Two Fundamental Transport Processes in Plants
Plants face a challenge that animals with hearts and blood vessels don’t: how to move water, minerals, and sugars across distances of meters — without any pumping organ. They solve this elegantly using two separate but interconnected systems.
Transpiration drives water and mineral transport upward through the xylem. Translocation moves sugars and organic solutes through the phloem to wherever they are needed. Together, these two processes make it possible for a 100-metre tall tree to deliver water to its topmost leaf and distribute the sugars made there to roots deep underground.
Transpiration — Water Loss from Leaves
Transpiration is the evaporation of water from the aerial parts of a plant, primarily through stomata in leaves, but also through the cuticle and lenticels.
Types of Transpiration
Stomatal transpiration: About 90–95% of all transpiration. Water evaporates from mesophyll cell surfaces into the leaf airspaces, then diffuses out through stomata when they are open.
Cuticular transpiration: Through the waxy cuticle of leaf surfaces. Normally very low (5–10%) but increases if the cuticle is damaged or thin.
Lenticular transpiration: Through lenticels (pores in the bark of woody stems). Negligible amount (<1%), continuous even when stomata close.
Stomata Structure and Function
Each stoma is surrounded by two guard cells. Guard cells control stomatal opening by changes in their turgor pressure.
Opening mechanism:
- Light activates proton pumps in guard cell membranes
- H⁺ is pumped out → electrical gradient drives K⁺ in
- K⁺ accumulation → water potential decreases → water enters by osmosis
- Guard cells become turgid and bulge outward → stomatal pore opens
- CO₂ for photosynthesis enters; water vapour exits
Closing mechanism:
- Darkness / drought / ABA (abscisic acid) triggers
- K⁺ exits guard cells → water potential increases → water leaves
- Guard cells lose turgor → stomatal pore closes
ABA (abscisic acid) is the “stress hormone” — it closes stomata during drought by triggering K⁺ efflux from guard cells. This is a classic NEET question connecting hormones to transpiration.
Factors Affecting Rate of Transpiration
| Factor | Effect |
|---|---|
| Temperature ↑ | Increases transpiration (more kinetic energy, higher water vapour pressure) |
| Humidity ↑ | Decreases transpiration (smaller gradient from leaf to air) |
| Light ↑ | Increases transpiration (stomata open) |
| Wind speed ↑ | Increases transpiration (removes humid air from leaf surface) |
| CO₂ ↑ | Decreases transpiration (stomata close when CO₂ is high — adequate photosynthesis) |
| Water availability ↓ | Decreases transpiration (stomata close via ABA mechanism) |
Transpiration Pull — How Water Reaches the Top
The mechanism by which water moves from roots to leaves is explained by the Cohesion-Tension Theory (proposed by Dixon and Joly, 1894):
- Transpiration from leaf cells creates a water deficit in mesophyll cells
- Water moves from xylem vessels into mesophyll cells
- This creates tension (negative pressure) in the xylem
- Water molecules are held together by cohesion (hydrogen bonding between water molecules)
- The cohesive chain of water is pulled upward — the “transpiration pull”
- Water is drawn up from the roots by adhesion (attraction of water to xylem cell walls)
This creates a continuous water column from root to leaf — the SPAC (Soil-Plant-Atmosphere Continuum).
Water potential gradient: Water moves from regions of higher water potential (soil, root) to lower water potential (leaf, atmosphere). The atmosphere has the lowest water potential — it is the ultimate “sink” driving the entire system.
Where:
- = Water potential (MPa)
- = Solute potential (always negative)
- = Pressure potential (positive in turgid cells, negative during transpiration pull in xylem)
Pure water: . Any dissolved solute lowers water potential below zero.
Significance of Transpiration
Despite being called a “necessary evil” (it’s unavoidable), transpiration provides several benefits:
- Creates the driving force (transpiration pull) for water and mineral transport
- Cools the leaf surface (evaporative cooling — like sweating)
- Helps distribution of minerals throughout the plant
- Maintains cell turgidity and plant shape
Translocation — Movement of Organic Solutes
Translocation is the movement of organic solutes (primarily sucrose, amino acids, and other assimilates) through the phloem from source to sink.
Source: Any organ that produces/exports sugars. Mature, photosynthetically active leaves are the main sources. Also: germinating seeds (mobilising stored starch), tubers being emptied.
Sink: Any organ that consumes/stores sugars. Growing regions (shoot tips, root tips), developing fruits, seeds, storage organs (roots, tubers).
Phloem Structure
Phloem consists of:
- Sieve tube elements: Long, narrow cells with perforated end walls (sieve plates) that allow mass flow of sap. Mature sieve tubes lack nucleus (enucleate) and most organelles.
- Companion cells: Closely associated with sieve tubes; have nucleus and dense cytoplasm; provide metabolic support to sieve tubes via plasmodesmata connections.
- Phloem fibres: Structural support
- Phloem parenchyma: Storage and lateral transport
Pressure Flow (Mass Flow) Hypothesis — Munch’s Theory
The most widely accepted model for phloem transport was proposed by Ernst Munch (1930):
At the source (e.g., leaf):
- Sucrose is actively loaded into sieve tubes (using ATP — active transport via sucrose-H⁺ symporters)
- This lowers water potential inside sieve tube
- Water enters from adjacent xylem by osmosis
- High turgor pressure (positive pressure) develops at the source end
At the sink (e.g., root):
- Sucrose is unloaded from sieve tubes (actively or passively)
- Water potential in sieve tube rises
- Water exits into adjacent tissues
- Low turgor pressure at the sink end
The pressure difference (high at source, low at sink) drives a bulk flow of phloem sap from source to sink through the sieve tubes — no energy needed for the actual movement once the gradient is established (though energy is needed for loading and unloading).
