7.13 End of Chapter Questions
Question 1
When sucrose is actively transported into a companion cell, what occurs in the cytoplasm of the companion cell in terms of water potential and hydrogen ion concentration?
Options:
A. Water potential decreases; hydrogen ion concentration decreases
B. Water potential decreases; hydrogen ion concentration increases
C. Water potential increases; hydrogen ion concentration decreases
D. Water potential increases; hydrogen ion concentration increases
Correct Answer: B (Water potential decreases; hydrogen ion concentration increases)
- Active Loading of Sucrose:
Sucrose is transported into the companion cell using a proton-sucrose symporter. This process relies on a proton gradient, which is established by pumping H⁺ ions out of the cell using ATP-driven proton pumps.- When sucrose accumulates inside the cell, it reduces the water potential (a higher solute concentration leads to a lower water potential).
- At the same time, the active transport of protons out of the cell increases the hydrogen ion concentration (proton concentration) outside the cell while reducing it inside. However, as protons return with sucrose, the cytoplasmic hydrogen ion concentration increases temporarily during active transport.
Explanation of Incorrect Options:
Option A (Water potential decreases; hydrogen ion concentration decreases):
While it is true that water potential decreases due to the accumulation of sucrose, the hydrogen ion concentration in the cytoplasm does not decrease. Active transport of sucrose is accompanied by an increase in cytoplasmic hydrogen ions.
Option C (Water potential increases; hydrogen ion concentration decreases):
This is incorrect because the active loading of sucrose lowers water potential, not increases it. A lower water potential allows water to move into the cell via osmosis. Additionally, the hydrogen ion concentration does not decrease as protons are pumped back into the cell with sucrose.
Option D (Water potential increases; hydrogen ion concentration increases):
This is wrong because water potential does not increase during sucrose loading. The increase in solute concentration (sucrose) lowers water potential. However, the hydrogen ion concentration in the cytoplasm does increase, which makes this option partially incorrect.
Question 2
Which of the following combinations accurately describes the pressure found in xylem vessel elements and phloem sieve tube elements?
Options:
A. Xylem: negative; Phloem: negative
B. Xylem: negative; Phloem: positive
C. Xylem: positive; Phloem: negative
D. Xylem: positive; Phloem: positive
Correct Answer: B (Xylem: negative; Phloem: positive)
- Xylem (Negative Pressure):
Xylem vessels transport water and minerals from roots to leaves through transpiration. Transpiration creates tension (negative pressure) in the xylem, as water molecules are pulled upwards due to cohesion and adhesion forces. This process relies on the generation of a suction-like effect. - Phloem (Positive Pressure):
Phloem sieve tube elements transport sugars (e.g., sucrose) from sources (like leaves) to sinks (like roots or growing tissues). This occurs through a mechanism known as pressure-flow hypothesis:- At the source, sucrose is actively loaded into the phloem, reducing water potential and causing water to flow in from adjacent xylem vessels via osmosis.
- The increased water volume creates positive hydrostatic pressure, pushing the sap through the sieve tubes towards the sink.
- At the sink, sucrose is unloaded, raising water potential, and water exits the phloem.
Explanation of Incorrect Options:
Option A (Xylem: negative; Phloem: negative):
While it is correct that the xylem operates under negative pressure due to transpiration pull, the phloem experiences positive pressure due to the pressure-flow mechanism.
Option C (Xylem: positive; Phloem: negative):
This is incorrect because xylem operates under negative pressure caused by transpiration, not positive pressure. Similarly, phloem does not experience negative pressure but positive pressure during sap transport.
Option D (Xylem: positive; Phloem: positive):
This is wrong because the xylem does not function under positive pressure. While some pressure may occur in root pressure scenarios, the dominant mechanism in xylem involves negative pressure. The phloem, however, does experience positive pressure.
Question 3
The figure shows transverse sections (TS) of two plant organs, X and Y, both containing vascular tissue.
Which of the following combinations correctly identifies the tissues in Figures X and Y?
