Phloem and Sugar Transport

Core Concepts

Phloem Structure

Phloem is a complex tissue. The sugar-conducting cells in angiosperms are sieve-tube elements. These are living cells arranged end-to-end to form sieve tubes. At maturity, sieve-tube elements lose their nucleus, ribosomes, and vacuole, but they retain a functional plasma membrane, mitochondria, endoplasmic reticulum, and plastids. The end walls between sieve-tube elements are called sieve plates — they contain large pores through which phloem sap flows.

Each sieve-tube element is closely associated with one or more companion cells — specialized parenchyma cells that retain their nucleus and full metabolic machinery. Companion cells are connected to sieve-tube elements by numerous plasmodesmata (cytoplasmic channels). They provide ATP, proteins, and other essential molecules to the enucleate sieve-tube elements and regulate sugar loading and unloading.

In gymnosperms, the sugar-conducting cells are called sieve cells (rather than sieve-tube elements) and are associated with albuminous cells (rather than companion cells). The functional principles are similar.

Sources and Sinks

A source is any plant organ that produces or releases more sugar than it consumes, exporting sugar into the phloem. Mature, photosynthesizing leaves are the most common sources. Storage organs (roots, tubers) can act as sources when they break down stored starch and export sugars — for example, a potato tuber in spring, sending sugars to growing shoots.

A sink is any plant organ that consumes or stores more sugar than it produces, importing sugar from the phloem. Growing shoot tips, developing leaves (before they become net exporters), roots, developing fruits and seeds, and storage organs all function as sinks at particular times.

The identity of sources and sinks can change with the season and developmental stage. A young leaf is a sink — importing sugars to fuel its growth. Once mature and photosynthesizing, the same leaf becomes a source — exporting sugars. The same root may be a sink in summer (storing sugars) and a source in spring (exporting sugars to support new shoot growth).

The Pressure-Flow Hypothesis

The pressure-flow hypothesis (also called the Münch pressure-flow model) explains how phloem sap moves from sources to sinks without a pump:

Step 1: Sugar loading at the source. Companion cells actively load sucrose (and other solutes) into sieve-tube elements at the source. This requires ATP — sucrose is transported against its concentration gradient using a proton cotransport mechanism. The accumulation of sugar in the sieve-tube element significantly lowers the water potential (makes it more negative).

Step 2: Water entry by osmosis. The low water potential in the sieve-tube element draws water from the adjacent xylem. Water enters the sieve-tube element by osmosis, increasing the hydrostatic pressure (turgor pressure) at the source end of the sieve tube.

Step 3: Pressure-driven flow. The high pressure at the source end pushes phloem sap toward regions of lower pressure. The sieve pores in the sieve plates allow sap to flow from one sieve-tube element to the next.

Step 4: Sugar unloading at the sink. At the sink, sucrose is actively or passively unloaded from the sieve-tube elements into sink cells. This raises the water potential (makes it less negative) in the sieve-tube elements at the sink end.

Step 5: Water exit by osmosis. The higher water potential in the sieve-tube element causes water to leave by osmosis, returning to the xylem. This reduces the hydrostatic pressure at the sink end of the sieve tube.

The pressure difference between the source (high pressure) and sink (low pressure) drives bulk flow of phloem sap. No pump is needed — the pressure gradient is established by the osmotic consequences of sugar loading and unloading. The energy investment is in active sugar loading at the source (and sometimes active unloading at the sink), not in propelling the sap through the sieve tubes.

Phloem Transport: Direction and Content

Unlike xylem transport, which is almost exclusively upward (roots to shoots), phloem transport can be bidirectional within a single stem. Different sieve tubes may carry sap in different directions simultaneously. The direction of flow in any given sieve tube depends on the relative locations of sources and sinks.

Phloem sap is a concentrated solution — typically 10–30% sucrose by weight — along with amino acids, hormones, inorganic ions, and even some proteins and RNA molecules that serve signaling functions. The composition reflects phloem’s dual role as a nutrient-distribution and information-transmission system.

Xylem vs. Phloem Comparison

Structure and Function

The structural features of phloem reflect its functional requirements:

• Sieve-tube elements lose their nucleus and vacuole → reduces resistance to flow.

• Sieve plates with large pores → allows sap to move between elements.

• Companion cells remain metabolically active → support the enucleate sieve-tube elements.

• Living cytoplasm with plasma membrane intact → maintains the osmotic gradients that drive pressure flow.

• Close association with xylem → facilitates water exchange between the two vascular tissues at sources and sinks.

ELI-10

Phloem is the plant’s food-delivery system. It moves sugar — the product of photosynthesis — from the leaves (where sugar is made) to every other part of the plant (where sugar is needed).

Here is how it works. The plant loads sugar into the phloem tubes in the leaf. Sugar makes the inside of the tubes very concentrated — like adding a lot of sugar to a glass of water. Water from the nearby xylem tubes rushes in by osmosis to dilute the sugar. That incoming water creates pressure — like a water balloon filling up. The pressure pushes the sugary solution down the tube toward places where sugar is being used or stored — growing tips, roots, fruits.

At the destination, the plant unloads the sugar. The inside of the tube becomes less concentrated. Water leaves by osmosis back into the xylem. Pressure drops. The sap moves from high pressure (at the source, where sugar is loaded) to low pressure (at the sink, where sugar is unloaded).

The whole system runs on sugar loading and unloading, not on pumps. It is like a conveyor belt driven by pressure differences — load sugar at one end, unload it at the other, and the belt moves on its own.

