Transport in Plants (Senior 5 Biology): free lessons | Ganzaa

Transport in Plants

Lessons

Senior 5 · Biology

Transport in Plants

4 lessons

Lesson 1 of 4

The need for transport: xylem and phloem

25 min

Why can a sugarcane stalk be chewed but not swallowed?

Chew a piece of sugarcane (igisheke) and the sweet juice runs out, but you are left with a mouthful of tough strands. Those strands are mostly the plant's vascular bundles and the fibres around them: the pipes that carried water up from the roots and sugar away from the leaves. A cell at the tip of a 30-metre eucalyptus tree in a Rwandan woodlot needs water from roots it will never touch, and a root cell deep in the soil needs sugar made in leaves it will never see. In Senior 1 you met xylem and phloem as tissues. This lesson asks why a large plant must have them, and how the structure of each tissue fits its job.

Definition

Vascular tissue: tissue that carries materials over long distances inside a plant. It has two parts: xylem and phloem. Plants that have it, such as ferns, conifers and flowering plants, are called vascular plants.

Xylem: the vascular tissue that carries water and dissolved mineral ions from the roots to the rest of the plant, and helps to support it. Its conducting cells are dead and lignified.

Phloem: the vascular tissue that carries dissolved organic substances, mainly sucrose and amino acids, from where they are made or stored to where they are used. Its conducting cells, the sieve tube elements, are alive.

Vascular bundle: a strand in which xylem and phloem run side by side, often with fibres around them. Example: the "veins" of a leaf.

Why diffusion is not enough

A single-celled alga gets everything it needs by diffusion across its surface, because its surface area to volume ratio is large and no part of it is far from the outside. A large plant cannot do this, for three reasons.

Distance. The time diffusion takes rises with the square of the distance. A useful estimate is $t \approx \frac{x^2}{2D}$ , where $x$ is the distance and $D$ is the diffusion coefficient of the substance.
Surface area to volume ratio. As an organism grows, its volume rises faster than its surface area, so the outer surface can no longer supply the inner cells.
Division of labour. Water and mineral ions enter at the roots, but sugars are made in the leaves. Each organ needs what the other one makes.

So vascular plants move materials by mass flow: a whole body of liquid moves along a tube, carrying its dissolved substances with it, far faster than diffusion could.

Worked example 1: how long would diffusion take?

Sucrose diffuses through water with $D \approx 5.2 \times 10^{-10}\,\mathrm{m^2\,s^{-1}}$ (an approximate value at 25 °C). Using $t \approx \frac{x^2}{2D}$ , estimate how long sucrose would take to diffuse (a) across one cell, 0.10 mm; (b) from a leaf to a root 1.0 m away.

(a) $x = 0.10\,\mathrm{mm} = 1.0 \times 10^{-4}\,\mathrm{m}$ . $t \approx \frac{(1.0 \times 10^{-4})^2}{2 \times 5.2 \times 10^{-10}} = \frac{1.0 \times 10^{-8}}{1.04 \times 10^{-9}} \approx 9.6\,\mathrm{s}$
(b) $x = 1.0\,\mathrm{m}$ . $t \approx \frac{1.0^2}{1.04 \times 10^{-9}} \approx 9.6 \times 10^{8}\,\mathrm{s}$ Dividing by the seconds in a year, $3.15 \times 10^{7}$ , gives about 30 years.

Interpretation: a distance 10 000 times longer takes $10\,000^2 = 10^8$ times longer. Diffusion is fine across one cell and useless across a plant, which is why phloem moves sucrose by mass flow, at roughly a metre per hour.

Where the vascular tissue lies

The arrangement differs between organs, and each arrangement suits the forces that organ faces.

Stem (dicot): vascular bundles form a ring near the outside, with phloem outside and xylem inside (Figure 1). A ring of stiff tissue near the surface resists bending in the wind, as a hollow tube does. Between the xylem and the phloem lies the cambium, a layer of dividing cells that makes new xylem and phloem as the stem thickens. In monocot stems such as maize and sugarcane, the bundles are scattered and there is no cambium.
Root: the xylem forms a central star, with the phloem between its arms (Figure 2). Tissue in the centre resists the pulling force on a root anchored in soil, as a rope does. Around the vascular tissue lie the pericycle and the endodermis; you will meet the endodermis again in the next lesson.
Leaf: the veins branch through the mesophyll so that no cell is more than a few cells from a vein. In the midrib, xylem lies towards the upper surface and phloem towards the lower surface.

