Unit 3 Flashcards
(201 cards)
1
Q
How do gas exchange mechanisms differ between single-celled organisms and larger multicellular organisms?
A
Single-celled organisms:
Rely on diffusion across their body surface
Have a large surface area to volume ratio
Have a short diffusion pathway, so diffusion is sufficient
Larger organisms:
Have a small surface area to volume ratio, so diffusion alone isn’t enough
Require specialised gas exchange surfaces (e.g., lungs, gills)
These have a large surface area, are thin (short diffusion pathway), and maintain a steep concentration gradient to allow efficient diffusion
2
Q
What is the name of the tubes through which oxygen enters an insect’s body?
A
Oxygen enters through spiracles into tracheae, which then branch into tracheoles.
3
Q
Why are the tracheae lined with rings of chitin?
A
To prevent them from collapsing during ventilation.
4
Q
Why are tracheoles the main site of gas exchange in insects?
A
Tracheoles are not lined with chitin, making them permeable for gas exchange.
5
Q
How do insects ensure a short diffusion pathway?
A
Tracheoles are in direct contact with body cells (and sometimes enter them)
The insect’s small size helps maintain a short diffusion distance from spiracles to tissues
Tracheole walls are thin
6
Q
How is a steep concentration gradient maintained for oxygen in insects?
A
Oxygen is used by respiring cells, keeping oxygen concentration low at the cells
Body movement by muscles can ventilate the system, moving air in and out to maintain the gradient
7
Q
How is a large surface area for gas exchange achieved in insects?
A
The tracheoles are highly branched, providing a large surface area for diffusion.
8
Q
What is the exoskeleton of insects made of, and how does it help reduce water loss?
A
It is made of chitin, which is impermeable and reduces water loss by preventing evaporation from the insect’s tissues.
9
Q
What is the function of the waxy cuticle that covers the insect exoskeleton?
A
The waxy cuticle is waterproof/impermeable and helps further reduce evaporation of water from the insect’s body surface.
10
Q
How do hairs around spiracles help reduce water loss in insects?
A
The hairs trap water vapour around the spiracle, which reduces the water potential gradient, so less water evaporates.
11
Q
What role do spiracles play in water conservation?
A
Spiracles can close to prevent water loss when gas exchange isn’t needed.
12
Q
During activity, why does gas exchange in insects become faster?
A
During activity, insect cells use anaerobic respiration, producing lactic acid, which lowers the water potential in the tissues.
This causes water to move by osmosis from the tracheoles into the tissues.
As fluid leaves the tracheoles, they become filled with more air, reducing the diffusion distance for gases.
Since diffusion through air is faster than through fluid, gas exchange becomes quicker and more efficient.
13
Q
Where does gas exchange occur in fish?
A
At the gills, where oxygen is absorbed from water and carbon dioxide is released.
14
Q
How do fish gills maintain a short diffusion pathway?
A
Gills have a single layer of epithelial cells
Capillaries within the gills also have a single layer of endothelial cells
15
Q
How do fish gills provide a large surface area for gas exchange?
A
Gills are folded into filaments
Filaments are further folded into lamellae, increasing the surface area
16
Q
How is a steep concentration gradient maintained across fish gills?
A
Gills have many blood capillaries
Blood flows in the opposite direction to water over the gills — this is called counter-current flow, which maintains a high oxygen concentration gradient
17
Q
Why do plants need carbon dioxide, and what gas do they release?
A
Plants need CO₂ for photosynthesis and release O₂ as a waste product.
18
Q
How does carbon dioxide enter a leaf?
A
It diffuses down a concentration gradient through pores called stomata.
19
Q
Where does gas exchange occur inside the leaf?
A
At the surface of the mesophyll cells, especially the palisade mesophyll cells.
20
Q
How do leaves have a large surface area for gas exchange?
A
Leaves are large and flat
Palisade mesophyll cells are tall and long, increasing the internal surface area
21
Q
What features of a leaf reduce the diffusion distance for gases?
A
Thin leaves
Presence of air spaces between mesophyll cells
22
Q
How is a concentration gradient maintained in the leaf?
A
Mesophyll cells use CO₂ in photosynthesis, keeping CO₂ levels low and encouraging more CO₂ to diffuse in.
23
Q
What controls the opening and closing of stomata?
A
Guard cells, which surround each stoma, control its opening and closing.
24
Q
Why do stomata open and close?
