Plants Need a Transport System

A large plant contains many cells that may be far from the substances they need.

Roots need sugars for respiration and growth, but they usually cannot photosynthesize.

Leaves produce sugars through photosynthesis, but they also need water and mineral ions from the soil.

Plants therefore contain specialized transport tissues:

xylem – transports water and mineral ions

phloem – transports sugars and other organic substances

Together, xylem and phloem form part of the plant's:

vascular tissue

---

What Is Phloem?

Phloem is vascular tissue that transports dissolved organic substances around a plant.

The most important transported sugar is:

sucrose

Phloem can also transport other substances, including some:

• amino acids
• hormones
• signaling molecules
• other organic compounds

The movement of these substances through phloem is called:

translocation

---

Why Transport Sucrose?

Photosynthesis produces glucose.

A simplified equation is:

carbon dioxide + water → glucose + oxygen

Plants can convert some glucose into:

sucrose

Sucrose is well suited for transport through the phloem.

The basic pathway is:

photosynthesis → glucose → sucrose → phloem transport

---

Why Don't All Plant Cells Make Their Own Food?

Photosynthesis requires:

light

Many plant tissues receive little or no light.

Examples include:

• roots
• underground storage organs
• internal tissues
• developing fruits
• growing buds

These tissues still require organic molecules for:

• respiration
• growth
• cell division
• storage
• production of new substances

Phloem allows sugars produced elsewhere to reach these tissues.

---

Sources and Sinks

Understanding translocation requires two important terms:

source

and:

sink

A source is an area that supplies sugars to the phloem.

A sink is an area that receives sugars from the phloem.

---

Sources

A source produces or releases more sugar than it currently requires.

A mature photosynthesizing leaf is a common example.

During photosynthesis:

leaf produces glucose

↓

some glucose is converted to sucrose

↓

sucrose enters phloem

↓

sucrose is transported elsewhere

Therefore, mature leaves are often:

sources

---

Sinks

A sink is a region that receives and uses or stores transported sugars.

Examples include:

• roots
• developing fruits
• seeds
• flowers
• growing shoots
• buds
• young leaves
• storage organs

Sugars arriving at a sink can be used for:

respiration

growth

storage

or:

making other biological molecules

---

Source-to-Sink Transport

The central idea of translocation is:

source → phloem → sink

For example:

mature leaf → phloem → root

or:

mature leaf → phloem → developing fruit

or:

storage organ → phloem → growing shoot

The interactive view below lets you trace this source-to-sink movement through the plant.

---

Sources and Sinks Can Change

A plant organ is not necessarily always a source or always a sink.

Its role can change.

For example, a young leaf may initially be a:

sink

because it is growing and receives sugars.

Later, when it becomes mature and photosynthesizes efficiently, it can become a:

source

This makes the plant transport system highly flexible.

---

A Storage Organ Can Also Change Roles

Consider a potato tuber.

During one part of the plant's life cycle, sugars may move:

leaves → tuber

The tuber acts as a:

sink

and carbohydrates are stored.

Later, when new shoots begin growing:

tuber → growing shoots

The stored material is mobilized, and the tuber can act as a:

source

So:

source and sink describe current function, not permanent identity.

---

Structure of Phloem

Phloem contains several types of specialized cells.

Two particularly important components are:

sieve tube elements

and:

companion cells

These cells work together to transport dissolved organic substances.

---

Sieve Tube Elements

Sieve tube elements are specialized cells arranged end-to-end.

Together they form long structures called:

sieve tubes

These tubes provide a pathway through which phloem sap can move.

Sieve tube elements are living cells, but mature sieve tube elements have a highly modified internal structure.

---

Sieve Plates

Where neighboring sieve tube elements meet, their end walls form:

sieve plates

Sieve plates contain many pores.

These pores allow phloem sap to move from one sieve tube element to the next.

The arrangement is:

sieve tube element

↓

sieve plate

↓

sieve tube element

↓

sieve plate

↓

sieve tube element

This creates a continuous transport pathway.

---

Companion Cells

Next to sieve tube elements are:

companion cells

Companion cells are living cells containing:

• a nucleus
• cytoplasm
• many mitochondria

They have an important role in supporting sieve tube elements.

Companion cells are involved in processes including the loading and unloading of sugars.

---

Why Do Companion Cells Have Many Mitochondria?

Loading sucrose into phloem can require:

active transport

Active transport requires energy supplied through:

ATP

Mitochondria are involved in aerobic respiration and ATP production.

