Biology (9610) | Mass transport systems in plants

Mass Transport Systems in Plants: The Biological Highway

Welcome to the fascinating world of plant transport! As you move through the curriculum section on Biological systems and disease, you learn how large organisms, both animal and plant, need efficient ways to move vital substances around.

Think about a redwood tree, towering over 100 meters tall. Simple diffusion is far too slow to move water from the roots all the way to the top leaves! This is where mass transport systems come in. These are specialized systems that move substances over long distances quickly and efficiently.

In this chapter, we will uncover how plants use their vascular tissue—the xylem and phloem—to circulate water, minerals, and sugars, keeping every cell alive and growing.

1. Xylem: The Water and Mineral Transport System

The xylem acts like the plant's plumbing network, dedicated primarily to moving water and dissolved mineral ions (like nitrates and phosphates) from the roots up to the leaves and stem.

1.1 Structure of Xylem Vessels

Xylem vessels are key adaptations for large organisms.

• Composition: Xylem vessels are essentially long, continuous, hollow tubes.
• Key Feature: When mature, these cells are dead and have no cytoplasm or end walls, which allows for minimal resistance to water flow.
• Strength: Their walls are thickened with lignin, a strong, waterproof substance. Lignin provides structural support, preventing the vessels from collapsing under tension.

Memory Aid: Xy-up! Xylem moves substances up the plant.

1.2 Water Movement in the Root

Before water enters the xylem, it must first cross the root tissues. Water absorbed by the root hairs (which greatly increase surface area) can take two main paths: the apoplastic pathway or the symplastic pathway.

A. The Apoplastic Pathway

This path is like water moving along the outside walls of houses on a street.

• Water moves through the cell walls and the intercellular spaces (spaces between cells).
• It involves minimal resistance and does not cross any plasma membranes.
• Analogy: The water flows freely through the porous structure of the cell walls.

B. The Symplastic Pathway

This path is like passing a bucket of water from person to person inside the houses.

• Water enters the cytoplasm of a root hair cell and moves from cell to cell via plasmodesmata (small channels connecting adjacent cytoplasms).
• It involves movement across the plasma membrane at the start.

C. The Role of the Endodermis and Casparian Strip

This is a critical checkpoint! The endodermis is a layer of cells surrounding the central vascular tissue (stele) of the root.

• The endodermis contains the Casparian strip, a waxy, waterproof band (made of suberin) in the cell walls.
• Function: When water moving via the apoplastic pathway reaches the Casparian strip, the strip blocks the flow. The water is forced to enter the cytoplasm of the endodermal cells, switching to the symplastic pathway.
• Why? This forces the water and dissolved minerals to pass through the cell membrane, allowing the plant to actively control which solutes (ions) are allowed to enter the xylem.

1.3 Mechanisms of Water Movement in Xylem

Water is moved against gravity through two key mechanisms: root pressure and the transpiration pull.

A. Root Pressure (The Push)

Root pressure is a relatively small, upward force generated in the roots.

• Ions are actively pumped into the xylem vessels in the roots.
• This lowers the water potential (\(\Psi\)) inside the xylem.
• Water moves into the xylem by osmosis down the water potential gradient, creating a positive pressure (the root pressure) that pushes water up a short distance.

B. Transpiration and the Cohesion-Tension Theory (The Main Pull)

This is the main mechanism responsible for pulling water up tall trees.

1. Transpiration (Evaporation): Water vapour evaporates from the surface of the mesophyll cells into the air spaces in the leaf, and then diffuses out through the stomata.
2. Tension: The loss of water from the mesophyll cells lowers their water potential, so water is drawn from the adjacent xylem vessels into the cells. This creates a powerful negative pressure, or tension, in the xylem column.
3. Cohesion: Water molecules are attracted to each other by hydrogen bonds (a property called cohesion). This means the water forms a continuous, unbroken column up the xylem vessel, like a long chain.
4. Adhesion: Water molecules are also attracted to the walls of the xylem vessel (a property called adhesion). This helps prevent the column from breaking and provides additional support.

Analogy: Imagine drinking through a very long straw. The act of sucking (transpiration) creates tension, and the cohesive forces keep the water column intact as it is pulled upwards.