A
Open to allow gas exchange (CO₂ in, O₂ out)
Close to reduce water loss
25
How do plants reduce water loss through stomata?
Stomata close when guard cells lose water and become flaccid
Most stomata are on the lower surface of the leaf to reduce evaporation
The leaf is covered in a hydrophobic, waxy cuticle that prevents water loss by evaporation
26
Why do humans need a gas exchange system?
To get oxygen into the blood for respiration and remove carbon dioxide as a waste gas.
27
Describe the pathway of air through the human gas exchange system.
Air enters the trachea, then moves into the bronchi, then into bronchioles, and finally reaches the alveoli, where gas exchange occurs.
28
How does the structure of alveoli help provide a large surface area?
There are many alveoli, each small but numerous, creating a large surface area for gas exchange.
29
How do alveoli maintain a short diffusion pathway?
Both the alveolar epithelium and the capillary endothelium are only one cell thick, allowing a short diffusion distance for O₂ and CO₂.
30
How is a steep concentration gradient maintained in the alveoli?
Many capillaries close to alveoli maintain good blood supply
Alveoli are well-ventilated, bringing in fresh air (O₂) and removing stale air (CO₂)
➡️ This maintains steep gradients for efficient gas exchange.
31
Why must air be constantly moved in and out of the lungs?
To maintain the diffusion of gases (O₂ and CO₂) across the alveolar epithelium by ensuring a steep concentration gradient.
32
Which muscles are involved in changing the volume of the thorax?
Diaphragm
External and internal intercostal muscles
33
What happens to the external intercostal muscles during inhalation?
They contract, moving the rib cage upwards and outwards.
34
What happens to the diaphragm during inhalation?
It contracts and flattens.
35
What is the effect of these muscle movements on thoracic volume and pressure?
Volume of thorax increases
Air pressure decreases below atmospheric pressure
36
What happens to the external intercostal muscles during exhalation?
They relax, and the rib cage moves down and inwards.
37
What happens to the diaphragm during exhalation?
It relaxes and becomes dome-shaped.
38
What is the effect of these muscle movements on thoracic volume and pressure?
Volume of thorax decreases
Air pressure increases above atmospheric pressure
39
What do the internal intercostal muscles do during forced exhalation?
They contract pulling ribcage further down and in
40
What happens to large biological molecules during digestion?
They are hydrolysed into smaller molecules that can be absorbed across cell membranes.
41
Which enzymes are involved in carbohydrate digestion in mammals?
Amylase (from salivary glands and pancreas); Membrane-bound disaccharidases (e.g., maltase, lactase, sucrase).
42
Where does amylase act, and what does it do?
In the mouth, salivary amylase hydrolyses glycosidic bonds in starch to produce maltose.
43
What enzyme is released by the pancreas, and where does it act?
The pancreas releases pancreatic amylase, which acts in the small intestine to hydrolyse starch into maltose.
44
What do membrane-bound disaccharidases do and where are they found?
They are enzymes found in the epithelial cells of the ileum; they hydrolyse glycosidic bonds in disaccharides (e.g., maltose, lactose, sucrose) into monosaccharides.
45
What is the full sequence of carbohydrate digestion, including different disaccharides?
Carbohydrates are hydrolysed in this sequence:
Starch → Maltose → Glucose
Starch is hydrolysed by amylase
Maltose is hydrolysed by maltase (a membrane-bound disaccharides)
Lactose → Glucose + Galactose
Lactose is broken down by lactase
Sucrose → Glucose + Fructose
Sucrose is broken down by sucrase
🧠 All disaccharides (maltase, lactase, sucrase) are membrane-bound enzymes found in the ileum.
46
What enzyme breaks down lipids?
Lipase.
47
Where is lipase produced and where does it work?
Lipase is produced in the pancreas and works in the small intestine.
48
What are the products of lipid digestion?
Monoglycerides and fatty acids.
49
Why does the pH decrease during lipid digestion?
Fatty acids are released
50
What is the role of bile salts in lipid digestion?
Bile salts emulsify large lipid droplets into smaller ones to increase the surface area for lipase.
51
How do bile salts affect the rate of lipid digestion?
They speed up digestion by increasing the surface area for lipase to act on.
52
Why should a pH buffer not be used in this lipid digestion experiment?
A pH buffer would prevent the pH from dropping, which normally occurs when fatty acids are produced during lipid digestion by lipase. This drop in pH is used as an indicator of lipase activity, so using a buffer would interfere with measuring the rate of lipid digestion accurately.
