Plant tissues & Plant organ system — AQA GCSE Biology

SPEC 4.2.3.1–4.2.3.2 • PLANT ORGANISATION

Plant Tissues, Transpiration & Translocation

From animals back to plants — the leaf's named tissues, how water moves up the xylem, how sugars move through the phloem, and why guard cells (not stomata) control water loss.

The "Stomata Trap"

A reliable 1-mark question on Paper 1 asks you to name the cells that control water loss in a leaf. Most students confidently write "stomata" and score zero.

The mark scheme answer is "guard cells" — and explicitly says "ignore stoma(ta)". Why? Because the stoma is the gap; the guard cells are the controllers on either side of it. The question asked which CELLS control the gap — that's the guard cells, not the gap itself. — pattern recurring across AQA plant questions

What You Get From This Lesson

The leaf's named tissues and where each one sits, the difference between xylem and phloem (with the mark-scheme phrases for both), root hair cell adaptations, and the four factors that change the rate of transpiration.

What You'll Learn

By the end of this lesson, you'll be able to:

Lesson Objectives

Identify and describe the named tissues in a leaf (epidermis, mesophyll layers, xylem, phloem, guard cells).

Explain the structure of xylem, phloem, and root hair cells and how each is adapted to its function.

Describe transpiration and translocation — and the four factors that affect the rate of transpiration.

Spec ref: 4.2.3.1 (Plant tissues) and 4.2.3.2 (Plant organ system). Paper 1, Higher tier.

The Leaf — A Plant Organ

A leaf is a plant organ — made of several specialised tissues working together for photosynthesis and gas exchange. The roots, stem and leaves together form a plant organ system for transport of substances around the plant.

Tissue vs Cell

When a question asks you to name a tissue, name a TISSUE: palisade mesophyll, spongy mesophyll, epidermis, xylem, phloem, meristem. Writing "stomata" or "guard cells" for a tissue question scores zero — those are structures within tissues, not tissues themselves.

The Five Layers — Top to Bottom

Tissue / LayerFunction / Adaptation

Waxy cuticleWaterproof layer on top of the leaf — reduces water loss by evaporation.

Upper epidermisTransparent — lets light pass through to the layers below.

Palisade mesophyllPacked with chloroplasts — the main site of photosynthesis. Tall column cells aligned to catch maximum light.

Spongy mesophyllIrregular cells with air spaces between them — for gas exchange (CO₂ in, O₂ out).

Lower epidermisContains the stomata (gaps) and guard cells — controls gas exchange and water loss. Far more stomata here than in upper epidermis (reduces water loss in direct sunlight on top of leaf).

Xylem & phloemRun through the leaf as veins . Xylem brings water up, phloem carries sugars away.

MeristemNot in the leaf itself — found at growing tips of shoots and roots. Unspecialised dividing cells.

Stomata & Guard Cells

Stomata are tiny pores (gaps) mainly on the underside of the leaf. Each stoma is bordered by two guard cells that change shape to open or close it.

Mechanism — Turgor Controls the Pore

Water plentiful → guard cells take up water by osmosis → become turgid (curved/bent) → stoma OPEN → CO₂ enters for photosynthesis; water vapour escapes.

Water scarce → guard cells lose water → become flaccid (straight) → stoma CLOSED → water loss reduced.

Stomata ≠ Guard Cells

The stoma is the gap; the guard cells are the cells on either side that control it. When a question asks for "cells that control water loss" → write guard cells — NEVER "stomata".

Xylem — Water Transport

Xylem transports water and dissolved mineral ions from the roots up to the stems and leaves. One-way flow only.

Structure → Function

Dead cells — no cytoplasm, no end walls. Forms continuous hollow tubes from roots to leaves.

Hollow — water can flow freely with little resistance.

Walls strengthened with lignin — strong and waterproof, so the tubes can withstand the tension of water being pulled up without collapsing.

"Substance" vs "Property"

When AQA asks "name the substance that strengthens xylem cell walls" — the answer is LIGNIN. "Hollow" and "dead" are PROPERTIES of xylem cells, not substances. Mark schemes are strict on this distinction.

Process Name

The movement of water (and mineral ions) up the xylem is called the transpiration stream.

Phloem — Sugar Transport

Phloem transports dissolved sugars (mainly sucrose) from the leaves — where they're made by photosynthesis — to the rest of the plant for immediate use or storage. Two-way flow.

