Y12 Mass Transport Flashcards
1
Q
Heme group
A
A component of haemoglobin that contains an iron ion and gives haemoglobin its red colour. Each haemoglobin molecule contains four heme groups, and one oxygen molecule binds to each iron ion
2
Q
Partial pressure
A
The pressure of a single type of gas in a mixture of gases. In the context of haemoglobin, it refers to the partial pressure of oxygen (pO2) and is measured in kilopascals (kPa).
3
Q
Fetal haemoglobin
A
A type of haemoglobin found in foetuses that has a higher affinity for oxygen than adult haemoglobin, allowing it to take oxygen from the mothers haemoglobin when the two blood supplies come close together
4
Q
Oxyhaemoglobin
A
Haemoglobin with oxygen loaded onto it. Forms when four oxygen molecules bind to one haemoglobin molecule in a reversible reaction
5
Q
Cooperative binding
A
The process where binding of one oxygen molecule to haemoglobin changed its tertiary structure, making it easier for subsequent oxygen molecules to bind. This explains the S-shape of the oxygen dissociation graph
6
Q
Unloading/Dissociating
A
Process by which haemoglobin releases oxygen, occurring primarily in respiring tissues where partial pressure of oxygen is low
7
Q
Haemoglobin
A
A large globular protein with quaternary structure made up of four polypeptide chains, each containing a heme group with an iron ion. It transports oxygen from the lungs to respiring tissues.
8
Q
Oxygen dissociation curve
A
A graph showing the loading and unloading of oxygen in relation to its partial pressure, or percentage saturation of haemoglobin versus partial pressure of oxygen.
9
Q
Loading/associating
A
The process by which haemoglobin binds with oxygen, occurring primarily in the lungs where partial pressure of oxygen is high.
10
Q
Affinity
A
The tendency of haemoglobin to bind with oxygen. Affinity depends on the partial pressure of oxygen, being higher in the lungs and lower in respiring tissues.
11
Q
Red blood cells
A
Specialised blood cells containing haemoglobin that transport oxygen from the lungs to body tissues
12
Q
Iron ion
A
The metal ion found in each heme group of haemoglobin that directly binds to oxygen molecules during transport
13
Q
Respiring tissues
A
Body tissues actively using oxygen for cellular respiration, characterised by low partial pressure of oxygen which promotes oxygen unloading from haemoglobin
14
Q
Quaternary structure of proteins
A
The arrangement of multiple polypeptide chains to form a functional protein complex
15
Q
Polypeptide chain
A
A long, unbranched sequence of amino acids joined by peptide bonds that forms part of a protein structure.
16
Q
Percentage saturation of haemoglobin
A
The proportion of haemoglobin binding sites occupied by oxygen molecules, expressed as a percentage of the maximum possible binding
17
Q
S-shaped curve
A
The characteristic shape of the oxygen-dissociation curve, resulting from the cooperative nature of oxygen binding to haemoglobin
18
Q
Tertiary Structure of Proteins
A
The overall three-dimensional folding of a single polypeptide chain, giving the protein its functional shape
19
Q
Hereditary Persistence of Fetal Haemoglobin (HPFH)
A
A condition where production of Fetal haemoglobin continues into adulthood alongside adult haemoglobin, usually without symptoms
20
Q
Aerobic Respiration
A
The metabolic process in cells that uses oxygen to produce energy, requiring haemoglobin to deliver oxygen
21
Q
Alveoli
A
Tiny air sacs in the lungs where gas exchange occurs, with high partial pressure of oxygen promoting oxygen loading onto haemoglobin
22
Q
Alveolar Epithelium
A
The thin layer of cells lining the alveoli in lungs through which oxygen diffuses before reaching the capillary endothelium
23
Q
Capillary Endothelium
A
The thin layer of cells lining blood capillaries through which oxygen diffuses from alveoli to red blood cells
24
Q
Anemia
A
A condition characterised by decreased concentration of haemoglobin in the blood, which can alter the oxygen dissociation curve
25
Myoglobin
An oxygen-binding protein found in muscle tissue that has a higher affinity for oxygen than haemoglobin but lacks cooperative properties
26
Structural protein
A protein that provides structural support or mechanical function rather than catalytic activity, with haemoglobin being an example that provides the function of oxygen transport
27
Gas exchange surface
The area where oxygen diffuses from air into the blood, such as the alveoli in lungs or gills in fish, characterised by high partial pressure of oxygen
28
Gills
The respiratory organs in aquatic animals like fish that serve the same function as lungs in terrestrial animals, where haemoglobin loads oxygen
29
Diffusion
The passive movement of molecules from an area of higher concentration to an area of lower concentration, as occurs with oxygen moving from alveoli into red blood cells
30
Shift to the right
The movement of the oxygen dissociation curve that occurs with increased carbon dioxide, resulting in haemoglobin being less saturated with oxygen at any given partial point of oxygen.