Think of it this way: xylem is the water pipe going up. Phloem is the food pipe going wherever food is needed — up, down, or sideways.

ELI Example

Imagine two rooms connected by a hallway. In Room A (the source leaf), workers keep dumping sugar into a tank of water. The tank becomes very sugary, so water from a neighboring pipe flows in to dilute it, making the tank overflow into the hallway. In Room B (the sink root), workers keep taking sugar out of the tank. The tank becomes less sugary, so water flows back out into the pipe. The pressure in Room A is high because the tank is full. The pressure in Room B is low because sugar is being removed. The sugary solution flows down the hallway from high pressure to low pressure. No one is pushing it — the flow is driven entirely by the difference in sugar concentration between the two rooms.

Do Not Confuse

• Xylem vs. Phloem (contents): Xylem = water + minerals. Phloem = sugars + other organic compounds. If you see a question about “what transports sugar,” the answer is phloem, not xylem.

• Source vs. Sink: A source exports sugar (net producer). A sink imports sugar (net consumer). The same organ can be a source at one time and a sink at another.

• Pressure-Flow vs. Cohesion-Tension: Pressure-flow (phloem) is driven by osmotic pressure differences from sugar loading/unloading. Cohesion-tension (xylem) is driven by transpirational pull. Both move fluids without pumps, but the mechanisms are different.

Lab Link

Phloem transport can be demonstrated using aphids — insects that feed by inserting their mouthparts directly into sieve-tube elements. When the aphid’s body is removed, phloem sap continues to exude from the embedded mouthpart and can be collected for analysis. This technique, originally developed by scientists studying phloem, confirmed the composition and pressure-driven nature of phloem sap. While aphid-based demonstrations are specialized, observing the sticky residue (“honeydew”) produced by aphids feeding on plants provides indirect evidence of phloem activity.

High-Yield Memory Anchors

• Phloem = living, enucleate sieve-tube elements + companion cells.

• Source → loads sugar → water enters → high pressure. Sink → unloads sugar → water leaves → low pressure. Sap flows high to low.

• Xylem: water up, dead cells, cohesion-tension. Phloem: sugar anywhere, alive cells, pressure-flow.

• Source and sink can change seasonally: summer leaves are sources; spring tubers are sources.

Quick Check

Q1: In the pressure-flow hypothesis, the initial event that drives phloem transport is:

A) Transpiration at the leaf surface

B) Active loading of sugar into sieve-tube elements at the source

C) Active pumping of sap through sieve tubes

D) Gravity pulling sugar-rich sap downward

Q2: In early spring, a potato tuber (an underground storage stem) begins to shrivel as new shoots emerge above ground. Explain the change in the tuber’s status — is it a source or a sink? What is happening in the phloem?

Q3: Compare the driving forces for xylem and phloem transport. Why is one mechanism “pull” (tension) and the other “push” (pressure)?

Quick Check Answers

A1: B. Active loading of sugar into sieve-tube elements at the source. Sugar loading lowers the water potential, causing water to enter by osmosis, generating hydrostatic pressure. This pressure gradient drives bulk flow toward the sink. No active pumping of sap occurs — the flow is passive.

A2: In early spring, the potato tuber is a source. Stored starch in the tuber is broken down to sucrose, which is actively loaded into phloem sieve-tube elements. The resulting high osmotic pressure drives phloem sap upward to the growing shoots (sinks), which need sugar for growth but are not yet photosynthesizing. The tuber shrivels because its stored reserves are being exported. Later, when the new shoots develop mature leaves and begin photosynthesizing, the direction of phloem flow will reverse — the leaves become sources, and new daughter tubers (forming underground) become sinks.

A3: Xylem transport is “pull” (tension): Transpiration at the leaf surface evaporates water, creating tension (negative pressure) at the top of the water column. Cohesion transmits this tension downward through the continuous water column. The water is pulled up from the roots. Phloem transport is “push” (pressure): Active sugar loading at the source creates a high osmotic concentration, drawing water in from xylem and generating positive hydrostatic pressure. This pressure pushes sap toward the sink, where sugar unloading reduces pressure. Xylem operates under negative pressure (tension); phloem operates under positive pressure. Both are passive at the level of bulk flow, but phloem requires ATP for the initial sugar-loading step.

Chapter Summary

Phloem transports sugars from sources to sinks through the pressure-flow mechanism. Active sugar loading at sources creates high osmotic pressure, drawing in water and generating hydrostatic pressure that drives bulk flow toward sinks, where sugar unloading reduces pressure. Unlike xylem (dead cells, tension-driven, upward flow), phloem consists of living sieve-tube elements and companion cells, operates under positive pressure, and can transport in any direction. Sources and sinks change with development and season, enabling plants to allocate resources flexibly across their bodies.

Common Mistakes

• “Phloem only moves sugar downward.” Phloem moves sugar from sources to sinks, which can be in any direction. Sugar from mature leaves at the top of the plant may move downward to roots. Sugar from storage roots in spring may move upward to growing shoots.

• “Phloem cells are dead like xylem cells.” Phloem sieve-tube elements are alive at maturity — they must be alive to maintain the plasma membrane that controls solute movement. They lack nuclei but retain functional cytoplasm.

• “Sugar moves through phloem by diffusion.” Diffusion is far too slow to account for phloem transport rates (which can exceed 1 meter per hour). Phloem transport is bulk flow driven by pressure differences.