Options:
A. X: 1 (Phloem); Y: 4 (Phloem)
B. X: 2 (Phloem); Y: 3 (Phloem)
C. X: 1 (Xylem); Y: 4 (Phloem)
D. X: 2 (Xylem); Y: 3 (Xylem)
Correct Answer: C (X: 1 is xylem; Y: 4 is xylem)
- Figure X (Dicot Root):
- In a dicot root, xylem forms an “X” shape at the center, with phloem located between the arms of the “X”.
- Therefore, 1 is the xylem, and 2 is the phloem.
- Figure Y (Dicot Stem):
- In a dicot stem, vascular bundles are arranged in a ring. The cambium (3) lies between the xylem and phloem in each bundle. The phloem (4) is closer to the outer edge.
- However, the question asks for identification of xylem, which would correspond to 4 in this context.
Explanation of Incorrect Options:
Option A (X: 1 is phloem; Y: 4 is phloem):
This misidentifies the central “X” in Figure X, which is xylem, not phloem. Similarly, the tissue labeled as 4 in Figure Y is xylem, not phloem.
Option B (X: 2 is phloem; Y: 3 is phloem):
While 2 in Figure X is indeed phloem, 3 in Figure Y is cambium, not phloem.
Option D (X: 2 is xylem; Y: 3 is xylem):
This misidentifies 2 in Figure X, which is phloem, not xylem. Similarly, 3 in Figure Y is cambium, not xylem.
Question 4
Why can’t the movement of water from a root hair to the xylem occur entirely through the apoplast pathway? Which layer of cells blocks this pathway?
Options:
A. Cortex
B. Endodermis
C. Epidermis
D. Pericycle
Correct Answer: B (Endodermis)
- Role of the Endodermis:
- The endodermis is a layer of cells surrounding the vascular bundle in roots. These cells have a specialized structure called the Casparian strip, a waxy band made of suberin.
- The Casparian strip is impermeable to water and solutes, blocking the apoplast pathway. Water and nutrients must cross the plasma membrane of endodermal cells to enter the symplast pathway, allowing selective absorption into the xylem.
Explanation of Incorrect Options:
Option A (Cortex):
The cortex is the tissue between the epidermis and the endodermis. Water can freely move through the apoplast pathway here, so the cortex does not block this movement.
Option C (Epidermis):
The epidermis is the outermost layer of the root, where root hairs are located. It allows water to enter freely and does not block the apoplast pathway.
Option D (Pericycle):
The pericycle is located inside the endodermis, surrounding the xylem and phloem. It is not involved in blocking the apoplast pathway; water has already crossed the Casparian strip before reaching the pericycle.
Question 5
a) How water moves from the soil into a root hair cell [3 marks]
Osmosis: Water moves from the soil into the root hair cell by osmosis, as the soil water has a higher water potential compared to the cytoplasm of the root hair cell.
Concentration Gradient: This gradient is created because the root hair cell has a high solute concentration (lower water potential) due to the active uptake of minerals from the soil.
Semi-permeable Membrane: The movement occurs through the selectively permeable membrane of the root hair cell, which allows water to pass but restricts large molecules.
b) How water moves from one root cortex cell to another [4 marks]
Apoplast Pathway: Water moves through the cell walls and intercellular spaces without crossing any membranes (non-living pathway).
Symplast Pathway: Water moves from one cell to another through the cytoplasm, connected by plasmodesmata (living pathway).
Osmosis in Symplast Pathway: Water moves due to differences in water potential between neighboring cells.
Cohesion and Diffusion: In the apoplast pathway, water molecules are held together by cohesion and move due to diffusion along a water potential gradient.
c) How water moves from a xylem vessel into a leaf mesophyll cell [3 marks]
Pathway: Water moves either through the apoplast pathway (cell walls) or the symplast pathway (cytoplasm via plasmodesmata) to reach mesophyll cells.
Transpiration Pull: Water moves up the xylem vessels due to negative pressure created by transpiration in the leaves.
Osmosis: Water moves into the leaf mesophyll cells from the xylem because mesophyll cells have a lower water potential (due to evaporation of water from their surfaces).
