Topic 3: Exchanging substances Flashcards
Pack 7, 8 and a tiny bit of 4
1
Q
What is digestion?
A
- The chemical digestion and physical breakdown of foods
- Large molecules are hydrolysed by enzymes to produce smaller molecules to be absorbed and assimilated
2
Q
Where is the enzyme amylase made?
A
Salivary glands and pancreas
3
Q
Where are maltase, sucrase and lactase (enzymes) found
A
On the epithelial cells of small intestine
4
Q
What is a hydrolysis reaction?
A
Addition of water to break a bond
5
Q
What are the three types of carbohydrate?
A
- Polysaccharides
- Disaccharides
- Monosaccharides
6
Q
Name 3 polysaccharides
A
- Starch
- Cellulose
- Glycogen
7
Q
Name 3 disaccharides
A
- Maltose
- Sucrose
- Lactose
8
Q
Name 3 monosaccharides
A
- (a+b) glucose
- Fructose
- Galactose
9
Q
What are the monosaccharides of Sucrose?
A
Glucose and fructose
10
Q
What are the monosaccharides of Lactose?
A
Glucose and galactose
11
Q
What are the monosaccharides of maltose?
A
Glucose and glucose
12
Q
What 2 enzymes digests starch in order to produce glucose?
A
Amylase and Maltase
Starch + AMYLASE -> Maltose
Maltose + MALTASE -> Glucose x2
13
Q
What enzyme digests sucrose?
A
Sucrase
14
Q
What enzyme digests lactose?
A
lactase
15
Q
What is the advantage of having maltase, Lactase and Sucrase on the epithelial cells of the small intestine?
A
- Always there
- Not needed to be regularly made in pancreas
16
Q
What is a triglyceride (lipid) made up of?
A
A glycerol and 3 fatty acids
17
Q
What enzyme digests lipids?
A
Lipase
18
Q
Where is lipase produced?
A
In the pancreas
19
Q
What role to bile salts play in the digestion of lipids?
A
Emulsifies fat to increase the SA
20
Q
What are the small droplets made during lipid emulsification called?
A
Micelles
21
Q
What is a protein made of?
A
Amino acids
22
Q
What enzymes digest proteins?
A
Proteases or peptidases (latter for polypeptides)
23
Q
Were are the enzymes proteases produced?
A
Stomach (pepsin) and pancreas
24
Q
Why do proteases need to be inactive when first synthesized?
A
They would hydrolyse the proteins in the cell that made them
25
What is the difference between endopeptidases and exopeptidases?
- Endopeptidases break polypeptides in the middle
- Exopeptidases work from the ends of polypeptides (proteins)
- Better for endo first as provides more ends for exo
26
What is the name of the transport protein that moves two molecules through a membrane in the same direction?
A symporter
27
What ion travels into cells with glucose?
Sodium Na+ ions
28
Is co-transport an active or passive process?
Passive, no ATP is required
29
How is the concetration gradient of Na+ ions maintained so that it is low in co-transport?
A sodium/potssium pump moves Na+ ions out of the cell
30
What is a micelle?
A single droplet of a phospholipid
31
How do bile salts form micelles?
Surround a lipid to form a hydrophillic outside
32
How do micelles help to absorb lipids?
Release the lipid soluble content when in contact with an epithelial cell membrane, so the content can diffuse in
33
What happens once fatty acids and monoglycerides are inside an epithelial cell?
They are transported to the endoplasmic reticulm and recombined into triglycerides
34
What are chylomicrons?
A combo of triglycerides and cholesterol/lipids which leave cells by exocytosis
35
Why are large molecules broken down in digestion?
So that they can be absorbed by epithelial cells and then blood
36
What factors affect diffusion?
- Concentration gradient
- Surface area
- Diffusion distance
- Temperature
37
What is the SA:V of small animals normally?
Small animals - LARGE SA:V
| i.e more SA of their size
38
What is the SA:V of large animals normally?
Large animals - SMALL SA:V
| i.e less SA for their size
39
Why do elephants need large ears?
Heat loss
40
Do smaller animals require a higher rate of metabolism? Why?
Have large SA:V so more heat loss, so respiration required to maintain body temp
41
Give the word equation for aerobic respiration.
Glucose + oxygen -> Carbon dioxide + water
42
Give the word equation for anaerobic respiration in animals.
Glucose -> lactate
43
Give the word equation for anaerobic respiration in plants.
Glucose -> ethanol + carbon dioxide
44
Which type of respiration produces the most ATP?
