14.2 Plant Transport Systems

Plant Transport Systems

Plants need to move water, minerals, and sugars throughout their bodies, sometimes across distances of over 100 meters in tall trees. Xylem carries water and minerals upward from the roots, while phloem distributes sugars from photosynthetic tissues to wherever they're needed. Understanding how these two systems work is central to understanding how plants grow, respond to their environment, and survive.

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Xylem and Phloem Transport

Vascular Tissue Types and Functions

Xylem transports water and dissolved minerals from roots to leaves. It also provides structural support, which is why wood (mostly dead xylem tissue) is rigid.

Phloem transports sugars and other organic compounds from leaves to the rest of the plant for growth and storage.

Two key processes drive these transport systems:

• Transpiration is the evaporation of water from leaf surfaces (mainly through stomata). This evaporation creates a pulling force called transpirational pull that draws water upward through the xylem.
• Translocation is the movement of sugars and other organic compounds through the phloem, from tissues that produce sugar (sources) to tissues that consume or store it (sinks).

Xylem and Phloem Structure

Xylem is made of two types of water-conducting cells, both of which are dead at maturity (their hollow interiors form open pipelines):

• Tracheids have tapered ends with pits that allow water to pass between cells. They're found in all vascular plants.
• Vessel elements are wider, with perforated end walls that create less resistance to flow. They're found only in angiosperms, which is one reason flowering plants can transport water more efficiently.

Phloem is made of living cells that work as a team:

• Sieve tube elements are the actual transport cells. They're alive but lack a nucleus and most organelles, leaving their interior open for flow. Their end walls, called sieve plates, have pores that connect adjacent cells into continuous tubes.
• Companion cells sit next to sieve tube elements and provide the metabolic energy (ATP) needed to load and unload sugars. Each companion cell is connected to its sieve tube element by plasmodesmata.

Water Transport Mechanisms

Cohesion-Tension Theory

This is the primary explanation for how water moves upward through the xylem, even against gravity in trees that are tens of meters tall. It relies on three properties of water working together:

1. Transpiration creates tension. As water evaporates from mesophyll cells in the leaf and exits through stomata, it lowers the water potential in those cells. This creates a "pull" (negative pressure) on the water in nearby xylem vessels.
2. Cohesion maintains the water column. Water molecules are attracted to each other through hydrogen bonds. This cohesion means that when molecules at the top of the column are pulled upward, they drag the molecules below them along, forming an unbroken chain from leaf to root.
3. Adhesion provides support. Water molecules are also attracted to the hydrophilic walls of xylem cells. This adhesion helps counteract gravity and prevents the water column from pulling away from the vessel walls.

The result is a continuous stream of water pulled from roots to leaves without the plant spending any metabolic energy on the transport itself. Transpiration does the work.

Root Pressure

Root pressure is a secondary mechanism that supplements cohesion-tension, especially at night when stomata are closed and transpiration slows.

• Root cells actively transport mineral ions into the xylem. This lowers the water potential inside the xylem, so water follows by osmosis.
• The influx of water creates a positive pressure that pushes water upward.
• Root pressure can cause guttation, where droplets of water are forced out of leaf edges in the early morning.

Root pressure is relatively weak. It can push water a short distance but cannot account for transport to the top of tall trees. Cohesion-tension remains the dominant mechanism.

Sugar Transport

Source-Sink Relationship

Sugar transport in phloem is driven by the relationship between sources and sinks:

• Source tissues produce or release sugars. Mature leaves are the most common source because they photosynthesize. Storage organs (like a potato tuber in spring) can also act as sources when they break down stored starch into sugar.
• Sink tissues consume or store sugars. These include roots, developing fruits, flowers, growing shoot tips, and young leaves that aren't yet photosynthesizing enough to support themselves.

The same organ can switch roles depending on the season. A root storing starch over winter is a sink, but when it releases that sugar in spring to fuel new growth, it becomes a source.

Translocation Mechanisms

Sugar moves through the phloem by pressure flow (also called mass flow). Here's how it works step by step:

1. Loading at the source. Companion cells use active transport (requiring ATP) to load sucrose into sieve tube elements near the source tissue. This raises the solute concentration inside the phloem.
2. Water enters by osmosis. The high sugar concentration lowers the water potential in the sieve tube, so water moves in from the nearby xylem by osmosis. This increases turgor pressure in the phloem at the source end.
3. Bulk flow toward the sink. The elevated pressure at the source pushes the sugar-water solution through the sieve tubes toward the sink, where pressure is lower.
4. Unloading at the sink. At the sink tissue, sucrose is actively or passively unloaded from the sieve tubes into surrounding cells. As sugar leaves the phloem, the solute concentration drops, water exits by osmosis, and turgor pressure decreases.
5. Pressure gradient is maintained. Because sugar is constantly being loaded at the source and unloaded at the sink, a pressure gradient persists, keeping the flow going.