101. Which of the following has the lowest water potential?
ⓐ. pure water
ⓑ. a dilute sugar solution
ⓒ. a concentrated sugar solution
ⓓ. pure water under slight positive pressure
Correct Answer: a concentrated sugar solution
Explanation: Water potential decreases as the concentration of dissolved solutes increases. Pure water has the highest reference value, which is zero under standard conditions. A dilute sugar solution has a lower water potential than pure water, but a concentrated sugar solution has an even more negative value. This is because more dissolved solute reduces the free energy of water more strongly. Positive pressure can raise water potential, not lower it. Therefore, among the given options, the concentrated sugar solution has the lowest water potential.
102. Why is water potential considered a better concept than simple concentration for explaining water movement in plants?
ⓐ. It ignores the role of pressure and focuses only on solutes
ⓑ. It applies only to laboratory solutions and not to living cells
ⓒ. It includes both solute effects and pressure effects on water movement
ⓓ. It is used only for dry seeds and woody tissues
Correct Answer: It includes both solute effects and pressure effects on water movement
Explanation: Water movement in plants is influenced not only by how much solute is present but also by physical pressure acting on the water. A simple concentration-based explanation often misses the effect of turgor and other pressure factors. Water potential is better because it combines these influences into one usable concept. This makes it possible to explain movement between cells, into vacuoles, and through tissues more accurately. Plant physiologists therefore use water potential instead of relying on concentration alone. It gives a fuller picture of the real driving forces. Hence, water potential is better because it includes both solute effects and pressure effects.
103. A plant cell has a solute potential of -0.7 MPa and a pressure potential of +0.4 MPa. Its water potential is:
ⓐ. -1.1 MPa
ⓑ. -0.3 MPa
ⓒ. +0.3 MPa
ⓓ. +1.1 MPa
Correct Answer: -0.3 MPa
Explanation: Water potential is obtained by adding solute potential and pressure potential. In this case, the solute potential is -0.7 MPa and the pressure potential is +0.4 MPa. When these are added, the result is -0.3 MPa. This shows how positive pressure can partly offset the lowering effect caused by dissolved solutes. The total still remains negative because the magnitude of the solute effect is greater than the pressure contribution. Such calculations are important in understanding the direction of water movement in cells. Therefore, the water potential of the cell is -0.3 MPa.
104. Two adjacent plant cells have water potentials of -0.2 MPa and -0.6 MPa. Water will move:
ⓐ. from the cell with -0.6 MPa to the cell with -0.2 MPa
ⓑ. equally in both directions with no net movement
ⓒ. only if both cells have zero solute potential
ⓓ. from the cell with -0.2 MPa to the cell with -0.6 MPa
Correct Answer: from the cell with -0.2 MPa to the cell with -0.6 MPa
Explanation: Water moves from higher water potential to lower water potential. Between -0.2 MPa and -0.6 MPa, the value -0.2 MPa is higher because it is less negative. The value -0.6 MPa is lower because it is more negative. Therefore, net water movement will occur from the first cell to the second cell. This principle is central to plant water relations and applies whether the difference arises from solute concentration, pressure, or both. The sign and relative value of water potential determine the direction. Hence, water moves from the cell with -0.2 MPa to the cell with -0.6 MPa.
105. A cell is placed in pure water and becomes fully turgid. At equilibrium, the water potential of the cell is most likely:
ⓐ. zero
ⓑ. strongly positive
ⓒ. equal to its solute potential only
ⓓ. more negative than the surrounding pure water
Correct Answer: zero
Explanation: Pure water has a water potential of zero under standard conditions. When a cell is placed in pure water, water enters because the cell initially has a lower water potential. As water enters, turgor pressure develops inside the cell and raises its pressure potential. Entry continues until the water potential of the cell becomes equal to that of the surrounding pure water. At that point, equilibrium is reached and there is no net movement of water. Thus, the final water potential of the fully turgid cell becomes zero. Therefore, the correct answer is zero.
106. Two solutions have the same pressure potential, but solution X has a more negative solute potential than solution Y. Which statement is correct?