Therefore:

many mitochondria → ATP supply → active transport processes

This connects phloem transport to cellular respiration.

---

Connections Between Companion Cells and Sieve Tubes

Companion cells and sieve tube elements are closely connected.

Small channels called:

plasmodesmata

connect their cytoplasm.

These connections allow substances and signals to move between cells.

The close relationship between companion cells and sieve tube elements is essential for normal phloem function.

---

What Is Phloem Sap?

The liquid moving through phloem is often called:

phloem sap

It contains water with dissolved substances.

An important transported substance is:

sucrose

Other substances may also be present.

Therefore, phloem does not transport solid sugar crystals.

It transports:

dissolved substances in solution

---

What Is Translocation?

Translocation is the movement of dissolved organic substances through the phloem from sources to sinks.

A simplified example is:

leaf → sucrose enters phloem → sucrose moves through sieve tubes → sucrose reaches root

The root can then use the sugar for:

• respiration
• growth
• storage

---

Step 1: Sugar Is Produced

In a photosynthesizing leaf:

carbon dioxide + water → glucose + oxygen

Some glucose is:

• used immediately in respiration
• stored as starch
• used to build cellulose
• converted into other substances
• converted into sucrose for transport

The transported sugar is mainly:

sucrose

---

Step 2: Sucrose Is Loaded into Phloem

At a source, sucrose moves into the phloem.

Companion cells play an important role in this process.

In many plants, energy-dependent transport mechanisms help concentrate sucrose in the phloem.

This process is called:

phloem loading

---

Step 3: Water Enters the Phloem

When sucrose concentration increases inside the phloem, the water potential becomes lower.

Water can then move into the phloem from nearby xylem by:

osmosis

This increases pressure inside the sieve tubes.

So at the source:

sucrose loaded

↓

water potential decreases

↓

water enters

↓

pressure increases

---

Step 4: Phloem Sap Moves

The higher pressure near the source helps push phloem sap toward areas of lower pressure.

This movement is often explained using the:

pressure-flow hypothesis

or:

mass-flow hypothesis

The basic idea is:

high pressure at source → bulk flow through phloem → lower pressure at sink

---

Step 5: Sucrose Is Unloaded

At the sink, sucrose leaves the phloem.

This is called:

phloem unloading

The sugar may then be:

• used in respiration
• used for growth
• converted into starch
• used to produce other molecules

For example, in a growing root:

sucrose → respiration → ATP

or:

sucrose → storage compounds

---

Step 6: Water Can Leave the Phloem

As sucrose is removed at the sink, water can move out of the phloem.

Some water may return to nearby:

xylem

This helps maintain the pressure difference between source and sink.

---

Pressure-Flow Model

A simplified sequence is:

1. Sucrose is loaded at the source.

↓

2. Water enters by osmosis.

↓

3. Hydrostatic pressure increases.

↓

4. Phloem sap moves through sieve tubes.

↓

5. Sucrose is unloaded at the sink.

↓

6. Water can leave the phloem.

This creates movement from:

source → sink

---

Translocation Requires Living Tissue

Phloem transport depends on living cells.

Companion cells carry out metabolic processes needed to support transport.

This is an important difference from mature xylem vessels, which are:

dead at maturity

Phloem sieve tube elements are:

living, highly specialized cells

---

Where Does the Sugar Go?

Plants distribute sugars according to demand.

A developing fruit may require sugar for:

• respiration
• growth
• production of new cells
• storage

A root may require sugar for:

• respiration
• active transport of mineral ions
• growth
• storage

A growing shoot requires sugar for:

• cell division
• new leaves
• new stems

---

Sugars Moving to Roots

Roots usually receive little or no light.

Therefore, most root cells cannot photosynthesize.

Yet root cells need ATP.

Sugars can be transported:

leaf → phloem → root

At the root:

sugar + oxygen → respiration → ATP

The ATP can support processes such as active transport and growth.

---

Sugars Moving to Fruits

Developing fruits are often strong:

sinks

Sugars move from photosynthesizing leaves to fruits.

The sugars may be:

• used for respiration
• incorporated into new tissues
• stored
• converted into other carbohydrates

This is one reason fruits can accumulate substantial quantities of sugars during development.

---

Sugars Moving to Seeds

Developing seeds require large amounts of organic material.

Transported sugars can contribute to the production of:

• starch
• oils
• proteins
• new cells

Seeds therefore often act as important sinks during reproduction.

