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Y12 Biology > 3. Exchange of Substances > Flashcards

3. Exchange of Substances Flashcards

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1
Q

Describe the relationship between the size and structure of an organism and its surface area to volume ration (SA:V)

A
  • As size increases, SA:V decreases
  • More thin/flat/folded/elongated structures increase SA:V
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2
Q

How is SA:V calculated?

A

Divide SA by V
- Surface area = size length x size width x number of sides
- Volume = length x width x depth

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3
Q

Suggest an advantage of calculating SA:mass for organisms instead of SA:V

A

Easier/quicker to find/more accurate because of irregular shapes

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4
Q

What is metabolic rate? Suggest how it can be measured

A
  • Metabolic rate = amount of energy used up by an organism within a given period of time
  • Often measured by oxygen uptake —> as used in aerobic respiration to make ATP for energy release
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5
Q

Explain the relationship between SA:V and metabolic rate

A

As SA:V increases (smaller organisms), metabolic rate increases because:
1. Rate of heat loss per unit body mass increases
2. So organisms need a higher rate of respiration
3. To release enough heat to maintain a constant body temperature, e.g replace lost heat

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6
Q

Explain the adaptations that facilitate exchange as SA:V reduces in larger organisms

A
  1. Changes to body shape (e.g long and thin)
    > increases the SA:V and overcomes long diffusion pathway
  2. Development of systems, such as specialised surface/organ for gaseous exchange e.g lungs
    > increases internal SA:V and overcomes long diffusion pathway
    > maintain a concentration gradient for diffusion e.g by ventilation/good blood supply
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7
Q

Explain how the body surface of a single-celled organism is adapted for gas exchange

A
  • Thin, flat shape and large surface area to volume ratio
  • Short diffusion distance to all parts of cell —> rapid diffusion e.g of O2/CO2
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8
Q

Describe the tracheal system of an insect

A
  1. Spiracles = pores on surface that can open/close to allow diffusion
    2.Trachea = large tubes full of air that allow diffusion
  2. Tracheoles = smaller branches from trachea, permeable so allow gas exchange with cells
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9
Q

Explain how an insects tracheal system is adapted for gas exchange (6)

A
  • Tracheoles have thin walls
    > so short diffusion distance to cells
  • High numbers of highly branched tracheoles
    > so short diffusion distance to cells
    > so large surface area
  • Trachea provide tubes full of air
    > so fast diffusion
  • Contraction of abdominal muscles changes pressure in body, causing air to move in/out
    > maintains a concentration gradient for diffusion
  • Fluid in end of tracheoles drawn into tissues by osmosis during exercises (lactate produced in anaerobic respiration lowers water potential of cells)
    > diffusion is faster through air to gas exchange surface
  • Tracheole walls are permeable to oxygen/air
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10
Q

Explain structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water loss

A
  • Thick waxy cuticle/exoskeleton —> increases diffusion distance so less water loss (evaporation)
  • Spiracles can open to allow gas exchange and close to reduce water loss (evaporation)
  • Hairs around spiracles —> trap moist air, reducing water potential gradient so less water loss (evaporation)
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11
Q

Explain how the gills of fish are adapted for gas exchange

A
  • Gills made of many filaments covered with many lamellae (90º to surface)
    > increases surface area for diffusion
  • Thin lamellae wall/epithelium
    > so short diffusion distance between water/blood
  • Lamellae have a large number of capillaries
    > remove CO2 and bring O2 quickly so maintains concentration gradient
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12
Q

Explain the counter current flow model

A
  1. Blood and water flow in opposite directions through/over lamellae
  2. So oxygen concentration is always higher in the water than nearby blood
  3. So maintains a concentration gradient of O2 between water and blood
  4. For diffusion along whole length of lamellae
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13
Q

What would happen if parallel flow occurred in the gills of fish?