53
What is the most accurate way to measure pH change in this experiment?
Using a pH meter.
54
How do bile salts affect the pH of a solution during lipid digestion?
The pH drops faster in the solution with bile salts because they increase the rate of hydrolysis, producing fatty acids more quickly.
55
What role do fatty acids play in the pH of the solution?
Fatty acids lower the pH of the solution, making it more acidic.
56
What is the process by which lipids are broken down?
Lipids are broken down (hydrolysed) by lipase into monoglycerides and fatty acids.
57
Why does the pH drop faster in the presence of bile salts?
The presence of bile salts causes faster hydrolysis of lipids, leading to the quicker production of fatty acids.
58
What are proteins broken down into during digestion?
Amino acids.
59
Which three types of enzymes are involved in protein digestion?
Endopeptidases, exopeptidases, and dipeptidases.
60
What is the role of endopeptidases in protein digestion?
They hydrolyse peptide bonds within the protein, producing shorter polypeptides.
61
Where are endopeptidases active?
In the stomach and small intestine.
62
What is the role of exopeptidases?
They remove single amino acids by hydrolysing bonds at the ends of polypeptides.
63
Where do exopeptidases act?
In the small intestine.
64
What is the role of dipeptidases?
They break down dipeptides into two amino acids.
65
Where are dipeptidases found?
On the cell membranes of epithelial cells in the small intestine (membrane-bound).
66
Where are the products of digestion absorbed?
Across the epithelial cells of the ileum in the small intestine.
67
How is the ileum adapted to maximise absorption?
It is very long and folded into villi to increase surface area.
68
What are villi?
Finger-like projections in the ileum that increase surface area for absorption.
69
What are microvilli and what is their role?
Tiny folds on the epithelial cell membranes that increase surface area further for absorption.
70
How does the ileum maintain a steep concentration gradient for absorption?
Each villus has a rich capillary network and a lacteal to rapidly carry away absorbed substances.
71
Why is the absorption pathway in the ileum short?
Because the epithelium and capillary walls are each only one cell thick.
72
How are epithelial cells adapted for active transport and co-transport?
They have many mitochondria to produce ATP.
73
How do epithelial cells increase the number of transport proteins?
They contain many ribosomes and a large rough ER and Golgi body to produce and modify membrane proteins.
74
What does the lacteal in a villus absorb?
Lipids and lipid-soluble substances like monoglycerides and fatty acids.
75
Where are glucose and amino acids absorbed?
Into epithelial cells lining the ileum.
76
What happens when glucose or amino acid concentration is high in the ileum?
They enter epithelial cells by facilitated diffusion.
77
What happens when glucose or amino acid concentration is low in the ileum?
They are absorbed via co-transport with sodium ions.
78
What is the first step in the co-transport of glucose?
Sodium ions are actively transported out of the epithelial cell into the blood using ATP.
79
Why is sodium actively transported out of epithelial cells?
To create a concentration gradient for sodium to diffuse back in from the ileum.
80
What is the second step in glucose co-transport?
Sodium diffuses back into the epithelial cell down its gradient, bringing glucose with it via a co-transporter protein.
81
What is the third step in glucose absorption?
Glucose moves from the epithelial cell into the blood by facilitated diffusion.
82
What happens if ATP is not available (e.g., due to respiratory inhibition)?
Sodium cannot be actively transported out of the epithelial cell (step 1).
83
What happens if sodium isn’t pumped out of the cell?
No sodium gradient is created, so sodium cannot diffuse in with glucose.
84
How does this affect glucose absorption?
Without sodium diffusion, glucose cannot be co-transported into the epithelial cell, so absorption decreases.
85
What do bile salts do during lipid digestion?
Micelles make the lipids water-soluable in the watery solution in the lumen of the ileum and transport the fatty acids and monoglycerides to the cell membrane of the epithelial cells
86
Why are micelles important for lipid absorption?
They carry fatty acids and monoglycerides to the epithelial cell membrane and make them more soluble.
87
How do fatty acids and monoglycerides enter the epithelial cells?
They are released from micelles and diffuse through the phospholipid bilayer.
88
What happens to fatty acids and monoglycerides once inside the epithelial cell?
Once inside the cytoplasm, they are transported to the smooth endoplasmic reticulum (SER).
