Transport Systems » Plant Transport - Phloem Function

What you'll learn this session

Study time: 30 minutes

Understand the structure and function of phloem tissue in plants
Learn how translocation moves sugars around the plant
Explore the mass flow hypothesis and how it works
Discover the relationship between sources and sinks in plants
Examine adaptations of phloem cells for transport

🔒 Unlock Full Course Content

Unlock This Course

Introduction to Phloem Function

Plants are amazing living factories that make their own food through photosynthesis. But how do they move this food around their body? Just like you need blood vessels to carry nutrients to every part of your body, plants have special transport tubes called phloem. These incredible biological highways carry sugary solutions from where they're made to where they're needed most.

Think of phloem as the plant's food delivery service - it's constantly moving sweet, energy-rich substances from the leaves (the kitchen) to growing roots, developing fruits and storage organs (the hungry customers).

Key Definitions:

Phloem: Living transport tissue that carries dissolved sugars and other organic compounds around the plant.
Translocation: The movement of dissolved substances through phloem tissue.
Source: Any part of the plant that produces or releases sugars (like photosynthesising leaves).
Sink: Any part of the plant that uses or stores sugars (like growing roots or developing fruits).

🌿 What Makes Phloem Special?

Unlike xylem (which carries water upwards), phloem can transport materials in any direction - up, down, or sideways. This flexibility allows plants to respond to changing needs throughout their growth and development.

Structure of Phloem Tissue

Phloem tissue is made up of several types of cells working together like a well-organised team. Each cell type has a specific job that helps the whole system function efficiently.

The Phloem Team

The main players in phloem tissue include sieve tube elements, companion cells, phloem fibres and phloem parenchyma. Let's meet each member of this transport team:

🔧 Sieve Tube Elements

These are the main transport tubes. They're living cells that have lost most of their internal structures to make room for flowing sap. They connect end-to-end through sieve plates with tiny holes.

🤖 Companion Cells

These are the 'life support' cells. They keep the sieve tube elements alive by providing them with energy and controlling what enters and exits the transport system.

💪 Support Cells

Phloem fibres provide structural strength, while phloem parenchyma cells store substances and help with lateral transport between different phloem tubes.

Amazing Adaptation

Sieve tube elements are unique because they're alive but have no nucleus! They rely completely on their companion cells for survival. It's like having a car (sieve tube) that needs a driver (companion cell) to function properly.

How Translocation Works

The movement of sugars through phloem is called translocation and it's powered by a clever system called the mass flow hypothesis. This process is like a biological conveyor belt that never stops moving.

The Mass Flow Hypothesis

Imagine a river flowing from a mountain (high pressure) to the sea (low pressure). The mass flow hypothesis suggests that phloem sap moves in a similar way, flowing from areas of high pressure to areas of low pressure.

Here's how it works step by step:

Loading at the Source: In photosynthesising leaves, sugars are actively loaded into phloem tubes by companion cells using energy (ATP).
Water Follows: As sugar concentration increases, water moves in by osmosis, creating high pressure.
Mass Flow Begins: The high pressure pushes the sugary solution along the phloem tubes towards areas of lower pressure.
Unloading at the Sink: At growing or storage areas, sugars are actively removed from phloem tubes.
Pressure Drops: As sugars are removed, water leaves too, reducing pressure and maintaining the flow.

🌱 Sources in Action

Mature leaves are the main sources during the day when photosynthesis is happening. But storage organs like bulbs or tubers can become sources when they release stored sugars to support new growth in spring.

Sources and Sinks in Different Seasons

The beauty of the phloem system is its flexibility. What acts as a source or sink can change depending on the plant's needs and the time of year.

Seasonal Changes

Plants are constantly adapting their transport priorities based on their growth stage and environmental conditions:

🌸 Spring Growth

Storage organs (sources) release stored sugars to support new leaf and shoot growth (sinks). Young leaves that aren't fully developed yet act as sinks.

☀ Summer Activity

Mature leaves become the main sources, sending sugars to growing fruits, developing seeds and expanding roots (all acting as sinks).

🍂 Autumn Preparation

Leaves (sources) send sugars to storage organs like roots and bulbs (sinks) to prepare for winter survival.

Case Study: Apple Tree Transport

In an apple tree, mature leaves act as sources during summer, sending sugars to developing apples (sinks). As autumn approaches, the leaves send more sugars to the roots and trunk for winter storage. When spring arrives, these storage areas become sources, supporting new leaf growth before the leaves can photosynthesise effectively.

Factors Affecting Phloem Transport

Several factors can influence how well phloem transport works. Understanding these helps explain why plants sometimes struggle to move nutrients effectively.

Environmental and Internal Factors

Phloem transport is affected by both the plant's internal conditions and external environmental factors:

Temperature: Higher temperatures generally increase the rate of translocation, but extreme heat can damage the delicate phloem cells.
Light Intensity: More light means more photosynthesis, creating more sugars to transport and stronger source activity.
Water Availability: Water is essential for maintaining pressure in the phloem system. Drought can severely reduce transport efficiency.
Plant Age: Young, actively growing plants often have faster translocation rates than older, more established plants.
Damage or Disease: Physical damage or infections can block phloem tubes, disrupting transport pathways.

💡 Transport Efficiency

Phloem transport is remarkably efficient. The sugary sap can move at speeds of up to 1 metre per hour - fast enough to supply a tall tree's entire canopy with nutrients from its roots in just a few hours!

Comparing Phloem and Xylem Transport

While both phloem and xylem are transport systems, they work in very different ways and carry different substances.

Key Differences

Understanding how phloem differs from xylem helps us appreciate the complexity of plant transport systems:

🌿 Phloem Characteristics

Transports sugars and organic compounds
Uses living cells (sieve tube elements)
Requires energy (active transport)
Can move in any direction
Driven by pressure differences

💧 Xylem Characteristics

Transports water and minerals
Uses dead cells (vessel elements)
Passive process (no energy needed)
Only moves upwards
Driven by transpiration pull

Working Together

Phloem and xylem work as a team. Xylem brings water needed for photosynthesis to the leaves, while phloem distributes the sugars produced by photosynthesis throughout the plant. It's like having both a water supply system and a food delivery network in the same building!

Importance of Phloem Function

Phloem transport is absolutely vital for plant survival and growth. Without efficient phloem function, plants would quickly die as their non-photosynthetic parts would starve.

Why Phloem Matters

The phloem system supports virtually every aspect of plant life:

Growth and Development: Growing tips, developing fruits and expanding roots all depend on sugars delivered by phloem.
Energy Storage: Phloem moves excess sugars to storage organs like bulbs, tubers and thick roots for future use.
Reproduction: Developing flowers, fruits and seeds require massive amounts of sugars transported via phloem.
Survival: During winter or drought, stored sugars moved by phloem keep plants alive when photosynthesis isn't possible.
Communication: Phloem also transports signalling molecules that help coordinate plant responses to environmental changes.

Understanding phloem function helps us appreciate how plants manage to grow so large and complex while making their own food. From the tiniest seedling to the mightiest oak tree, phloem transport is working 24/7 to keep every cell supplied with the energy it needs to survive and thrive.

🔒 Test Your Knowledge!