Structure → Function

Tubes of elongated cells — cells are living (unlike xylem).

Sieve plates with pores in the end walls — cell sap (with dissolved sugars) flows from one phloem cell to the next through these pores.

Note: detailed phloem structure and the mechanism of sugar transport are NOT required by the spec — just know the cells are living, elongated, and have pores in their end walls.

Process Name

The movement of dissolved sugars through the phloem is called translocation.

Translocation ≠ Transpiration

These are easy to mix up. Memorise:

TRANSPIRATION = water vapour lost from the leaves (xylem-related)

TRANSLOCATION = dissolved sugars moved in the phloem

AQA accepts ONLY "translocation" for sugar transport — "transportation", "active transport", "osmosis" all score zero.

Xylem vs Phloem — Side By Side

FeatureXylemPhloem

TransportsWater + mineral ionsDissolved sugars (sucrose)

DirectionRoots → leaves (one-way)Leaves → rest of plant (two-way)

CellsDEAD , hollow tubesLIVING , elongated tubes

StrengthWalls strengthened with ligninPores in end walls (sieve plates)

ProcessTranspiration (water loss)Translocation (sugar movement)

Root Hair Cells

Found on the surface of plant roots — these cells are specialised for taking up water and mineral ions from the soil. They start the journey that ends with water being lost from the leaves.

Two Adaptations — Match Each To Its Process

Long extension (the "hair") → massively increases surface area for absorption.

Thin cell wall → short diffusion distance.

Many mitochondria → provide the energy (ATP) needed for active transport.

What's absorbedHow (process)Why

WaterOsmosisSoil has a higher water potential than the root cell — water moves down a gradient. No energy needed.

Mineral ionsActive transportSoil has a LOWER concentration of mineral ions than the root cell — moving against the gradient requires energy (ATP from mitochondria).

Transpiration — The Process

Transpiration is the loss of water vapour from the surface of the leaves, mainly through the stomata. It's not just water loss — it's what pulls water up the xylem.

The Transpiration Stream — 4-Step Sequence

Water enters root hair cells from the soil by osmosis.

Water moves up the plant through the xylem.

In the leaf, water evaporates from the surfaces of spongy mesophyll cells into the air spaces.

Water vapour diffuses out through the stomata — creating a "pull" that draws the next column of water up the xylem.

Why The Plant Allows This

The transpiration stream also delivers mineral ions dissolved in the water from the roots to where they're needed (leaves, growing tips). Losing some water through stomata is the cost of moving these resources upwards.

Four Factors Affecting Transpiration

The spec lists four factors. Each one's effect must be memorised — and the reason behind each one, since the longer questions often ask "explain why".

Temperature

↑ faster

Higher temperature → more kinetic energy → faster evaporation.

Humidity

↓ slower

Higher humidity → smaller concentration gradient → less water vapour can diffuse out.

Air movement

↑ faster

Wind removes water vapour from around stomata → maintains the gradient.

Light intensity

↑ faster

Brighter light → stomata open wider for photosynthesis → more water vapour escapes.

Memory Trick

A hot, dry, windy, sunny day = fastest transpiration. A cool, humid, still, dark night = slowest.

Required Skills — Practical & Maths

The spec requires you to handle data from transpiration and stomata investigations.

1. Rate of Transpiration (MS 1a, 1c)

Rate = quantity ÷ time. Transpiration rate is measured by tracking water uptake by a leafy shoot using a potometer. Units: usually cm³ per minute or mm per minute.

Example: A potometer shows water moving 36 mm along the capillary tube in 12 minutes. Rate = 36 ÷ 12 = 3 mm/min.

2. Stomata Distribution (AT 6, 7 + MS 2d)

You'll need to investigate the distribution of stomata and guard cells — typically by counting stomata per mm² on upper vs lower epidermis using a microscope and graticule. Sampling principles apply: take multiple counts from different areas of the leaf, then find the arithmetic mean to get a reliable result.

3. Surface Area & Volume (MS 5c)

When comparing transpiration rates across leaves of different sizes, you may need to calculate the surface area of a leaf (length × width approximation, or counting squares on graph paper). Rates can then be expressed per unit area (e.g. cm³ per cm² per minute) for fair comparison.

Worked Example

Compare the structure and function of xylem and phloem tissue in a plant. 4 marks

POINT

"Compare" means every statement must contrast the two using paired language. Aim for 4 distinct contrasts.

