Question 1

Plant gas exchange — By what physical process do CO2 and O2 normally enter and leave plant tissues through stomata?

Accepted Answer

Introduction / Context: Gas exchange in plants underpins photosynthesis and respiration. Carbon dioxide must diffuse in for photosynthesis, and oxygen diffuses out (or in for respiration). Understanding the dominant transport mechanism across stomata and intercellular spaces is foundational in plant physiology. Given Data / Assumptions: Stomata regulate pore aperture but do not actively pump gases. CO2 and O2 are small, nonpolar molecules that readily diffuse down concentration gradients. Leaf internal spaces provide short diffusion paths to mesophyll cells. Concept / Approach: Diffusion is the random molecular motion leading to net movement from higher to lower partial pressure. Stomatal opening sets the boundary conditions; once open, CO2 and O2 move by diffusion driven by concentration and partial pressure differences between ambient air and the moist internal leaf air spaces. Step-by-Step Solution: Recognize that plants do not expend ATP to import CO2.Stomatal guard cells modulate aperture, not the mode of gas transport.Therefore, the physical process is diffusion across the stomatal pore and through intercellular spaces. Verification / Alternative check: Fick’s law describes flux proportional to gradient and conductance; empirical measurements of stomatal conductance and intercellular CO2 concentration align with diffusion models. Why Other Options Are Wrong: Osmosis: applies to water movement, not gases. Active transport: no ATP-dependent gas pumps operate for CO2/O2 in leaves. Evaporation: describes water vapor loss, not CO2/O2 influx. Facilitated diffusion via aquaporins: aquaporins move water, not CO2/O2 as a primary mechanism. Common Pitfalls: Equating stomatal opening with active import. The guard cells use energy to change aperture, but gas molecules move passively by diffusion. Final Answer: Diffusion

Question 2

Phloem transport — What is the widely accepted mechanism explaining long-distance movement of sugars in the phloem of vascular plants?

Accepted Answer

Introduction / Context: Products of photosynthesis (primarily sucrose) must be distributed from sources (mature leaves) to sinks (developing tissues, roots, storage organs). The pressure flow hypothesis, proposed by Münch, explains how this bulk movement occurs efficiently through the phloem sieve-tube system. Given Data / Assumptions: Sucrose is loaded into sieve-tube elements at sources, often via active processes. Loading reduces water potential in phloem, causing water to enter osmotically from xylem. Sinks unload sucrose, raising water potential and allowing water exit. Concept / Approach: Osmotically generated pressure differences between source and sink regions drive a mass flow of phloem sap. This does not rely on transpiration pull (a xylem phenomenon), nor on cytoplasmic streaming over long distances. It is a bulk flow under pressure gradients along sieve tubes with companion cells maintaining loading/unloading. Step-by-Step Solution: At sources: active loading of sucrose lowers psi, water enters, pressure rises.Along sieve tubes: pressure differential drives sap toward sinks.At sinks: sucrose unloading raises psi, water leaves, pressure drops.Net result: mass flow from source to sink. Verification / Alternative check: Experiments show turgor pressure differences along phloem and rapid movement rates consistent with bulk flow; girdling and aphid stylet studies support the hypothesis. Why Other Options Are Wrong: Root pressure: insufficient and episodic; mainly affects xylem. Transpiration: drives xylem ascent, not phloem flow. Cytoplasmic streaming: local phenomenon, too slow for long-distance transport. Electro-osmosis: not the dominant mechanism in intact plants. Common Pitfalls: Confusing xylem and phloem drivers; transpiration pull is not responsible for sugar transport in phloem. Final Answer: Pressure flow (mass flow) hypothesis

Question 3

Mineral nutrition — The uptake of most mineral ions, including potassium (K+), into root cells occurs primarily by which mechanism?

Accepted Answer

Introduction / Context: Plants require mineral ions for metabolism and structure. Many essential cations and anions occur at low concentrations in soil solution. Cells must therefore accumulate them against steep electrochemical gradients, a task that hinges on membrane transport proteins and energy input. Given Data / Assumptions: Root epidermal and cortical cells express specific ion transporters and channels. Proton pumps (H+-ATPases) establish membrane potential and pH gradients. Potassium is a key macronutrient with tightly regulated uptake. Concept / Approach: Active transport uses metabolic energy (ATP) to drive ions into the symplast against gradients, often via H+-coupled co-transporters or channels energized by the proton motive force. Passive diffusion alone cannot achieve the high intracellular concentrations commonly observed for K+ and other nutrients. Step-by-Step Solution: H+-ATPase exports protons, generating negative membrane potential inside.K+ uptake occurs through selective channels or symporters driven by the electrochemical gradient.Cations/anions are further distributed via symplastic pathways to the stele.Ions cross the endodermis (Casparian strip) via symplastic routes to reach xylem. Verification / Alternative check: Pharmacological inhibitors of H+-ATPase reduce ion uptake; electrophysiology shows voltage-dependent K+ channels facilitating uptake consistent with active mechanisms. Why Other Options Are Wrong: Osmosis: water movement, not ion accumulation. Apoplastic diffusion alone: cannot explain uphill transport into cells. Bulk flow: moves ions in xylem long-distance, not into living root cells across membranes. Endocytosis: not the primary route for inorganic ion uptake. Common Pitfalls: Assuming that because ions are small they enter passively; physiological concentrations demand active, regulated uptake systems. Final Answer: Active transport across cell membranes