A

Equilibrium would be reached so oxygen wouldn’t diffuse into the blood along the whole gill plate

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14
Q

Explain how the leaves of dicotyledonous plants are adapted for gas exchange

A
  • Many stomata (high density) —> large surface area for gas exchange (when opened by guard cells)
  • Spongy mesophyll contains air spaces —> large surface area for gases to diffuse through
  • Thin —> short diffusion pathway
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15
Q

Explain structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss

A

Xerophyte = plant adapted to live in very dry conditions
- Thicker, waxy cuticle
> increases diffusion distance so less evaporation
- Stomata sunken in pits/rolled leaves/hairs
> trap water vapour/protect stomata from wind
> so reduced water potential gradient between leaf/air
> so less evaporation
- spines/needles
> reduces surface area to volume ratio

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16
Q

Describe the gross structure of the human gas exchange system

A

Trachea
Bronchi
Bronchioles
Lungs
Alveoli
Capillary network surrounding alveoli

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17
Q

Explain the essential features of the alveolar epithelium that make it adapted as a surface for gas exchange (5)

A
  • Flattened cells/1 cell thick —> short diffusion pathway
  • Folded —> large surface area
  • Permeable —> allows diffusion of O2/CO2
  • Moist —> gases can dissolve for diffusion
  • Good blood supply from large network of capillaries —> maintains concentration gradient
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18
Q

Describe how gas exchange occurs in the lungs

A
  • Oxygen diffuses from alveolar air space into blood down its concentration gradient
  • Across alveolar epithelium then across capillary endothelium
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19
Q

Explain the importance of ventilation

A
  • Brings in air containing higher concentration of O2 and removes air with a lower concentration of oxygen
  • Maintaining concentration gradients
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20
Q

Explain how humans breathe in and out (ventilation)

A

Inspiration (breathing in)
1. Diaphragm contracts and flattens
2. External intercostal muscles contract, internal intercostal muscles relax, rib cage is pulled upwards and outwards
3. Increasing volume and decreasing atmospheric pressure in thoracic cavity
4. Air moves into lungs down pressure gradient

Expiration (breathing out)
1. Diaphragm relaxes and moves upwards
2. External intercostal muscles relax, internal intercostal muscles may contract, rib cage moves downwards and inwards
3. Decreasing volume and increasing pressure in thoracic cavity
4. Air moves out of lungs down pressure gradient

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21
Q

Suggest why expiration is normally passive at rest

A
  • Internal intercostal muscles do not normally need to contract
  • Expiration aided by elastic recoil in alveoli
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22
Q

Suggest how different lung diseases reduce the rate of gas exchange

A
  • Thickened alveolar tissue (e.g fibrosis) —> increases diffusion distance
  • Alveolar wall breakdown —> reduces surface area
  • Reduce lung elasticity —> lungs expand/recoil less —> reduces concentration gradients of O2 and CO2
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23
Q

Suggest how different lung diseases affect ventilation

A
  • Reducing lung elasticity (e.g fibrosis, build up of scar tissue) —> lungs expand/recoil less
    > reducing volume of air in each breath (tidal volume)
    > reducing maximum volume of air breathed out in one breath (forced vital capacity)
  • Narrow airways/reduce airflow in and out of lungs (e.g asthma/inflamed bronchi)
    > reducing maximum volume of air breathed out in 1 second (forced expiratory volume)
  • Reduced rate of gas exchange —> increased ventilation rate to compensate for reduced oxygen in blood
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24
Q

Suggest why people with lung disease experience fatigue

A

Cells receive less oxygen —> rate of aerobic respiration reduced —> less ATP made

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25
Q

Suggest how you can analyse and interpret data to the effects of pollution, smoking and other risk factors on the incidence of lung disease

A
  • Describe overall trend —> e.g positive/negative correlation between risk factor and incidence of disease
  • Manipulate data —> e.g calculate percentage change
  • Interpret standard deviations —> overlap suggests differences in means are likely due to be due to chance
  • Use statistical tests —> identify whether difference/correlation is significant of due to chance
    > correlation coefficient = examining an association between 2 sets of data
    > students t-test = comparing means of 2 sets of data
    > chi-squared test = for categorical data (expected and observed results)
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26
Q