In the SER, enzymes catalyse the condensation reaction between:
One monoglyceride and 2 fatty acids (on monoglyceride is made up of one glycerol and one fatty acid)
One monoglyceride and Two fatty acids → to form triglycerides.They are recombined to form triglycerides.
89
Where are triglycerides reformed in the epithelial cell?
In the smooth endoplasmic reticulum (SER).
90
What happens to triglycerides after being reformed in the SER?
They are transported to the Golgi apparatus.
91
What does the Golgi do with triglycerides?
Where the triglyceride is added to a protein modifying it into a structure called a chylomicron.
92
What are chylomicrons?
Lipid-protein complexes used to transport lipids (glycoprotein)
93
How are chylomicrons transported out of the epithelial cell?
They are packaged into vesicles and exported by exocytosis.
94
Where do chylomicrons go after leaving the epithelial cell?
They enter the lacteals in the villi and are transported via the lymph.
95
What type of protein is haemoglobin and what is its role in the body?
Haemoglobin is a quaternary structured protein found in red blood cells. It functions in the loading, transporting, and unloading of oxygen by forming oxyhaemoglobin.
96
What is cooperative binding in haemoglobin?
When the first oxygen molecule binds, haemoglobin changes shape, which increases its affinity and makes it easier for the next oxygen molecules to bind. This is known as cooperative binding.
97
What does a left-shifted oxygen dissociation curve indicate about haemoglobin’s affinity?
A left-shifted curve means haemoglobin has a higher affinity for oxygen. It loads oxygen more readily at lower pO₂ but only unloads it at very low pO₂, making it efficient in low oxygen environments.
98
Why do animals like llamas or diving mammals have haemoglobin with a curve to the left?
Llamas and diving mammals live in low oxygen environments (e.g., high altitude or underwater). Their haemoglobin has a higher affinity, enabling efficient oxygen loading at low partial pressure (pO₂) and slow unloading to conserve oxygen.
99
What is the Bohr effect and how does it impact haemoglobin?
The Bohr effect is when carbon dioxide dissolves, forming acid and lowering pH, which changes haemoglobin’s shape. This reduces its affinity for oxygen, making unloading easier and shifting the dissociation curve to the right.
100
What does a right-shifted dissociation curve tell us?
A right-shifted curve indicates a lower oxygen affinity, so haemoglobin unloads oxygen more readily — useful in active animals that need fast aerobic respiration.
101
How does carbon dioxide affect haemoglobin’s oxygen affinity?
Carbon dioxide lowers pH (via the Bohr effect), which reduces haemoglobin’s affinity for oxygen. This means oxygen is unloaded more readily, supporting aerobic respiration in active tissues.
102
Why is Species B more active than Species A in the oxygen dissociation graph?
Species B has a right-shifted curve, meaning its haemoglobin has lower affinity for oxygen. It unloads oxygen more efficiently to cells, enabling greater/faster aerobic respiration for higher activity levels.
103
What type of wall do arteries have and why?
Thick muscular walls to resist pressure
104
Why do veins have thinner walls than arteries?
Blood in veins is under low pressure, so less resistance is needed.
105
Why do arteries have high pressure?
Due to contraction of the ventricles forcing blood into them.
106
Why do veins contain valves?
To prevent backflow of blood under low pressure.
107
What is the function of the elastic tissue in arteries?
It stretches under pressure and then recoils, pushing the blood forward and smoothing flow.
108
Why do arteries have a narrow lumen?
Helps maintain high pressure and smooth blood flow.
109
Why do veins have a wide lumen?
To allow large volumes of blood to return with less resistance.
110
How is pressure maintained in arteries even after the ventricles relax?
Elastic recoil of artery walls maintains relatively high pressure.
111
What feature of veins helps blood return to the heart despite low pressure?
Skeletal muscle contraction and breathing movements squeeze veins to push blood.
112
What is the structure of a capillary wall?
One cell thick endothelium for short diffusion distance.
113
Why are capillaries so small in diameter?
Ensures no body cell is far from a capillary
To reduce diffusion distance
Increase surface area to volume ratio to speed up gas exchange
114
What is the function of capillary pores?
Allow certain substances (e.g. water, white blood cells) to pass through the wall.
115
How does the endothelium of capillaries help exchange?
Thin and smooth, allowing efficient exchange by diffusion.
116
Why is a narrow lumen in capillaries useful for diffusion?
Reduces diffusion distance between haemoglobin in red blood cells and the capillary wall.
117
How is tissue fluid formed at the arteriole end?