Aerobic respiration
45
What type of system could you say organisms are?
An open system.
| Matter and energy (heat) exchanged with environment
46
What will happen to rate of diffusion if thickness of diffusion pathway increases?
Rate decreases (based on Fick's Law)
47
What will happen to rate of diffusion if t
| SA increases?
Rate increases
48
What will happen to rate of diffusion if concentration difference/gradient increases?
Rate increases
49
How do single-celled organisms get O2 in and CO2 out?
Simple diffusion through external membrane
50
Why do single-celled organisms not need a specialised gas exchange system?
Only one cell, small; so one cell thick
51
What is a problem regarding gas exchange as organisms increase in size?
- Cells are further away from surfaces, receive inadequate oxygen
- Increased metabolic rate (increasing demand)
52
How have larger organisms evolved to become efficient in gas exchange?
- Specialist respiratory surfaces (lungs, gills etc)
- Transport system/medium (blood)
- Respiratory pigments (haemoglobin)
53
What is the top layer of a leaf called?
Waxy cuticle
54
What are the "upper most" cells of a leaf called?
Upper epidermis cells
55
What cells in a leaf do the majority of photosynthesis?
The mesophyll cells (spongy and palisade)
56
What is the spongy mesophyll in leaf made up of irregular cells?
Increases SA
57
What 2 reactions take place in plant cells?
Respiration and photosynthesis
58
What is the word equation for photosynthesis?
Carbon dioxide + water -> glucose + oxygen
59
At what time of day does photosynthesis occur?
In daylight
60
At what time of day does respiration occur in palnts
All the time
61
What processes use stomata?
- Photosynthesis
- Respiration
- Transpiration
62
What are stomata?
Pores often located on the bottom side of leaves (not exclusively). Regulates gas exchange
63
What cells surround stomata?
Guard cells
64
What is the region of the leaf inside of stomata called?
Stomatal space - surrounded by spongy mesophyll cells
65
What happens to guard cells when there isn't much water?
They close to reduce water loss by transpiration
66
What effect does closing of guard cells have on gas exchange in plants?
- O2 and CO2 cannot get into the sub-stomatal spaces
| - No photosynthesis/respiration occurs
67
What is transpiration?
Evaporation of water from the leaves of a plant
68
What factors increase the rate of transpiration?
- High temp (increases Ke)
- Airflow (maintains WP)
- Low humidity (high WP)
- Light (stomata open)
69
Does water in transpiration leave the leaf via stomata with osmosis or diffusion?
Diffusion - it is WATER VAPOUR, so a gas
70
Why do guard cells open when water enters?
- Water moves in by osmosis
- Increases volume of guard cells, becomes turgid
- Turgidity opens stomatal pores
71
What is a xerophyte?
Plants which live in environments with low water availability
72
Give 6 ways that xerophyte plants have adapted to reduce water loss.
1. Thick cuticle
2. Rolled up leaves
3. Hairy leaves
4. Stomata in pits
5. Reduced SA:V
6. Extensive root systems
73
How does having a thick cuticle reduce water loss in plants?
Increases the diffusion distance
74
How does having rolled up leaves reduce water loss in plants?
Lowers concentration gradient (a micro atmosphere) and reduces effect of wind
75
How does having hairy leaves reduce water loss in plants?
Lowers concentration gradient
76
How does having stomata in pits reduce water loss in plants?
Lowers concentration gradient
77
How does having extensive root systems reduce water loss in plants?
More water is absorbed, counter acts lost water
78
Where do gases enter insects?
In spiracles
79
How is a concentration gradient for oxygen established in insects?
Oxygen is continually used up in cellular respiration
80
Give the order of tubes gases travel through to reach cells in insects.
1. Spiracles
2. Tracheae
3. Tracheoles
4. Cells
81
Why are insects small?
In order to be efficient diffusion pathways must be short
82
How do respiratory gases move in/out of the tracheole system in insects?
- Conc gradient
| - Ventilation due to breathing rhythms
83
Do insects have a circulatory system?
Yes. BUT oxygen is not transported in it, just sugars etc
84
How does CO2 leave the tracheole system of insects?
Moves down its own concentration gradient to outside
85
How does water effect gas exchange in insects
- Water moves in my capillarity
- Gases diffuse more slowly in a liquid than a gas
- Water slows diffusion of O2 and CO2
86
How is water removed from the trachiole system in insects?