Suggest how you can evaluate the way in which experimental data led to statutory restrictions on the sources of risk factors

A
  • Analyse and interpret data and identify what does and doesn’t support the statement
  • Evaluate method of collecting data:
    > sample size - large enough to be representative of whole population
    > participant diversity - age, sex, ethnicity & health status
    > control groups - enables comparisons
    > control variables - validity?
    > duration of study - long enough to show long-term effects?
  • Evaluate context —> broad generalisation made from data?
  • Other risk factors?
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27
Q

Explain the difference between correlations and casual relationships

A
  • Correlation = change in 1 variable reflected by a change in another variable - scatter diagram
  • Causation = change in 1 variable causes a change in another variable
  • Correlation does not mean causation —> may be other factors involved
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28
Q

Explain what happens in digestion

A
  • Large insoluble biological molcules hydrolyses to smaller soluble molecules
  • That are small enough to be absorbed across cell membranes into blood
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29
Q

Describe the digestion of starch in mammals

A
  • Amylase (produced by salivary glands/pancreas) hydrolyses starch to maltose —> acts in small intestine
  • Membrane-bound maltase (attached to cells lining the ileum) hydrolyses maltose to glucose
  • Hydrolysis of glycosidic bond
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30
Q

Describe the digestion of disaccharides in mammals

A
  • Membrane-bound disaccharidases hydrolyse disaccharides into 2 monosaccharides:
    > maltase - maltose —> glucose + glucose
    > sucrase - sucrose —> glucose + fructose
    > lactase - lactose —> glucose + galactose
  • Hydrolysis of glycosidic bond
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31
Q

Describe the digestion of lipids in mammals, including action of bile salts

A
  • Bile salts (produced by the liver) emulsify lipids causing them to form smaller lipid droplets
  • This increases the surface area of lipids for increases/faster lipase activity
  • Lipase (made in the pancreas) hydrolyses lipids (e.g triglycerides) —> monoglycerides + fatty acids
  • Hydrolysis of ester bonds
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32
Q

Describe the digestion of proteins by a mammal

A
  • Endopeptidases —> hydrolyse internal peptide bonds within a polypeptide - smaller peptides
    > so more ends/surface area for exopeptidases
  • Exopeptidases —> hydrolyse terminal peptide bonds at ends of polypeptide - single amino acids
  • Membrane-bound dipeptidases —> hydrolyse peptide bonds between a dipeptide - 2 amino acids
  • Hydrolysis of peptide bonds
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33
Q

Suggest why membrane-bound enzymes are important in digestion

A
  • Membrane-bound enzymes are located on cell membranes of epithelial cells lining the ileum
  • By hydrolysing molecules at the site of absorption they maintain concentration gradients for absorption
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34
Q

Describe the pathway for absorption of products of digestion in mammals

A

Lumen (inside) of ileum —> cells lining ileum (part of small intestine) —> blood

35
Q

Describe the absorption of amino acids and monosaccharides in mammals

A

Co-transport:
1. Na+ actively transported from epithelial cells lining ileum to blood (by Na+/K+ pump), establishing a concentration gradient of Na+ (higher in lumen than epithelial cell)
2. Na+ enters epithelial cell down concentration gradient with glucose against its concentration gradient, via a cotransporter protein
3. Glucose moves down a concentration gradient into the blood via facilitated diffusion

36
Q

Describe the absorption of lipids by a mammal, including the role of micelles

A
  • Micelles contain bile salts, monoglycerides and fatty acids
    > make monoglycerides and fatty acids more soluble in water
    > carry/release fatty acids and monoglycerides to cell/lining of ileum
    > maintain a high concentration of fatty acids to cell/lining
  • Monoglycerides/fatty acids absorbed (into epithelial cell) by diffusion (lipid soluble)
  • Triglycerides reformed in epithelial cells and aggregate into globules
  • Globules coated with proteins forming chylomicrons which are then packaged into vesicles
  • Vesicles move to cell membrane and leave via exocytosis
    > enter lymphatic vessels and eventually return to blood circulation
37
Q