31
Haemoglobin Affinity
The tendency of haemoglobin to combine with oxygen, which varies based on factors such as partial pressure of oxygen, carbon dioxide levels, and pH
32
Carbonic Acid
Formed when carbon dioxide reacts with water in blood plasma, leading to increasing acidity and lower pH, which affects haemoglobin affinity
33
Cooperative Binding
The process where binding of oxygen to one heme group in haemoglobin increases the affinity of the remaining heme groups for oxygen, opposite to the effect caused by increased carbon dioxide
34
Bohr Effect
The effect of increased partial pressure of carbon dioxide resulting in a lower blood pH and haemoglobin lower affinity for oxygen, causing the oxygen dissociation curve to shift to the right
35
Oxygen Dissociation Curve
A graph showing the relationship between partial pressure of oxygen and the percentage saturation of haemoglobin with oxygen
36
High Metabolic Rate Adaptions
Smaller animals and active species have haemoglobin with lower affinity for oxygen to unload it more readily at tissues, shifting their oxygen dissociation curve to the right
37
Low pO2 Adaptions
Animals living in environments with low oxygen availability (high altitudes, waterlogged sand) have haemoglobin with higher affinity for oxygen, shifting their oxygen dissociation curve to the left.
38
Species-Specific Haemoglobin
Different species have haemoglobin with slightly different amino acid sequences (primary structures) that affect their affinity for oxygen based on their environmental and metabolic needs
39
Lactic Acid
An acid produced during vigorous exercise when oxygen is limited, contributing to lower blood pH and enhancing the Bohr effect
40
Oxygen Consumption rate
The volume of oxygen used by an organism per unit of time, which varies with body size, metabolic rate, and activity level
41
Oxygen Unloading
The release of oxygen from haemoglobin to tissues, which occurs more readily in the presence of high carbon dioxide levels and lower pH
42
Surface area to volume ratio
The relationship between an organisms external surface and its internal volume that affects heat loss and metabolic rate, influencing oxygen requirements and haemoglobin properties
43
Hydrogen Ions
Positively charged ions released from carbonic acid that disrupt ionic bonds in haemoglobin, changing its tertiary structure and reducing its affinity for oxygen based
44
Anaerobic Environment
Habitat with little or no oxygen, requiring organisms there to have haemoglobin with higher oxygen affinity or alternate respiratory adaptions
45
Partial Pressure of Carbon Dioxide
The measure of carbon dioxide concentration in blood that affects haemoglobin affinity for oxygen; higher pCO2 leads to lower affinity for oxygen and increased oxygen unloading at respiring tissues
46
Respiratory Rate
The frequency of breathing, which affects gas exchange and can be influenced by factors such as body size, activity level and environmental conditions.
