Transport in plants

Unit 7 Transport in plants

Key unit competence: Analyse plant transport tissues and the movement of substances within plants.

Key ideas

• Large plants need a transport system because their surface area to volume ratio is small and distances are too great for diffusion. Plants have two transport tissues in vascular bundles: xylem (water and mineral ions, upwards) and phloem (organic solutes such as sucrose and amino acids, up and down).
• Xylem contains vessel elements (dead, hollow, no end walls, joined end to end, walls thickened with lignin in annular, spiral, reticulate or pitted patterns; pits allow lateral movement), tracheids, xylem fibres (support) and xylem parenchyma. Lignin makes vessels waterproof, strong and able to withstand tension.
• Phloem contains sieve tube elements (living, no nucleus, few organelles, end walls form sieve plates), companion cells (dense cytoplasm, nucleus, many mitochondria; linked by plasmodesmata; load sucrose by active transport), phloem fibres and parenchyma.
• Water uptake: root hair cells (long extensions, large surface area, thin walls) have a lower water potential than soil water, so water enters by osmosis. Water crosses the cortex by three pathways: apoplast (through cell walls and spaces, fastest), symplast (through cytoplasm and plasmodesmata) and vacuolar. At the endodermis, the waxy Casparian strip (suberin) blocks the apoplast, forcing water into the symplast so the plant controls which ions enter the xylem.
• Water moves up the xylem by: root pressure (active pumping of ions into xylem lowers its ψ, water follows; causes guttation), capillarity, and mainly the cohesion-tension mechanism: water evaporates from leaf mesophyll (transpiration), creating tension (negative pressure) that pulls a continuous column of water held together by cohesion (hydrogen bonds) and to vessel walls by adhesion.
• Transpiration is the loss of water vapour from the aerial parts, mostly through stomata (also cuticle and lenticels). Significance: transpiration pull, transport of mineral ions, cooling the leaf, keeping cells turgid. Disadvantage: water loss causing wilting; regulated by stomatal closure (abscisic acid) and leaf structure.
• Factors affecting transpiration: light (opens stomata), temperature (increases evaporation and diffusion), humidity (high humidity lowers the water potential gradient), wind (removes the diffusion shell of humid air), soil water; leaf area, stomatal number and cuticle thickness. A potometer measures the rate of water uptake (≈ transpiration).
• Translocation: movement of organic solutes in phloem from sources (photosynthesising leaves, storage organs being used) to sinks (roots, growing tips, fruits, seeds, storage organs being filled). Mass flow hypothesis (Münch): sucrose is actively loaded into sieve tubes at the source (companion cells, H⁺-sucrose co-transport), lowering ψ; water enters from xylem by osmosis, raising hydrostatic pressure; sap flows along the pressure gradient to the sink, where sucrose is unloaded and used, lowering pressure.
• Evidence for mass flow: sap flows out of cut phloem under pressure; aphid stylets collect sap rich in sucrose; radioactive ¹⁴CO₂ labels sugars moving in phloem; ringing experiments; metabolic poisons stop translocation (active loading). Objections: different solutes move in different directions at different speeds; sieve plates would resist flow.
• Xerophytes reduce transpiration: thick waxy cuticle, sunken stomata, hairs, rolled leaves (marram grass), reduced leaves/spines (cactus), fewer stomata, CAM. Excretion in plants: plants excrete little; O₂ and CO₂ diffuse out; wastes are stored in vacuoles, bark and leaves that fall, or as crystals, tannins, resins, latex and alkaloids.

Common misconceptions

• "Water is pushed up tall trees mainly by root pressure." No: root pressure is small; the main force is transpiration pull (cohesion-tension).
• "Phloem sieve tubes are dead cells like xylem vessels." No: sieve tube elements are living, though they lack a nucleus, and depend on companion cells.
• "The Casparian strip stops all water entering the xylem." No: it blocks only the apoplast route, forcing water through living cells (symplast).
• "Translocation always moves food downwards." No: phloem moves solutes from sources to sinks, which can be up or down.

Worked examples

Practice questions

1. Explain why large plants need a transport system.

   Show answer

   Small surface area to volume ratio; long distances from roots to leaves; diffusion is too slow; leaves and roots are specialised and must exchange water, minerals and sugars.

   Common mistake: Saying plants have hearts.

