state that assimilates dissolved in water, such as sucrose and amino acids, move from sources to sinks in phloem sieve tubes

Transport in Higher Plants – Cambridge IGCSE/A‑Level Biology (Topic 7)

1. Overview

Plants transport two fundamentally different types of material:

• Water and mineral ions – move upward from the roots to the leaves in the xylem.
• Organic nutrients (assimilates) – mainly sucrose and amino acids, move from sites of production (sources) to sites of use or storage (sinks) in the phloem.

Both transport systems rely on differences in water potential (Ψ) but use different physical mechanisms.

2. Xylem Transport – Water & Mineral Ions

2.1 Water Potential

Ψ = Ψ_s (solute potential) + Ψ_p (pressure potential).

Water moves from regions of higher Ψ to lower Ψ.

2.2 Root‑Pressure

• Active uptake of mineral ions (especially K⁺) into the root cortex creates a high solute concentration in the xylem parenchyma.
• The resulting osmotic influx of water raises the pressure potential in the root xylem to 0.1–0.2 MPa, generating a positive pressure that can push water upward (most noticeable in seedlings and at night).

2.3 Cohesion‑Tension Theory (Transpiration Pull)

1. Transpiration from stomata produces a negative pressure (tension) in the leaf mesophyll.
2. Water molecules are cohesive (hydrogen‑bonded) and form a continuous column in the xylem.
3. The tension is transmitted down the xylem, pulling water from the roots to the leaves.

2.4 Comparison of Root‑Pressure and Transpiration Pull

2.5 Pathways for Water & Mineral Ions in Roots

Two parallel routes allow water and dissolved minerals to reach the xylem:

3. Phloem Transport – Assimilate Movement

3.1 Source and Sink

• Source – tissue that produces or releases assimilates (e.g., mature photosynthetic leaves, green stems).
• Sink – tissue that consumes, stores or grows using assimilates (e.g., roots, developing fruits, young leaves, tubers, seeds).

3.2 Loading of Sucrose (and Other Assimilates)

Two main strategies are recognised in the syllabus:

1. Active (energy‑requiring) loading – typical of many herbaceous plants.

   • H⁺‑ATPase in the companion‑cell plasma membrane pumps H⁺ out of the cell (uses 1 ATP per H⁺).
   • The electrochemical H⁺ gradient drives a sucrose‑H⁺ symporter, importing sucrose against its concentration gradient.
   • Result: high solute concentration in the sieve‑tube sap.

2. Passive (facilitated‑diffusion) loading – occurs in some trees and grasses where the concentration of sucrose in the mesophyll already exceeds that in the phloem.

   • Sucrose moves through plasmodesmata or carrier proteins without ATP expenditure.
   • Often associated with a high density of plasmodesmata between mesophyll, companion cells and sieve elements.

3.3 Energy Cost of Loading

Each H⁺‑ATPase cycle consumes one molecule of ATP; typically 1–2 ATP are required to import one sucrose molecule via the symporter. This cost is offset by the large amount of carbon transported.

3.4 Pressure‑Flow (Mass‑Flow) Hypothesis – Quantitative View

Loading raises the solute potential (Ψ_s) at the source, causing water to enter by osmosis and increase the pressure potential (Ψ_p). Unloading at the sink has the opposite effect.

The pressure difference (ΔP ≈ 0.4 MPa) drives bulk flow of phloem sap. The flow rate can be expressed as:

Q = ΔP / R

where Q is the volume flow rate and R is the hydraulic resistance of the sieve‑tube system.

3.5 Worked Example (AO2 Skill)

Problem: A source leaf generates a turgor pressure of 0.55 MPa, while a sink root has a turgor pressure of 0.15 MPa. The hydraulic resistance of the pathway is 2 MPa·s L⁻¹. Calculate the sap flow rate (Q) and state the direction of movement.

1. ΔP = 0.55 MPa – 0.15 MPa = 0.40 MPa
2. Q = ΔP / R = 0.40 MPa / (2 MPa·s L⁻¹) = 0.20 L s⁻¹
3. Direction: from the source leaf (high pressure) to the sink root (low pressure).

This simple calculation demonstrates how the pressure‑flow hypothesis can be applied quantitatively – a skill required for AO2 questions.

3.6 Steps in Phloem Transport (Summarised)

1. Photosynthesis in mesophyll produces sucrose.
2. Sucrose is loaded into companion cells and then into sieve‑tube elements (active or passive loading).
3. Water follows osmotically, raising turgor pressure in the source sieve tube.
4. The pressure gradient drives bulk flow of sap toward regions of lower pressure (the sink).
5. At the sink, sucrose is unloaded (active uptake or diffusion).
6. Water exits the sieve tube, lowering pressure and completing the cycle.

3.7 Comparison of Sources and Sinks

4. Xerophytic Leaf Adaptations (Optional – Exam Topic 7.3)

• Thick, waxy cuticle – reduces non‑stomatal water loss.
• Sunken stomata or hypostomatous leaves – lower transpiration rate.
• Reduced leaf surface area (needle‑like leaves) – less area for evaporation.
• Dense trichomes or hairs – create a still‑air layer, decreasing evaporation.
• Leaf rolling or vertical orientation – minimises exposure to intense sunlight.

5. Key Points for Examination (AO1–AO2)

1. State that assimilates dissolved in water move from sources to sinks in phloem sieve tubes.
2. Explain the role of companion cells: H⁺‑ATPase creates a proton gradient that powers sucrose‑H⁺ symport (active loading) and note the alternative passive loading strategy.
3. Describe how the resulting osmotic increase draws water in, raising turgor pressure and establishing a pressure gradient (pressure‑flow hypothesis). Include a brief quantitative example (ΔP ≈ 0.4 MPa, Q = ΔP/R).
4. Identify typical source (mature leaf) and sink (root, fruit, young leaf, storage organ) tissues in a flowering plant.
5. Summarise the two main mechanisms of water transport in the xylem: root‑pressure (positive pressure generated by ion uptake) and transpiration pull (cohesion‑tension). Contrast when each dominates.
6. Distinguish apoplastic and symplastic pathways for water/mineral uptake in roots, explicitly naming the Casparian strip and its function.
7. Recall xerophytic leaf adaptations that help conserve water in arid environments.