Table of Contents
Overview
Plants, unlike animals, are rooted in one place. They cannot move to find water, minerals, or food. Instead, they have evolved efficient transport systems that move water, dissolved minerals, and organic (carbon‑containing) substances over long distances within the body of the plant. These processes link root, stem, and leaves into an integrated whole and are essential for photosynthesis, growth, and reproduction.
This chapter provides a general overview of transport processes in plants and prepares for the more detailed subchapters on:
- Basic components of the plant
- Uptake of water and mineral salts
- Water and ion transport in the stem
- Regulation of transpiration
- Transport of organic substances
Here, the focus is on what all these aspects have in common: the basic principles, structures, and driving forces behind plant transport.
The Two Main Transport Systems: Xylem and Phloem
Vascular (higher) plants possess two specialized tissues for long‑distance transport:
- Xylem
- Transports water and dissolved mineral ions.
- Main direction: from roots to leaves (unidirectional).
- Cells are dead at maturity, with thick, lignified cell walls.
- Forms continuous tubes (tracheids and vessels) that act like microscopic pipes.
- Provides both transport and mechanical support (wood).
- Phloem
- Transports mainly organic substances (e.g. sucrose, amino acids, some hormones).
- Direction: from “sources” to “sinks”; can be up or down depending on where substances are needed.
- Cells are living but highly specialized.
- Sieve tube elements: elongated cells joined end‑to‑end with sieve plates (perforated end walls).
- Companion cells: metabolically active partner cells that maintain and control sieve tube elements.
These two tissues run together in vascular bundles in stems and leaves, and form a continuous network throughout the plant, connecting roots, stem, leaves, flowers, and fruits.
Transport Pathways at the Cellular Level
Before substances enter xylem or phloem for long‑distance transport, they move from cell to cell within tissues. There are three main pathways:
- Apoplastic pathway
- Movement through cell walls and intercellular spaces.
- The apoplast is everything outside the plasma membrane: cell walls plus spaces between cells.
- Transport here is mainly by diffusion and bulk flow, not crossing cell membranes.
- Especially important for water and mineral movement toward the root xylem, until interrupted by the Casparian strip in the endodermis (covered later).
- Symplastic pathway
- Movement through the cytoplasm of living cells.
- Neighboring cells are connected by plasmodesmata (fine cytoplasmic channels through cell walls).
- Substances cross the plasma membrane once to enter the symplast and then move cell‑to‑cell via plasmodesmata.
- Transmembrane (or transcellular) pathway
- Substances repeatedly cross plasma membranes and move through both cytoplasm and vacuoles.
- Combines membrane transport (channels, carriers, pumps) with diffusion inside cells.
In reality, plant tissues use combinations of these pathways at the same time. Which pathway dominates can vary with tissue type, developmental stage, and the particular substance being transported.
Driving Forces Behind Transport
Transport in plants is not powered by a circulating pump like an animal heart. Instead, several physical and physiological forces act together:
Water Potential as a Driving Concept
Movement of water in plants can be understood using water potential ($\Psi$):
- Water moves from regions of higher water potential (less negative $\Psi$) to regions of lower water potential (more negative $\Psi$).
- $\Psi$ is influenced mainly by:
- Solute concentration ($\Psi_s$; solute or osmotic potential).
- Pressure ($\Psi_p$; pressure potential, e.g. turgor or tension).
- Matrix effects ($\Psi_m$; binding to surfaces), important in soils and cell walls.
The total water potential is:
$$
\Psi = \Psi_s + \Psi_p + \Psi_m
$$
In many simple situations, $\Psi_m$ is small or constant, so water flow is driven by a combination of solutes and pressure.
Osmosis and Turgor
- Osmosis is the passive movement of water across a semipermeable membrane from higher to lower water potential.
- Plant cells maintain internal turgor pressure when water enters and presses the plasma membrane against the cell wall.
- Turgor:
- Keeps tissues rigid (e.g. leaves upright).
- Drives cell expansion and growth.
- Provides a hydrostatic “push” component for short‑distance transport between cells.
Bulk Flow vs. Diffusion
Two fundamentally different modes of movement operate in plants:
- Diffusion
- Random thermal motion of molecules from high to low concentration.
- Effective over microscopic distances (within cells or between neighboring cells).
- Too slow for long‑distance transport in tall plants.
- Bulk flow (mass flow)
- Movement of water and solutes together as a fluid, driven by pressure differences.
- Occurs in xylem and phloem.
- Much faster than diffusion and allows rapid transport over meters.
Xylem transport is primarily bulk flow driven by a pressure gradient (tension at the top, higher pressure at the root). Phloem transport is bulk flow driven by pressure differences generated osmotically between sources and sinks.
Source–Sink Relationships
Organic substances (e.g. sugars) do not move in a fixed direction like water in xylem. Instead, they follow changing patterns of production and use:
- Source: any organ that produces more of a substance than it needs.
- Mature leaves: major sources of sugars produced by photosynthesis.
- Storage organs (tubers, roots, seeds) when they are mobilizing stored reserves (e.g. during germination or early growth).