High hydrostatic pressure from the left ventricle forces water and small molecules (e.g. glucose, amino acids) out of the capillaries.
## Footnote
Plasma proteins and blood cells remain, lowering the water potential in the capillary.
118
How does water re-enter the capillary at the venous end?
Water re-enters the capillary by osmosis due to low water potential.
## Footnote
This process occurs at the venous end of the capillary.
119
What happens to excess tissue fluid?
Any excess tissue fluid is collected by the lymphatic system and returned to the blood.
120
How does high blood pressure cause oedema?
It increases hydrostatic pressure at the arteriole end, forcing more water out of the capillary. The water potential gradient remains unchanged, so less water is reabsorbed at the venous end. This causes fluid build-up in tissues.
121
How does plasma protein deficiency cause oedema?
There is less plasma protein in the blood, so the water potential of the blood is not lowered enough. The water potential gradient is reduced, and less water moves back into the capillaries by osmosis at the venous end. Fluid accumulates in the tissues.
122
How does a blocked lymph vessel cause oedema?
Excess tissue fluid cannot be drained by the lymphatic system. This causes water to accumulate in the tissues, leading to swelling.
123
What supplies oxygenated blood to the heart muscle itself?
The coronary artery.
124
What separates the left and right sides of the heart?
A septum that prevents mixing of oxygenated and deoxygenated blood.
125
Which chambers receive blood from veins?
The atria.
126
Which chambers pump blood into arteries?
The ventricles.
127
Why are ventricular walls thicker than atrial walls?
Ventricles pump blood further distances and need to generate higher pressure.
128
Why is the left ventricle wall thicker than the right?
It needs to pump blood to the whole body at high pressure, while the right only pumps to the lungs.
129
Why is high pressure dangerous in pulmonary circulation?
It could force fluid out of capillaries in the lungs and impair gas exchange.
130
Which side of the heart pumps blood to the lungs?
The right side.
131
Which side of the heart pumps blood to the body?
The left side.
132
What does systole mean?
Contraction of the heart muscle.
133
What does diastole mean?
Relaxation of the heart muscle.
134
What happens during ventricular diastole?
Heart relaxes, pressure drops, and blood fills atria from veins (vena cava and pulmonary vein).
135
What happens during atrial systole?
Atria contract, increasing pressure, opening AV valves and forcing blood into ventricles.
136
Why does blood not flow backwards into veins during atrial systole?
Valves in the veins stop backflow into the vena cava/pulmonary vein.
137
What happens during ventricular systole?
Ventricles contract, pressure increases, AV valves shut, semi-lunar valves open, blood is forced into arteries.
138
What causes AV valves to shut?
Ventricular pressure becomes higher than atrial pressure.
139
What causes semi-lunar valves to open?
Ventricular pressure becomes higher than aortic or pulmonary artery pressure.
140
What prevents backflow into ventricles after ventricular systole?
Semi-lunar valves close when arterial pressure exceeds ventricular pressure.
141
How do elastic arteries help maintain blood flow during diastole?
They stretch during systole and recoil during diastole, maintaining pressure and forward blood flow.
142
Q:
Using the graph below, explain the changes in pressure and volume during the cardiac cycle and how they relate to valve opening and closing.
Atrial systole: atria contract, atrial pressure > ventricular → AV valve opens → ventricle fills (volume ↑, point 5a).
Ventricular systole: ventricle contracts → pressure ↑ → AV valve closes (point 2), then semilunar valve opens (point 3) → blood ejected, volume ↓ (point 6).
Diastole: ventricle relaxes → pressure falls below aortic pressure → semilunar valve closes (point 4), blood starts to fill ventricle again (point 5b).
Pressure and volume changes are controlled by valve operation depending on pressure differences between chambers and vessels.
143
What is an atheroma?
A build-up of white blood cells, lipids, and connective tissue within the artery wall that hardens and narrows the lumen.
144
What initially causes atheroma formation?
Damage to endothelial cells, allowing lipids and immune cells to build up and form fatty streaks.
145
What can atheromas lead to?
Restricted blood flow, increased blood pressure, aneurysms, or thrombosis.
146
What is an aneurysm?
Swelling of an artery due to pressure pushing through weakened blood vessel walls, which may burst and cause internal bleeding.
147
How does an aneurysm form?
High blood pressure pushes through damaged walls and causes balloon-like swelling of the artery.
148
What is thrombosis?
