Transport in Plants Practice Questions

Correct answer: Xylem

Xylem is a specialized complex permanent tissue that forms a continuous system of tubes running throughout the plant. It transports water and inorganic ions unidirectionally from the roots to the aerial parts of the plant. Phloem, by contrast, is responsible for the multidirectional transport of organic food molecules.

Correct answer: Transpiration pull

Transpiration pull is driven by the evaporation of water vapor from the stomata of leaves into the atmosphere, creating a negative pressure or tension within the xylem vessels. This tension acts like a straw, pulling a continuous column of water upward from the roots. While root pressure and capillary action assist water movement in short plants, they are insufficient to reach the canopy of tall trees.

Correct answer: Translocation

Translocation is the precise term used for the movement of organic nutrients, or photosynthates, through the sieve tubes of the phloem tissue. This process is highly dynamic and multidirectional, delivering energy-rich sugars to active metabolic sinks like growing roots, fruits, and storage organs. Transpiration refers specifically to the loss of water vapor from leaves.

Correct answer: ATP

Loading sucrose against its concentration gradient into phloem companion cells and sieve elements requires active transport powered by ATP. The cell uses ATP to pump hydrogen ions out, creating a gradient that drives sucrose in via cotransport proteins. Once loaded, this accumulation of sugar pulls water in by osmosis, generating the high hydrostatic pressure needed to push the sap through the phloem.

Correct answer: Xylem parenchyma

Xylem parenchyma cells are the only living components found within mature xylem tissue, serving primarily to store carbohydrates and fats and assist in short-distance lateral water transport. Vessels, tracheids, and fibers all undergo programmed cell death at maturity. Their hollow, lignified remnants are essential for minimizing resistance to water flow and providing structural support.

Correct answer: Cohesion

Cohesion is the attraction between like molecules, which binds water molecules tightly together to resist pulling forces. Adhesion is the attraction between water molecules and the hydrophilic walls of the xylem vessels, preventing the water column from slipping downward under gravity. Together, cohesion and adhesion allow water columns to remain unbroken under immense tension.

Correct answer: Guard cells

Guard cells are pairs of specialized crescent-shaped cells flanking each stomatal pore. When water flows into these cells by osmosis, they turgidly swell and bow outward due to their unevenly thickened cell walls, opening the pore. When they lose water, they flaccidly collapse inward, closing the pore to prevent excessive dehydration.

Correct answer: Apoplastic pathway

The apoplastic pathway is a fast, passive route where water and solutes move strictly through the non-living matrix of cell walls and extracellular spaces. The symplastic pathway, conversely, requires water to enter the cytoplasm of a cell and travel through thin cytoplasmic bridges called plasmodesmata. The apoplastic route is blocked when it reaches the endodermis layer of the root.

Correct answer: Casparian strip

The Casparian strip is a belt-like deposit of suberin located within the transverse and radial walls of endodermal cells. It acts as a waterproof seal that blocks passive apoplastic water flow into the central vascular cylinder. This block forces water and solutes to pass through the plasma membrane into the symplast, allowing the root to filter out toxic compounds or excess minerals.

Correct answer: Sieve tube elements

Sieve tube elements are elongated cells joined end-to-end to form the functional conduits of phloem translocation. To maximize internal space for fluid flow, they shed their nuclei and most major organelles during development. They remain metabolically alive through close structural connections with highly specialized companion cells via abundant plasmodesmata.

Correct answer: The Pressure-Flow hypothesis

The Pressure-Flow hypothesis states that the active loading of sugars at a source (like leaves) lowers water potential, drawing water from adjacent xylem into the phloem via osmosis. This creates high turgor pressure at the source end. At the sink (like roots), sugars are unloaded, water leaves the phloem, and pressure drops, creating a hydrostatic pressure gradient that drives mass flow.

Correct answer: Atmospheric humidity

High atmospheric humidity decreases the water vapor concentration gradient between the saturated interior of the leaf and the outside air. Because diffusion slows down when a concentration gradient is shallow, transpiration rates drop. Increases in light, wind, and temperature all accelerate transpiration by altering stomatal opening or clearing away stagnant moisture layers.

Correct answer: Active transport

When plants must absorb mineral nutrients against a concentration gradient, they cannot rely on passive diffusion. Instead, specialized transport proteins embedded in the plasma membranes of root hair cells utilize active transport, expending ATP energy to pump ions into the cell. Osmosis applies to the movement of water molecules, not dissolved minerals.

Correct answer: Guttation

Guttation is driven by positive root pressure that builds up during the night when soil moisture is high but stomata are closed and transpiration is at a standstill. The roots continue to pump minerals and absorb water, forcing excess sap upward and out through pore openings called hydathodes. Transpiration involves invisible water vapor rather than visible liquid droplets.

Correct answer: Lignin rings or spirals

Lignin is a highly rigid, complex polymer deposited within the secondary cell walls of xylem vessels and tracheids. It hardens the walls, allowing them to withstand the strong crushing forces of negative hydrostatic pressure generated by transpiration. Without lignification, the water conduits would collapse inward, stopping transport.

Correct answer: The roots starve first, since sugars from the leaves can no longer reach them.

Girdling cleanly severs the phloem network located just beneath the outer bark while leaving the deeper, interior xylem intact. Because sugars can no longer travel downward from the leaves, the roots are starved of energy, leading to their death and the eventual decline of the entire tree. Water transport continues initially because the xylem is untouched.

Correct answer: The membrane shrinks away from the rigid wall, a process known as plasmolysis.

In a hypertonic environment, water moves out of the vacuole and cytoplasm via osmosis to balance solute concentrations. As water leaves, the cell loses its internal turgor pressure, and the flexible plasma membrane pulls away from the structural cell wall, a condition known as plasmolysis. This loss of turgor causes the plant to wilt physically.

Correct answer: Plasmodesmata

Plasmodesmata are microscopic channels that penetrate the cell walls of adjacent plant cells, connecting their cytoplasms into a continuous system known as the symplast. This allows small molecules, ions, and water to pass easily from cell to cell without facing the barrier of plasma membranes. Stomata and lenticels are structures used for gas exchange with the outer atmosphere.

Correct answer: Lenticels

Lenticels are lens-shaped, aerating pores that rupture the cork layer of woody stems and roots, providing a direct pathway for internal tissues to exchange oxygen and carbon dioxide with the environment. Hydathodes are structures involved in liquid water release during guttation, while sieve plates are the perforated end-walls of phloem elements.

Correct answer: By extending thin tubular outgrowths that increase the root's absorptive surface area

Root hairs are fine, elongated extensions of individual root epidermal cells. By projecting into the microscopic crevices of the soil, they dramatically expand the total surface area available for contact with thin film water and minerals. This expansion optimizes the efficiency of both passive osmosis and active nutrient transport.