47
Blood Plasma
The liquid component in which red blood cells are suspended and where most carbon dioxide is dissolved and transported
48
Respiring Tissues
Body cells actively undergoing cellular respiration, producing carbon dioxide and requiring increased oxygen delivery due to their metabolic activity
49
Coronary Arteries
Blood vessels that branch off the aorta and supply the heart muscle with oxygen and nutrients needed for contraction
50
Atrioventricular Valves
Heart valves located between the atria and ventricles that ensure unidirectional blood flow
51
Pulmonary Circuit
The loop of circulation that carries blood between the heart and lungs for gas exchange
52
System Circuit
The loop of circulation that carries blood between the heart and the rest of the body
53
Renal Arteries
Blood vessels that carry blood from the circulatory system to the kidneys
54
Mass flow
The movement of fluid (usually water, dissolved solutes and suspended objects) in a particular direction due to a force, requiring energy to pump the fluid but being much faster than diffusion
55
Double Circulation
A circulatory system where blood passes through the heart twice per complete circuit, consisting of pulmonary and systemic circuits
56
Semi-lunar valves
Herat valves located between the ventricles and major arteries (left ventricle to aorta, right ventricle to pulmonary artery) that prevent back flow of blood
57
Renal Veins
Blood vessels that carry blood away from the kidneys back to the circulatory system
58
Closed circulatory system
A system where blood is contained entirely within the heart and blood vessels, as found in humans
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Vena Cava
The large vein that carries deoxygenated blood from the heart to the right atrium of the heart
60
Left Ventricle
The chamber of the heart with the thickest muscular wall, responsible for pumping oxygenated blood to the entire body through the aorta
61
Artificial Heart
A mechanical device used to replace to assist a failing heart, often connected to major blood vessels to improve circulation
62
Cardiac Cycle
The sequence of events that occur during one heartbeat, including the opening and closing of valves and and the contraction and relaxation of heart chambers
63
Left side of the heart
The portion of the heart that receives oxygenated blood from the lungs and pumps it to the body, consisting of the left atrium and left ventricle
64
Right side of the heart
The portion of the heart that receives deoxygenated blood from the body and pumps it to the lungs, consisting of the right atrium and right ventricle
65
Pulmonary Vein
The blood vessel that carries oxygenated blood from the lungs to the left atrium of the heart
66
Pulmonary Artery
The blood vessel that carries deoxygenated blood form the right ventricle to the lungs
67
Oxygenated blood
Blood rich in oxygen after passing through the lungs, typically carried in the pulmonary veins, left side of the heart, and arteries of the systemic circulation
68
Aorta
The largest artery in the body that carries oxygenated blood from the left ventricle to the rest of the body
69
Deoxygenated blood
Blood that is low in oxygen after delivering it to body tissues, typically carried in veins of the systemic circulation, right side of the heart, and pulmonary arteries
70
Ventricular Septum
The wall that separates the left and right ventricles of the heart, preventing mixing of oxygenated at deoxygenated blood
71
Unidirectional blood flow
The one-way movement of blood through the heart and blood vessels, ensures by valves that prevent backflow
72
Atria
The upper chambers of the heart that receive blood entering the heart and have thinner walls then ventricles as they only pump blood to adjacent chambers
73
Ventricles
The lower chambers of the heart with thick muscular walls that generate pressure to pump blood over longer distances
74
Right Ventricle
The chamber of the heart with thinner walls than the left ventricle, responsible for pumping oxygenated blood to the lungs through the pulmonary artery
75
Heart Failure
Condition where the heart is not pumping blood as effectively as it should, potentially requiring interventions such as artificial hearts
76
Structures in multicellular organisms that facilitate the exchange of materials with the environment, such as lungs and kidneys
77
Valve function in the heart
Mechanism where higher pressure on one side of a valve causes it to open, and higher pressure on the other sides causes it to close, ensuring unidirectional blood flow
78
Coronary Artery Function
Role of coronary Arteries in supplying heart tissue with oxygen and glucose for respiration, enabling muscle contraction
79
Cardiac Pressure Differences
Variations in blood pressure generated in different chambers of the heart, with the left ventricle producing the highest pressure to pump blood throughout the body
80
Congenital Heart Defects
Structural abnormalities of the heart present at birth, such as holes between ventricles, which can affect oxygen delivery to tissues
81
Systole
Phase of the cardiac cycle when heart muscles contract, increasing pressure and pushing blood through the heart chambers and into blood vessels
82
Stroke volume
The volume of blood pumped out of the ventricle in one contraction, typically measured in ml/beat or dm^3
83
Atrioventricular Valves
Heart valves located between the atria and ventricle that:
Open when atrial pressure exceeds ventricular pressure; close when ventricular pressure exceeds atrial pressure, ensuring unidirectional blood flow
84
Semilunar Valves
Heart valves located between the ventricles and major arteries that open when ventricular pressure exceeds arterial pressure; close when ventricular pressure falls below arterial pressure, preventing backflow of blood