---

Sugars Moving to Growing Shoots

Young shoots grow rapidly.

Growth requires:

• energy
• carbon-containing molecules
• amino acids
• new cell material

Young leaves may not yet photosynthesize enough to meet their own needs.

Therefore, they can receive substances through the phloem.

---

Storage

Plants can convert transported sugars into storage substances.

One important storage carbohydrate is:

starch

A simplified relationship is:

sucrose transported → glucose molecules → starch storage

Storage organs include:

• roots
• tubers
• bulbs
• seeds

---

Why Transport Sucrose Instead of Starch?

Starch is:

insoluble

This makes it useful for storage.

Sucrose is:

soluble

This makes it suitable for transport in solution.

Therefore:

sucrose → transport

starch → storage

This is an important distinction.

---

Xylem and Phloem

Plants contain two major transport tissues.

Xylem

Primarily transports:

• water
• mineral ions

Phloem

Transports:

• sucrose
• amino acids
• other organic substances

---

Comparing Xylem and Phloem

---

Direction of Xylem Transport

Xylem transport is mainly:

roots → stems → leaves

Water absorbed from the soil moves upward through the plant.

Therefore, xylem transport is commonly described as primarily:

upward

---

Direction of Phloem Transport

Phloem transport is more flexible.

Sugars move:

from source to sink

A sink might be:

• above the source
• below the source

Therefore, phloem transport can occur:

upward or downward in different sieve tubes

depending on the locations of sources and sinks.

---

Does Sugar Move Both Directions in One Sieve Tube?

It is useful to be precise.

Phloem as a whole can transport substances in different directions because different source-sink pathways may operate simultaneously.

However, movement within an individual sieve tube pathway at a particular time is generally directed from its:

source toward its sink

So "phloem moves both ways" does not mean molecules are simply moving randomly up and down the same tube.

---

Xylem and Phloem Work Together

Although xylem and phloem perform different functions, they are connected.

For example:

xylem supplies water to leaves

↓

water participates in photosynthesis

↓

photosynthesis produces glucose

↓

glucose can be converted to sucrose

↓

phloem transports sucrose

The two transport systems therefore support one another.

---

Water Can Move Between Xylem and Phloem

The relationship is even closer during translocation.

At a source:

water may move from xylem → phloem

At a sink:

water may move from phloem → xylem

This movement helps create the pressure differences involved in mass flow.

---

Vascular Bundles

Xylem and phloem are commonly arranged together in:

vascular bundles

These bundles run through:

• roots
• stems
• leaves

A vascular bundle is somewhat like a transport network connecting different parts of the plant.

---

Vascular Bundles in Stems

In a stem, vascular bundles contain both:

xylem

and:

phloem

Their exact arrangement depends on the type of plant.

The tissues provide continuous transport pathways connecting:

roots ↔ stems ↔ leaves ↔ flowers ↔ fruits

---

Vascular Tissue in Leaves

The visible veins in a leaf contain vascular tissue.

Xylem delivers:

water and minerals

Phloem carries away:

sugars produced by photosynthesis

This explains why leaf veins are essential for both supplying and exporting materials.

---

Connecting Phloem to Photosynthesis

Photosynthesis occurs mainly in leaves.

It produces glucose.

Some glucose is converted into sucrose.

Therefore:

photosynthesis → glucose → sucrose → phloem → sinks

Without phloem, sugars produced in leaves could not be efficiently distributed throughout a large plant.

---

Connecting Phloem to Respiration

Non-photosynthetic cells still require energy.

For example, root cells require ATP for:

• growth
• active transport
• cell division
• metabolism

Sugars transported by phloem provide respiratory substrates.

Therefore:

phloem transport → sugar reaches root → respiration → ATP

---

Connecting Phloem to Growth

Growing tissues are often sinks.

Growth requires:

• energy
• carbon compounds
• amino acids
• new cell structures

Therefore, sugars are often transported toward:

• root tips
• shoot tips
• developing leaves
• flowers
• fruits
• seeds

Phloem allows resources produced in one part of the plant to support growth elsewhere.

---

Connecting Phloem to Seasonal Growth

Consider a deciduous plant early in spring.

New leaves may not yet photosynthesize enough to support themselves.

Stored carbohydrates in roots or stems can be mobilized.

The pathway might be:

storage tissue → phloem → developing buds

Later, mature leaves begin photosynthesizing.

Then:

mature leaves → phloem → storage tissues

The source-sink relationship changes with the season.