ⓐ. Solution X has the higher water potential
ⓑ. Solution Y has the lower water potential
ⓒ. Solution X has the lower water potential
ⓓ. Both solutions must have equal water potential
Correct Answer: Solution X has the lower water potential
Explanation: Water potential is the sum of solute potential and pressure potential. If the pressure potential is the same in both solutions, then the deciding factor becomes the solute potential. A more negative solute potential lowers the total water potential further. Therefore, the solution with the more negative solute potential will also have the lower water potential. This is why concentrated solutions usually attract water more strongly than less concentrated ones when pressure conditions are equal. The comparison depends on relative negativity, not just on the presence of solutes. Hence, solution X has the lower water potential.
107. A turgid cell and a flaccid cell contain the same amount of dissolved solute. Why does the turgid cell usually have a higher water potential?
ⓐ. The turgid cell has a more negative solute potential
ⓑ. The turgid cell has positive pressure potential that raises total water potential
ⓒ. The flaccid cell always has positive pressure potential
ⓓ. The turgid cell contains no vacuole
Correct Answer: The turgid cell has positive pressure potential that raises total water potential
Explanation: If both cells contain the same amount of dissolved solute, their solute potentials are similar. The important difference is that the turgid cell has developed internal pressure against the cell wall, whereas the flaccid cell has little or no such pressure. This positive pressure potential adds to the total water potential of the turgid cell. As a result, the turgid cell has a higher water potential than the flaccid one under similar solute conditions. This example clearly shows why pressure must be included when explaining plant water relations. Therefore, the higher water potential of the turgid cell is due to its positive pressure potential.
108. A student says, "Water always moves from dilute solution to concentrated solution, so water potential is unnecessary." What is the best correction?
ⓐ. The statement is fully correct because pressure never affects water movement
ⓑ. Water movement is better explained by total water potential, which includes both solute and pressure effects
ⓒ. Water potential is used only for dry seeds, not for living cells
ⓓ. Water moves only according to solute potential and never according to total water potential
Correct Answer: Water movement is better explained by total water potential, which includes both solute and pressure effects
Explanation: The idea that water moves simply from dilute to concentrated solution is useful in some simple cases, but it is incomplete for living plant cells. In real plant systems, pressure inside cells can significantly affect water movement. A turgid cell, for example, may contain solutes yet stop taking in water because pressure has raised its water potential. Water potential provides a fuller explanation by combining solute potential and pressure potential. This makes it a more accurate concept for understanding movement between cells and tissues. Therefore, the best correction is that total water potential, not concentration alone, explains water movement properly.
109. Two neighboring cells have identical water potentials, but one has a more negative solute potential and a more positive pressure potential than the other. What will happen?
ⓐ. Water will move into the cell with the more negative solute potential
ⓑ. Water will move into the cell with the more positive pressure potential
ⓒ. There will be no net movement of water between the two cells
ⓓ. Water will move equally only if both cells contain no solutes
Correct Answer: There will be no net movement of water between the two cells
Explanation: Net water movement depends on the difference in total water potential, not on one component alone. If two cells have the same total water potential, then neither cell has a higher tendency to lose or gain water overall. Even if one cell has a more negative solute potential, that effect can be balanced by a more positive pressure potential. This is why total water potential is more informative than looking at only one component separately. The balance of the two components determines equilibrium. Therefore, if the total water potentials are identical, there will be no net movement of water between the two cells.
110. A wilting herbaceous plant regains firmness after watering mainly because:
ⓐ. its solute potential becomes zero immediately
ⓑ. its pressure potential increases as cells regain turgor
ⓒ. its water potential becomes lower than that of dry soil
ⓓ. its dissolved solutes are removed from the vacuole
Correct Answer: its pressure potential increases as cells regain turgor
Explanation: Wilting occurs when cells lose water and their turgor falls, causing pressure potential to drop. After watering, water enters the cells again because the soil water potential becomes relatively higher than that of the dehydrated cells. As water enters, the vacuole expands and the protoplast presses against the cell wall. This restores turgor and raises the pressure potential. The return of this positive pressure is what makes the plant firm again. The explanation is therefore based on the pressure component of water potential. Hence, the plant regains firmness mainly because its pressure potential increases as cells regain turgor.
