Study Guide

Plant transport tissues

BiologyΒ· Unit 7: Transport in PlantsΒ· 15 min read

1. Structure and Function of Xylemβ˜…β˜…β˜†β˜†β˜†β± 5 min

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.

πŸ“˜ Definition

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.

πŸ“ Worked Example

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

  1. 1
    1. Vessel elements are dead at maturity with no cytoplasm or organelles.
  2. 2

    This leaves the entire lumen of the cell completely hollow, with no obstruction to bulk water flow.

  3. 3
    1. Cell walls are heavily impregnated with lignin.
  4. 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. 5
    1. Vessel elements are joined end-to-end with no end walls between adjacent cells.
  6. 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.

2. Structure and Function of Phloemβ˜…β˜…β˜…β˜†β˜†β± 6 min

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.

πŸ“˜ Definition

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.

πŸ“ Worked Example

Explain why sieve tube elements cannot function without companion cells.

  1. 1
    1. When sieve tube elements mature, they break down most of their internal organelles, including the nucleus, ribosomes, and most mitochondria.
  2. 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. 3
    1. Companion cells are directly connected to sieve tube elements via channels called plasmodesmata, which allow sharing of cytoplasm.
  4. 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.

3. Comparing Xylem and Phloemβ˜…β˜…β˜†β˜†β˜†β± 4 min

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)

E

n

d

w

a

l

l

s

b

e

t

w

e

e

n

c

e

l

l

s

N

o

e

n

d

w

a

l

l

s

P

e

r

f

o

r

a

t

e

d

s

i

e

v

e

p

l

a

t

e

s

C

o

m

p

a

n

i

o

n

c

e

l

l

s

p

r

e

s

e

n

t

N

o

Y

e

s

βœ“ Quick check

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

    Reveal answer
    Sieve plates β€”

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

4. Common Pitfalls

Wrong move:

Stating that xylem transports sucrose or other organic solutes.

Why:

This is a very common mix-up between the functions of xylem and phloem.

Correct move:

Xylem only transports water and dissolved mineral ions; phloem transports organic solutes like sucrose.

Wrong move:

Claiming mature sieve tube elements have a nucleus.

Why:

Mature sieve tube elements lose their nucleus to create more space for solute flow.

Correct move:

Mature sieve tube elements lack a nucleus; their metabolic needs are fully supported by adjacent companion cells.

Wrong move:

Saying phloem transport is always unidirectional (only downwards from leaves).

Why:

Transport direction changes based on the location of sources and sinks, which varies by season and growth stage.

Correct move:

Phloem transport is bidirectional, while xylem transport is strictly unidirectional upwards from roots to shoots.

Wrong move:

Claiming lignin makes xylem walls permeable to water.

Why:

Lignin is waterproof, which is a key part of its function in xylem.

Correct move:

Lignin is waterproof and provides structural support; water moves between xylem vessels through small unlignified pits in the walls.

5. Quick Reference 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

6. Frequently Asked

Why are xylem cells dead at maturity?

Dead cells lose all internal content, creating a hollow, unobstructed lumen for unimpeded water flow. Lignified cell walls provide structural support even after cell death.

What is the main difference between xylem and phloem function?

Xylem transports water and dissolved mineral ions only upwards from roots to shoots. Phloem transports organic solutes (like sucrose) bidirectionally between sources (production sites) and sinks (use/storage sites).

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 1

    Xylem structure identification

  • 2021 Β· 2

    Compare xylem and phloem

  • 2023 Β· 2

    Adaptations for transport function

Going deeper

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.