Topic 6 of 17Cambridge A Levels

Transport in Plants

Plants transport water, minerals, and sugars through specialized xylem and phloem tissues.

Multicellular plants require a specialized transport system to move substances over distances far greater than what diffusion alone can achieve. This system consists of two complex vascular tissues: xylem and phloem, which are bundled together in vascular bundles throughout the roots, stem, and leaves.

### Transport of Water and Minerals: The Xylem

The xylem is responsible for the unidirectional transport of water and dissolved mineral ions from the roots up to the rest of the plant. It also provides mechanical strength and support.

Structure of Xylem:

Xylem tissue is composed primarily of two types of cells: xylem vessels and tracheids. Both are dead at functional maturity, meaning they have no protoplast, which creates a hollow, uninterrupted tube called a lumen for water flow. Their cell walls are thickened with lignin, a strong, waterproof polymer. This lignification prevents the vessels from collapsing under negative pressure and makes the walls impermeable to water. Pits, which are non-lignified areas, allow for the sideways movement of water between adjacent vessels.

The Cohesion-Tension Theory:

This is the widely accepted mechanism for water movement up the xylem against gravity.

Water Uptake by Roots: Water enters root hair cells from the soil via osmosis, moving down a water potential gradient. It then travels across the root cortex to the xylem via two routes: the apoplast pathway (through cell walls) and the symplast pathway (through the cytoplasm and plasmodesmata).

The Casparian Strip: Before entering the xylem, water must pass through the endodermis. Here, the apoplast pathway is blocked by the Casparian strip, a waterproof band of suberin in the cell walls. This forces water and dissolved minerals to enter the symplast pathway, allowing the cell membrane to selectively control which minerals enter the xylem.

Transpiration: This is the loss of water vapour from the plant's surface, mainly through pores on the leaf surface called stomata. This water loss from the mesophyll cells lowers their water potential.

Tension and the Transpiration Stream: The lowered water potential in the leaf creates a pulling force, or tension, on the water in the xylem vessels. Because water molecules are cohesive (they stick together due to hydrogen bonds) and adhesive (they stick to the lignified xylem walls), this tension pulls the entire column of water up from the roots. This continuous flow is known as the transpiration stream.

### Transport of Assimilates: The Phloem

Phloem is responsible for transporting the products of photosynthesis, known as assimilates (mainly sucrose), from their site of production (source) to regions of use or storage (sink). This process is called translocation and it can be bidirectional.

Structure of Phloem:

Phloem consists of living cells. The main conducting cells are sieve tube elements, which are arranged end-to-end to form a continuous tube. These cells have very little cytoplasm, no nucleus, and few organelles at maturity to maximise space for transport. The end walls are perforated to form sieve plates, which allow sap to flow through. Each sieve tube element is closely associated with a companion cell. The companion cell is metabolically active, containing a nucleus and dense cytoplasm, and it provides the metabolic support (e.g., ATP) for the sieve tube element via numerous connections called plasmodesmata.

The Mass Flow Hypothesis (Pressure Flow Hypothesis):

This model explains the movement of sap in the phloem.

Loading at the Source: Sucrose produced in photosynthesising leaves (the source) is actively loaded into the companion cells and then into the sieve tube elements. This active transport requires energy in the form of ATP.

Creation of a Water Potential Gradient: The high concentration of sucrose in the sieve tube at the source significantly lowers its water potential.

Hydrostatic Pressure Buildup: Water moves by osmosis from the adjacent xylem into the sieve tube, down the water potential gradient. This influx of water creates a high hydrostatic pressure at the source.

Unloading at the Sink: At a sink (e.g., roots, fruits, flowers), sucrose is actively removed from the sieve tube for use in respiration or for conversion into storage molecules like starch. This process also requires ATP.

Mass Flow: The removal of sucrose at the sink raises the water potential inside the sieve tube, causing water to move out by osmosis. This leads to a low hydrostatic pressure at the sink. The difference in hydrostatic pressure between the source and the sink creates a pressure gradient that drives the mass flow of phloem sap along the sieve tube.

Key Points to Remember

1Xylem transports water and minerals unidirectionally from roots to leaves; phloem transports sugars bidirectionally.
2The **Cohesion-Tension Theory** explains water movement in xylem, driven by a transpiration-induced water potential gradient.
3**Lignified** xylem vessels are dead, hollow tubes providing a continuous column for the transpiration stream.
4The **Casparian strip** in the root endodermis forces selective mineral uptake by blocking the apoplast pathway.
5**Translocation** is the transport of assimilates (sucrose) in the phloem from a **source** to a **sink**.
6The **Mass Flow Hypothesis** explains translocation, driven by an active, ATP-dependent hydrostatic pressure gradient.
7Phloem consists of living **sieve tube elements** (for transport) and metabolically active **companion cells** (for support).
8**Water potential gradients**, established by osmosis, are fundamental to movement in both xylem and phloem.

Pakistan Example

Source-Sink Dynamics in Sugarcane Cultivation

The cultivation of sugarcane (Gan'na) in Pakistan's Punjab and Sindh provinces is a perfect real-world example of the source-to-sink principle of translocation. The large, green leaves of the sugarcane plant act as a powerful **source**, carrying out photosynthesis at a high rate in the intense sunlight. The sucrose produced is actively loaded into the phloem. The primary **sink** is the plant's stem (the cane), which is a modified storage organ. The **mass flow** of sap, driven by the high hydrostatic pressure from the source, transports vast quantities of sucrose to the stem, where it is stored in parenchyma cells. This highly efficient transport and storage mechanism is what makes sugarcane a major cash crop, forming the backbone of Pakistan's sugar industry.

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Biology — Quick Revision

Transport in Plants

Key Concepts

1Xylem transports water and minerals unidirectionally from roots to leaves; phloem transports sugars bidirectionally.

2The **Cohesion-Tension Theory** explains water movement in xylem, driven by a transpiration-induced water potential gradient.

3**Lignified** xylem vessels are dead, hollow tubes providing a continuous column for the transpiration stream.

4The **Casparian strip** in the root endodermis forces selective mineral uptake by blocking the apoplast pathway.

5**Translocation** is the transport of assimilates (sucrose) in the phloem from a **source** to a **sink**.

6The **Mass Flow Hypothesis** explains translocation, driven by an active, ATP-dependent hydrostatic pressure gradient.

Pakistan Example

Source-Sink Dynamics in Sugarcane Cultivation

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