GCSE Biology structure function of xylem vessel tubes, phloem vessels, transporting materials throughout a flowering plants work as two separate transport systems which must reach all parts of the plant, diffusion, osmosis & active transport, function of vascular bundles, Types of plant cells organisation into tissues organs function of epidermal tissue meristem tissue palisade spongy mesophyll tissue waxy cuticle veins vascular bundles phloem tubular cell systems xylem Doc Brown's GCSE biology exam revision study notes

GCSE level Biology exam revision notes on transport in plants

Transport in flowering plants: Part 1.

(1) Types of plant cells and their organisation into tissues and organs in the context of movement of substances around a plant

AND (2) Flowering plant transport systems structure, function of plant xylem and phloem cells (on this page too)

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[Key points and learning objectives for this page, after the main body of notes]

Sub-index of biology notes on transport systems in plants

(1) Types of plant cells and their organisation into tissues and organs

Plant cells are organised into tissues and these form plant organs such as leaves, roots and stems.

These organs must function together in such way to work as an organ system - to ensure the plant gets all its needs to survive and grow into a mature plant.

Transportation of e.g. nutrients or waste products is one of the most important function of a plant organ system.

Plant organs are made of tissues

(A summary of those you must know are briefly described below and in more detail later where necessary)

Epidermal tissue

Epidermal tissue covers the whole surface of the plant - its the equivalent of our 'skin'!

Meristem tissue

Meristem cells are found in the growing tips of shoots and roots.

They can differentiate into all the different types of plant cell needed for growth and reproduction.

Palisade mesophyll tissue

Most photosynthesis occurs in the palisade mesophyll tissue, part of the leaves.

Spongy mesophyll tissue

Spongy mesophyll tissue forms part of the leaf and contains lots of air spaces to let gases diffuse in and out of the leaf structure.

Xylem and phloem

Xylem and phloem are tubular cell networks that allow the transportation of mineral ions, food e.g. sugars and water around the plant - the leaves, roots and stems must be all connected together.

Both xylem and phloem tissue networks are held together in vascular bundles which give strength and protection to plant tissues.

Waxy cuticle

The cuticle is a water repellent protective layer covering the epidermal cells of leaves and other parts and limits water loss.

and this is how some of the above fit together in the structure of a leaf ...

More on the tissue structures of a leaf and their functions - adaptations of leaves

These descriptions apply to dicotyledonous plants.

So, starting from the top layers, and all marked on the above diagram ...

The epidermal tissue on the upper side of the leaf are covered with a waxy cuticle layer which is water repellent - this helps water loss by evaporation.

The upper epidermal layer is transparent to visible light, so light can penetrate to the palisade cell layer where it is needed for photosynthesis.

The palisade mesophyll layer is made of the palisade cells which are packed with lots of chloroplasts - the sites of photosynthesis - note that the palisade cells are near the upper surface to receive the most light.

The xylem and phloem are networks of vascular sheathed bundles of cells that are the backbone veins of the plant's transport system .

The details of how all these function is described in detail in Part 2.

The leaf tissues are adapted for efficient gas exchange.

The broad flat green leaves of plants exposed to light, provide a large surface area for the light absorbing sites of photosynthesis - more than the thinner stem.

The leaves are thin so the absorbed carbon dioxide has only a short distance to diffuse to the photosynthesising cells.

Leaves have veins (vascular bundles of xylem and phloem cells) that support the leaf and transport water and minerals to the leaves and glucose away from the leaves.

The lower epidermal tissue is full of tiny holes (stomata, pores) which allow carbon dioxide to diffuse into the leaf for photosynthesis.

Reminder: carbon dioxide + water == light/chlorophyll ==> glucose + oxygen

The opening and closing of stomata is controlled by guard cells which respond to changes in environmental (ambient) conditions including the movement of water in and out of leaves.

The spongy mesophyll tissues contain air spaces which increase the rate of diffusion gases in (carbon dioxide) and out (oxygen).

Transport in plants: 2. Flowering plants have two separate transport systems which must reach all parts of the plant

Sub-index of biology notes on transport systems in plants

(2) Flowering plants have two separate transport systems which must reach all parts of the plant

Reminder of the two types of transportation

Other than for active transport, diffusion, naturally occurs in gases and liquids, because all the particles (molecules or ions) have sufficient kinetic energy to move around freely at random from a region of higher concentration to a region of lower concentration.

