Biology • Year 11 • Module 2 • Lesson 16
Plant Transport Systems: Xylem and Phloem
Lock in the core vocabulary and the structural contrast between xylem and phloem, the two vascular tissues that carry different cargoes in different directions through every plant.
1. Term–definition match
Match each term on the left to the correct definition. Write the letter of the definition in the right-hand column. Each definition is used once. 11 marks
| Term | Answer | Definitions (A–K) |
|---|---|---|
| xylem | A. A waxy suberin band in the cell walls of root endodermal cells that blocks the apoplast pathway, forcing water and minerals through cell membranes for selective uptake. | |
| phloem | B. The attraction of water molecules to the hydrophilic walls of xylem vessels, helping maintain an unbroken water column. | |
| transpiration pull | C. Vascular tissue in plants composed of dead, hollow, lignified cells that transports water and dissolved minerals upward from roots to leaves. | |
| cohesion | D. The route water takes through cell walls and intercellular spaces, not crossing any membranes, blocked at the Casparian strip. | |
| tension | E. The attraction between water molecules via hydrogen bonds that keeps the xylem water column intact under tension. | |
| adhesion | F. A plant organ that produces or releases sucrose into the phloem, such as a photosynthesising leaf. | |
| Casparian strip | G. The driving force for xylem water movement, generated by evaporation of water from leaf mesophyll cells creating tension in the water column. | |
| apoplast pathway | H. A plant organ that consumes or stores sucrose received from the phloem, such as a growing root or ripening fruit. | |
| symplast pathway | I. Vascular tissue in plants composed of living sieve tubes and companion cells that transports sugars bidirectionally from source to sink. | |
| source | J. The route water takes through the cytoplasm and plasmodesmata of connected cells, crossing membranes and subject to cellular regulation. | |
| sink | K. The negative pressure (below atmospheric) in the xylem water column that pulls water upward from the roots. |
2. Complete the comparison table
Fill in every blank cell. Each answer must be precise, use the lesson's exact terminology. 16 marks (1 per cell)
| Feature | Xylem | Phloem |
|---|---|---|
| Cell types | Tracheids and _______________ | Sieve tube elements and _______________ |
| Living or dead at maturity? | ||
| Cell wall | Thick, _______________ walls | Thin, _______________ walls |
| Contents transported | Sucrose, amino acids, _______________ | |
| Direction of flow | _______________ (roots to leaves only) | _______________ (source to sink, any direction) |
| Driving mechanism | _______________ theory, passive, no ATP | _______________ hypothesis, ATP for active loading |
| Pressure in vessel | _______________ (tension) | _______________ (turgor) |
| End walls between cells? | No (vessel elements) or _______________ (tracheids) | _______________, perforated end walls |
3. True or false, with correction
For each statement, circle T or F. If the statement is false, rewrite it correctly on the line provided. 10 marks (1 for T/F, 1 for each correction)
3.1 Xylem vessels are composed of living cells that actively pump water upward using ATP. T / F
3.2 Phloem transport is bidirectional because it moves from source (high-sucrose) to sink (low-sucrose) which can be above or below the source leaf. T / F
3.3 The Casparian strip blocks the symplast pathway in root endodermal cells, allowing selective mineral uptake. T / F
3.4 Cohesion refers to the attraction between water molecules via hydrogen bonds, which keeps the xylem water column intact under tension. T / F
3.5 A developing fruit is an example of a phloem source because it receives and stores sucrose. T / F
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 10 marks (2 each)