Formation of a blood clot due to rough surface from an atheroma triggering platelets and fibrin to build up.
149
What are the dangers of a thrombus (blood clot)?
It can completely block the artery or dislodge and block another vessel.
150
What happens if an atheroma forms in the coronary arteries?
It causes coronary heart disease (CHD), reducing blood flow to the heart muscle.
151
Why does CHD cause heart attacks (myocardial infarction)?
Less blood = less oxygen and glucose = less aerobic respiration and ATP, causing the heart muscle to die.
152
What makes water a polar molecule?
Water has uneven charge distribution. The shared electrons are more attracted to the oxygen atom, making it slightly negative (δ⁻), and hydrogen slightly positive (δ⁺). This causes water to be polar.
## Footnote
The difference in electronegativity between oxygen and hydrogen atoms leads to this charge distribution.
153
How does water's polarity lead to cohesion?
Water's slight positive and negative charges cause hydrogen bonding between molecules. The slightly negative oxygen of one water molecule attracts the slightly positive hydrogen of another, creating cohesion.
## Footnote
This hydrogen bonding is crucial for many of water's unique properties, including its high surface tension.
154
What is cohesion in water, and why is it important in the xylem?
Cohesion is the attraction between water molecules due to hydrogen bonding. It allows an unbroken column of water to form and be pulled up the xylem during transpiration.
## Footnote
This unbroken column is essential for transporting water from roots to leaves in plants.
155
Which type of bond causes cohesion between water molecules?
Hydrogen bonds, formed due to water’s polarity (δ⁺ and δ⁻ regions).
## Footnote
Hydrogen bonds are relatively weak individually but collectively strong enough to influence water's behavior.
156
What is the function of xylem vessels in plants?
To transport water and ions in stems and leaves as part of mass flow.
157
What are xylem vessels made of?
Dead cells with no cytoplasm or organelles and no end walls — forming long, hollow, continuous tubes.
158
Why is it important that xylem vessels have no end walls?
It allows water to move as a continuous column with no resistance or interruption in flow.
159
What is the role of lignin in xylem vessels?
Lignin provides structural strength to prevent inward collapse and helps water adhere to the vessel walls.
160
Why is the absence of cytoplasm in xylem important?
It reduces resistance to water flow, allowing unimpeded mass transport.
161
What are pits in the xylem wall and what do they allow?
Pits are small gaps in the xylem walls that allow lateral (horizontal) movement of water between vessels.
162
What is the cohesion-tension theory of water transport?
A theory explaining how water moves up the xylem: water evaporates (transpiration) from leaves, creating tension. Cohesion between water molecules allows them to be pulled up in a continuous column from the roots.
163
What is transpiration?
The evaporation of water from leaves, mainly through the stomata, creating tension in the xylem.
164
What is tension in the xylem and what causes it?
A negative pressure caused by water evaporation in the leaves. This pulls water upward from the roots.
165
What is cohesion and how does it help water move in the xylem?
Cohesion is the attraction between water molecules due to hydrogen bonding, helping form a continuous water column up the xylem.
166
What is adhesion in water transport?
Attraction between water molecules and the hydrophilic xylem walls, helping maintain upward movement.
167
How does water enter the xylem from the roots?
By osmosis from soil into root hair cells, following a water potential gradient.
168
What happens to the water potential of the leaf cells during transpiration?
The water potential decreases because water vapour diffuses out through the stomata. This causes water to move by osmosis from the xylem into the leaf cells.
169
How is tension created in the xylem?
As water evaporates from the leaves, it is drawn up the xylem to replace it. This creates negative pressure (tension) in the xylem.
170
What is meant by a continuous column of water in the xylem?
Water molecules stick together (cohere) by hydrogen bonding, forming an unbroken column up the xylem.
171
What causes adhesion in the xylem?
Water molecules are attracted to the hydrophilic xylem cell walls, helping them stick and resist gravity.
172
How does water enter the roots from the soil?
By osmosis, following a water potential gradient. It then travels across the root cortex and into the xylem.
173
What does the graph show about the relationship between transpiration rate and xylem pressure?
As transpiration rate increases, tension in the xylem increases, causing the pressure inside the xylem to become more negative. This is due to the cohesion-tension mechanism, where water is pulled up as a continuous column.
## Footnote
The lower the pressure, the greater the upward pull, increasing water flow rate.
174
Why does the pressure inside the xylem become more negative when transpiration rate increases?