- Intense activity causes lactate to build up in cells
- Lactate decreases water potential (more negative)
- Water moves in by osmosis
- Frees up the trachioles
87
How do insects reduce water loss?
- Waxy cuticle + exoskeleton
- Close spiracles
- Setae (hairs) near spiracles
88
What material surrounds trachioles?
Chitin
89
What causes lactate to increase in the cells of insects?
- More exercise
| - Anaerobic respiration occurs in absence of O2
90
How does the removal of water from the tracheole system increase gas exchange in insects?
- Less water, more gas so diffusion of gases is quicker
- Water moves into cells with osmosis, causes low pressure, sucking in air
- Also causes a deeper penetration of air
- Increases rate of respiration
91
Summarise the adaptions of insects for gas exchange.
- Use of tracheole system
- Tracheoles are only a thin layer of cells
- Tracheoles found throughout insect, penetrate all tissues
- Chitin for structural support
- Muscle contraction to force air in/out
92
Describe the mechanisms used by fish to move water over the gill.
1. Mouth opens, volume increases, pressure decreases
2. Water enters
3. Mouth closes, floor of mouth rises
4. Volume decreases, pressure increases
5. Water forced over gills and out via operculum
93
Why do fish not have lungs?
- Water denser than air
- More work required to physically move it
- Work requires respiration, uses precious O2
94
What system do fish use in gas exhange
Counter-current (or flow) system
95
Why is counter-current in fish more efficient?
Blood is constantly meeting oxygenated water
96
What is the gas exchange surface of gills called?
Lamella(e)
97
As distance across lamellae increases, what effect does it have on O2 concentration in blood and water?
- O2 in water decreases
| - O2 in blood increases
98
Does blood flow in "different" or "opposite" directions in counter-current?
OPPOSITE
99
What does the counter-current system used by fish ensure?
Concentration gradient is maintained across the whole lamellae
100
Why is counter-current used by bony fish?
Water has a very small concentration of O2 dissolved within it. Counter current ensure maximum uptake
101
Give adaptations of fish to ensure efficient gas exchange.
- Thin diffusion pathway
- Large SA of many lamellae
- Counter current system for constant concentration gradient along the length of gill
102
How is ATP produced?
In cell respiration
103
Name 5 characteristics of a successful respiratory surface.
1. Permeable to some gases
2. Large SA
3. Thin
4. Ventilation (maintain conc gradient)
5. Movement of internal medium (maintain conc gradient)
104
Describe the route air takes as it enters the lungs.
1. Mouth
2. Trachea, bronchus
3. bronchioles
4. Alvioli
5. Capillaries, blood
105
Name the muscles which act on the lungs.
- Diaphragm
- Internal intercostal muscles
- External intercostal muscles
106
What type of cell are alvioli and capillary cells made of?
Squamous epithelium (very THIN+ FLAT)
107
What do capillaries do to increase gas exchange?
- The lumen is very small
- Red blood cells (RBCs) squeeze
- Slows down RBCs
- More time for gas exchange
108
What is "type" of squamous cell surrounds alvioli?
Squamous EPIthelium
109
What is "type" of squamous cell surrounds capillaries (next to alvioli)?
Squamous ENDOthelium
110
What cells are also present in alvioli? Why?
Macrophage to prevent airborne infections
111
What is the air which does not reach the alvioli called?
Dead space
112
What is partial pressure?
Equivalent to concentration
113
Why does oxygen diffuse from Alvioli into cappilaries?
The partial pressure of oxygen is higher in alvioli than in the blood (which is deoxygenated)
114
Give the 6 stages in the diffusion of oxygen from alviolus to red blood cells.
1. Diffuses through alviolar space
2. Dissolves in mucus
3. Diffuses through alvioli epithelium cells
4. Diffuses through capillary endothelium cells
5. Diffuses through blood plasma
6. Diffuses through semi-permiable membrane of RBCs
115
How does exhalation occur?
- Diaphragm relaxes, domes
- Internal intercostal muscles contract
- External intercostal muscles relax
- Volume of thorax + lungs decreases
- Pressure increases
- Air forced out
116
How does inhalation occur?
- Diaphragm contracts, flattens
- External intercostal muscles contract
- Internal intercostal muscles relax
- Volume of thorax + lungs increases
- Pressure decreases
- Air forced in
117
What other mechanism aids exhalation?
The natural elasticity of the lungs
118
What are the short term effects of exercise on breathing?