2. Relate the structure of xylem vessels to their function.

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   Dead and hollow with no end walls: continuous tube for mass flow; lignified walls: strong, waterproof, resist tension; pits: lateral movement; narrow: helps capillarity and adhesion.

   Common mistake: Saying xylem vessels have nuclei.

3. Describe the apoplast and symplast pathways and the role of the endodermis.

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   Apoplast: water moves through cell walls and spaces; symplast: through cytoplasm and plasmodesmata. At the endodermis the Casparian strip blocks the apoplast, so water and ions must pass through cell membranes, allowing selective uptake.

   Common mistake: Saying the symplast is through cell walls.

4. Explain the cohesion-tension theory.

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   Water evaporates from mesophyll cells and diffuses out of stomata; this lowers water potential in leaves and creates tension in xylem; cohesion between water molecules and adhesion to vessel walls keep a continuous column that is pulled up from the roots.

   Common mistake: Leaving out cohesion.

5. State four factors affecting transpiration and explain one.

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   Light, temperature, humidity, wind speed, soil water. E.g. temperature increases evaporation and diffusion of water vapour and lowers relative humidity outside the leaf, increasing the gradient.

   Common mistake: Saying humidity increases transpiration.

6. Describe the mass flow hypothesis.

   Show answer

   Sucrose actively loaded into sieve tubes at the source lowers water potential; water enters from xylem by osmosis; hydrostatic pressure rises; sap flows to the sink where sucrose is unloaded and used, lowering pressure there.

   Common mistake: Saying sugars diffuse slowly along phloem.

7. Give three pieces of evidence for and one against mass flow.

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   For: sap exudes from cut phloem under pressure; aphid stylet sap is sugar-rich; ¹⁴C labelled sugars move in phloem; ringing causes swelling above the ring. Against: solutes move in different directions at different speeds; sieve plates resist flow.

   Common mistake: Giving only evidence for.

8. State four adaptations of xerophytes.

   Show answer

   Thick waxy cuticle; sunken stomata; hairs; rolled leaves; reduced leaves (spines); fewer stomata; deep or wide roots; water storage tissue; CAM.

   Common mistake: Listing hydrophyte features.

Matching and drag-and-drop

1. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. Sunken stomata help xerophytes by:	a. collect sap directly from sieve tubes
   2. Water molecules sticking to each other is:	b. xylem
   3. Aphids are used to study phloem because their stylets:	c. trapping humid air near the pores
   4. Water and mineral ions are carried in the:	d. cohesion

   Show answer

   1 → c (trapping humid air near the pores) · 2 → d (cohesion) · 3 → a (collect sap directly from sieve tubes) · 4 → b (xylem)

   • The diffusion gradient is reduced.
   • Due to hydrogen bonds.
   • The sap can be analysed.
   • Mostly upwards.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   loading of sucrose requires ATP  ·  water loss by transpiration  ·  sieve tubes are dead  ·  lignin
   1. Rolled leaves of marram grass reduce .
   2. Metabolic poisons stop translocation. This shows that .
   3. Xylem vessel walls are strengthened with .
   Show answer

   1: water loss by transpiration · 2: loading of sucrose requires ATP · 3: lignin

   • Stomata face a humid inner space.
   • Companion cells use active transport.
   • It resists tension.

3. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. Living cells that load sucrose into sieve tubes are:	a. guttation
   2. The waxy band in endodermal cell walls is the:	b. reduces the water potential gradient
   3. Loss of liquid water droplets from leaf margins due to root pressure is:	c. companion cells
   4. High humidity reduces transpiration because it:	d. Casparian strip

   Show answer

   1 → c (companion cells) · 2 → d (Casparian strip) · 3 → a (guttation) · 4 → b (reduces the water potential gradient)

   • They have many mitochondria.
   • It is made of suberin.
   • It happens on humid nights.
   • Air is already full of water vapour.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   active H⁺-sucrose co-transport  ·  apoplast pathway  ·  symplast pathway  ·  osmosis
   1. The water pathway through cell walls is the .
   2. Sucrose is loaded into sieve tubes by .
   3. Root hair cells absorb water by .
   Show answer

   1: apoplast pathway · 2: active H⁺-sucrose co-transport · 3: osmosis

   • It is the fastest route.
   • It needs ATP.
   • Their ψ is lower than soil water.

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