tissue fluid? which substances remain?

A
  • forced out: water molecules, dissolved minerals and salts, glucose, small protein and amino acids, fatty acids, oxygen
  • what remains: red blood cells, platelets, large proteins
98
Q

how are substances in tissue fluid reabsorbed?

A
  • large molecules remain in capillaries, lowering water potential
  • towards the venule end of the capillaries, the hydrostatic pressure is lower due to the loss of liquid, but the water potential is very low (highly negative)
  • water enters by osmosis
  • dissolved waste molecules eg CO2 + urea are reabsorbed back into blood to be removed
99
Q

what is transpiration?

A

loss of water vapour from the stomata by evaporation

100
Q

why won’t all liquid from the tissue fluid be reabsorbed into the capillaries by osmosis? where does the rest go?

A
  • eqm will be reached
  • rest of tissue fluid is absorbed into lymphatic system + eventually drains back into the bloodstream near the heart
101
Q

list the factors affecting transpiration

A
  • light intensity
  • temperature
  • humidity
  • wind
102
Q

how does light intensity affect transpiration?

A
  • positive correlation
  • more light causes more stomata to be open, resulting in a larger surface for evaporation
103
Q

how does temperature affect transpiration?

A
  • positive correlation
  • more heat = more kinetic energy, so molecules move faster, resulting in more evaporation
104
Q

how does humidity affect transpiration?

A
  • negative correlation
  • more water vapour in air will make the water potential outside the leaf more positive, reducing the water potential gradient
105
Q

how does wind affect transpiration?

A
  • positive correlation
  • more wind will blow away humid air containing water vapour, therefore maintaining the water potential gradient and allowing more air to evaporate out
106
Q

what 3 factors make up cohesion tension theory?

A
  • cohesion
  • capillarity/adhesion
  • root pressure
107
Q

how is cohesion involved in cohesion tension theory?

A
  • water is a dipolar molecule, which enables hydrogen bonds to form between hydrogen + oxygen of different water molecules
108
Q

how is capillarity involved in cohesion tension theory?

A
  • adhesion of water is when water sticks to other molecules
  • water adheres to the xylem walls
  • narrower xylem = bigger impact of capillarity
109
Q

how is root pressure involved in cohesion tension theory?

A
  • as water moves into roots by osmosis, it increases the volume of liquid inside the root + therefore the pressure inside the root increases
  • increase in pressure in the roots forces water above it upwards (positive pressure)
110
Q

describe the movement in water up the xylem

A
  1. water evaporates out of stomata on leaves - loss in water volume creates a lower pressure
  2. water is pulled up by the xylem to replace it due to the negative pressure
  3. cohesion created by hydrogen bonds causes water to form a column within the xylem
  4. water molecules adhere to the walls of the xylem which helps to pull the water column upwards
  5. pulling up of water creates tension, pulling the xylem in to become narrower
111
Q

what is a potometer used for? why?

A
  • measure rate of water uptake from a plant
  • near impossible to measure rate of transpiration
  • potometer can be used to investigate effect of a named variable on rate of transpiration as water uptake is proportional to transpiration
112
Q

describe how a potometer is used?

A
  • sample of plant cut underwater (prevents any air entering xylem + breaking water column)
  • potometer filled with water + all air bubbles removed
  • plant is then attached to potometer using rubber seals + petroleum jelly to make equipment air tight
  • one air bubble introduced + distance moved by bubble towards plant is recorded
  • distance used to work out volume of water in tube that evaporated
  • volume is divided by time taken to get rate of transpiration
113
Q

when using a potometer, why must the plant be cut underwater?

A
  • due to cohesion tension creating negative pressure in xylem, if it was cut in air it would draw air into the xylem tube
  • would break the continuous water column + prevent transpiration from taking place
  • cut underwater = only water drawn into xylem
114
Q

when using a potometer, why must all the joints in the apparatus be covered in petroleum jelly?

A
  • petroleum jelly waterproof
  • prevents ant water leaking out + ensures all water can leave by evaporation out of the stomata only
115
Q

when using a potometer, which variables would have to be controlled when performing the experiment on 2 different plant species?

A

surface area of leaves (number/size)

116
Q

what is the role of the phloem?

A

transports organic substances to all cells in a plant

117
Q

list 2 components of phloem tissue

A
  • sieve tube elements
  • companion cells
118
Q

describe the properties of sieve tube elements

A
  • end walls are perforated, allowing for a continuous flow of sugars
  • living cells
  • contain no nucleus and have few organelles (creates a larger volume for sugar to flow)
119
Q

what is the role of companion cells?

A

provide ATP required for active transport of organic substances

120
Q

what is the mass flow hypothesis?

A

mass flow from source of production (e.g. leaves) to the sink (where organic substances e.g. glucose are used up in respiring tissues)

121
Q

outline the translocation of sucrose

A
  • sucrose lowers water potential of source cell so water enters by osmosis which increases the hydrostatic pressure in the source cell
  • higher hydrostatic pressure than sink cell so solution is forced towards sink via phloem
  • respiring cells use up sucrose, so water potential is more positive + leaves by osmosis, decreasing the hydrostatic pressure in the sink cell
122
Q

during the translocation of sucrose, explain how sucrose is transported from source to sieve tube element

A
  1. photosynthesis creates organic substances, inc sucrose - creates high conc of sucrose at site of production so sucrose diffuses down conc gradient into companion cell by facilitated diffusion
  2. active transport of H+ occurs from companion cells into spaces within cell walls - creates conc gradient via carrier proteins into sieve tube element
  3. cotransport of sucrose with H+ ions occurs via protein cotransporters to transport sucrose into sieve tube element
123
Q

during the translocation of sucrose, explain how sucrose is transported within the phloem sieve tube element

A
  1. increase of sucrose in sieve tube element lowers the water potential so water enters sieve tube element from the surrounding xylem vessels by osmosis
  2. increase in water volume in the sieve tube element increases the hydrostatic pressure causing liquid to be forced towards sink
124
Q

during the translocation of sucrose, explain how sucrose is transported to the sink

A
  1. sucrose used in respiration at stink or stored as starch
  2. more sucrose actively transported into sink cell, causing water potential to decrease - results in osmosis of water from sieve tube element into sink cell, some water also returns to xylem
  3. removal of water decreases the volume in the sieve tube element + therefore hydrostatic pressure decreases
  4. movement of soluble organic substances is due to difference in hydrostatic pressure between the source + sink end of sieve tube element
125
Q

explain how tracers can be used to investigate translocation

A
  • CO2 containing radioactive isotope 14C used as radioactive tracer
  • labelled CO2 is fixed into sugars formed during photosynthesis
  • movement of sugars by translocation can be tracked using autoradiography
126
Q

explain how ringing experiments can be used to investigate translocation

A
  • ring of bark + phloem are peeled + removed off a tree trunk, causing trunk to swell above removed section
  • fluid from section has higher conc of sugars than fluid below ring
  • shows that when phloem removed, sugars can’t be transported + therefore proves the phloem transports sugars

A-Level Biology (6 decks)

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