- Rate increases
- Depth of breathing increases
- Increase in O2 consumption
- Increase of CO2
119
What are the long term effects of regular exercise?
- diaphragm and intercostal muscles become larger and stronger
- More capillaries and alvioli
120
What is a risk factor?
Something that increases the chance
121
What causes emphysema?
- Smoking
- Air pollution
- Enzyme deficiency
- Dust exposure
122
What are the symptoms of emphysema?
- Reduced elasticity of alvioli (causes shortness of breath)
- Less O2 for cells
- Chronic cough reflex
123
What are the long term consequences of emphysema?
- Lungs are permanently stretched
- SA of alvioli reduced
- Thicker walls of alvioli
124
What are the effects of TB?
- Immune system builds a wall around bacterial infections
- Tidal volume decreases
- Causes fibrosis
- Causes reduced oxygen uptake
125
What is fibrosis?
- Scar tissue in the lungs as a result of an infection/substances
- Creates thicker, less elastic lung tissue
126
What causes asthma?
- Airways become inflamed due to pollen or allergic reaction
| - Bronchioles constrict
127
How are red blood cells adapted to perform mass transport?
- Large SA, biconcave shape (greater SA:V)
- Lots of haemoglobin
- No nuclei (so more haemoglobin)
- Flexible plasma membrane (fit through narrow capillaries)
- Selectively permeable to O2 and CO2
128
What protein structure is haemoglobin?
Quaternary, with a haem prosthetic group (contains Fe2+)
129
What is it called when O2 combines with haemoglobin?
Oxyhaemoglobin
130
Under what conditions does haemoglobin load oxygen?
When there is a high partial pressure of oxygen
131
Haemoglobin has a ___ _______ for oxygen
Haemoglobin has a high affiity for oxygen
132
Where in the body do you find the highest O2 pp?
The lungs
133
Why are oxygen dissociation curves S-shaped?
As one molecule of oxygen binds to one of the Fe2+ the haemoglobin changes shape, exposing the second haem group and so forth...
Called the co-operative nature of oxyge binding
134
How does the Bohr effect ensure that sufficient oxygen is available for respiring tissues?
- More respiration releases more CO2
- pH of blood is lowered
- Shape of haemoglobin cahnges
- More O2 is subsequently unloaded
- More O2 avaliable for respiration
135
Where would you expect Llamas (who live at 5000m) oxygen dissociation curve to be relative to humans?
To the left of humans because there is a lower oxygen pp at higher altitudes
136
Myoglobin is an O2 store in muscle cells. Where will its ODC lie?
Left of haemoglobin, has to be able to take O2 from haemoglobin
137
Why is carbon monoxide dangerous?
It can combine with haemoglobin in place of oxygen
138
Why do large organisms need a mass transport system?
- Bigger, so lower SA:V
- Substances required cannot be absorbed through external surfaces
- Waste not excreted fast enough
139
What are the main features of a mass transport system?
- Suitable medium to carry materials
- Arteries and veins systems
- Branches for even distribution to all parts of organism
- Mechanism to move medium (heart)
140
What is mass flow?
- Movement of medium together
- At same speed
- In same direction
- Over large distances
- Due to pressure differences
141
What is a double circulatory system?
2 parts. Eg One part to heart, one part to lungs
142
What does blood transport around the body?
Respiratory gases, products of digestion, metabolic wastes and hormones
143
What supplies the heart with blood?
The coronary arteries
144
What blood vessel leaves from the left ventricle?
Aorta
145
Where does the Aorta go?
To body tissues from the heart
146
What blood vessel leaves from the right ventricle?
Pulmonary artery
147
Where does the pulmonary artery go?
To lungs from the heart
148
What blood vessel enters the right atrium?
Vena cava
149
Where does the vena cava go?
From body tissues to the heart
150
What blood vessel enters the left atrium?
Pulmonary vein
151
Where does the pulmonary vein go?
From lungs to heart
152
What separates the chambers on the left side of the heart from the right?
The septum
153
What valve lies between the right atrium and right ventricle?
The right atrio ventricular valve
154
What valve lies at the base of the aorta?
Semi-lunar valves (same for pulmonary artery)
155
Why is the wall of the left ventricle of the heart thicker than the right one?
- Left ventricle pumps blood all around the body
- Requires more forceful contractions...
- ...To produce a higher pressure than right ventricle
156
Why are atria of heart thin-walled?
Only need to pump blood into ventricles
157
What role do tendons play in the heart?