Xylem: dead pipes built for water under tension

Xylem contains four kinds of cell:

Vessel elements: wide, dead cells joined end to end. Their end walls break down during development, so they form long continuous tubes called vessels. They are the main water pipes of flowering plants.
Tracheids: long, narrow, dead cells with tapering ends. Water passes from one to the next through pits. They are the only water-conducting cells in most conifers.
Xylem fibres: long, thick-walled dead cells that give support. Timber is mostly xylem.
Xylem parenchyma: living cells that store starch and help to load ions into the vessels.

The walls of vessels and tracheids are impregnated with lignin, a tough, waterproof polymer. It is laid down in rings, spirals or networks, or covers the whole wall except for thin areas called pits. Pits let water move sideways into neighbouring vessels and living cells.

Phloem: living tubes with a helper

Phloem also contains four kinds of cell:

Sieve tube elements: long living cells joined end to end. At maturity each loses its nucleus, its vacuole and most organelles, leaving a thin layer of cytoplasm against the wall. The end walls become sieve plates, perforated by pores through which the sap flows.
Companion cells: small cells beside each sieve tube element, with a nucleus, dense cytoplasm, many ribosomes and many mitochondria. They keep the sieve tube element alive and supply the ATP for loading sucrose. Many plasmodesmata, strands of cytoplasm through the cell wall, join the two cells.
Phloem parenchyma and phloem fibres, for storage and support.

Feature	Xylem vessel	Phloem sieve tube
Living or dead at maturity	Dead: no cytoplasm	Living, but no nucleus
Wall	Thick, lignified	Thin, cellulose; not lignified
End walls	Broken down: a continuous tube	Sieve plates with pores
What it carries	Water and mineral ions	Sucrose, amino acids, some hormones
Direction of flow	Upwards only, from root to leaf	Up or down, from source to sink
Energy needed by the tissue	None: the flow is driven from outside	ATP for loading, supplied by companion cells

How the structure fits the function

No end walls and no cytoplasm in a vessel means almost nothing slows the flow of water.
Lignin stops a vessel collapsing inward when the water in it is pulled up under tension, and it makes the wall waterproof.
Pits let water bypass a vessel blocked by an air bubble.
Sieve plates let sap flow between elements but help to seal a damaged tube: the protein in phloem sap and a carbohydrate called callose plug the pores.
Companion cells do the metabolic work that the stripped-down sieve tube element cannot do for itself.

Worked example 2: how wide is a xylem vessel?

In a photomicrograph of a sunflower stem at a magnification of $\times 400$ , a xylem vessel measures 18 mm across. Calculate its actual diameter in micrometres, and say whether it could be seen with the naked eye (about 0.1 mm at best).

$\text{actual size} = \frac{\text{image size}}{\text{magnification}} = \frac{18\,\mathrm{mm}}{400} = 0.045\,\mathrm{mm}$
Convert: $1\,\mathrm{mm} = 1\,000\,\mathrm{\mu m}$ , so $0.045\,\mathrm{mm} = 45\,\mathrm{\mu m}$ .
$45\,\mathrm{\mu m} = 0.045\,\mathrm{mm}$ , which is less than half of 0.1 mm.

Answer: about 45 µm wide, too narrow to see as a single tube without a microscope, although a whole vascular bundle, made of many cells, can be seen with a hand lens.

Warning

Common mistakes

"Xylem is a living tissue that pumps water." The vessels are dead and do no pumping; water is pulled up by evaporation from the leaves (lesson 3). Only xylem parenchyma is alive.
"Phloem carries food downwards." Phloem carries sucrose from a source to a sink, and a sink can be above the source: a growing shoot tip or a ripening fruit.
"Sieve tube elements are dead because they have no nucleus." They are alive: they have a cell membrane and cytoplasm, and depend on their companion cells.
Do not confuse pits (thin places in a xylem wall) with pores (holes in a sieve plate).