Describe the role of red blood cells and haemoglobin in oxygen transport

A
  • Red blood cells contain lots of haemoglobin (Hb) —> no nucleus, biconcave, high SA:V, short diffusion pathway
  • Hb associates with/binds/loads O2 at gas exchange surfaces where partial pressure of O2 is high
  • This forms oxyhaemoglobin which transports O2 (each can carry 4O2 - one at each Haem group)
  • Hb dissociates from/unloads O2 near cells/tissues where pO2 is low
38
Q

Describe the structure of haemoglobin

A
  • Protein with quaternary structure
  • Made of 4 polypeptide chains
  • Each chain contains a Haem group containing an iron ion (Fe2+)
39
Q

Describe the loading, transport and unloading of oxygen in relation to the oxyhaemoglobin dissociation curve

A

Areas with low pO2:
- Hb has a low affinity for O2
- So O2 readily unloads/dissociates with Hb
- So % saturation is low

Areas with high pO2:
- Hb has a high affinity for O2
- So O2 readily loads/associates with Hb
- So % saturation is high

40
Q

Explain how the cooperative nature of oxygen binding results in an S-shaped oxyhaemoglobin dissociation curve

A
  1. Binding of the first oxygen changes tertiary/quaternary structure of Hb
  2. This uncovers Haem group binding sites, making further binding of oxygens easier
41
Q

Describe evidence for the cooperative nature of oxygen binding

A
  • At low pO2, as oxygen increases there is little/slow increase in % saturation of Hb with oxygen
    > when oxygen is first binding
  • At higher pO2, as oxygen increases there is a big/rapid increase in % saturation of Hb with oxygen
    > showing it has got easier for oxygens to bind
42
Q

What is the Bohr effect?

A

Effect of CO2 concentration on dissociation of oxyhaemoglobin —> curve shifts to the right

43
Q

Explain effect of CO2 concentration on the dissociation of oxyhaenoglobin.
How does the curve provide evidence for this?

A
  1. Increasing blood CO2 e.g due to increased respiration rate
  2. Lowers blood pH (more acidic)
  3. Reduces Hb’s affinity for oxygen as shape/tertiary/quaternary structure changes slightly
  4. So more/faster unloading of oxygen to respiring cells at a given pO2

At a given pO2, % saturation of Hb is lower

44
Q

Explain the advantage of the Bohr effect (e.g during exercise)

A

More dissociation of oxygen —> faster aerobic respiration/less anaerobic respiration —> more ATP produced

45
Q

Explain why different types of haemoglobin can have different oxygen transport properties

A
  • Different types of Hb are made of polypeptide chains with slightly different amino acid sequences
  • Resulting in different tertiary/quaternary structures/shape —> different affinities for oxygen
46
Q

Explain how organisms can be adapted to their environment by having different oxygen transport properties

A

Curve shifts left -> Hb has a higher affinity for O2
- More O2 associates with Hb more readily
- At gas exchange surfaces where pO2 is lower
- e.g organisms in low O2 environments - high altitudes, underground or foetuses

Curve shifts right -> Hb has a lower affinity for O2
- More O2 dissociates from Hb more readily
- At respiring tissues where more O2 is needed
- e.g organisms with high metabolic rates/high rates of respiration

47
Q

Describe the general pattern of blood circulation in a mammal

A

Closed double circulatory system - blood passes through the heart twice for every circuit round the body
1. Deoxygenated blood in the right side of the heart pumped to lungs; oxygenated returns to left side
2. Oxygenated blood in left side of heart pumped to rest of body; deoxygenated returns to right side

48
Q

Suggest the importance of a double circulatory system

A
  • Prevents mixing of oxygenated/deoxygenated blood
    > so blood pumped to body is fully saturated with oxygen for aerobic respiration
  • Blood can be pumped to body at higher pressure (after being lower from lungs)
    > substances taken to/removed from body cells quicker/more efficiently
49
Q

Draw a diagram to show the general pattern of blood circulation in a mammal, including the names of key blood vessels