Question 6
Arrange the following components in decreasing order of water potential, using the symbol > to represent “greater than”:
Dry air, mesophyll cell, root hair cell, soil solution, xylem vessel contents.
The correct order is:
Soil solution > Root hair cell > Xylem vessel contents > Mesophyll cell > Dry air
Explanation:
Dry air: Dry air has the lowest water potential due to its very low water vapor content, driving the process of transpiration.
Soil solution: Water in the soil has the highest water potential because it is relatively dilute and not under tension.
Root hair cell: Water potential is slightly lower than the soil due to the active absorption of solutes, creating a gradient for water uptake.
Xylem vessel contents: The water potential decreases in the xylem due to the tension created by transpiration, which pulls water upward.
Mesophyll cell: Water potential in mesophyll cells is lower because water evaporates from their surfaces during transpiration, creating a gradient.
Question 7
Figure A illustrates how the atmospheric relative humidity varies during the daylight hours of a 24-hour period.
Figure B displays the changes in xylem tension within a tree during the same time frame. The xylem tension is measured in kilopascals (kPa), where higher tension is represented by increasingly negative pressure values.
a) Describe and explain the relationship between relative humidity and xylem tension. [4 Marks]
- Observation: As relative humidity decreases, xylem tension becomes more negative, particularly between 9:50 and 13:00 hours when the relative humidity drops steeply from around 88% to 35%. After 13:00, as relative humidity begins to increase, xylem tension becomes less negative.
- Explanation:
- Humidity and Transpiration: Lower relative humidity increases the water vapor deficit between the leaf and the surrounding air. This enhances transpiration rates.
- Xylem Tension: Higher transpiration rates create a greater pull on the water column in the xylem, leading to an increase in tension (more negative pressure).
- Reversal After 13:00: As humidity rises again, transpiration rates decrease, reducing the pull on the water column and making the tension less negative.
b) Describe and explain the differences in xylem tension between the top and bottom of the tree. [3 Marks]
Water Column Continuity: Greater tension is needed at the top to maintain the continuous water column as it experiences the strongest transpiration pull.
Observation: The xylem tension is more negative at the top of the tree compared to the bottom at all times. For example, at 13:00, tension is approximately -2250 kPa at the top and -1500 kPa at the bottom.
Explanation:
Height Difference: The top of the tree is farther from the roots, requiring a higher tension to pull water upwards against gravity.
Transpiration Rates: The top of the tree is exposed to higher light intensity and wind, increasing transpiration rates and, consequently, the tension.
Question 8
A dendrogram is a device used to monitor slight variations in the diameter of a tree trunk. It typically indicates that the tree trunk’s diameter is smallest during the daytime and largest at night. Propose a reason for these changes. [4 marks]
Answer:
Tissue Relaxation: With less tension in the xylem, the vessels and surrounding tissues relax and expand, leading to an increase in trunk diameter.
Daytime (Smallest Diameter):
Transpiration Rates: During the day, transpiration is at its highest due to sunlight and lower relative humidity. This increases the pull on the water column in the xylem, creating greater tension (more negative pressure).
Xylem Contraction: The high tension in the xylem causes the vessels and surrounding tissues to contract slightly, reducing the trunk’s diameter.
Nighttime (Largest Diameter):
Reduced Transpiration: At night, transpiration slows or stops due to the absence of sunlight and higher relative humidity. This reduces the tension in the xylem.
A dendrogram is a device used to monitor slight variations in the diameter of a tree trunk. It typically indicates that the tree trunk’s diameter is smallest during the daytime and largest at night. Propose a reason for these changes. [4 marks]
Question 9
a) Define the term transpiration. [2 marks]
Transpiration is the process by which water vapor is lost from the aerial parts of a plant, primarily through stomata on the leaves. It involves the movement of water from the xylem to the atmosphere.
b) Suggest two environmental factors likely responsible for the changes in transpiration rate. [2 marks]
- Light intensity: Transpiration rates increase with light because stomata open during the day for photosynthesis.