The Munch pressure flow hypothesis is tested every year in CBSE Class 11 and NEET. Know: source = high turgor, sink = low turgor, transport is from high to low pressure. Also know that translocation requires ATP (for active loading at source) unlike the passive transpiration pull in xylem.
Evidence for Phloem Transport
- Ringing/girdling experiment: Removing a ring of bark (phloem) from a tree trunk causes swelling above the ring (sugars accumulate) and wilting below — shows sugars move downward through phloem
- Radioactive tracer (¹⁴C): ¹⁴CO₂ fed to a leaf is incorporated into sugars which are then traced moving through phloem to other parts
- Aphid stylet experiments: Aphids insert stylets into sieve tubes; when stylets are cut, phloem sap continues to exude — showing positive pressure in phloem
Comparison: Xylem vs Phloem Transport
| Feature | Xylem (Transpiration) | Phloem (Translocation) |
|---|---|---|
| Substance transported | Water, minerals | Sugars, amino acids, organic solutes |
| Direction | Unidirectional — root to shoot | Bidirectional — source to sink |
| Driving force | Transpiration pull (negative pressure) | Pressure flow (positive pressure) |
| Energy requirement | No direct ATP (passive) | Yes — for active loading/unloading |
| Vessel type | Dead cells (vessels, tracheids) | Living cells (sieve tubes) |
| Speed | Fast (up to 15 m/hr) | Slow (0.3–1.5 m/hr) |
Common Mistakes to Avoid
Mistake 1: Saying xylem transport requires energy. The transpiration pull is passive — energy from the sun drives evaporation, which creates the tension. The plant does not spend ATP on pulling water up through xylem. Contrast this with phloem loading, which does require ATP.
Mistake 2: Confusing the direction of phloem transport. Phloem transport is bidirectional — not always downward. Sucrose can move from a leaf downward to roots AND upward to growing shoot tips. “Source to sink” can be in any direction.
Mistake 3: Saying guard cells are “big cells.” Guard cells are actually kidney-shaped (in dicots) or dumbbell-shaped (in monocots like grasses). Their unique shape means that when they become turgid, the thickened inner wall pulls outward, opening the pore.
Practice Questions
Q1. Why is transpiration called a “necessary evil”?
Transpiration is “necessary” because it creates the transpiration pull that drives water and mineral transport from root to leaf — without which tall plants could not function. It is “evil” (or a cost) because 99% of water absorbed by roots is lost through transpiration rather than used in metabolism. However, since the same stomata needed for CO₂ entry also allow water vapour exit, some transpiration is unavoidable for any photosynthetically active leaf.
Q2. What is the role of potassium ions in stomatal opening?
When stomata open, K⁺ ions are actively pumped into guard cells (potassium ion channels open in response to light or ABA-independent signals). The increased K⁺ concentration lowers water potential inside guard cells → water enters by osmosis → guard cells become turgid and bulge outward → pore opens. Closing is the reverse: K⁺ exits → water leaves → guard cells become flaccid → pore closes.
Q3. What would happen to phloem transport if all the companion cells of a plant were killed?
Phloem transport would stop. Companion cells provide the metabolic support (ATP, proteins, cellular machinery) that sieve tube elements lack (sieve tubes are enucleate). Companion cells are essential for the active loading of sugars into sieve tubes at the source and unloading at the sink. Without companion cells, the sieve tubes cannot maintain their function.
Q4. A tree trunk is girdled (bark removed in a ring). What happens to the roots after some time?
The roots will eventually die. Girdling removes the phloem, which carries sugars from leaves to roots. Roots cannot photosynthesise — they depend entirely on phloem-transported sugars for energy. Initially, the roots may continue to function using stored reserves, but once these are exhausted, root cells starve and die. The xylem (wood) inside the bark is unaffected, so water and minerals continue to reach the leaves for some time.
FAQs
What is the ascent of sap? Ascent of sap refers to the upward movement of water and dissolved minerals through the xylem from roots to leaves. It is driven by the transpiration pull (cohesion-tension theory) and aided by root pressure (a minor contribution).
Can plants lose too much water through transpiration? Yes. Excessive water loss causes wilting, stomatal closure, and eventually death. Plants in arid regions (xerophytes) have adaptations to reduce transpiration: thick waxy cuticle, sunken stomata, CAM photosynthesis (open stomata only at night), small leaf area, or leaf orientation (parallel to sun rays to reduce heating).
Why is phloem transport bidirectional but xylem transport is unidirectional? Xylem transport is driven by transpiration pull, which always goes from the water source (soil) to the water sink (atmosphere via leaves) — always upward. Phloem transport is driven by source-to-sink pressure gradients. Sources and sinks can be anywhere — a fruit forming high up in the tree is a sink receiving sugars from leaves, while the roots below are also sinks receiving sugars from those same leaves. Different parts of the phloem can be transporting in different directions simultaneously.
What is root pressure and when is it significant? Root pressure is a positive pressure generated in roots due to active ion uptake, which draws water in by osmosis. It is significant mainly at night or in conditions of low transpiration (high humidity). Root pressure causes guttation — drops of water secreted from leaf tips through special pores called hydathodes. Root pressure alone cannot account for water reaching the tops of tall trees.