Question 4

Water relations — By which process does water primarily move from soil into the root cells of plants across the plasma membrane?

Accepted Answer

Introduction / Context: Water uptake is fundamental to plant survival, supporting cell turgor, nutrient transport, and photosynthesis. Understanding the immediate mechanism by which water crosses membranes into root cells clarifies how plants maintain internal water status despite changing soil conditions. Given Data / Assumptions: Root epidermal cells contact soil solution and have semi-permeable membranes. Solute accumulation in cells lowers water potential relative to soil water potential (under non-saline conditions). Aquaporins facilitate water flow but do not use ATP directly. Concept / Approach: Osmosis is the passive movement of water across a semi-permeable membrane from higher water potential (or lower solute concentration) to lower water potential (higher solute concentration). Ion pumps set up solute gradients; water follows those gradients osmotically into living cells of the root cortex and epidermis. Step-by-Step Solution: Root cells accumulate solutes via transporters.Water potential inside the cells drops relative to soil solution.Water crosses the plasma membrane through lipid bilayer and aquaporins down its potential gradient.Symplastic/apoplastic pathways converge toward the xylem after crossing the endodermis. Verification / Alternative check: Blocking aquaporins reduces hydraulic conductivity; however, water still moves passively down gradients, confirming an osmotic mechanism rather than direct ATP-driven pumping of water. Why Other Options Are Wrong: Active transport: there are no ATP-driven water pumps in plants. Diffusion (for solutes): water’s membrane movement is termed osmosis. Pressure flow: a phloem mechanism, not for membrane-level water entry. Capillary action: acts in cell walls/xylem conduits, not across the lipid membrane. Common Pitfalls: Confusing xylem bulk flow (cohesion–tension) with membrane-level water entry; both are important but distinct processes. Final Answer: Osmosis

Question 5

Stomatal physiology — What happens to stomatal aperture when guard cells swell (become turgid)?

Accepted Answer

Introduction / Context: Stomata regulate gas exchange and water loss. Guard cell turgor changes are central to stomatal opening and closing, integrating environmental and hormonal signals such as light, CO2, humidity, and abscisic acid (ABA). Given Data / Assumptions: Guard cells form a pore whose width depends on their turgor pressure. K+, Cl−, and malate^2− fluxes alter osmotic status of guard cells. Cell wall architecture of guard cells promotes bending when turgor rises. Concept / Approach: Blue light and low internal CO2 promote ion uptake into guard cells. Water follows osmotically, increasing turgor pressure. Due to differential wall thickening and cellulose microfibril orientation, turgid guard cells bow outward, widening the pore and opening the stomata. Step-by-Step Solution: Environmental cue triggers ion uptake (e.g., K+ influx).Osmotic potential decreases; water enters guard cells.Guard cells swell and bow apart.Stomatal aperture increases (stomata open) for gas exchange. Verification / Alternative check: Direct microscopy shows aperture increases with guard cell turgor; ABA induces ion efflux, water loss, and closing, confirming the mechanism is reversible and ion-driven. Why Other Options Are Wrong: Closing occurs when turgor drops, not when it rises. Photosynthesis does not stop upon opening; CO2 entry facilitates it. Transpiration does not cease; it typically increases when stomata open. Plasmolysis is loss of turgor, opposite of swelling. Common Pitfalls: Assuming swelling always closes pores. Due to guard cell wall anatomy, swelling opens the pore; shrinking closes it. Final Answer: Stomata open

Question 6

Xylem transport — The cohesion–tension theory primarily explains water movement through which conducting tissue in vascular plants?