85
Cardiac Output
The volume of blood pumped out of the ventricle in one minute, calculated as stroke volume multiplied by heart rate
86
Heart rate
Number of heart contractions (beats) per minute, typically 75bpm in an average human
87
Cardiac Cycle
The sequence of events in the heart during which the atria and ventricles contract and relax to pump blood and keep it circulating round the body, lasting about 0.8 seconds in humans
88
Cardiac Cycle order
1. Diastole (Relax)
2. Atrial Systole (Contract)
3. Ventricular Systole (Contract)
89
Atrial Systole
• Atria contract simultaneously
• Atrial volume decreases while pressure increases
• Blood is pushed from atria into ventricles
90
Ventricular Systole
• Ventricles contract simultaneously from the base upwards
• Ventricular volume decreases while pressure increases
• When ventricular pressure exceeds atrial pressure, atrioventricular valves close, preventing backflow
• Tendons attached to atrioventricular valves prevent them from turning inside out
• As ventricular pressure continues to rise and exceeds pressure in the aorta and pulmonary artery, semilunar valves open
• Blood is pushed into the aorta and pulmonary artery, flowing to the body and lungs
91
Diastole (Initial Phase)
• Both atria and ventricles are relaxed, causing reduced pressure
• Blood fills the atria when atrial pressure is lower than pressure in the vena cava and pulmonary veins
• As atrial pressure increases and exceeds ventricular pressure, atrioventricular valves open
92
Return to Diastole
• Ventricles relax, causing pressure reduction
• When ventricular pressure falls below pressure in the aorta and pulmonary artery, semilunar valves close
• Blood returns to the atria through major veins
• The next cardiac cycle begins
93
Cardiac Output Formula
Cardiac output = Stroke volume × Heart rate
Where:
• Stroke volume: Volume of blood pumped out of the ventricle in one contraction
• Heart rate: Number of contractions (beats) per minute (bpm)
94
Route which blood circulates the body:
Ventricles → Artery → Arteriole → Capillary → Venule → Vein → Atria
95
Arteries
• Carry blood away from the heart
• Generally carry oxygenated blood (exception: pulmonary arteries carry deoxygenated blood)
Structure:
• Thick muscular walls containing elastic tissue
• Smooth endothelium lining with folds
• Adapted to withstand high pressure and pressure surges
• Elastic tissue allows stretching and recoiling to even out blood flow
96
Arterioles
• Smaller vessels that branch from arteries
• Have relatively thick muscular walls for their size
Function:
• Control blood flow to capillaries through vasoconstriction and vasodilation
• Direct more or less blood to different body parts as needed
97
Capillaries
• Smallest blood vessels
• Primary site of exchange between blood and cells
Adaptations for efficient exchange:
• Very large number increases surface area
• Located near cells to decrease diffusion distance
• Narrow lumen slows blood flow, allowing more time for exchange
• Endothelium only one cell thick for short diffusion pathway
• Small gaps between endothelial cells aid in tissue fluid formation
98
Venules
• Small vessels that collect blood from capillaries
• Join together to form veins
99
Veins
• Carry blood back to the heart
• Generally carry deoxygenated blood (exception: pulmonary veins)
Structure:
• Wider lumen than arteries
• Less elastic and muscle tissue
• Contain valves to prevent backflow
• Blood flow assisted by contractions of adjacent skeletal muscles
100
Blood flow dynamics
-Pressure changes
-Flow Speed
-Oxygenation pattern
101
Pressure changes (Blood flow dynamic)
• Blood pressure gradually decreases as blood flows away from the heart
• Highest in arteries with fluctuations due to ventricular contractions
• Decreases through arterioles and reaches lowest point in capillaries
102
Flow speed (Blood flow dynamic)
• Speed decreases as blood moves from arteries to capillaries
• Slowest in capillaries despite their small individual diameter
• This occurs because the total cross-sectional area of all capillaries is larger than that of the supplying arteries
• Slow flow in capillaries allows more time for diffusion and exchange
103
Oxygenation pattern (Blood flow dynamic)
• Oxygenated blood flows through most arteries
• Deoxygenated blood flows through most veins
• Exceptions: pulmonary arteries (deoxygenated) and pulmonary veins (oxygenated)
104
Tissue fluid
• Forms from gaps between endothelial cells in capillaries
• Plays important role in exchange of substances between blood and body cells
105
Tissue fluid contains:
• Water
• Dissolved ions (Na+, K+, Cl-)
• Dissolved monomers (amino acids, glucose)
• Dissolved gases (oxygen, carbon dioxide)
• Some hormones
• Some white blood cells
106
Blood consists of 4 main components
• Red blood cells: Biconcave shaped cells without a nucleus, packed with hemoglobin for oxygen transport
• White blood cells: Various types involved in immunity
• Platelets: Cell fragments involved in blood clotting
• Plasma: Straw-colored liquid that suspends blood cells and platelets, transporting dissolved molecules including ions, glucose, amino acids, hormones, and blood proteins like fibrinogen
107
4 stages of tissue fluid formation:
1. Ultrafiltration
2. Exchange of substances
3. Reabsorption
4. Lymphatic system
108
Ultrafiltration (Tissue fluid formation)
Blood enters the arteriole end of capillaries where hydrostatic pressure inside the capillaries exceeds the pressure in surrounding tissues. This pressure difference forces fluid out through gaps between endothelial cells (fenestrations). Blood cells and plasma proteins remain in the capillary because they are too large to pass through these gaps.