Prevents valves turning inside out
158
Why are the chambers of the heart lined with endothelium?
To reduce friction
159
What do atrio ventricular valves do?
Stop blood flowing back into the atria when the ventricles contract
160
What happens in diastole? (part of cardiac cycle)
- Atria and ventricles relax
- Semi lunar shut
- Atrioventricular open ("DUB")
- Blood enters the ventricles from the atria
161
What happens in atrial systole? (part of cardiac cycle)
- Atria contract
- Atrial volume decreases, pressure increases
- Remaining blood forced through AV valves into ventricles
162
What happens in ventricular systole? (part of cardiac cycle)
- Ventricles contract
- Atria relax
- Pressure in ventricles increases above that of atria
- AV valves close ("LUB")
- Ventricular pressure increases above that of pulmonary artery and aorta
- Semi lunar open
- Blood leaves heart
163
What is the purpose of valves in the circulatory system?
Ensures that flow is only in one direction. No backflow
164
What are pocket valves? How do they operate?
- Valves found in veins
- Close when veins are squeezed by contracting muscles
- Ensures blood returns to heart
165
Where is blood stored?
In liver and spleen
166
Name some risk factors of heart disease.
- smoking
- High blood pressure
- Blood cholesterol
- Diet
167
How does smoking cause heart disease?
- More CO in blood/on haemoglobin
- Less O2 in blood, heart works harder
- Nicotine increases rate
- Causes blood pressure to increase
168
How can a poor diet cause heart disease?
- Salt can increase blood pressure
| - Sat fats can increase LDL cholesterol
169
What are the 4 main layers of arteries + veins from outer-most layer to the inner-most layer
1. Tough outer layer
2. Muscle layer
3. Elastic lining
4. Lumen lining layer (endothelium)
5. Lumen (not really a layer, but a hole)
170
Why are arteries and veins lined with an endothelium?
To reduce friction to blood flow
171
What are some properties of arteries?
- Thick, muscular walls
| - elastic tissue to stretch and recoil in response to pressure changes (caused by heart beat), maintains pressure
172
True of false?
| Blood in veins is under high pressure.
FALSE. Low pressure
173
What are some properties of veins?
- Wider lumen
- Not much elastic/muscle tissue
- Valves to stop back flow
- Body muscle contraction aids flow
174
What are some properties of capillaries?
- Made of one layer of squamous endothelium cells
- Very leaky, allowing for exchange of materials
- Thin lumen to restrict blood flow, increasing time for diffusion
- Thin lumen also causes RBCs to cram up to walls, reducing diffusion distance
- Lots of capillaries to increase SA (as "capillary beds")
175
Why doesn't the exchange of substances with tissues occur in arteries/veins?
Unlike capillaries, the walls of arteries + veins are too thick, so diffusion distance is too far
176
What is the benefit of elastic tissue in arteries?
Helps even out pressure by:
- stretching when blood pressure is high, reduces pressure
- recoiling when blood pressure is low, increases pressure
177
What muscles are used to control blood flow through veins?
Skeletal muscles.
| Contraction squeezes the vein, forcing blood along
178
How does breathing affect blood flow in the vena cava?
- Inhalation causes volume of thorax to increase, pressure to decrease
- Diaphragm contracts (and flattens) causing abdominal volume to decrease and pressure to increase
- Together these force blood up
.
179
What two variables are involved in the formation of tissue fluid?
1. Water potential
| 2. Hydrostatic pressure
180
What is plasma?
Mostly water, with dissolved proteins, ions, glucose and other substances
181
What is hydrostatic pressure?
The pressure of a liquid (Blood)
182
What is ultrafiltration (in terms of tissue fluid)?
Small molecules (inc water) forced out of blood due to pressure (out of capillaries), forming tissue fluid
183
What is tissue fluid?
- A fluid which surrounds all cells in the body
- Similar to blood plasma, lacks proteins + blood cells
- Supplies nutrients to cells and removes waste
184
What is the hydostatic pressure in the arteriole end of capillary compared to tissue fluid?
Greater than in tissue fluid
185
What happens at the arteriole end of capillaries (in terms of tissue fluid)?
- Hydrostatic pressure in capillary higher than in tissue fluid
- Water and dissolved substances are forced out of capillaries, forming tissue fluid
186
What happens when water and dissolved substances leave the capillary?
The water potential in the capillary deceases (relative to the tissue fluid)
187
What causes some of the tissue fluid to re-enter the capillary at the venial end?