Accepted Answer

Introduction / Context: The cohesion–tension theory is the dominant model for ascent of sap, describing how water moves in a continuous column from roots to leaves driven by transpiration and supported by cohesive and adhesive forces within xylem conduits. Given Data / Assumptions: Water evaporates from mesophyll cell walls, creating tension (negative pressure) in leaf xylem. Hydrogen bonding leads to cohesion between water molecules and adhesion to xylem walls. Xylem tracheids and vessel elements are dead, lignified tubes optimized for tension transmission. Concept / Approach: Transpiration at the leaf surface pulls on the water column. Because the xylem network is continuous from roots to leaves and water molecules cohere, this tension is transmitted downward, drawing water upward from the roots and ultimately from the soil. The theory specifically pertains to the xylem pathway, not to phloem or guard cell mechanics. Step-by-Step Solution: Transpiration increases curvature of water menisci in leaves.This generates negative pressure (tension) in leaf xylem.Cohesion transmits tension down the water column through xylem.Water is pulled from roots and soil to replace losses. Verification / Alternative check: Cavitation/vulnerability curves and pressure chamber measurements support tension-based transport; cutting stems breaks the continuous column and can cause embolisms, consistent with the model. Why Other Options Are Wrong: Phloem: transports sugars via pressure flow, not cohesion–tension. Guard/companion cells: regulate stomata or phloem loading, not bulk water ascent. “From leaf to root only”: water flow direction is root to leaf in xylem under transpiring conditions. Common Pitfalls: Assuming root pressure drives most ascent; it is minor compared with transpiration-driven tension in most conditions. Final Answer: Xylem

Question 7

Leaf water loss — The process describing evaporation of water from plant leaves (primarily through stomata) is called what?

Accepted Answer

Introduction / Context: Water loss from leaves is a central driver of the soil–plant–atmosphere continuum. Naming this process correctly is essential before tackling quantitative topics like transpiration rates, stomatal conductance, and water-use efficiency. Given Data / Assumptions: Most leaf water loss occurs through stomata, with a small fraction via cuticular evaporation. Transpiration is influenced by vapor pressure deficit, light, temperature, wind, and stomatal aperture. Transpiration indirectly drives mineral uptake and xylem flow. Concept / Approach: Transpiration is the evaporation of water from plant aerial parts, especially leaves. It creates tension that pulls water upward through xylem. Cohesion and tension are physical principles underlying movement but are not themselves the evaporative process. Pressure flow pertains to phloem transport and is unrelated to evaporative water loss. Step-by-Step Solution: Identify the phenomenon: water evaporates from moist cell walls to leaf air spaces and out stomata.Recognize its name: transpiration.Connect to function: drives xylem sap ascent and leaf cooling.Differentiate from other terms (cohesion, tension, pressure flow). Verification / Alternative check: Gas-exchange measurements (porometry, infrared gas analysis) quantify transpiration concurrently with photosynthesis, correlating with environmental drivers. Why Other Options Are Wrong: Cohesion/tension: mechanisms within water columns, not the evaporative process. Pressure flow: refers to phloem sugar transport. Guttation: exudation of liquid water at hydathodes, distinct from vapor loss. Common Pitfalls: Equating guttation (night or high humidity) with transpiration; guttation is liquid exudate, not vapor loss driven by VPD. Final Answer: Transpiration

Question 8

Phloem anatomy — Which cell types make up the functional conducting tissue of phloem in angiosperms?

Accepted Answer

Introduction / Context: Phloem transports photosynthates and signaling molecules throughout the plant. Understanding its cellular composition is vital to grasp loading/unloading, pressure flow, and response to injury (callose deposition at sieve plates). Given Data / Assumptions: Angiosperm phloem comprises sieve-tube elements associated closely with companion cells. Sieve plates connect sieve-tube elements into long conducting files. Companion cells support metabolic functions lost by sieve-tube differentiation. Concept / Approach: Sieve-tube elements are living, enucleate conduits specialized for mass flow of sap. Companion cells (derived from the same mother cell) maintain the metabolic machinery, regulate loading/unloading, and communicate via plasmodesmata. Xylem cells (tracheids, vessel elements) are dead at maturity and are not part of phloem. Step-by-Step Solution: Identify the conducting cells: sieve-tube elements.Recognize support cells: companion cells intimately connected.Exclude xylem elements (tracheids, vessels) from phloem.Select the correct pair: sieve-tube elements + companion cells. Verification / Alternative check: Microscopy reveals sieve plates and dense companion cell cytoplasm; functional studies show companion cell roles in sucrose loading and signaling. Why Other Options Are Wrong: Vessel elements/tracheids: xylem components. Tracheids with sieve-tube elements: mixes xylem with phloem. Parenchyma/fibers alone: lack sieve function. Common Pitfalls: Confusing sieve cells/albuminous cells (gymnosperms) with angiosperm sieve-tube/companion cell pairs. Final Answer: Sieve-tube elements and companion cells

Transport of Molecules in Plant Questions