4.1 What is the function of lignin in the walls of xylem vessel elements?
4.2 What is the function of companion cells in phloem transport?
4.3 What is the function of the Casparian strip in relation to mineral uptake?
4.4 What is the function of cohesion in the cohesion-tension theory?
4.5 What is the function of active loading in the pressure-flow hypothesis of phloem transport?
5. Sequence the water pathway
The steps below describe water moving from soil to the atmosphere, but they are out of order. Number them 1–8 to show the correct sequence. 7 marks (1 off for each step out of order)
| Order (#) | Step |
|---|---|
| Water vapour diffuses through open stomata to the dry external atmosphere. | |
| Water moves through root cortex via the apoplast and symplast pathways toward the central vascular tissue. | |
| Water evaporates from the surfaces of mesophyll cells into air spaces inside the leaf. | |
| Water enters root hair cells by osmosis, driven by the lower water potential inside the cell compared to soil solution. | |
| Water is forced through the plasma membrane of endodermal cells at the Casparian strip. | |
| Water moves by osmosis from leaf xylem into mesophyll cells. | |
| Water travels upward through xylem vessels in the stem, pulled by tension transmitted via the cohesive water column. | |
| Water enters root xylem vessels, where the tension created at the leaf end of the column lowers the water potential of xylem below that of surrounding endodermal cells. |
Q1, Term–definition matches
xylem → C • phloem → I • transpiration pull → G • cohesion → E • tension → K • adhesion → B • Casparian strip → A • apoplast pathway → D • symplast pathway → J • source → F • sink → H.
Q2, Comparison table
Cell types: xylem, vessel elements; phloem, companion cells. Living/dead: xylem, dead at maturity; phloem, living (sieve tubes lack a nucleus but are alive; companion cells are fully living). Cell wall: xylem, lignified; phloem, unlignified. Contents: xylem, water and dissolved minerals (inorganic ions); phloem, hormones (accept also amino acids already given). Direction: xylem, unidirectional; phloem, bidirectional. Mechanism: xylem, cohesion-tension; phloem, pressure-flow. Pressure: xylem, negative; phloem, positive. End walls: xylem, pits (for tracheids); phloem, sieve plates.
Q3, True/false with correction
3.1 False. Xylem vessels are composed of dead cells. Water movement is passive, driven by cohesion-tension, not ATP-powered pumping.
3.2 True.
3.3 False. The Casparian strip blocks the apoplast pathway (cell wall route), not the symplast pathway. This forces water and ions through the plasma membrane of endodermal cells, allowing selective mineral uptake.
3.4 True.
3.5 False. A developing fruit is a phloem sinkit consumes (or stores) sucrose received from the phloem. The source is the photosynthesising leaf that produces and loads the sucrose.
Q4.1, Function of lignin
Lignin in xylem vessel walls provides structural strength, enabling the vessel to resist the inward force of negative pressure (tension) generated during transpiration. Without lignin, the vessel walls would collapse inward as the water column is pulled under tension.
Q4.2, Function of companion cells
Companion cells maintain the living sieve tube elements (which lack a nucleus and most organelles) by providing metabolic support and supplying the ATP needed for active loading of sucrose into sieve tubes at the source end of the phloem.
Q4.3, Function of the Casparian strip
The Casparian strip (a waxy suberin band) blocks the apoplast pathway at the endodermis, forcing all water and dissolved minerals to cross the plasma membrane of endodermal cells. This membrane crossing allows selective mineral uptake: only ions with matching transport proteins in the endodermal membrane can enter the vascular tissue.
Q4.4, Function of cohesion
Cohesion (hydrogen bonding between water molecules) holds the xylem water column together as a continuous chain. When transpiration creates tension at the leaf end, cohesion transmits this tension all the way down the column to root xylem without the column breaking, allowing water to be pulled upward without any pump.
Q4.5, Function of active loading
Active loading (ATP-powered pumping of sucrose from mesophyll cells into sieve tubes at the source) raises the solute concentration in source sieve tubes. This lowers their water potential, causing water to enter by osmosis, which raises turgor pressure. The pressure gradient between high-pressure source and low-pressure sink then drives bulk flow of phloem sap to the sink.
Q5, Water pathway sequence
Correct order: 1water enters root hair cells by osmosis; 2water moves through cortex via apoplast/symplast; 3water forced through endodermal cell membrane at Casparian strip; 4water enters root xylem; 5water travels upward through xylem; 6water moves by osmosis from leaf xylem into mesophyll cells; 7water evaporates from mesophyll cells into leaf air spaces; 8water vapour diffuses through stomata to atmosphere.