Cognito: "A-Level Biology - Xylem & Phloem - Structure, Function, Distribution". Watch for how the lignin and the missing end walls of vessels suit them to carrying water, and for the roles of sieve plates and companion cells; then check the positions of the tissues against Figures 1 and 2.

Tip

Memory hook: Phloem carries food (both say "f"), and it can go both ways, like a two-way road. Xylem carries water on a one-way road up the trunk. In a root the xylem is the "X" in the middle; in a stem the phloem is on the outside, nearest the bark.

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Practice: the need for transport, xylem and phloem

Name the two tissues that make up a vascular bundle, and state what each carries.
Give two reasons why a tall tree cannot rely on diffusion to supply its cells.
State three ways in which a sieve tube element differs from a xylem vessel element.
Explain why companion cells contain many mitochondria.
Using $t \approx \frac{x^2}{2D}$ with $D = 5.2 \times 10^{-10}\,\mathrm{m^2\,s^{-1}}$ , how long would sucrose take to diffuse 1.0 mm? Give your answer in seconds to 2 significant figures.
A tracheid is 2.4 mm long and 30 µm wide. In a drawing its length is 120 mm. What is the magnification, and how wide should it be drawn?
Explain how the position of the xylem in a root suits the forces a root experiences.
A gardener removes a ring of bark, which contains the phloem, from a tree trunk but leaves the wood. Predict what happens to the roots over the following months, and explain why the leaves stay green at first.

Show answersHide answers

Xylem carries water and mineral ions; phloem carries organic solutes such as sucrose and amino acids.
Any two: diffusion time rises with the square of distance, so it would take years over metres; the surface area to volume ratio is too small for the surface to supply inner cells; water enters at the roots but sugar is made in the leaves, far apart.
Any three: it is living (a vessel element is dead); it has cytoplasm (a vessel has none); its walls are cellulose, not lignified; its end walls are sieve plates, not broken down; it carries organic solutes, not water and ions; flow can be in either direction.
Companion cells actively load sucrose into the sieve tubes, which needs ATP; mitochondria make ATP by aerobic respiration.
$x = 1.0 \times 10^{-3}\,\mathrm{m}$ : $t = \frac{1.0 \times 10^{-6}}{1.04 \times 10^{-9}} \approx 960\,\mathrm{s}$ , about 16 minutes.
Magnification $= \frac{120\,\mathrm{mm}}{2.4\,\mathrm{mm}} = \times 50$ . Width drawn $= 30\,\mathrm{\mu m} \times 50 = 1\,500\,\mathrm{\mu m} = 1.5\,\mathrm{mm}$ .
A root is pulled by the shoot above it and anchored in soil, so it is under tension. Strong lignified xylem in the centre resists pulling, like the core of a rope.
Sucrose can no longer reach the roots, so the roots use up their stored food and eventually die, and then the whole tree dies. The leaves stay green at first because the xylem, in the wood, is intact and still brings them water and ions.

Key points

Large plants need transport because diffusion time rises with the square of distance, the surface area to volume ratio is small, and roots and leaves supply each other.
Xylem: dead, lignified vessels and tracheids carry water and ions upwards and give support.
Phloem: living sieve tube elements, with sieve plates, and companion cells carry sucrose and amino acids from source to sink, in either direction.
Stem: bundles in a ring, phloem outside; root: central xylem star with phloem between the arms.

Sources and credits

Written by Ganzaa. Facts were checked against 30.2 Stems and 30.3 Roots, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax, licensed under CC BY 4.0. No text was copied.
Photo: Herbaceous Dicot Stem Vascular Bundles Young Helianthus, Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons.
Video: A-Level Biology - Xylem & Phloem - Structure, Function, Distribution, Cognito, embedded from YouTube.

Figures 1 to 3, the table, the worked examples and the practice questions are by Ganzaa. The diffusion coefficient in Worked example 1 is an approximate textbook value.

1Why can a sugarcane stalk be chewed but not swallowed?

2Why diffusion is not enough

3Where the vascular tissue lies

4Xylem: dead pipes built for water under tension

5Phloem: living tubes with a helper

6How the structure fits the function