111. Which structure is primarily responsible for absorption of water from the soil in young roots?
ⓐ. root cap
ⓑ. root hair
ⓒ. xylem vessel
ⓓ. apical bud
Correct Answer: root hair
Explanation: Root hairs are thin, delicate extensions of epidermal cells found in the region of maturation of young roots. They greatly increase the surface area available for contact with soil particles and the thin film of water around them. Because of this close contact, they serve as the main absorbing structures for water in most roots. Their walls are thin and their plasma membrane remains active, which supports water uptake. The root cap mainly protects the tip, while xylem is involved in later transport rather than initial absorption. Thus, root hairs are the principal structures responsible for water absorption from soil.
112. A major advantage of root hairs in water absorption is that they:
ⓐ. secrete ATP directly into the soil
ⓑ. convert minerals into sugars for transport
ⓒ. increase the absorbing surface area of the root
ⓓ. form lignified channels for upward conduction
Correct Answer: increase the absorbing surface area of the root
Explanation: Root hairs are numerous and very slender, so together they produce an enormous surface area for the root system. This enlarged area allows more contact between the root and the water films surrounding soil particles. Greater contact means a greater opportunity for water to enter the plant. Their structure is therefore highly suited for efficient absorption. This adaptation is especially important because water is unevenly distributed in soil spaces. The increase in area is the chief reason root hairs are effective. Therefore, their main advantage is that they increase the absorbing surface area of the root.
113. Water enters a root hair cell from the soil mainly because:
ⓐ. the water potential of the soil solution is higher than that of the root hair cell
ⓑ. the root hair cell pushes water inward using ATP pumps
ⓒ. the soil particles force water directly into the xylem
ⓓ. the cell wall of the root hair is completely impermeable to solutes
Correct Answer: the water potential of the soil solution is higher than that of the root hair cell
Explanation: Water moves from a region of higher water potential to a region of lower water potential. In normal conditions, the cell sap of a root hair has a lower water potential than the surrounding soil solution because it contains dissolved substances. As a result, water enters the root hair by osmosis across the plasma membrane. This movement does not require pumping of water by ATP. The difference in water potential is the essential driving force. This explains why living root hair cells can absorb water effectively from the soil. Hence, water enters mainly because the soil solution has a higher water potential than the root hair cell.
114. Root hairs are usually found in the:
ⓐ. region of cell division near the root tip
ⓑ. region of elongation only
ⓒ. region of maturation of the root
ⓓ. permanently woody portion of the root
Correct Answer: region of maturation of the root
Explanation: The region of maturation lies behind the regions of division and elongation in a growing root. In this zone, epidermal cells differentiate and many of them develop into root hairs. These hairs are temporary absorbing structures and are most abundant where cells have completed elongation. Their position is advantageous because this region is actively involved in water and mineral absorption. The root tip itself is protected by the root cap and is not the main absorbing region. Therefore, the region of maturation is the correct location of root hairs.
115. Which statement best defines the apoplast pathway of water movement?
ⓐ. Water moves only through the cytoplasm of connected living cells
ⓑ. Water moves through cell walls and intercellular spaces without crossing membranes initially
ⓒ. Water is transported only through sieve tubes into the cortex
ⓓ. Water moves directly from soil to xylem through vacuoles only
Correct Answer: Water moves through cell walls and intercellular spaces without crossing membranes initially
Explanation: The apoplast pathway is the route in which water travels through the non-living continuum of cell walls and intercellular spaces. Along this path, water can move freely without repeatedly crossing plasma membranes in the outer root tissues. Because cell walls are porous, this route offers relatively little resistance to movement. It is therefore an important pathway for water as it moves through the cortex. However, this route is later blocked at a certain point in the root. Thus, the apoplast pathway is correctly described as movement through walls and intercellular spaces without initial membrane crossing.