Osmosis is a particular spontaneous diffusion through a membrane from a higher water potential region (solution less concentrated in solutes) to a lower water potential region (solution more concentrated in solutes).

Active transport is the movement of particles (molecules or ions) through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Within cell membranes there are carrier proteins which use energy from respiration to transport molecules or ions across the membrane, against the natural concentration gradient, therefore cells that use active transport usually have more mitochondria for respiration compared to other cells.

The structure and function of plant xylem and phloem cells - the transport tissues

Plants have two networks of 'fine tubes' to transport molecules and ions (xylem tubes and phloem tubes).

The xylem vessel tissue transports water and minerals from the root hairs to all the rest of plant i.e. through the roots and stem to the very tips of all the leaves.

This xylem vessel process is driven by transpiration.

The phloem vessel tissue transports dissolved sugars from the leaves (where they are made from photosynthesis) to all parts of the plant e.g. for growth of new cells or to storage tissue where they are converted to starch.

This function of the phloem to move sugar molecules (mainly dissolve sucrose), amino acid molecules and mineral ions around the plant is called translocation.

These systems are essential for a plant to be healthy.

In some trees the transport systems run through the bark.

Unfortunately, some animals like to chew this bark.

The transport systems are disrupted and the tree sadly dies!

Consider the xylem first

The xylem tissue transports water and mineral ions from the roots to the stem and leaves.

Xylem tubes are made of lignified dead cells joined together 'end to end' in such a way they form a complete fine tube through which water and mineral ions are freely transported from the roots, up through the stem to the leaves.

The xylem cells have no end wall but a hole down the middle (lumen) allows the free movement of fluid - but in only one direction.

The strong xylem cell walls are made from cellulose and are strengthened-stiffened by a material called lignin - these give the plant support.

The movement of water from the roots through the xylem and out of the leaves is called the transpiration stream and is caused by the diffusion of water and its subsequent evaporation. The transpiration stream only flows in one direction - up through the plant.

Water evaporating through the stomata in the leaves, causes it to be replaced by water absorbed by the roots and this water moves up via the xylem tube system through the stem and to all the leaves - from the roots to ALL of the plant and carrying essential mineral ions too.

If the stomata pores are open, evaporation of water will always take place because the concentration of water in the air is less than the concentration off water in outer layers of a leaf.

The diffusion and evaporation of water from the leaves produces a water deficiency in the plant, so water is automatically (if available) drawn up through the xylem tube system, so the transpiration stream is driven by this evaporation of moisture from the leaves.

Water is essential to the plant for both transportation and photosynthesis.

Evolution adaptation notes:

Transpiration is a necessary adaptation to work in conjunction with photosynthesising leaves - the stomata allow the gas exchange - carbon dioxide in, and water vapour and oxygen out.

Even the narrow roots of plants are further covered in tiny root hairs that greatly increase the surface area even more, and hence increase the efficiency of water absorption.

This adaptation means the water has only got to move a short distance to the xylem to be transported up through the whole of the plant.

Now consider the phloem cells and compare their structure and function with xylem cells

Phloem cells are elongated living cells and the phloem tube tissues carry dissolved sugars (food - glucose, sucrose) from the leaves to the rest of the plant, including the tissue growing regions and the storage organs.

The phloem cells also transport other important materials like amino acids for protein synthesis and mineral ions for the function of chlorophyll and enzymes.

This process is called translocation and can operate fluid flow in both directions - another useful adaptation.

Two of the principal substances transported by translocation are sucrose and amino acids from regions of production to regions of storage, respiration (sugars) or growth (e.g. protein synthesis).

Note that some parts of a plant may act as a source or a sink at different times during the life of a plan

The sugars from photosynthesis enter the phloem system by active transport and transported around by water which enter the phloem cells by osmosis.

Reminder: Osmosis as the net movement of water molecules from a region of higher water potential (from a more dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane (concentrated refers to dissolved molecules or ions).

Phloem cells are elongated with end walls that have pores in them to allow fluids to flow through and the phloem system allows transportation in both directions.

Phloem tubes are sometimes called sieve tube elements and the perforated end-plates allow fluids to pass through.

The phloem cells (sieve tube elements) have no nucleus and can't survive on their own, so each one has a companion cell (not shown, but has nucleus) that controls the living functions for both cells.

