7. Transport in Plants: The Plant's Delivery Service

Hello Biologists! Welcome to one of the most fascinating topics in plant physiology: how plants manage their internal transport. Unlike us, plants don't have hearts, but they perform massive long-distance transport, moving water 100 meters up against gravity and shipping energy molecules around their bodies. Understanding this chapter is key to grasping photosynthesis, mineral nutrition, and plant survival!


7.1 Structure of Transport Tissues

Plants rely on a vascular system, which is essentially two parallel pipeline systems: the Xylem and the Phloem. We need to know the specific cells that make up these tissues and how their unique structure enables their function.

Key Transport Tissues: Xylem and Phloem

1. Xylem Tissue (The Water Highway)

  • Function: Transport of water and dissolved mineral ions (from roots up to the stem and leaves). Also provides mechanical support.
  • Key Components: Xylem vessel elements.
  • Structure-Function Relationship:
    • These cells are dead and hollow at functional maturity (no cytoplasm, nucleus, or organelles). This provides minimal resistance to water flow.
    • They are arranged end-to-end to form continuous, narrow tubes.
    • Walls are thickened and supported by lignin (a tough, woody substance), which prevents the vessels from collapsing under the tension forces involved in water transport.
    • Pits (areas without lignin) allow lateral movement of water between adjacent vessels.

2. Phloem Tissue (The Food Delivery System)

  • Function: Transport of assimilates (mainly sucrose, but also amino acids) from the site of production (source) to the site of use or storage (sink).
  • Key Components: Sieve tube elements and companion cells.
  • Sieve Tube Elements:
    • Are living but lack a nucleus, ribosome, and large vacuole at maturity. This reduces obstruction but means they are not fully independent.
    • Joined end-to-end, separated by porous partitions called sieve plates (which allow cytoplasm to link up).
  • Companion Cells:
    • Located next to sieve tube elements and are metabolically highly active.
    • Contain a dense cytoplasm, nucleus, and many mitochondria (essential for providing ATP for active transport).
    • They control the metabolism and function of the non-nucleated sieve tube element.

Quick Tip: Remember which is which! Xy-Lo (Xylem is Low pressure/Liquid water); Ph-Hi (Phloem is High pressure/Hydrostatic gradient).

Distribution of Vascular Bundles in Herbaceous Dicots

You must be able to recognise and draw the distribution of xylem and phloem in transverse sections (TS) of roots, stems, and leaves of herbaceous dicotyledonous plants.

  • Root (TS): The vascular bundle is central, forming a distinct star shape (an 'X' shape) made of xylem. The phloem fills the spaces between the arms of the xylem star.
  • Stem (TS): Vascular bundles are arranged in a ring near the edge of the stem. In each bundle, the xylem is towards the inside (centre of the stem), and the phloem is towards the outside (cortex).
  • Leaf (TS/Vein): The vascular tissue is found within the veins. The xylem is typically found on the upper side, and the phloem on the lower side.

Key Takeaway for 7.1: Xylem (dead tubes, lignin) moves water up using tension. Phloem (sieve tubes + companion cells, living) moves sugar via pressure gradient. Their location differs drastically depending on the organ.


7.2 Transport Mechanisms for Water and Minerals

How does water get from the soil into the xylem, and then how does it defy gravity to reach the leaves?

Water Uptake and Movement to the Xylem

Water and mineral ions are absorbed by root hairs and travel through the cortex towards the central xylem vessels. There are two main pathways:

1. The Apoplast Pathway

  • Water moves through the non-living parts of the cell: the cellulose cell walls and the intercellular spaces.
  • It's the fastest route because there is no resistance from membranes or cytoplasm.

2. The Symplast Pathway

  • Water moves through the living parts of the cell: the cytoplasm, passing from cell to cell via plasmodesmata (tiny pores connecting adjacent cytoplasms).
  • It is slower because it involves passing through cell surface membranes (which offer resistance).
The Role of the Endodermis and Casparian Strip

In the root, before water can enter the xylem, it encounters a specialized layer of cells called the endodermis. The walls of the endodermal cells contain a waterproof strip made of suberin and lignin, known as the Casparian strip.

  • Function of the Casparian Strip: It acts like a checkpoint. It blocks the apoplast pathway.
  • Result: All water and dissolved mineral ions are forced to move into the cytoplasm (the symplast pathway) of the endodermal cells. This allows the plant to regulate which ions enter the xylem via active transport mechanisms in the endodermis membrane, before the water moves through osmosis.
Water Movement up the Xylem: Cohesion-Tension Theory

This is the universally accepted model for water transport, relying on three key phenomena:

1. Transpiration Pull (The Driving Force)

Transpiration is the ultimate driver. It involves:

  1. Evaporation of water from the internal surfaces (mesophyll cell walls) of the leaf into the intercellular air spaces.
  2. Diffusion of water vapour through the stomata to the atmosphere.

As water evaporates from the mesophyll cell walls, the remaining water molecules exert a pulling force (tension) on the water column below them.