- Sink: any organ that consumes or stores more of a substance than it produces.
- Growing tissues: shoot tips, young leaves, flowers, developing fruits, roots.
- Storage organs when they are accumulating reserves (e.g. during late summer).
Transport in the phloem goes from sources to sinks. Importantly:
- One and the same organ can switch between being a source and a sink at different developmental stages.
- Multiple sources and sinks can be active at the same time.
- The pattern of flow in the phloem network is dynamic and can change rapidly with light, temperature, damage, or developmental signals.
This flexible source–sink system allows the plant to allocate resources where they are needed most, for example to growing fruits or recovering tissues.
Short‑Distance vs. Long‑Distance Transport
Plant transport can be divided into two scales:
- Short‑distance transport (local)
- Between cells within a tissue or organ.
- Uses diffusion, osmosis, active transport across membranes, and local pressure differences.
- Operates via apoplastic, symplastic, and transmembrane pathways.
- Crucial in roots (movement from soil to xylem), in leaves (distribution of water and CO₂ to photosynthesizing cells), and in phloem loading/unloading regions.
- Long‑distance transport (systemic)
- Over large distances through xylem and phloem.
- Uses bulk flow driven by pressure gradients.
- Links distant parts of the plant (e.g. roots and leaves meters apart).
Successful functioning of the plant depends on tight coordination between these two levels. For example, active loading of sucrose into phloem at a leaf (short‑distance, energy‑dependent) creates high osmotic pressure that drives bulk flow along sieve tubes to fruits (long‑distance).
Energy Use in Plant Transport
Not all plant transport is passive. Plants invest metabolic energy in specific steps:
- Active transport across membranes
- Uses membrane proteins (pumps, carriers) and ATP to move ions and molecules against their electrochemical gradients.
- Creates ion gradients (especially for $H^+$) that are later used to drive the secondary active transport of other substances (e.g. sucrose, nitrate).
- Secondary active transport
- Uses existing gradients (e.g. proton gradients) to power co‑transport of other substances.
- Common in:
- Root uptake of mineral ions from the soil.
- Loading and unloading of sucrose into and out of the phloem.
- Passive transport
- Once gradients (pressure, concentration) exist, movement by diffusion and bulk flow does not require further direct energy input.
As a result, the plant’s overall energy cost for long‑distance movement per unit of transported material is relatively low. Most ATP is spent at key interfaces: roots–soil and phloem sources–sinks.
Coordination With Leaf Gas Exchange and Root Uptake
Transport processes are closely tied to other plant functions:
- Gas exchange and transpiration in leaves
- Opening of stomata allows CO₂ to enter leaves for photosynthesis.
- At the same time, water vapor is lost by transpiration.
- This water loss generates tension in the leaf xylem, which pulls water upward from roots and through the stem.
- Water and mineral uptake in roots
- Water drawn upward by transpiration must be replaced from the soil.
- Mineral ions taken up from the soil travel through root tissues and into xylem for delivery to leaves.
- Root growth and branching adapt to resource availability, influencing the capacity of the transport system.
Thus, local processes at the plant surface (stomata in leaves, root hairs in soil) feed into, and depend on, the internal transport network.
Functional Significance of Transport Processes
Effective transport in plants supports multiple vital processes:
- Supply of water for photosynthesis and cell turgor
- Water is a reactant in photosynthesis and a solvent for biochemical reactions.
- Turgor, sustained by water transport, is essential for growth, leaf orientation, and movement (e.g. opening and closing of leaves or flowers).
- Distribution of mineral nutrients
- Essential elements (e.g. nitrogen, phosphorus, potassium, iron) taken up by roots are transported to growing tissues and photosynthetic organs.
- Deficiencies often manifest first in leaves where transport cannot keep up with demand.
- Allocation of photosynthetic products
- Sugars and other assimilates produced in leaves support respiration and growth everywhere in the plant.
- They are also stored in specialized organs (seeds, fruits, roots, tubers) for later use or for reproduction.
- Communication within the plant
- Hormones and signaling molecules move through xylem, phloem, or both.
- This allows different organs to coordinate responses to light, gravity, drought, injury, or attack by herbivores and pathogens.
Disturbances in any part of the transport system (e.g. drought, freezing, disease that blocks xylem or phloem) can quickly affect the entire plant because water, minerals, and carbohydrates can no longer be adequately distributed.
Overview of the Following Subchapters
The subsequent subchapters will examine particular aspects of plant transport in more detail:
- Basic Components of the Plant
- Structural organization of roots, stems, and leaves and how these structures support transport.
- Uptake of Water and Mineral Salts
- How roots absorb water and ions and the role of root hairs, membranes, and the endodermis.
- Water and Ion Transport in the Stem
- The path and driving forces of upward water movement in xylem.
- Regulation of Transpiration
- How stomata and environmental factors control water loss and indirectly regulate xylem flow.
- Transport of Organic Substances
- Mechanisms of loading, transport, and unloading of sugars in phloem and the consequences for growth and storage.
Together, these topics build a complete picture of how plants move materials within their bodies and maintain internal balance while rooted in changing environments.