Transport in Plants is Cambridge International Biology 9700 Topic 7. It links the distribution and structure of xylem and phloem to water movement, transpiration, xerophytic leaves and assimilate translocation. Theory owns tissue recognition, structure-function reasoning and transport mechanisms; microscope handling, slide work, biological drawing execution and plant transport investigations remain in the dedicated practical hub.
1. Distribution of xylem and phloem
In a transverse section of a herbaceous dicotyledonous stem, vascular bundles form a ring. Xylem lies toward the centre of each bundle and phloem lies toward the outside. This arrangement supports the upright stem and connects roots with leaves and growing regions.
In a typical young dicot root, xylem commonly forms a central star or cross, with phloem between its arms. In a typical dorsiventral leaf, xylem lies toward the upper surface of a vascular bundle and phloem toward the lower surface. These arrangements are models, not universal patterns for every root or leaf. The precise outline changes with the plane and position of the section, so identify tissues from several features rather than memorising one perfect diagram.
A plan diagram shows the distribution and relative proportions of tissues without individual cells. A cellular drawing shows selected cells and their visible boundaries. These are different representations and should not be mixed.
2. Xylem vessel elements
Mature xylem vessel elements are dead and joined end to end. Their end walls are partly or completely lost, forming a continuous, low-resistance lumen for water and dissolved mineral ions. They contain no cytoplasm that would obstruct flow.
Lignin is deposited in the walls. It strengthens the vessel, resists collapse under the tension generated by transpiration and waterproofs wall regions. Uneven lignification can allow some flexibility during organ growth. Pits are regions where the thick secondary wall is interrupted. In an intact paired pit, a porous pit membrane derived from primary walls remains; a pit is not simply a completely open hole through all wall layers. Pits permit lateral water movement between vessels and surrounding tissues.
The vessel is not a living pump. Its structure supports passive bulk movement driven by a pressure difference established elsewhere in the plant.
3. Sieve tube elements and companion cells
Phloem transports assimilates such as sucrose and amino acids. Sieve tube elements are living cells joined end to end. Their end walls form sieve plates with pores through which phloem sap moves. Mature sieve tube elements have little cytoplasm and no nucleus, reducing resistance but limiting their independent metabolic control.
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Attempt the matching topic bank before reopening the notes. Use each missed idea to decide what to review next.
Companion cells lie beside sieve tube elements and are connected to them by plasmodesmata. They contain a nucleus, many mitochondria and membrane transport proteins. Their metabolism supports sieve-tube function and assimilate exchange. Loading and unloading mechanisms vary; the proton-driven loading model below is one example, not a universal requirement that both processes are active. A sieve tube element and its companion cell therefore act as a functional partnership.
4. Water movement across a root
Water enters root tissues and moves toward the xylem through apoplast and symplast pathways. In the apoplast pathway, water moves through cellulose cell walls and intercellular spaces without crossing a cell-surface membrane at each cell. Cellulose is hydrophilic and its wall spaces provide a route for water.
In the symplast pathway, water moves through the cytoplasm of living cells and between cells through plasmodesmata. It crosses a cell-surface membrane when entering the symplast.
At the endodermis, the Casparian strip forms a wall barrier that blocks a continuous apoplastic bypass. Entry into the symplast requires crossing a selectively permeable membrane, either earlier along the route or at the endodermis; membrane transport contributes selectivity. Do not equate the strip with later suberin lamellae. Primary research in Arabidopsis identifies the strip as lignin-based and distinguishes it from later suberin deposition. This species-specific evidence does not establish every plant's barrier chemistry or the required official examination wording.
5. Transpiration
In the leaf-transpiration model, water evaporates from the moist internal surfaces of leaf cells into intercellular air spaces. Water vapour then diffuses through stomata to the atmosphere down a water-vapour concentration gradient.
Evaporation lowers the water potential of leaf cells, so water moves into them from nearby xylem. The resulting pull is transmitted down the xylem water column. Transpiration is therefore evaporation followed by diffusion, not simply water moving up a stem.
Stomata provide a variable route between internal air spaces and the atmosphere. The practical consequences of changing environmental conditions and methods for estimating water uptake belong in plant transport investigations, while the theory note retains the causal mechanism.
6. Cohesion-tension and adhesion
Water molecules form hydrogen bonds with one another. Cohesion maintains a continuous water column in xylem. Evaporation from leaves creates tension, meaning negative pressure, and pulls this column upward as a mass. This is the cohesion-tension mechanism of transpiration pull.
Water also forms hydrogen bonds with cellulose in xylem walls. This adhesion helps the water column remain associated with the vessel wall. Lignified walls resist collapse while the contents are under tension.
The mechanism does not require xylem vessel elements to spend ATP. The energy driving evaporation ultimately comes from the environment, while the pressure gradient transmits the pull through the column.
7. Xerophytic leaf adaptations
Xerophytes are adapted to habitats where water conservation is important. A thick waxy cuticle reduces evaporation from the epidermal surface. Sunken stomata place pores in pits where humid air can accumulate, reducing the water-vapour concentration gradient. Hairs can trap a relatively still, humid boundary layer and reduce air movement near stomata.