2. Cohesion and Tension

  • Water molecules are polar and form hydrogen bonds with each other. This causes strong cohesion (attraction between like molecules).
  • This cohesion means that when water molecules evaporate at the leaf, they pull the next molecule along, creating a continuous, unbroken column of water extending all the way down the stem and into the roots.
  • This continuous pull exerted by transpiration is called the transpiration pull, which generates tension (negative pressure) in the xylem vessels.

3. Adhesion

  • Water molecules are also attracted to the hydrophilic inner surfaces of the xylem vessel walls (which contain cellulose). This is called adhesion.
  • Adhesion helps prevent the water column from breaking (cavitation) and provides extra support against the downward pull of gravity.

Analogy: Imagine drinking through a very long, very thin straw. The suction you create (transpiration pull/tension) pulls the entire column of liquid up (cohesion).

Quick Review: Water Movement
  • In Roots: Apoplast $\rightarrow$ Casparian Strip $\rightarrow$ Symplast $\rightarrow$ Xylem.
  • In Stem/Leaves: Transpiration $\rightarrow$ Tension $\rightarrow$ Cohesion $\rightarrow$ Adhesion.

7.3 Adaptation to Water Stress: Xerophytes

Xerophytes are plants adapted to dry habitats (deserts, high altitudes, hot coasts) where water loss through transpiration is a significant problem.

You must be able to describe and use annotated drawings to show these adaptations:

Adaptations to Reduce Water Loss:

  • Thick Cuticle: A thick, waxy layer covering the epidermis reduces water evaporation from the leaf surface. (The thicker the cuticle, the longer the diffusion pathway).
  • Rolled Leaves: Rolling the leaf (e.g., marram grass) traps a layer of moist air inside the curl. This increases the humidity next to the stomata, reducing the water potential gradient between the leaf and the outside air, thus slowing diffusion.
  • Hairs or Trichomes: Dense layers of tiny hairs (trichomes) on the leaf surface trap moist air, similar to rolled leaves, lowering the water potential gradient.
  • Sunken Stomata: Stomata are located in pits or grooves, which also trap moist air, reducing the rate of diffusion.
  • Small Surface Area: Features like small, needle-like leaves (e.g., conifers) reduce the total surface area available for transpiration.

7.4 Transport Mechanisms for Assimilates (Translocation)

Translocation is the movement of organic substances, primarily sucrose (the transport carbohydrate) and amino acids, within the phloem.

Source and Sink

Assimilates move from a source (where they are made or stored) to a sink (where they are used or stored).

  • Source Examples: Photosynthesising leaves (during the day), or storage organs (like potatoes) when they are releasing starch.
  • Sink Examples: Growing points (root tips, shoot tips), fruits, seeds, or storage organs (like potatoes) when accumulating starch.
Step 1: Loading Sucrose into the Phloem (Source)

Sucrose is actively transported into the sieve tube elements at the source region. This process requires ATP supplied by the companion cells.

The Role of Companion Cells (Proton Pumps and Co-transporters):

  1. Proton Pump Activation: Companion cells use ATP to actively pump hydrogen ions (\(H^+\)) out of the cytoplasm and into the surrounding tissue (e.g., cell walls).
  2. Gradient Creation: This creates a high concentration gradient of \(H^+\) outside the companion cell.
  3. Co-transport: The \(H^+\) ions flow back into the companion cell (and potentially into the sieve tube) down their concentration gradient, via specialized membrane proteins called co-transporter proteins.
  4. Sucrose Entry: The co-transporter protein simultaneously carries the \(H^+\) ion and a sucrose molecule into the phloem unit (sieve tube element and companion cell), against the sucrose concentration gradient.

Did you know? This is an example of secondary active transport—energy is initially used to pump H+, but the subsequent movement of sucrose is powered by the H+ gradient, not directly by ATP.

Step 2: Generating the Hydrostatic Pressure Gradient (Mass Flow)
  1. Osmotic Water Movement: The high concentration of sucrose actively pumped into the sieve tube elements causes the water potential inside the phloem to decrease (become more negative).
  2. High Pressure at Source: Water then moves from the adjacent xylem (or surrounding cells) into the sieve tube by osmosis. The influx of water increases the volume and therefore increases the hydrostatic pressure at the source end.
  3. Mass Flow: This high pressure forces the phloem sap (sucrose and water) to flow along the sieve tube towards the sink, which has lower pressure. This bulk movement is called mass flow.
Step 3: Unloading Sucrose from the Phloem (Sink)
  1. Sucrose Unloading: At the sink, sucrose is actively or passively removed from the sieve tube elements and transferred to the sink cells (e.g., for respiration, conversion to starch, or growth).
  2. Water Movement Out: As sucrose leaves the phloem, the water potential increases (becomes less negative) inside the sieve tube.
  3. Low Pressure at Sink: Water moves out of the sieve tube by osmosis, usually back into the xylem. This removal of water lowers the hydrostatic pressure at the sink end.

The difference in hydrostatic pressure between the high-pressure source and the low-pressure sink drives the continual flow of assimilates in the phloem.

Key Takeaway for 7.2:

Water moves by Cohesion-Tension (a passive process driven by the environment). Assimilates move by Mass Flow, driven by a hydrostatic pressure gradient created by active loading (using proton pumps/co-transporters) and passive unloading.