A

LV —> aorta —> renal artery —> kidneys —> renal vein —> vena cava —> RA —> RV —> pulmonary artery —> lungs —> pulmonary vein —> LA

50
Q

Name the blood vessels entering and leaving the heart and lungs

A
  • Vena cava: transport deoxygenated blood from respiring body tissues —> heart
  • Pulmonary artery: transport deoxygenated blood from heart —> lungs
  • Aorta: transport oxygenated blood from heart —> respiring tissues
  • Pulmonary vein: transports oxygenated blood from lungs —> heart
51
Q

Name the blood vessels entering and leaving the kidneys

A

Renal arteries —> oxygenated blood —> kidneys
Renal vein —> deoxygenated blood —> to vena cava from kidneys

52
Q

Name the blood vessels that carry oxygenated blood to the heart muscle

A

Coronary arteries - located on surface of heart, branching from aorta

53
Q

Label a diagram to show the gross structure of the human heart

A

look over notes

54
Q

Suggest why the wall of the left ventricle is thicker than that of the right

A
  • Thicker muscle to contract with greater force
  • To generate higher pressure to pump blood around entire body
55
Q

What happens during atrial systole?

A
  • Atria contract —> volume decreases, pressure increases
  • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles
  • Semilunar valves remain shut as pressure in arteries exceeds pressure in ventricles
  • So blood is pushed into the ventricles
56
Q

What happens during ventricular systole?

A
  • Ventricles contract —> volume decreases, pressure increases
  • Atrioventricular valves shut when pressure in ventricles exceeds pressure in atria
  • Semilunar valves open when pressure in ventricles exceeds pressure in arteries
  • So blood pushed out of heart through arteries
57
Q

What happens during diastole?

A
  • Atria and ventricles relax —> volume increases, pressure decreases
  • Semilunar valves shut when pressure in arteries exceeds pressure in ventricles
  • Atrioventricular valves open when pressure in atria exceeds pressure in ventricles
  • So blood fills atria via veins and flows passively into ventricles
58
Q

Explain how graphs showing pressure or volume changes during the cardiac cycle can be interpreted, e.g do identify when valves are open/closed

A

AV valves open:
> pressure in atrium is higher than pressure in ventricle
> so blood flows from atrium to ventricle
AV valves closed:
> pressure in ventricle higher than atrium
> to prevent backflow of blood from ventricles to atrium
SL valves open:
> pressure in ventricles higher than in [named] artery
> so blood flows from ventricle to artery
SL valves closed:
> pressure in [named] artery higher than in ventricle
> to prevent backflow of blood from artery to ventricles

59
Q

Describe the equation for cardiac output

A

Cardiac output (volume of blood pumped out of heart per min) = stroke volume (volume of blood pumped in each heart beat) x heart rate (number of beats per min)

60
Q

How can heart rate be calculated from cardiac cycle data?

A

Heart rate (beats per minute) = 60 (seconds) / length of one cardiac cycle (seconds)

61
Q

Explain how the structure of arteries relates to their function (5)

A

Function - carry blood away from the heart at high pressure
- Thick, smooth muscle tissue —> can contract and control/maintain blood flow/pressure
- Thick elastic tissue —> can stretch as ventricles contract and recoil as ventricles relax, to reduce pressure surges/even out blood pressure/maintain high pressure
- Thick wall —> withstand high pressure/stop bursting
- Smooth/folded endothelium —> reduces friction/can stretch
- Narrow lumen —> increases/maintains high pressure

62
Q

Explain how the structure of arteriole relates to their function

A

Function: (division of arteries to smaller vessels which can) direct blood to different capillaries/tissues
- Thicker smooth muscle layer than arteries
> Contracts —> narrows lumen (vasoconstriction) —> reduces blood flow to capillaries
> Relaxes —> widens lumen (vasodilation) —> increases blood flow to capillaries
- Thinner elastic layer —> pressure surges are lower (as further from heart/ventricles)

63
Q

Explain how the structure of veins relates to their function (3)

A

Function: carry blood back to heart at a lower pressure
- Wider lumen than arteries —> less resistance to blood flow
- Very little elastic & muscle tissue —> blood pressure lower
- Valves —> prevent backflow of blood