- Temperature: Higher temperatures increase water evaporation from leaf surfaces, raising the transpiration rate.
c) Describe the relationship between the rate of transpiration and the rate of water uptake. [2 marks]
The rate of water uptake closely follows the rate of transpiration but is slightly delayed. Transpiration peaks at midday, while water uptake peaks later in the afternoon. Water uptake starts and ends at higher levels than transpiration.
d) Explain the relationship. [4 marks]
Environmental Influence: Transpiration rate is directly affected by external factors like light and temperature, while water uptake is influenced by the water potential gradient created by transpiration.
Transpiration Drives Water Uptake: Transpiration creates negative pressure in the xylem, pulling water from the roots to replace the water lost through evaporation.
Lag Between Processes: The delay in water uptake is due to the time needed for water to move from the soil to the leaves through the plant’s vascular system.
Higher Water Uptake at Night: Water uptake remains higher at night to rehydrate tissues and replenish the water lost during the day.
Question 10
Explain how the active transport of sucrose into companion cells results in the following observations:
a) The cytoplasm of companion cells has a high pH of approximately 8. [1]
b) The inside of companion cells is negatively charged compared to the outside, with an electrical potential difference of about -150 mV. [2]
c) Companion cells contain a high concentration of ATP. [1]
a) Relatively high pH in the cytoplasm (about pH 8): [1 mark]
- Active transport of sucrose into companion cells involves proton pumps (H⁺-ATPases) expelling hydrogen ions (H⁺) out of the cytoplasm into the cell wall space.
- The removal of H⁺ increases the pH of the cytoplasm, making it less acidic (more alkaline).
b) Negative electrical potential inside companion cells: [2 marks]
- Proton Pump Activity: The active transport of H⁺ ions out of the cytoplasm creates an electrochemical gradient, leaving behind more negatively charged ions inside the cell.
- Membrane Potential Difference: This gradient establishes a difference in electrical potential across the cell membrane, with the inside becoming negatively charged relative to the outside (-150 mV).
c) High ATP concentration inside companion cells: [1 mark]
- The active loading of sucrose relies on ATP-driven proton pumps to maintain the proton gradient.
- High energy demand for these pumps results in companion cells containing large amounts of mitochondria and producing high concentrations of ATP to fuel these processes.
Question 11
Organic solutes are transported between sources and sinks in plants.
a) Briefly explain the circumstances under which the following plant parts can act as sinks or sources:
i) A seed as a sink [1 mark]
ii) A seed as a source [1 mark]
iii) A leaf as a sink [1 mark]
iv) A leaf as a source [1 mark]
v) A storage organ as a sink [1 mark]
vi) A storage organ as a source [1 mark]
b) Suggest two possible roles for glucose in the following sinks:
i) A storage organ [2 marks]
ii) A growing bud [2 marks]
Answers:
a) Circumstances for sinks and sources:
i) A seed as a sink:
During germination or development, seeds act as sinks because they require organic solutes like sucrose for growth and energy storage.
ii) A seed as a source:
Once a seed matures, it becomes a source as it mobilizes stored nutrients (e.g., starch converted to glucose) for seedling growth.
iii) A leaf as a sink:
Young or developing leaves act as sinks because they are not fully photosynthetically active and rely on organic solutes from other parts of the plant for growth.
iv) A leaf as a source:
Mature leaves act as sources when they photosynthesize and produce sugars, which are transported to sinks like roots or fruits.
v) A storage organ as a sink:
Storage organs (e.g., tubers) act as sinks during the growing season when sugars are transported to them for storage.
vi) A storage organ as a source:
Storage organs become sources during periods of active growth (e.g., spring), mobilizing stored carbohydrates to supply energy and nutrients to developing parts of the plant.
b) Roles of glucose in sinks:
i) In a storage organ:
- Energy Storage: Glucose is converted into starch or other carbohydrates for long-term energy storage.
- Structural Components: Glucose can be used to synthesize cellulose or other structural polysaccharides for the storage organ’s development.
ii) In a growing bud:
Synthesis of Biomolecules: Glucose is a precursor for the synthesis of nucleotides, proteins, and other molecules essential for the development of the growing bud.
Energy Supply: Glucose is metabolized during cellular respiration to provide ATP for cell division and elongation.