# Plant transport tissues

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u7-plant-transport-tissues/

This sub-topic covers the structure, cell types, and functional adaptations of the two main plant vascular transport tissues: xylem and phloem. You will learn how their structure directly matches their roles in water and solute transport.

**Prerequisites:** [Basic plant tissue organisation](https://www.owlsprep.com/study/cie-9700-u5-plant-tissues/)

## Learning objectives

- Distinguish between the structure and function of xylem and phloem tissue
- Relate structural adaptations of xylem and phloem to their transport roles
- Identify key cell types in each vascular tissue

## Structure and Function of Xylem

Xylem is the vascular tissue responsible for two key roles in plants: it transports water and dissolved mineral ions from the roots up to the leaves and growing shoots, and provides mechanical structural support to the plant stem.

**Xylem** — Vascular tissue consisting of dead, lignified cells that transport water and mineral ions upwards through the plant, and provide structural support.

*Example:* Mature xylem forms the wood of tree trunks, supporting the entire tree.

- **Vessel elements**: Elongated dead cells that join end-to-end to form long continuous hollow tubes, the main route for water flow in angiosperms.
- **Tracheids**: Dead elongated cells with tapered ends, that also transport water, found in both gymnosperms and angiosperms.
- **Xylem parenchyma**: Living packing cells that store starch and assist with lateral water movement.
- **Xylem fibres**: Sclerenchyma cells that provide additional mechanical support.

**Worked example:** Explain how three structural features of xylem vessel elements adapt them to their function of water transport.

1. 1. Vessel elements are dead at maturity with no cytoplasm or organelles.
2. This leaves the entire lumen of the cell completely hollow, with no obstruction to bulk water flow.
3. 2. Cell walls are heavily impregnated with lignin.
4. Lignin is a rigid, waterproof polymer. It waterproofs the vessel wall to prevent water leaking out, and provides strength to stop the vessel collapsing under negative pressure from transpiration pull.
5. 3. Vessel elements are joined end-to-end with no end walls between adjacent cells.
6. This forms a continuous unbroken tube from the roots all the way to the leaves, allowing water to flow freely with minimal resistance.

> **Exam tip:** CIE always rewards explicit links between structure and function: never just list a feature, always explain how it helps the tissue carry out its role.

## Structure and Function of Phloem

Phloem transports organic solutes (primarily sucrose and amino acids) from regions of production or storage called sources to regions of use or storage called sinks, in a process called translocation. Unlike xylem, phloem is made up almost entirely of living cells.

**Phloem** — Living vascular tissue that transports organic solutes between sources and sinks via translocation.

*Example:* Phloem transports sucrose from photosynthetic leaves to developing fruits and growing roots.

- **Sieve tube elements**: Living cells that form the main transport tubes. They lose most organelles at maturity, and have perforated end walls called sieve plates between cells.
- **Companion cells**: Small metabolically active cells adjacent to each sieve tube element. They retain all organelles and supply the sieve tube with energy and nutrients.
- **Phloem parenchyma**: Packing cells for storage and support.
- **Phloem fibres**: Sclerenchyma cells that provide structural support.

**Worked example:** Explain why sieve tube elements cannot function without companion cells.

1. 1. When sieve tube elements mature, they break down most of their internal organelles, including the nucleus, ribosomes, and most mitochondria.
2. This creates more space for solute flow through the lumen, but means the sieve tube cannot carry out essential metabolic processes like protein synthesis or ATP production.
3. 2. Companion cells are directly connected to sieve tube elements via channels called plasmodesmata, which allow sharing of cytoplasm.
4. Companion cells retain all organelles, so they produce ATP for loading and unloading of solutes, and supply all required proteins and nutrients to the sieve tube element, allowing it to function.

> **Exam tip:** A common exam question asks for the role of companion cells: always mention they provide ATP for translocation and metabolic support.

## Comparing Xylem and Phloem

Direct comparison questions between xylem and phloem are extremely common in CIE A-Level Biology, appearing in both multiple choice and structured papers. Use the table below to summarise the key differences:

| Feature | Xylem | Phloem |
| --- | --- | --- |
| State of cells at maturity | Dead | Living |
| Cell wall composition | Lignified walls | Cellulose only, no lignin |
| Main transported substance | Water and mineral ions | Organic solutes (sucrose, amino acids) |
| Direction of transport | Unidirectional (only upwards) | Bidirectional (any direction) |

**Check your understanding**

Check your understanding of key differences:

1. Which of the following features is unique to phloem?

   - Lignified cell walls
   - Dead cells at maturity
   - Sieve plates
   - Unidirectional transport

   *Why:* Lignified walls, dead cells and unidirectional transport are all features of xylem. Only phloem has perforated sieve plates between sieve tube elements.

## Common pitfalls

- **Wrong:** Stating that xylem transports sucrose or other organic solutes.
  - Why it fails: This is a very common mix-up between the functions of xylem and phloem.
  - Correct: Xylem only transports water and dissolved mineral ions; phloem transports organic solutes like sucrose.
- **Wrong:** Claiming mature sieve tube elements have a nucleus.
  - Why it fails: Mature sieve tube elements lose their nucleus to create more space for solute flow.
  - Correct: Mature sieve tube elements lack a nucleus; their metabolic needs are fully supported by adjacent companion cells.
- **Wrong:** Saying phloem transport is always unidirectional (only downwards from leaves).
  - Why it fails: Transport direction changes based on the location of sources and sinks, which varies by season and growth stage.
  - Correct: Phloem transport is bidirectional, while xylem transport is strictly unidirectional upwards from roots to shoots.
- **Wrong:** Claiming lignin makes xylem walls permeable to water.
  - Why it fails: Lignin is waterproof, which is a key part of its function in xylem.
  - Correct: Lignin is waterproof and provides structural support; water moves between xylem vessels through small unlignified pits in the walls.

## Cheatsheet

| Feature | Xylem | Phloem |
| --- | --- | --- |
| Main Function | Water + minerals up | Organic solutes any direction |
| Cell Maturity | Dead | Living |
| Cell Walls | Lignified | Non-lignified cellulose |
| Key Cells | Vessels, tracheids | Sieve tubes + companion cells |
| End Walls | None, continuous tube | Perforated sieve plates |

## What's next

The structural features of xylem and phloem you learned here form the foundation for understanding the mechanisms of transport in plants, the next core topics in Unit 7. These concepts are heavily tested in CIE exams, especially extended response questions that require you to link structure to mechanism. Mastering this sub-topic makes it much easier to learn how water moves from root hair cells to leaves via the transpiration stream, and how translocation of solutes occurs via the mass flow hypothesis.

- [Translocation in phloem](https://www.owlsprep.com/study/cie-9700-u7-translocation/)
- [Water uptake and movement](https://www.owlsprep.com/study/cie-9700-u7-water-uptake-and-movement/)
- [Transpiration](https://www.owlsprep.com/study/cie-9700-u7-transpiration/)

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