Plant Structure and Function: KS3 Biology Guide

Flowering plants are made of distinct organs — roots, stems, leaves, and flowers — each with a precise structure that suits a particular set of functions. Understanding how plant organs work, and how they are connected, is a key part of KS3 biology, usually covered in Year 7 or Year 8.

What are the main organs of a flowering plant?

Organ | Main functions
Roots | Anchor the plant in the soil; absorb water and dissolved mineral ions
Stem | Supports the plant; transports water, minerals, and sugars between roots and leaves
Leaves | Site of photosynthesis; gas exchange (CO₂ in, O₂ out)
Flowers | Sexual reproduction; produce pollen and ovules
Seeds / fruit | Protect and disperse the embryo plant

How are roots adapted to their function?

Roots are adapted to absorb water and minerals as efficiently as possible:

Root hair cells extend outwards from the root surface, enormously increasing the surface area in contact with soil water. A single plant can have billions of root hairs.

Thin cell walls and no cuticle — water can enter easily by osmosis (movement of water from an area of high water concentration to an area of lower water concentration).

Large permanent vacuole — helps maintain the osmotic gradient that draws water in.

Roots also anchor the plant against wind and provide structural support. Mineral ions (such as nitrates, needed to make proteins) are absorbed by active transport — a process that requires energy because the ions move against their concentration gradient.

How does the stem transport materials?

The stem contains two types of transport tissue arranged in vascular bundles:

Tissue | Carries | Direction
Xylem | Water and dissolved mineral ions | Up from roots to leaves (one-way)
Phloem | Dissolved sugars (mainly sucrose) | Up and down, to all growing parts

Xylem vessels are dead, hollow tubes with thick, waterproofed walls reinforced with a substance called lignin. Water moves up the xylem by transpiration pull — as water evaporates from leaves, it pulls the water column up from the roots (like water rising through a straw when you suck).

Phloem is made of living cells with pores between them. Sugar produced in the leaves during photosynthesis is loaded into the phloem and transported (a process called translocation) to growing regions (shoot tips, flowers) and storage organs (roots, fruit).

What is the structure of a leaf and why is it important?

The leaf is the main organ of photosynthesis. Its structure is finely adapted to capture light and exchange gases efficiently.

Layer / Feature | Function
Waxy cuticle | Waterproof coat on the upper surface; reduces water loss
Upper epidermis | Transparent layer; allows light through to the mesophyll
Palisade mesophyll | Tightly packed cells with many chloroplasts; main site of photosynthesis
Spongy mesophyll | Loosely packed cells with air spaces; allows CO₂ and O₂ to diffuse
Lower epidermis | Outer layer on underside; contains stomata
Stomata (singular: stoma) | Pores that open and close to control gas exchange and water loss
Guard cells | Pairs of cells surrounding each stoma; control whether the stoma is open or closed
Vascular bundle (midrib and veins) | Contains xylem and phloem throughout the leaf

Key adaptation: leaves are broad and flat to maximise surface area for light absorption, and thin so that CO₂ does not have far to diffuse from the stomata to the palisade cells.

How do stomata control gas exchange and water loss?

Stomata are typically found on the lower surface of leaves, away from direct sunlight, which reduces water loss. Each stoma is surrounded by two guard cells. When light falls on the leaf:

Guard cells absorb water by osmosis and become turgid (swollen).

Their uneven cell walls cause them to curve outward, opening the pore.

CO₂ diffuses in; O₂ and water vapour diffuse out.

At night, when photosynthesis stops, guard cells lose water and become flaccid, closing the stoma and reducing water loss. On hot, dry days stomata may partly close even in the light to prevent the plant from wilting — this limits CO₂ entry and slows photosynthesis.

Worked example: tracing water through a plant

Question: Describe the route taken by a water molecule from the soil to the atmosphere via a leaf.

Answer:

Water is absorbed from the soil by a root hair cell through osmosis.

It passes through the root cortex cells (again by osmosis) into the xylem of the root.

Water travels up the xylem in the root, through the xylem in the stem, and into the xylem of the leaf veins.

Water moves from the xylem into the spongy mesophyll cells and evaporates into the air spaces.

Water vapour diffuses out through the stomata into the atmosphere — this is transpiration.

The rate of transpiration increases in warm, dry, windy conditions and decreases in cool, humid, still conditions.

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to understand the structure and functions of flower-bearing plants, including the role of xylem and phloem, and how leaves are adapted for photosynthesis and gas exchange. BBC Bitesize KS3 Biology covers plant organs, leaf structure, stomata, and transpiration with labelled diagrams and test-yourself activities.

Frequently asked questions

What is the difference between xylem and phloem?

Xylem carries water and dissolved mineral ions upward from the roots to the rest of the plant. Its vessels are made of dead, hollow cells with lignified walls that provide structural support as well as a channel for water flow. Phloem carries dissolved sugars produced by photosynthesis (mainly sucrose) both up and down the plant to wherever energy or building materials are needed. Unlike xylem, phloem is made of living cells. Both types of tissue travel together in vascular bundles throughout the stem, roots, and leaves.

Why do plants need mineral ions as well as water?

Water alone is not enough for a plant to grow. Mineral ions dissolved in soil water are essential raw materials for making important biological molecules. Nitrate ions (NO₃⁻) are needed to make amino acids, which are the building blocks of proteins — without nitrates, plants show stunted growth and yellowing leaves. Magnesium ions (Mg²⁺) are needed to make chlorophyll — without magnesium, leaves turn yellow because the plant cannot produce enough of the green pigment to photosynthesise properly. These ions are absorbed by active transport in root hair cells.

What are guard cells and why are they important?

Guard cells are pairs of specialised cells that surround each stoma on a leaf. They control whether the stoma (pore) is open or closed by changing shape. When the plant is well-watered and it is daytime, guard cells fill with water and become turgid — their asymmetric cell walls cause them to bow outward, opening the stoma so CO₂ can enter for photosynthesis and O₂ and water vapour can leave. When the plant is short of water, guard cells lose water and become flaccid, closing the stoma to prevent further water loss. Guard cells therefore balance the need for photosynthesis against the need to conserve water.

How does transpiration benefit a plant?

Transpiration — the evaporation of water from leaves through stomata — provides three benefits. First, it drives the transpiration stream: water evaporating from leaves creates a tension that pulls more water up through the xylem from the roots, delivering minerals dissolved in that water to all parts of the plant. Second, it cools the leaf in bright sunlight, much like sweating cools the human body. Third, the movement of water through root hair cells by osmosis requires a water potential gradient, and continuous transpiration keeps that gradient in place. The cost of transpiration is potential water loss on hot days, which is why stomata can partly close.

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