109
Exchange of substances (Tissue fluid formation)
Small dissolved substances like oxygen and glucose move with the fluid into body cells. Waste substances such as carbon dioxide and urea move out of body cells and into the tissue fluid.
110
Reabsorption (Tissue fluid formation)
As fluid leaves the capillaries, hydrostatic pressure decreases along the length of the capillary until it equals tissue fluid pressure at the venule end. Due to fluid loss and increasing concentration of plasma proteins (which remain in the capillary), the water potential at the venule end becomes lower than in the tissue fluid. This causes water to re-enter the capillary from the tissue fluid by osmosis.
111
Lymphatic system (Tissue fluid formation)
Approximately 95% of the water that left the capillary is reabsorbed. The remaining 5% is drained into the lymphatic system and eventually re-enters the blood at the superior vena cava.
112
Kwashiorkor
A condition resulting from inadequate protein intake seen in developing countries. A common symptom is a large, protruding belly related to tissue fluid accumulation.
113
The lymphatic system functions
• Transporting infection-fighting white blood cells
• Helping to remove toxins, waste, and other unwanted materials
• Draining excess tissue fluid
114
Cardiovascular Disease
Cardiovascular disease (CVD) refers to a range of problems affecting blood vessels and the heart, primarily the arteries.
115
Cause and Correlation
• Correlation: When a change in one variable is reflected by a change in another variable
• Cause: Can only be established through experimental evidence showing direct causation
• Important distinction: Correlation does not prove causation; other factors may be involved
116
Risk Factors for CVD
• High blood cholesterol
• Cigarette smoking
• High blood pressure
• Uncontrollable factors: age, sex, and genetic factors
117
Forms of Cardiovascular Disease
-Atheroma Formation
-Thrombosis
-Heart Attack (Myocardial Infarction)
-Aneurysms
118
Atheroma Formation
• Fibrous fatty plaques build up in artery walls
• Gradually blocks the lumen, restricting blood flow
119
Thrombosis
• Formation of blood clots in arteries
• Often occurs in arteries damaged by atheromas
• Can block arteries or move until lodged in smaller arteries
120
Heart Attack (Myocardial Infarction)
• Occurs when coronary arteries become blocked
• Heart muscle doesn't receive enough oxygen and glucose
• Results in heart muscle death
121
Aneurysms
• Weakened artery walls allow arteries to swell outward
• Can burst, causing internal bleeding
122
Respiration and Energy
• Aerobic respiration occurs with oxygen and happens in the mitochondria
• Respiration releases energy from high energy molecules
• ATP is primarily produced from aerobic respiration in the mitochondria
• Energy from respiration powers active transport processes
123
Transport Across Membranes
• Active transport is the movement against concentration gradient (low to high)
• Active transport requires energy and transport proteins embedded in the membrane
Examples of active transport include:
• Movement of solutes in plants
• Co-transport of glucose with ions in the ileum (gut)
124
Xylem Transport
• Xylem vessels transport water and dissolved mineral ions upward through plants
• Xylem consists of hollow tubes made of dead cells with little resistance to water movement
• Xylem walls are strengthened with lignin to prevent collapse under tension
125
Cohesion-Tension Theory
• Explains how water moves through the xylem
Key terms:
• Hydrogen bonds: Weak bonds between hydrogen and electronegative atoms like oxygen
• Cohesion: Attraction between molecules of the same substance (water molecules stick together)
• Adhesion: Attraction between molecules of different substances (water molecules stick to xylem walls)
126
Process of Water Movement
1. Water evaporates from mesophyll cell surfaces (transpiration)
2. Creates lower water potential in leaf cells
3. Water moves from cell to cell down a water potential gradient by osmosis
4. Cohesive properties of water create tension in the xylem
5. This tension pulls the entire water column upward
6. Water enters the roots from soil by osmosis
127
Evidence for Cohesion-Tension Theory
• Tree trunk diameter decreases during high transpiration rates
• Broken xylem vessels prevent water movement upward
• Water doesn't leak out of broken vessels, suggesting a pulling force
128
Factors Affecting Transpiration
• Temperature: Higher temperatures increase transpiration rate
• Wind: Air movement increases transpiration by maintaining steeper water potential gradient
• Humidity: Low humidity (dry air) increases transpiration rate