- Low water potential in capillary
- High (less negative) WP in tissue fluid
- Water and dissolved substances move into capillaries by osmosis
188
Why is the absorption of tissue fluid not 100% at venial ends of capillaries?
Some enters the lymphatic system (which re-enters blood at vena cava)
189
How does transpiration cause water to be transported up plant stems?
Water leaves via stomata, which results in negative pressure, drawing water up from roots
190
What controls the rate of transpiration?
Size and number of stomata
191
What are the two ways that water moves from cell to cell in leaves?
1. Apoplastic (cell wall pathway) - diffusion
| 2. Symplastic (through cell cytoplasm) - osmosis
192
What are xylem vessels made of?
Dead cells (without nucleus, thickened with lignin)
193
What does the xylem transport?
Water and mineral ions in solution; up plant, to leaves
194
What part of xylem tubes allow for lateral movement?
Pits. Areas where lignin is not present to water proff the otherwise permeable cellulose
195
What property of water causes it to move up xylem?
Water cohesion and adhesion
196
Are there end walls on xylem tubes?
No, xylem vessels are uninterrupted tubes
197
What 4 factors affect the rate of transpiration?
1. Light (lighter = more photosynthesis)
2. Temperature (hotter = more evaporation)
3. Humidity
4. Wind (increases conc gradient)
198
What apparatus can be used to estimate the rate of transpiration?
Potometer
199
Why do potometers give an estimate of the rate of transpiration?
Actually measures water uptake by plant, but assumed to be directly related to water loss by leaves
200
What precautions must be made when setting up a potometer?
1. Water tight joints
2. Cut shoot underwater
3. Cut shoot at slant
4. Dry leaves
5. Assemble underwater
201
What does a potometer really measure?
Water uptake, accounts for;
- Transpiration
- Leaks
- Respiration (water produced)
- Photosynthesis (water used)
- Structural support (vacuoles)
202
What causes the diameter of plant stems to vary?
Negative pressure of water varies due to fluctuations in rate of transpiration
203
True or False?
| Transpiration is the movement of water in both directions up/down stem.
FALSE. Only up stem (root -> leaf)
204
What substances could the phloem transport?
Glucose, starch and sucrose (generally solutes)
205
Define translocation?
Transportation of organic molecules and mineral ions from one part of a plant to another
206
What is another word for phloem?
Sieve tube elements
207
What is the site of sugar production from photosynthesis called?
Source
208
What is the "sink" in terms of translocation?
- Site sugars are relocated for use/storage
| - Can be above/below source
209
Why are companion cells found next to sieve tube elements?
Sieve tube elements have no nucleus and few organelles so companion cells support them
210
True of False?
| Translocation is a passive process.
FALSE. Requires energy
211
What theory is used to explain translocation?
Mass flow theory
212
Describe (in detail) translocation of sucrose.
1. Sucrose loaded into seive tubes at source by co-transport (ATP needed)
2. Sucrose enters sieve tube, lowers WP
3. Water moves into sieve tubes from xylem by osmosis
4. Sucrose is moved in sieve tube under high hydrostatic pressure
5. Sucrose moved into sink by active transport (ATP)
6. WP of sieve tube increases (/less negative), water moves to xylem by osmosis
213
How does sucrose enter sieve tube?
By co-transport with H+ ions/protons (H+ ions actively transported into cell wall of sieve tube to set up conc gradient)
214
What is needed for active transport?
ATP (Adenosine Tri Phosphate)
215
Why does the WP lower (more neg) in Sieve tube at source?
Sucrose moves into the sieve tube
216
What causes hydrostatic pressure to increase in the sieve tubes?
Water moving in by osmosis from the xylem (from high to low WP)
217
Does water move into the sink after moving in sieve tubes?
Yes. Sucrose moves into companion cells, lowers WP, so water moves in by osmosis
218
Is mass flow a passive process?
Yes (and no). Mass flow itself is passive, however it relies on the active transport of sucrose
219
Give some evidence which supports the mass flow theory.
- Sap is released from sieve tubes when cut
- Glucose concentration higher in leaves than roots
- Companion cells have many mitochondria
- Ringed stems have a bulge build up
- Tracer experiments show that CO2 (as sucrose) moves down plant over time (not up with xylem)
220
Give some evidence which opposes mass flow theory.
- Wouldn't sieve plates hinder mass flow?
- Not all solutes move at same speed
- Sucrose is delivered to all regions at same rate, should be vary due to different sucrose concentrations
