Plant Transport » Phloem and Translocation

What you'll learn this session

Study time: 30 minutes

The structure and function of phloem tissue
How translocation works in plants
The pressure flow hypothesis
Factors affecting translocation rates
The importance of translocation for plant survival
Differences between xylem and phloem transport

Introduction to Phloem and Translocation

Plants need to move food (sugars) from where they're made to where they're needed. This movement of sugars through the phloem tissue is called translocation. It's just as important as water transport, but works in a completely different way!

Key Definitions:

Phloem: Plant tissue that transports dissolved sugars and other organic compounds.
Translocation: The movement of sugars and other organic compounds through the phloem.
Source: Where sugars are made or released (like leaves during photosynthesis).
Sink: Where sugars are used or stored (like roots, fruits, or growing tips).

🌱 The Phloem Highway

While xylem only transports water upwards, phloem can move sugars in any direction - up, down, or sideways! This means food can travel from leaves to roots, or from leaves to fruits, or wherever it's needed most.

🌾 Why Plants Need Translocation

Plants make food through photosynthesis in their leaves, but they need to share this food with non-photosynthetic parts like roots, flowers and fruits. Without translocation, these parts would starve!

Structure of Phloem Tissue

Phloem tissue is made up of several specialized cell types that work together to transport sugars efficiently.

Phloem Cell Types

📊 Sieve Tube Elements

These are the main conducting cells. They're living but have no nucleus when mature. Their end walls have holes (sieve plates) that allow sap to flow through.

🧠 Companion Cells

These cells sit alongside sieve tubes and control their activity. They have lots of mitochondria to provide energy and contain the nuclei that sieve tubes lack.

🌳 Supporting Cells

Phloem also contains fibre cells for strength and parenchyma cells for storage. These help maintain the structure of the phloem tissue.

Did You Know?

Sieve tube elements are one of the few living plant cells that don't have a nucleus when mature. They rely completely on their companion cells for genetic instructions and protein synthesis!

How Translocation Works

Translocation is explained by the pressure flow hypothesis (also called the mass flow hypothesis). Here's how it works:

The Pressure Flow Hypothesis

This is a bit like squeezing toothpaste from one end of the tube to the other!

Loading sugars at the source: Sugars (usually sucrose) are actively transported into the phloem at source regions using energy from ATP.
Creating pressure: As sugar concentration increases in the phloem, water follows by osmosis, creating high pressure.
Flow to the sink: At sink regions, sugars are removed from the phloem and used or converted to starch for storage.
Reducing pressure: As sugars leave, water follows by osmosis, creating lower pressure at the sink.
Mass flow: The pressure difference pushes the phloem sap from high pressure (source) to low pressure (sink).

🚀 Active Transport

Loading sugars into the phloem requires energy (ATP) because the sugars move against their concentration gradient. This is active transport and is crucial for creating the pressure that drives translocation.

💧 Water Movement

Water moves into and out of the phloem by osmosis. It follows the sugar concentration gradient, moving into the phloem at sources and out at sinks.

Comparing Xylem and Phloem Transport

Feature	Xylem	Phloem
Direction of transport	One-way (roots to leaves)	Multi-directional (source to sink)
What's transported	Water and minerals	Sugars and other organic compounds
Cell status	Dead cells	Living cells
Energy requirement	Passive (no energy needed)	Active (requires ATP)
Driving force	Transpiration pull and root pressure	Pressure flow (from source to sink)

Factors Affecting Translocation

Several factors can speed up or slow down the rate of translocation in plants:

🌞 Temperature

Higher temperatures (up to an optimum) increase translocation rates by speeding up metabolic processes and active transport.

🌱 Plant Growth Stage

Rapidly growing parts and developing fruits become strong sinks, increasing translocation towards them.

☀️ Light Intensity

More light means more photosynthesis, creating stronger sources and increasing sugar availability for translocation.

Case Study: Fruit Development

When fruits are developing, they become powerful sinks that draw sugars from all over the plant. This is why fruit trees often grow more slowly during fruiting - much of their energy is being directed to the fruits! Farmers sometimes thin out excess fruits to maintain tree health and ensure the remaining fruits grow to a good size.

Practical Applications

Understanding translocation helps us in many practical ways:

Agricultural Applications

Farmers and gardeners use knowledge of translocation to:

Apply systemic pesticides: These chemicals enter the phloem and travel throughout the plant, protecting all parts.
Improve fruit production: By manipulating source-sink relationships, farmers can increase yield and quality.
Time harvesting: Knowing when sugars are highest in crops helps determine the best harvest time.

🔬 Experimental Evidence

Scientists have used radioactive carbon dioxide (C¹⁴O₂) in experiments to track the movement of sugars through plants. The plants photosynthesize using the radioactive CO₂ and then the movement of the radioactive sugars can be tracked as they move through the phloem.

🍰 Sweet Success

When harvesting crops like sugar cane or maple syrup, timing is everything! Farmers tap into the translocation system to collect sugars when they're most concentrated in the phloem.

Common Misconceptions

Let's clear up some common confusions about phloem and translocation:

Misconception: Phloem only transports sugars downward.
Reality: Phloem can transport sugars in any direction - wherever there's a sink!
Misconception: Translocation is just like water transport but with sugar.
Reality: Translocation is fundamentally different from water transport in xylem. It requires energy and living cells.
Misconception: Phloem cells are dead like xylem vessels.
Reality: Phloem sieve tube elements are living cells, though they lack some organelles.

Exam Tip!

In exams, you might be asked to compare xylem and phloem transport. Remember the key differences: living vs. dead cells, direction of transport, active vs. passive processes and what's being transported.

Summary

Phloem and translocation form a sophisticated transport system that allows plants to move sugars from sources to sinks. The pressure flow hypothesis explains how this works through a combination of active transport and osmosis. Unlike xylem transport, translocation requires energy and can move substances in any direction based on the plant's needs. Understanding this process helps us grow better crops and appreciate the complex systems that keep plants alive.

🧠 Test Your Knowledge!