1.4 Factors Affecting the Rate of Transpiration

Transpiration is essentially evaporation, so any factor that affects the rate of evaporation will affect the rate of water movement. Plants must balance the need for CO₂ uptake (requiring open stomata) against the risk of water loss.

• Light Intensity: Increased light causes stomata to open (for photosynthesis), increasing the rate of water loss.
• Temperature: Higher temperature increases the kinetic energy of water molecules, increasing the rate of evaporation from the leaf surface and the diffusion rate of water vapour.
• Humidity: High external humidity means the water potential gradient between the leaf air space and the external atmosphere is shallower, slowing the rate of diffusion/evaporation. Low humidity increases the rate.
• Air Movement (Wind): Increased air movement (wind) sweeps away the layer of humid air (boundary layer) surrounding the leaf, maintaining a steep water potential gradient, thus increasing the rate of transpiration.

2. Phloem: Transport of Organic Substances (Translocation)

The phloem is responsible for translocation, which is the movement of organic substances, primarily the sugar sucrose, from a source (where it is made, like a leaf) to a sink (where it is used or stored, like a root, fruit, or growing bud).

2.1 Structure of Phloem Tissue

Unlike xylem, phloem tissue is alive, but highly specialized.

• Sieve Tube Elements: These are the main transport vessels. They have very few organelles, resulting in minimal obstruction to flow. Their end walls are perforated (the sieve plates), allowing cytoplasm to connect adjacent cells.
• Companion Cells: These cells are located next to the sieve tube elements. They are metabolically active and packed with mitochondria to provide the ATP needed for active loading of sucrose. They essentially keep the sieve tube elements alive.

Memory Aid: Pha-food! Phloem moves food (sugars) around the plant.

2.2 The Mass Flow Hypothesis (Mechanism of Translocation)

The accepted theory explaining translocation is the mass flow hypothesis. This movement is driven by differences in hydrostatic pressure generated by active transport and osmosis.

Step 1: Loading at the Source

Sucrose (produced during photosynthesis in the source leaf) is moved into the phloem tissue (specifically the companion cells and then the sieve tube elements).

• This loading process requires Active Transport (using ATP supplied by the companion cells).
• The active pumping of sucrose drastically lowers the water potential (\(\Psi\)) within the sieve tube elements at the source.

Step 2: Water Movement and Pressure Flow

Because the sucrose concentration is now high in the sieve tubes:

• Water moves rapidly from the nearby xylem into the sieve tube elements by osmosis, following the water potential gradient.
• This influx of water creates a high hydrostatic pressure at the source end.

Step 3: Unloading at the Sink

At the sink (where sugars are needed, e.g., for growth or storage), the process is reversed.

• Sucrose is actively or passively removed from the sieve tube elements into the sink cells.
• This removal raises the water potential (\(\Psi\)) in the sieve tube elements.
• Water then moves back out of the sieve tubes, often re-entering the xylem, by osmosis.
• This outflow of water reduces the hydrostatic pressure at the sink end.

The high pressure at the source and the low pressure at the sink create a pressure gradient, driving the mass flow of sucrose solution through the phloem.

2.3 Evidence Related to Mass Transport

The syllabus requires you to be able to analyse and interpret experimental evidence related to these mechanisms.

Evidence Supporting the Xylem:

Experiments like using a potometer measure the rate of water uptake, which is a good proxy for the rate of transpiration. If you cut the xylem, the continuous water column breaks, proving the importance of cohesion.

Evidence Supporting the Phloem (Mass Flow):

• Ringing Experiments: Removing a ring of bark (which contains the phloem) causes the area above the ring to swell as sugars accumulate, confirming that sugars travel downwards in the phloem.
• Radioactive Tracers: Using radioactive CO₂ (\(^{14}\text{CO}_2\)) shows that the radioactive carbon (now incorporated into sugars) appears in the phloem and travels rapidly from the source leaf to the sink.
• Aphid Studies (Relevant Link): Aphids insert their sharp mouthparts (stylets) directly into sieve tube elements. The high hydrostatic pressure in the phloem forces the sap into the aphid's gut, providing direct evidence of positive pressure flow in the phloem.

You have now completed the key mechanisms plants use to survive and grow. Understanding the relationship between active loading, osmosis, and pressure gradients is essential for success in this topic!