---

How Could Scientists Investigate Phloem?

One classic investigation involves removing a ring of bark from around a woody stem.

This is sometimes called:

ringing or girdling

The phloem is located toward the outer part of the stem.

Removing a complete ring can interrupt phloem transport while leaving much of the xylem functioning temporarily.

---

The Ringing Experiment

If phloem transport is interrupted:

sugars cannot move normally past the removed region

Sugars may accumulate above the cut.

Tissues below may receive less transported sugar.

This provides evidence that sugars are transported through tissue associated with the bark:

phloem

---

Aphids and Phloem

Scientists have also studied phloem using:

aphids

Aphids have specialized mouthparts that can penetrate plant tissue and reach phloem.

Phloem sap can sometimes be sampled from these feeding structures.

Analysis shows that phloem sap contains substantial amounts of:

sugars, especially sucrose

This provides additional evidence about what phloem transports.

---

Why Does Translocation Require Energy?

Not every part of the pressure-driven flow requires ATP directly.

However, energy is required for important processes such as:

active loading and unloading of sucrose

Companion cells provide much of the metabolic support for these processes.

Therefore:

translocation depends on living, metabolically active tissue

---

What Happens If Respiration Is Reduced?

If companion cells cannot produce sufficient ATP:

active transport processes may decrease

↓

phloem loading can be disrupted

↓

normal translocation can be reduced

This demonstrates a connection between:

respiration and transport

---

Source-Sink Example

Imagine a tomato plant.

Mature leaves:

sources

Developing tomatoes:

sinks

Roots:

sinks

Growing shoot:

sink

Sugars may therefore move from mature leaves toward several different regions.

The distribution depends on the plant's current requirements.

---

Example: Germinating Seed

A germinating seed initially cannot depend on mature photosynthesizing leaves.

Stored food inside the seed provides energy and materials.

As the young plant grows:

stored food → growing root and shoot

Later, once leaves become established:

photosynthesis → sugars → other plant tissues

The direction of resource movement changes during development.

---

Example: Fruit Development

Suppose a fruit begins growing.

Its cells are:

• dividing
• expanding
• respiring
• accumulating materials

The developing fruit becomes a strong:

sink

Sugars can move:

mature leaves → phloem → fruit

As fruit development increases, demand for transported sugars may also increase.

---

Example: Root Storage

Some plants store large quantities of carbohydrate underground.

Examples include storage roots and tubers.

During the growing season:

leaves → phloem → storage organ

Sugars can be converted into:

starch

Later:

stored carbohydrate → sugars → phloem → growing tissues

---

Why Is Translocation Important?

Without translocation, photosynthetic products would remain concentrated near the cells that produced them.

Other tissues could be deprived of:

• respiratory substrates
• carbon for growth
• materials for storage

Phloem allows the plant to function as:

one integrated organism

rather than a collection of isolated organs.

---

Common Misconception: Phloem Transports Glucose Only

Photosynthesis produces glucose, but the major carbohydrate transported through phloem is usually:

sucrose

Remember:

glucose produced → sucrose transported

---

Common Misconception: Phloem Carries Food Only Downward

Phloem does not simply transport sugars:

leaves → roots

It transports:

source → sink

If the sink is above the source, transport can occur upward.

If the sink is below the source, transport can occur downward.

---

Common Misconception: Xylem and Phloem Do the Same Job

They are both vascular tissues, but they transport different substances.

Xylem:

water + mineral ions

Phloem:

sugars + other organic substances

They also differ in structure and transport mechanisms.

---

Common Misconception: Sieve Tubes Are Hollow Dead Pipes

Mature xylem vessels are dead and hollow.

Phloem sieve tube elements are:

living

although they have reduced internal structures.

They depend closely on:

companion cells

This distinction is important.

---

Common Misconception: Roots Only Receive Water

Roots absorb water and minerals from the soil.

But roots also receive sugars from other parts of the plant through:

phloem

Therefore, roots participate in two different transport systems:

soil → root → xylem

and:

leaf → phloem → root

---

Challenge: Identify the Source and Sink

A mature leaf produces more sugar than it requires.

A developing strawberry receives sugar.

Source:

mature leaf

Sink:

developing strawberry

Direction:

leaf → fruit

---

Challenge: Spring Growth

A plant has stored starch in its roots during winter.

In spring, new shoots begin growing before many mature leaves are present.

Where is the source?

storage tissue in the roots

Where is the sink?

growing shoots

This demonstrates why source and sink are not permanent labels.

