---
title: "Sieve-Tube Cells | General Biology I"
description: "Sieve-tube cells are living phloem cells that move sugars through plants, working with companion cells and sieve plates to feed roots, fruits, and growing tissue."
canonical: "https://fiveable.me/college-bio/key-terms/sieve-tube-cells"
type: "key-term"
subject: "General Biology I"
unit: "Unit 30"
---

# Sieve-Tube Cells | General Biology I

## Definition

Sieve-tube cells are the main sugar-conducting cells in phloem. In General Biology I, they explain how plants move sucrose from leaves to roots, stems, and growing parts even though the cells lose their nuclei at maturity.

## What It Is

Sieve-tube cells are the conducting cells of phloem in General Biology I, the plant tissue that moves sugars and other organic nutrients around the plant. They are elongated, living cells that line up end to end, forming tubes that can carry sap from photosynthetic tissues to places that need fuel or storage.

What makes them unusual is that they are alive but simplified. Mature sieve-tube cells lose their nucleus and many of their internal organelles, which leaves more room for flow inside the cell. Since they cannot fully run their own metabolism, they depend on nearby companion cells to keep them functioning. That partnership is a classic plant biology example of division of labor between cells.

The end walls between sieve-tube cells are called sieve plates. These plates have pores, so dissolved sugars can move from one cell to the next with less resistance than if the walls were solid. Think of the phloem as a pipeline and the sieve plates as perforated connectors that let the transport stream pass through the stem efficiently.

In a stem, sieve-tube cells are part of the vascular system alongside xylem. Xylem moves water and minerals upward, while phloem moves products of photosynthesis, especially sucrose, from source tissues like leaves to sinks such as roots, developing fruits, young leaves, or storage tissues. That means phloem transport is not just upward or downward, it moves wherever the plant needs resources.

The movement through sieve-tube cells follows pressure flow. Sugars are loaded into the phloem near a source, water follows by osmosis, and the resulting pressure pushes the sugary sap through the tube to a sink where the sugars are unloaded. This is why damage to sieve-tube cells or their companion cells can disrupt the whole distribution network, not just one small region of the plant.

## Why It Matters

Sieve-tube cells show up whenever General Biology I shifts from plant parts to plant function. If you are studying stems, vascular tissue, or photosynthesis, this term explains how the food made in leaves reaches the rest of the plant instead of staying trapped where it was produced.

They also help you connect several plant ideas at once. Photosynthesis makes sugar in leaf cells, but growth in roots, buds, fruits, and storage organs depends on transport through phloem. Sieve-tube cells are the route that links those source and sink tissues, so they sit at the center of the plant’s internal supply chain.

This term is useful for comparing transport systems too. Xylem and phloem are often taught together because they move different materials in different ways. If you can tell sieve-tube cells apart from xylem vessels, you can better explain stem structure, plant response to damage, and why vascular tissue is organized into bundles.

It also reinforces a big biology pattern: structure fits function. Mature sieve-tube cells lose their nuclei, have sieve plates, and work with companion cells because those features make long-distance sugar transport possible. That kind of structure-function reasoning comes up again and again in plant biology labs, diagram questions, and short-answer explanations.

## Connections

### Phloem

Sieve-tube cells are the main conducting cells inside phloem. If phloem is the whole transport tissue, sieve-tube cells are the tube-like units that actually carry the sugary sap. When you identify phloem in a stem, you are often also identifying where sieve-tube cells are located and how sugars move through the plant.

### [Companion Cells](/college-bio/key-terms/companion-cells)

Companion cells keep sieve-tube cells alive and functioning after the sieve-tube cells lose their nuclei. They supply energy, proteins, and metabolic support that sieve-tube cells can no longer make efficiently on their own. If a question asks how phloem transport works despite the lack of a nucleus, companion cells are part of the answer.

### Sieve Plates

Sieve plates are the porous end walls between sieve-tube cells. Their pores let dissolved sugars and water move from one sieve-tube element to the next, so transport does not stop at each cell boundary. If you see a diagram of phloem, the sieve plates are the clue that the cells are connected into a transport pathway.

### [dicot](/college-bio/key-terms/dicot)

Dicot stems often show vascular bundles arranged in a ring, which makes it easier to locate phloem and the sieve-tube cells inside it. In many textbook cross sections, the outer side of each vascular bundle contains phloem, while xylem is closer to the center. That layout helps you identify sieve-tube cells on stem slides or diagrams.