• Light: High light intensity causes stomata to open for photosynthesis, increasing water loss
129
Measuring Transpiration
• Potometers measure water uptake by cut stems
• Setup includes a capillary tube with an air bubble to track water movement
• Rate of water uptake can be calculated using the formula:
• Rate of water uptake (mm³/s) = cross-sectional area of capillary tube (mm²) × distance bubble moves (mm/s)
130
Phloem Transport
• Phloem tissue contains sieve plates and companion cells
• Transport process involves source and sink cells
• Source cells (e.g., photosynthetic cells) produce glucose that is converted to sucrose
• Sucrose is actively transported into the phloem (loading)
131
Mass Flow in Phloem
1. Active transport loads sucrose into phloem at source cells
2. High solute concentration lowers water potential
3. Water enters phloem by osmosis
4. Creates high pressure in the phloem
5. At sink cells, sucrose moves out of phloem
6. Water potential increases, water exits by osmosis
7. Creates low pressure at sink end
8. Pressure gradient drives mass flow of dissolved solutes from source to sink
132
Adaptations in Plants
• Some xerophytic plants have sunken stomata to reduce water loss
• Plants regulate stomatal opening based on environmental conditions
• Stomatal area changes throughout the day, affecting water flow rates
133
Structure of Phloem
• Phloem transports organic substances such as sugars (particularly sucrose), amino acids, and growth factors throughout plants
• Phloem is composed of sieve tube elements with porous end walls called sieve plates
• Unlike xylem cells, phloem cells have walls made of cellulose and are living cells
• Sieve tube elements have few organelles and a thin layer of cytoplasm near their walls
• Companion cells support sieve tube elements by respiring on their behalf and aiding in transport
• Companion cells contain many mitochondria and play a crucial role in transporting sucrose into the phloem
134
Translocation Process
• Translocation is the transport of solutes from "sources" (where solutes are produced, e.g., leaves) to "sinks" (where solutes are used or stored, e.g., roots)
• There is always a high concentration of solutes in source cells and a low concentration in sink cells
135
Mass Flow Hypothesis
1. Solutes are actively transported from source cells to companion cells to sieve tubes (active loading)
2. As solutes enter the phloem, solute concentration increases and water potential decreases
3. This causes water to enter the sieve tubes by osmosis from the xylem
4. Water entry creates high pressure at the source end of the sieve tubes
5. At the sink end, solutes are removed for storage or use, decreasing solute concentration and increasing water potential
6. Water leaves the sieve tube by osmosis, creating lower pressure at the sink end
7. This establishes a pressure gradient between source and sink
8. Water and solutes move down this pressure gradient from high pressure (source) to low pressure (sink)
136
Ringing Experiment
• A ring of bark is removed from a branch, deep enough to cut the phloem but not the xylem
After several days:
• A swelling appears above the ring as solutes continue to move downward but are blocked
• Reduced growth occurs below the ring as solutes for growth cannot reach this area
• Leaves remain unaffected as they continue to photosynthesiz
137
Tracer Experiment
• Radioactive isotopes are used to trace the movement of substances through plants
• A plant leaf is exposed to carbon dioxide containing radioactive isotope 14C
• The 14C is incorporated into glucose and then sucrose through photosynthesis
• Autoradiographs (images on photographic film) show the location of compounds containing 14C
• Repeating autoradiographs at different time intervals reveals the route and speed of 14C movement
138
Evidence Supporting Mass Flow Hypothesis
• Higher sucrose concentration in leaves (source) than in roots (sink)
• Increases in sucrose concentration in leaves are followed by similar increases in the phloem
• Downward flow in phloem occurs in daylight but not in darkness or when leaves are shaded
• Aphids can be used to provide evidence by piercing the phloem with their mouthparts
139
Evidence Questioning Mass Flow Hypothesis
• Sucrose is delivered at approximately the same rate to all areas of the plant, not more quickly to areas with lowest concentration
• Sieve plates would seem to impede transport through the phloem
• Not all solutes move at the same speed through the phloem
140
Effects of Metabolic Inhibitors
• Respiratory inhibitors prevent or reduce respiration rate
• These would affect active transport of solutes into the phloem
• This would impact the overall translocation process
141