---

Challenge: Compare the Transport Systems

Suppose water enters a root from the soil.

Pathway:

root → xylem → leaf

Suppose sucrose is produced in that leaf and transported to a growing root tip.

Pathway:

leaf → phloem → root tip

The two vascular systems therefore transport different materials in different ways.

---

Did You Know?

Phloem transport can redistribute carbon from one region of a plant to another over surprisingly large distances.

In a large tree, sugars produced high in the canopy can eventually supply:

• roots
• growing tissues
• developing fruits or seeds
• storage tissues

The vascular system allows the entire plant to share resources.

---

Key Terms

• Phloem: Vascular tissue that transports sugars and other dissolved organic substances.
• Translocation: Movement of dissolved organic substances through phloem from sources to sinks.
• Sucrose: Soluble sugar commonly transported through phloem.
• Source: Plant region that supplies transported organic substances.
• Sink: Plant region that receives and uses or stores transported organic substances.
• Sieve tube element: Specialized living phloem cell involved in transport.
• Sieve tube: Column of sieve tube elements forming a transport pathway.
• Sieve plate: Porous end wall between neighboring sieve tube elements.
• Companion cell: Metabolically active cell associated with a sieve tube element.
• Phloem sap: Liquid containing dissolved substances transported through phloem.
• Phloem loading: Movement of sugars into phloem at a source.
• Phloem unloading: Movement of sugars out of phloem at a sink.
• Pressure flow: Pressure-driven movement of phloem sap from source toward sink.
• Vascular tissue: Plant transport tissue consisting principally of xylem and phloem.
• Vascular bundle: Group of vascular tissues within a plant organ.
• Xylem: Tissue transporting water and mineral ions, mainly from roots toward shoots.
• Plasmodesmata: Microscopic cytoplasmic connections between neighboring plant cells.
• Active transport: Movement of substances using cellular energy.
• Osmosis: Net movement of water across a partially permeable membrane down a water-potential gradient.

---

Key Relationships

Photosynthesis and transport:

photosynthesis → glucose → sucrose → phloem

Basic translocation:

source → phloem → sink

At the source:

sucrose loading → water enters → pressure increases

Transport:

higher pressure → mass flow → lower-pressure region

At the sink:

sucrose unloading → sugar used or stored → water can leave

Sugar use:

sucrose → respiration → ATP

or:

sucrose → storage compounds

Xylem:

roots → water + mineral ions → shoots

Phloem:

source → sugars + organic substances → sink

---

Key Takeaways

• Phloem is the vascular tissue responsible for transporting sugars and other organic substances.
• The major sugar transported through phloem is sucrose.
• Movement of organic substances through phloem is called translocation.
• Phloem transports substances from sources to sinks.
• Mature photosynthesizing leaves are common sources.
• Roots, fruits, seeds, buds, storage organs, and growing tissues can act as sinks.
• Sources and sinks can change as a plant grows or as seasons change.
• Phloem contains sieve tube elements and companion cells.
• Sieve tube elements are arranged end-to-end to form sieve tubes.
• Sieve plates contain pores that allow phloem sap to move between sieve tube elements.
• Sieve tube elements are living cells with specialized internal structures.
• Companion cells provide metabolic support for sieve tube elements.
• Companion cells contain many mitochondria and participate in energy-dependent transport processes.
• Sucrose is loaded into phloem at sources.
• Water can enter the phloem by osmosis.
• Increased pressure at a source helps drive phloem sap toward sinks.
• Sucrose is unloaded from phloem at sinks.
• Sugars arriving at sinks may be used for respiration, growth, or storage.
• Phloem transport can occur upward or downward through the plant depending on source and sink locations.
• Xylem mainly transports water and mineral ions from roots toward shoots.
• Phloem transports sugars and other organic substances from sources to sinks.
• Mature xylem conducting cells are dead, while phloem transport depends on living cells.
• Xylem and phloem often occur together in vascular bundles.
• Leaf veins contain both xylem and phloem.
• Xylem supplies water needed for photosynthesis, while phloem distributes products of photosynthesis.
• Roots depend on phloem for sugars because most root tissues cannot photosynthesize.
• Developing fruits and seeds can be strong sinks for transported sugars.
• Storage organs can switch between acting as sinks and sources.
• Translocation links photosynthesis, respiration, osmosis, active transport, growth, and storage.
• The central idea is: leaves and other sources produce or release sugars → phloem transports them → sinks use or store them.