The companion cell, with a nucleus, has lots of mitochondria to provide energy for the phloem's transportation function.

They allow the transport of water and dissolved substances to all part of the plants where nutrients are needed for immediate use in growth or converted to starch for storage

The phloem tubes mainly carry the sugars made in the leaves from photosynthesis up and down the stem to all parts of the plant for immediate use in respiration, new growth or to the food storage organs to form starch.

Phloem cells contain a little cytoplasm, but no nucleus and have very few sub-cellular structures, this adaptations increase the efficiency of transportation - also aided by the sieve plates of adjoining cells.

Vascular bundles

Throughout the plant the chains of xylem and phloem cells are held together in vascular bundles.

You find them in the roots, stem and in the leaf - where you see them as the veins in leaves.

Key points on transport of materials throughout a flowering plant

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Transport Systems in Flowering Plants

Plants have specialised tissues and processes that move essential substances (like water, minerals, and sugars) to where they’re needed.

1. Xylem Vessels

Function: Transport water and mineral ions from roots to leaves (upward only).
Structure:
- Made of long, dead cells with no internal contents.
- Strengthened with lignin for support and water resistance.
- Form hollow tubes for uninterrupted flow.
Direction: One-way only (roots → stems → leaves).

2. Phloem Vessels

Function: Transport sugars (mainly sucrose) and amino acids from leaves to all parts of the plant.
Structure:
- Made of living cells.
- Contains sieve tube elements and companion cells.
- Sieve plates have pores to allow movement between cells.
Direction: Two-way (bidirectional), called translocation.

3. Vascular Bundles

Groups of xylem and phloem tissues bundled together.
Found in roots, stems, and leaves.
Also provide structural support to the plant.

4. Stomata and Guard Cells

Stomata: Tiny pores (usually on the underside of leaves).
- Allow gas exchange (CO₂ in, O₂ out).
- Also let water vapour escape (transpiration).
Guard Cells:
- Control the opening and closing of stomata.
- Become turgid (full of water) to open stomata.
- Become flaccid (lose water) to close stomata.

5. Epidermal Tissue

Outer layer of cells covering leaves, stems, and roots.
Functions:
- Protects against water loss and infection.
- Upper epidermis is often transparent to let light through.
- May be covered with a waxy cuticle to reduce water loss.

Transport Processes in Plants

Osmosis

Movement of water molecules from a region of high water concentration to low water concentration, across a partially permeable membrane.
Used to absorb water from the soil into root hair cells.

Diffusion

Movement of molecules from high to low concentration.
In plants, diffusion moves gases (like CO₂ and O₂) through stomata.

Transpiration

Evaporation of water from plant leaves (mainly through stomata).
Creates a transpiration pull that draws water upward through the xylem.
Affected by:
- Temperature
- Humidity
- Light intensity
- Wind speed
See (5) More on leaf adaptations to aid photosynthesis and gas exchanges and factors affecting the rate of transpiration and the function of the stomata and guard cells

Summary Table

Component/Process	Function	Special Features
Xylem	Water/mineral transport	Dead cells, lignified walls, upward flow
Phloem	Sugar and amino acid transport	Living cells, sieve plates, bidirectional
Vascular Bundles	Transport + support	Xylem + Phloem grouped
Stomata	Gas exchange/transpiration	Opened/closed by guard cells
Guard Cells	Control stomata opening	Change shape with water levels
Epidermal Tissue	Protection, water regulation	Transparent, waxy cuticle
Osmosis	Water uptake	Passive, in root hairs
Diffusion	Gas exchange	Occurs in leaf spaces
Transpiration	Pulls water up from roots	Involves stomata and xylem

Keywords, phrases and learning objectives for this part on transport systems in plants

Be able to describe the types of plant cells and their organisation into tissues and organs in the context of transferring materials out, in and around a plant.

Know the function of epidermal tissue, meristem tissue, palisade cells, spongy mesophyll tissue, xylem and phloem tubular cell systems, the waxy cuticle, veins and vascular bundles.

Be able to describe the structure and function of xylem vessel tubes and phloem vessels in transporting materials throughout a flowering plant.

Know that they work as two separate transport systems which must reach all parts of the plant.

Know the role of diffusion, osmosis and active transport in movement of substances around a plant.

Know what vascular bundles are structures that hold the xylem and phloem cell tubular networks together,.

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