116. Which statement best defines the symplast pathway of water movement?
ⓐ. Water moves through the interconnected cytoplasm of living cells
ⓑ. Water moves only through xylem vessels and tracheids
ⓒ. Water moves through air spaces between cortical cells
ⓓ. Water moves through cellulose fibers of dead tissues only
Correct Answer: Water moves through the interconnected cytoplasm of living cells
Explanation: The symplast consists of the continuous network of living cell cytoplasm connected by plasmodesmata. In the symplast pathway, water first crosses a plasma membrane to enter a living cell and then travels from cell to cell through these cytoplasmic connections. Because it passes through living protoplasts, this route is more controlled than movement through the apoplast. It is especially important after water has crossed the outer cell membrane of the root. This route allows selective participation of living tissues in water movement. Therefore, the symplast pathway is correctly defined as movement through interconnected cytoplasm of living cells.
117. The Casparian strip in the endodermis mainly functions to:
ⓐ. increase photosynthesis in root cortex cells
ⓑ. block apoplastic movement and force water into the symplast
ⓒ. produce root hairs near the vascular cylinder
ⓓ. pump sucrose from phloem into the soil
Correct Answer: block apoplastic movement and force water into the symplast
Explanation: The Casparian strip is a waxy band present in the radial and transverse walls of endodermal cells. It is impregnated with substances such as suberin, making it impermeable to water moving through the cell wall route. As a result, water traveling by the apoplast pathway cannot continue straight into the vascular tissue at this point. Instead, it must cross a plasma membrane and enter the symplast of endodermal cells. This helps the plant regulate what enters the xylem. The strip therefore plays a selective control role rather than a photosynthetic or secretory one. Hence, its main function is to block apoplastic movement and force water into the symplast.
118. Which tissue forms the innermost boundary of the cortex and regulates entry of water into the vascular tissue?
ⓐ. epidermis
ⓑ. pericycle
ⓒ. endodermis
ⓓ. pith
Correct Answer: endodermis
Explanation: The endodermis is the innermost layer of the cortex and surrounds the vascular cylinder. It is especially important because of the presence of the Casparian strip in its cell walls. This structure regulates the passage of water and dissolved substances into the stele by preventing unrestricted apoplastic flow. Because of this, the endodermis acts as a checkpoint between the cortex and the vascular tissues. Its regulatory role is central to root absorption. Therefore, the tissue forming this controlled boundary is the endodermis.
119. Which comparison between apoplast and symplast pathways is correct?
ⓐ. Both pathways occur only through dead cells of the cortex
ⓑ. Apoplast involves cell walls, whereas symplast involves cytoplasm connected by plasmodesmata
ⓒ. Apoplast requires repeated membrane crossing, whereas symplast never enters living cells
ⓓ. Symplast occurs only in xylem, whereas apoplast occurs only in phloem
Correct Answer: Apoplast involves cell walls, whereas symplast involves cytoplasm connected by plasmodesmata
Explanation: The two pathways differ mainly in the structures through which water moves. In the apoplast route, water passes through cell walls and intercellular spaces, which are non-living components. In the symplast route, water moves through the cytoplasm of living cells and passes from one cell to another via plasmodesmata. This makes the symplast pathway membrane-associated and more selective. The apoplast route is generally less controlled until it reaches the endodermis. Therefore, the correct comparison is that apoplast involves cell walls, whereas symplast involves cytoplasm connected by plasmodesmata.
120. Water moving by the apoplast pathway is finally compelled to cross a membrane at the:
ⓐ. root cap
ⓑ. epidermis
ⓒ. endodermis
ⓓ. pericycle
Correct Answer: endodermis
Explanation: Although water can move through cell walls and intercellular spaces in the cortex, this movement cannot continue unchecked into the vascular cylinder. At the endodermis, the Casparian strip blocks further passage through the apoplast. Because of this barrier, water must cross the plasma membrane of an endodermal cell to continue inward. This shifts the route from the non-living apoplast to the living symplast. The endodermis therefore serves as the point where membrane crossing becomes compulsory. Hence, the correct location is the endodermis.