Rolled leaves enclose stomata and humid air, while reduced leaf area lowers the exposed surface available for water loss. Several adaptations can occur together, but each annotation must connect a visible feature to a mechanism. Naming a feature without explaining how it reduces evaporation or diffusion is incomplete.
8. Sources, sinks and assimilates
A source releases assimilates into phloem. A photosynthesising leaf is a common source because it exports sucrose. A storage organ can also become a source when reserves are mobilised. A sink removes or uses assimilates. Growing tissues, roots, fruits and storage organs can be sinks.
Source and sink are functional roles, not permanent organ labels. A developing leaf may be a sink before it becomes photosynthetically productive, and a storage organ can switch between receiving and releasing assimilates.
Assimilates move dissolved in water through sieve tubes. Translocation can occur in different directions in different sieve tubes because each tube connects particular source and sink regions.
9. Phloem loading and mass flow
In the active apoplastic loading model described here, proton pumps in companion-cell membranes at a source use ATP to move hydrogen ions out of the cell. This creates an electrochemical gradient. Hydrogen ions return through cotransporter proteins together with sucrose, loading sucrose into the companion cell. Sucrose then passes into sieve tube elements through connections between the cells.
Initially, at the same pressure, increasing solute concentration lowers water potential in the source phloem. Water enters from nearby xylem by osmosis, raising hydrostatic pressure. At a sink, assimilates are removed for use or storage. Water potential rises, water can leave, and hydrostatic pressure is lower.
Phloem sap therefore moves by mass flow from higher hydrostatic pressure at the source toward lower hydrostatic pressure at the sink. In this loading model, active transport helps establish the source conditions that generate a pressure difference. Other species can load passively, and unloading routes also vary. Long-distance mass flow does not mean that every sucrose molecule is carried along the sieve tube by a motor protein.
Worked application: explaining two transport observations
Suppose source leaves are above a stem ring and the relevant sinks are below it. A ring of bark containing phloem is removed while most xylem remains intact, with no effective alternate phloem route across the interruption. Sucrose accumulates above the ring because assimilates from source leaves cannot continue through the interrupted sieve tubes to sinks below. Water can still rise initially through the remaining xylem, so the observation distinguishes phloem translocation from xylem transport. In a separate leaf section, sunken stomata occur beneath hairs. The hairs trap humid air and reduce air movement, while the pit lengthens the diffusion pathway. Both features reduce the water-vapour gradient or effective diffusion rate, so they support water conservation without stopping gas exchange completely.
Common misconceptions and corrections
Putting xylem outside phloem in a dicot stem bundle. Xylem is toward the centre.
Putting leaf phloem above xylem. Xylem is normally toward the upper surface.
Drawing individual cells in a plan diagram. A plan shows tissue regions.
Calling mature xylem vessel elements living pumps. They are dead conduits.
Calling pits damage or fully open holes through all wall layers. They are patterned interruptions in secondary walls, with a pit membrane remaining in an intact paired pit.
Giving mature sieve tube elements a nucleus. Companion cells retain nuclei.
Saying companion cells are disconnected from sieve tubes. Plasmodesmata connect them.
Calling the apoplast the cytoplasm route. It uses walls and spaces.
Calling the symplast a wall route. It uses cytoplasm and plasmodesmata.
Equating the Casparian strip with suberin lamellae. Arabidopsis research identifies a lignin-based strip distinct from later suberin deposition; do not generalise its chemistry without evidence.
Saying transpiration is water uptake by roots. It is evaporation followed by diffusion from leaves.
Saying water molecules are pulled by covalent bonds. Hydrogen bonding produces cohesion.
Ignoring adhesion. Water also bonds to cellulose in walls.
Saying ATP pumps xylem water to the canopy. Transpiration pull drives the mass flow.
Listing xerophyte features without mechanisms. Each feature needs a water-loss explanation.
Calling every leaf permanently a source. Roles change with development and conditions.
Saying phloem always moves downward. Direction follows source-sink relationships.
Calling sucrose loading passive in the stated mechanism. Proton pumping uses ATP.
Saying mass flow is active transport along the entire sieve tube. Loading establishes a hydrostatic pressure gradient.
Assessment guidance
For tissue questions, identify the organ and orientation before naming xylem or phloem, and distinguish plan diagrams from cellular drawings. Structure-function answers should pair each feature with a transport consequence. Root transport explanations must state where water travels, when a membrane is crossed and why the Casparian strip redirects the apoplast route. A transpiration answer needs evaporation, diffusion, water potential and cohesion-tension in a causal sequence. Xerophyte annotations must connect visible features to gradients or diffusion resistance. For the active-loading translocation model, identify source and sink, explain proton pumping and cotransport, then derive water entry, hydrostatic pressure and mass flow. Distinguish this model from loading and unloading routes in other species.
Retrieval practice
Sketch stem, root and leaf transverse sections and place xylem and phloem correctly. Compare vessel elements, sieve tube elements and companion cells feature by feature. Trace one water molecule through apoplast, endodermis, symplast, xylem and leaf air spaces. Explain cohesion-tension from hydrogen bonding. Annotate four xerophyte adaptations. Finally, turn companion-cell proton pumping into a complete source-to-sink pressure-flow sequence and predict the effect of interrupting phloem.