Transpiration causes water to evaporate from the stomata, creating tension that pulls more water up the xylem, reducing pressure inside the xylem.
175
How does temperature affect transpiration rate?
Higher temperature increases kinetic energy of water molecules, increasing evaporation from mesophyll cells and diffusion through stomata, steepening the water potential gradient and increasing transpiration.
176
How does humidity affect the rate of transpiration?
High external humidity decreases the water potential gradient between the leaf and air, so transpiration decreases. Lower humidity steepens the gradient and increases transpiration.
177
How does air movement (wind) affect transpiration rate?
Wind removes the layer of saturated air near the stomata, reducing local water potential, steepening the water potential gradient, and increasing diffusion of water vapour, thus increasing transpiration.
178
How does light intensity affect transpiration rate?
Increased light opens stomata for gas exchange, allowing more water vapour to diffuse out, which increases transpiration rate.
179
What is the role of the phloem in plants?
The phloem transports organic substances like sucrose from source to sink using living cells.
180
What are sieve tube elements?
They are living cells with no nucleus and few organelles, allowing low resistance flow. They contain sieve plates for solute transport.
181
What are companion cells and their role?
They are metabolically active cells that support sieve tubes. They have many mitochondria to produce ATP for active transport of sucrose.
182
How are sieve tube elements and companion cells connected?
Via plasmodesmata (small cytoplasmic connections in cell walls).
183
Where is the source in the mass flow hypothesis?
The leaf cells (where photosynthesis occurs and glucose is converted into sucrose).
184
Where is the sink in the mass flow hypothesis?
Areas that use or store sucrose, e.g., root cells for storage or respiration.
185
Describe step 1 of the mass flow hypothesis.
Glucose from photosynthesis is converted to sucrose at the source.
186
Describe step 2 of the mass flow hypothesis.
Sucrose is actively transported into phloem by companion cells using ATP.
187
Describe step 3 of the mass flow hypothesis.
Water moves in from the xylem by osmosis due to lower water potential, increasing volume and pressure.
188
Describe step 4 of the mass flow hypothesis.
High hydrostatic pressure pushes the sucrose solution along the phloem via mass flow to the sink.
189
Describe step 5 of the mass flow hypothesis.
At the sink, sucrose is actively transported out of the phloem into sink cells, which lowers the water potential of the sink cells.
As a result, water moves into the sink cells by osmosis from the phloem.
190
Describe step 6 of the mass flow hypothesis.
Water moves from phloem back into xylem by osmosis at the sink.
191
What drives the mass flow of sucrose?
A pressure gradient from source (high pressure) to sink (low pressure).
192
What is the purpose of the ringing experiment in plant stems?
To investigate the transport of organic substances (e.g., sucrose and amino acids) by removing the phloem to see if transport is disrupted.
193
What happens to the plant when a ring of bark containing the phloem is removed?
A bulge forms above the ring because sugar accumulates there. This indicates that sugars cannot move past the ring due to the removal of phloem.
194
What causes the swelling above the ring in the ringing experiment?
Swelling is caused by a build-up of sugar solution. The sugar cannot travel downward because the phloem has been removed.
195
Why does the bulge only form above the ring and not below?
This shows that translocation in the phloem is unidirectional—sugar moves downwards in the phloem and cannot pass the removed section.
196
How does the ringing experiment provide evidence for the mass flow hypothesis?
The accumulation of sugars above the removed phloem supports the idea that solutes in the phloem move in one direction (from source to sink), consistent with mass flow theory.
197
How does the aphid experiment support the mass flow hypothesis?
It demonstrates a pressure gradient in the phloem, with higher pressure at the leaf (source) end, which is necessary to push solutes down to the sink.
198
What does the autoradiography experiment with 14CO2 show about phloem transport?
It shows that sugars (produced from 14CO2) move from the leaf (source) to the root (sink), proving directional transport in the phloem.
199
What does the use of metabolic inhibitors suggest about phloem transport?
When metabolic inhibitors are used, translocation stops, indicating ATP is required, supporting that active transport is involved in loading organic molecules into the phloem.
200
Give two pieces of evidence against the mass flow hypothesis.
1) Different organic substances move at different rates. 2) Some substances move in opposite directions in the same sieve tube (bidirectional flow).
201
Why do sieve plates challenge the mass flow hypothesis?
Sieve plates would obstruct mass flow. A high pressure would be needed to push solutes through them at a reasonable rate.