64
Q

Explain how the structure of capillaries relates to their function (4)

A

Function: allow efficient exchange of substances between blood and tissue fluid (exchange surfaces)
- Wall is a thin layer (one cell thick) of endothelial cells —> reduces diffusion distance
- Capillary bed is a large network of branched capillaries —> increases surface area for diffusion
- Small diameter/narrow lumen —> reduces blood flow rate so more time for diffusion
- Pores in walls between cells —> allow larger substances through

65
Q

Explain the formation of tissue fluid

A

At the arteriole end of capillaries:
1. Higher blood/hydrostatic pressure inside capillaries (due to contraction of ventricles) than tissue fluid (so net outward force)
2. Forcing water (and dissolved substances e.g inorganic ions, salts, glucose and amino acids) out of capillaries
3. Large plasma proteins remain in the capillary

66
Q

Explain the return of tissue fluid to the circulatory system

A

At the venule end of capillaries:
1. Hydrostatic pressure reduces as fluid leaves the capillary
2. (Due to water loss) an increasing concentration of plasma proteins lowers the water potential in the capillary below that of the tissue fluid
3. Water enters capillaries from tissue fluid by osmosis down a water potential gradient
4. Excess water taken up by lymph capillaries and returned to circulatory system through veins

67
Q

Suggest and explain causes of excess tissue fluid accumulation

A
  • Low concentration of protein in blood plasma OR high salt concentration
    > water potential in capillary not as low —> reduces water potential gradient
    > so more tissue fluid formed at arteriole end/less water absorbed at venule end by osmosis
  • High blood pressure —> high hydrostatic pressure
    > increases outward pressure from arterial end AND reduces inward pressure at venule end
    > so more tissue fluid formed at arteriole end/less water absorbed at venule end by osmosis
    > lymph system may not be able to drain excess fast enough
68
Q

What is a risk factor? Give examples for cardiovascular disease

A
  • An aspect of a persons lifestyle or substances in a persons body/environment
  • That have been shown to be linked to an increased rate of disease
  • EXAMPLES: age, diet high in salt or saturated fat, smoking, lack of exercise, genes
69
Q

Describe the function of xylem tissue

A

Transports water (and mineral ions) through the stem, up the plant to leaves of the plant

70
Q

Suggest how xylem tissue is adapted for its function (4)

A
  • Cells joined with no end walls forming a long continuous tube —> water flows as a continuous column
  • Cells contain no cytoplasm/nucleus —> easier water flow/no obstructions
  • Thick cells walls with lignin —> provides support/withstand tension/prevent water loss
  • Pits in side walls —> allow lateral water movements
71
Q

Explain the cohesion-tension theory of water transport in the xylem

A
  1. Water is lost from the leaf by transpiration - water evaporates from spongy mesophyll cells into air spaces and water vapour diffuses through (open) stomata
  2. Reducing water potential of mesophyll cells
  3. So water drawn out of xylem down a water potential gradient
  4. Creating tension in xylem
  5. Hydrogen bonds result in cohesion between water molecules (stick together) so water is pulled up as a continuous column
  6. Water also adheres (sticks) to walls of xylem
  7. Water enters roots via osmosis
72
Q

Describe how to set up a potometer

A
  1. Cut a shoot underwater at a slant —> prevent air entering xylem
  2. Assemble potometer with capillary tube end submerged in a beaker of water
  3. Insert shoot underwater
  4. Ensure apparatus is watertight/airtight
  5. Dry leaves and allow time for shoot to acclimatise
  6. Shut tap to reservoir
  7. Form an air bubble - quickly remove end of capillary tube from water
73
Q

Describe how a potometer can be used to measure the rate of transpiration

A

Estimates transpiration rate by measuring water uptake:
1. Record position of air bubble
2. Record distance moved in a certain amount of time (e.g 1 minute)
3. Calculate volume of water uptake in a given time:
> use radius of capillary tube to calculate cross-sectional area of water (pixr^2)
> multiply this by distance moved by bubble
4. Calculate rate of water uptake - divide volume by time taken