EVIDENCE

Pair each xylem feature with its phloem counterpart: transport material, direction, cell state, process name.

EXPLAIN

Model answer:

Xylem transports water and mineral ions; phloem transports dissolved sugars. ✓

Xylem flow is one-way (roots to leaves); phloem flow is two-way (from leaves to rest of plant). ✓

Xylem cells are dead and hollow; phloem cells are living. ✓

Xylem walls are strengthened with lignin; phloem cells have pores in their end walls (sieve plates) for sap to flow through. ✓

Every statement is COMPARATIVE — each xylem feature is paired with its phloem counterpart. Vague answers like "they're both transport tissues" don't compare; they just describe what's the same. The command word "compare" needs paired contrasts.

Three Examiner Alerts

Alert 1 — Guard cells (not stomata)

When asked "name the cells that control water loss/gas exchange", the answer is guard cells. AQA mark schemes explicitly ignore "stomata" — stomata are the GAPS, guard cells are the CELLS controlling them.

Alert 2 — "Lignin" is the substance

For "what substance strengthens xylem walls", the answer is LIGNIN. AQA examiner reports flag students who write "hollow" or "dead" — those are properties of xylem, not substances. Lignin is the specific chemical answer.

Alert 3 — Translocation vs Transpiration

For "name the process that moves dissolved sugars through phloem", the answer is translocation. AQA mark schemes reject "transpiration" (wrong process — that's water loss), "transportation" (not the spec term), "active transport", and "osmosis". Memorise translocation verbatim.

Common Errors — Wrong vs Right

Wrong

"Stomata control water loss."

Right

"Guard cells control water loss (by opening/closing the stomata)."

Wrong

"Xylem cells are alive."

Right

"Xylem cells are dead and hollow; phloem cells are alive."

Wrong

"Translocation moves water through the plant."

Right

"Transpiration moves water (in xylem); translocation moves dissolved sugars (in phloem)."

Wrong

"Wind decreases the rate of transpiration."

Right

"Wind increases transpiration — it removes water vapour from around the stomata, maintaining the concentration gradient."

Wrong

"Stomata are on the top of the leaf."

Right

"Stomata are mainly on the underside of the leaf — reduces water loss in direct sunlight."

Wrong

"Root hair cells take in mineral ions by osmosis."

Right

"Root hair cells take in water by osmosis and mineral ions by active transport (needs energy)."

Three Things To Remember

1. Leaf Tissues & The "Guard Cells" Trap

A leaf is an organ. Named tissues (top to bottom): waxy cuticle, upper epidermis, palisade mesophyll (photosynthesis), spongy mesophyll (gas exchange), lower epidermis with stomata, plus xylem & phloem in veins. Guard cells (not stomata) control opening/closing of stomata.

2. Xylem vs Phloem

Xylem: water + mineral ions, roots→leaves, dead hollow cells, walls strengthened with lignin, process = transpiration. Phloem: dissolved sugars, two-way flow, living cells with sieve plates + pores, process = translocation. Root hair cells: water by osmosis, mineral ions by active transport.

3. Transpiration Rate — Four Factors

Temperature ↑ → rate ↑ (more KE, faster evaporation). Humidity ↑ → rate ↓ (smaller gradient). Air movement ↑ → rate ↑ (removes water vapour). Light ↑ → rate ↑ (stomata open wider). Hot + dry + windy + sunny = fastest.

Next Lesson

Lesson 2.09 — Lifestyle Risk Factors & Disease Overview. Going deeper into the data and skills side of risk factors — interpreting graphs, identifying correlation vs causation, and the data-analysis questions on smoking/cardiovascular disease that traditionally trip students up on Paper 1.

Done — Plant Transport Locked In

Quick Self-Check

Before moving on, you should be able to answer these without looking:

What's the substance that strengthens xylem cell walls? (Lignin.)

What's the difference between transpiration and translocation? (Transpiration = water loss from leaves via xylem; translocation = sugars moved in phloem.)

Name the cells that control gas exchange in a leaf. (Guard cells — NOT stomata.)

How are mineral ions absorbed by root hair cells? (Active transport — uses energy from mitochondria.)

What happens to transpiration rate on a hot, windy day? (Both factors increase the rate — fastest transpiration.)

Practice Next

Work through the end-of-lesson questions, then move to Lesson 2.09 to bring together the disease + risk factor data work that sits across spec 4.2.2.5–4.2.2.6.