## On the AP Exam

A stem cross-section question may ask you to identify the phloem and explain how sugar moves through it. That is where sieve-tube cells come in: you would point to the living, tube-like cells with sieve plates and connect them to sucrose transport from leaves to roots or growing tissues. If the item includes companion cells, recognize that they support sieve-tube cells because mature sieve-tube cells lack nuclei.

On lab practicals, you may need to label phloem versus xylem in a dicot stem or explain why damage to phloem can stunt plant growth. In short-answer questions, the best move is to trace the path of sugars, source to sink, and name sieve-tube cells as the conducting cells that make that movement possible.

## Key Takeaways

- Sieve-tube cells are the sugar-transporting cells of phloem in plants.
- They stay alive at maturity, but they lose their nucleus and depend on companion cells.
- Sieve plates connect sieve-tube cells end to end and let phloem sap flow through the stem.
- Their job is to move sucrose and other organic nutrients from sources like leaves to sinks like roots, fruits, and growing tissues.
- If phloem or sieve-tube cells are damaged, the plant can lose its ability to distribute food efficiently.

## FAQs

### What is sieve-tube cells in General Biology I?

Sieve-tube cells are the main conducting cells of phloem, the plant tissue that transports sugars. They form living tubes that carry sucrose from photosynthetic parts of the plant to tissues that need energy or storage. Even though they lose their nuclei at maturity, they keep working with help from companion cells.

### How are sieve-tube cells different from xylem?

Sieve-tube cells move organic nutrients, especially sugars, through phloem. Xylem moves water and dissolved minerals from the roots upward. They are often found together in vascular bundles, but they do opposite transport jobs and have very different cell structures.

### Why do sieve-tube cells not have a nucleus?

They lose their nucleus as they mature so the inside of the cell has less clutter and sap can move more easily. That makes the transport pathway more efficient, but it also means the cells cannot fully maintain themselves. Companion cells provide the support they need to keep functioning.

### What do sieve plates do?

Sieve plates are porous end walls between sieve-tube cells. Their holes let phloem sap pass from one cell to the next, turning individual cells into a continuous transport route. Without sieve plates, sugar movement through the phloem would be much slower.

## Related Study Guides

- [30.2 Stems](/college-bio/unit-30/2-stems/study-guide/QMFgE4Vimg5iLrcB)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
- [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/college-bio/key-terms/sieve-tube-cells#resource","name":"Sieve-Tube Cells | General Biology I","url":"https://fiveable.me/college-bio/key-terms/sieve-tube-cells","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/college-bio/key-terms/sieve-tube-cells#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:11.425Z","isPartOf":{"@type":"Collection","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/college-bio/key-terms/sieve-tube-cells#term","name":"sieve-tube cells","description":"Sieve-tube cells are the main sugar-conducting cells in phloem. In General Biology I, they explain how plants move sucrose from leaves to roots, stems, and growing parts even though the cells lose their nuclei at maturity.","url":"https://fiveable.me/college-bio/key-terms/sieve-tube-cells","inDefinedTermSet":{"@type":"DefinedTermSet","name":"General Biology I Key Terms","url":"https://fiveable.me/college-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is sieve-tube cells in General Biology I?","acceptedAnswer":{"@type":"Answer","text":"Sieve-tube cells are the main conducting cells of phloem, the plant tissue that transports sugars. They form living tubes that carry sucrose from photosynthetic parts of the plant to tissues that need energy or storage. Even though they lose their nuclei at maturity, they keep working with help from companion cells."}},{"@type":"Question","name":"How are sieve-tube cells different from xylem?","acceptedAnswer":{"@type":"Answer","text":"Sieve-tube cells move organic nutrients, especially sugars, through phloem. Xylem moves water and dissolved minerals from the roots upward. They are often found together in vascular bundles, but they do opposite transport jobs and have very different cell structures."}},{"@type":"Question","name":"Why do sieve-tube cells not have a nucleus?","acceptedAnswer":{"@type":"Answer","text":"They lose their nucleus as they mature so the inside of the cell has less clutter and sap can move more easily. That makes the transport pathway more efficient, but it also means the cells cannot fully maintain themselves. Companion cells provide the support they need to keep functioning."}},{"@type":"Question","name":"What do sieve plates do?","acceptedAnswer":{"@type":"Answer","text":"Sieve plates are porous end walls between sieve-tube cells. Their holes let phloem sap pass from one cell to the next, turning individual cells into a continuous transport route. Without sieve plates, sugar movement through the phloem would be much slower."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"General Biology I","item":"https://fiveable.me/college-bio"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/college-bio/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 30","item":"https://fiveable.me/college-bio/unit-30"},{"@type":"ListItem","position":4,"name":"sieve-tube cells"}]}]}
```