74
Q

Describe how a potometer can be used to investigate the effect of a named environmental variable on the rate of transpiration

A
  • Carry out the normal practical, change one variable at a time (wind, humidity, temperature or light)
    > e.g set up a fan OR spray water in a plastic bag and wrap around plant OR change distance of light source OR change temperature of room
  • Keep all other variables constant
75
Q

Suggest limitations in using a potometer to measure rate of transpiration

A
  • Rate of water uptake might not be same as rate of transpiration
    > water used for support/turgidity
    > water used in photosynthesis and produced during respiration
  • Rate of movement through shoot in potometer may not be the same as rate of movement through shoot of whole plant
    > shoot in potometer has no roots whereas a plant does
    > xylem cells are very narrow
76
Q

Suggest how different environmental variables affect transpiration rate

A

Light intensity —> increases rate of transpiration
> stomata open in light to let in CO2 for photosynthesis
> allowing more water to evaporate faster
> stomata close when its dark so theres a low transpiration rate
Humidity —> decreases rate of transpiration
> more water in the air so has a higher water potential
> decreasing water potential gradient from leaf to air
> water evaporates slower
Temperature —> increases rate of transpiration
> water molecules gain kinetic energy as temperature increases
> so water evaporates faster
Wind intensity —> increases rate of transpiration
> wind blows away water molecules from around stomata
> decreasing water potential of air around stomata
> increasing water potential gradient so water evaporates faster

77
Q

Describe the function of phloem tissue

A

To transport organic substances e.g sucrose in plants

78
Q

Suggest how phloem tissue is adapted for its function

A
  1. Sieve tube elements
    > no nucleus/few organelles —> maximise space for/easier flow of organic substances
    > end walls between cells perforated (sieve plate)
  2. Companion cells
    > many mitochondria —> high rate of respiration to make ATP for active transport of solutes
79
Q

What is translocation?

A
  • Movement of assimilates/solutes such as sucrose
  • From source cells (where made e.g leaves) to sink cells (where used/stored e.g roots) by mass flow
80
Q

Explain the mass flow hypothesis for translocation in plants

A
  1. At source, sucrose is actively transported into phloem into sieve tubes/cells
  2. By companion cells
  3. This lowers the water potential in sieve tubes so water enters (from xylem) by osmosis
  4. This increases the hydrostatic pressure in sieve tubes (at source)/creates a hydrostatic pressure gradient
  5. So mass flow occurs - movement from source to sink
  6. Now a higher water potential at sink than in xylem so water moves back into the xylem via osmosis
  7. At sink, sucrose is removed by active transport to be used by respiring cells or stored in storage organs
81
Q

Describe the use of tracer experiments to investigate transport in plants

A
  1. Leaf supplied with a radioactive tracer e.g CO2 containing radioactive isotope 14C
  2. Radioactive carbon incorporated into organic substances during photosynthesis
  3. These move around the plant by translocation
  4. Movement tracked using auto radiography or a Geiger counter
82
Q

Describe the use of ringing experiments to investigate transport in plants

A
  1. Remove/kill phloem e.g remove a ring of bark
  2. Bulge forms on source side of ring
  3. Fluid from bulge has a higher concentration of sugars than below - shows sugar is transported in phloem
  4. Tissues below ring die as cannot get organic substances
83
Q

Suggest some points to consider when interpreting evidence from tracer & ringing experiments and evaluating evidence for/against the mass flow hypothesis

A
  • Is there evidence to suggest the phloem (not xylem) is involved?
  • Is there evidence to suggest respiration/active transport is involved?
  • Is there evidence to show movement is from source to sink? What are these?
  • Is there evidence to suggest movement if from high to low hydrostatic pressure?
  • Could movement be due to another factor e.g gravity?
84
Q

What is pulmonary ventilation? Write out the equation

A
  • the total volume of air moved into the lungs in 1 minute
    PVR = tidal volume x breathing rate

Y12 Biology (4 decks)

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