Transport in Plants

Learning ObjectivesFeatures of effective transport systems

in plants.Nature of waste products and excretory

mechanisms and systems in plants.Features of effective surfaces of gaseous

exchange; mechanisms and systems of gaseous exchange in plants; process of diffusion.

Structure of a Stem

Xylem

Cambium

Phloem

Pith

Cortex

Epidermis

Vascular bundle

PhloemFood substances such as sucrose and amino

acids travel in the phloem.The sucrose is made in the leaves, through

photosynthesis, and travels to the rest of the plant to be used as food for cellular respiration.

The cells in the phloem tissue form elongated tubes called sieve tubes.

The cells in the phloem remain alive, although they lose their nuclei.

The end walls of each cell become perforated to allow substances to pass through. These are called sieve plates.

XylemWater and mineral salts are transported in

the xylem from the roots to the rest of the plant.

The cells become vessels – long cells joined end to end (like a straw). The end walls break down and the cells die when they are mature.

Cell walls in xylem vessels become strengthened with a substance called lignin. This makes the cell walls impermeable.

Xylem vessels are further strengthened by many elongated, lignified cells called fibres.

QuestionsWhat are the differences between

xylem and phloem:in structure?in function?

RootsInternal structure with vascular bundles are

set out differently compared with stems.Vascular bundle is in centre of the root.Xylem carries salts and water from roots to

stem.Phloem will bring food from the stem to the

roots.

Root HairsThe cells of the outer layer of the root just

above the root cap produce tiny, tube-like outgrowths called root hairs.

Root hairs grow between soil particles and stick closely to them.

Root hairs take up water by osmosis.They absorb mineral salts by active

transport.The large number of root hairs greatly

increases the surface area for absorption.

TranspirationEvaporation of water from leaves of a plant.Water moves from inside the cell walls of the cells

in the leaf to the air spaces between the cells.The water evaporates from here and passes by

diffusion through the air spaces in the mesophyll and out of the stomata.

This creates a force that draws water up from the soil through the plant – it causes a tension up the stem, which ‘sucks’ water molecules along.

The transpiration stream is the flow of water up the vascular bundles.

Rate of TranspirationCan be altered by reducing or increasing

evaporation from leaves.What processes might affect the rate of

transpiration?Light intensityHumidityAir movementTemperature

How does the water move from roots to leaves?Draw a flowchart or annotated diagram to

illustrate and summarise how water moves through the plant.