AQA GCSE Biology (8461)

4.2.3 Plant tissues, organs and systems

This page covers AQA GCSE Biology 4.2.3: plant tissues (epidermis, palisade + spongy mesophyll, xylem, phloem, meristem), the leaf as an organ, and transpiration + translocation as the two main transport processes. The exam asks the same questions every year — how xylem and phloem differ, why the leaf has palisade ABOVE spongy mesophyll, what causes water to move up through xylem. Master the structure-fits-function logic and the precise marking phrases here.

Why this matters

Plants face a unique challenge: roots are buried in soil collecting water and minerals, leaves are high in the air collecting sunlight, but the chemistry of photosynthesis needs both. The solution is two specialised transport tissues — xylem to lift water from roots to leaves, phloem to carry sugars (made by photosynthesis) from leaves to growing or storage parts. The leaf itself is an organ — a flat, thin structure with layers of different tissues each doing one job: epidermis protects, palisade mesophyll absorbs light, spongy mesophyll lets gases diffuse, xylem and phloem move substances in and out. Master the layered structure of the leaf and you've nailed half the GCSE plant biology marks.

How to learn this topic

Build on what you already know

GCSE 4.2.1: levels of organisation; tissue = group of cells with similar function.
GCSE 4.1.1: chloroplasts are the site of photosynthesis; root hair cells are adapted for absorption.
KS3: photosynthesis: CO₂ + water + light → glucose + oxygen.

Recap the plant tissues — epidermis, palisade mesophyll, spongy mesophyll, xylem, phloem, meristem.
Build the leaf layer by layer (from top to bottom).
Xylem vs phloem — the canonical exam comparison.
Transpiration — the water flow through the plant.
Translocation — phloem moves sugars from source to sink.
Stomata + guard cells — opening + closing to balance gas exchange against water loss.

Key terms

palisade mesophyll: Tall, tightly-packed cells in the upper part of a leaf, full of chloroplasts. The main site of photosynthesis. (Always state the LOCATION (just below upper epidermis) AND the FUNCTION (main site of photosynthesis) — both score marks.)
spongy mesophyll: A loose layer of cells in the lower part of a leaf, with large air spaces between them. Allows CO₂ to diffuse in and O₂ + water vapour to diffuse out. ('Air spaces' is the marking phrase examiners want — not just 'loosely packed'.)
xylem: Plant transport tissue made of dead, hollow, lignified tubes. Carries water and dissolved mineral ions from roots to leaves in one direction (up). (Three required features: DEAD, HOLLOW, LIGNIFIED. Don't just say 'tubes' — the lignin gives strength and waterproofing.)
phloem: Plant transport tissue made of living sieve tubes with companion cells. Carries sugars and amino acids from source (leaves) to sink (growing parts). (Phloem is LIVING — that's the contrast with xylem. Companion cells provide ATP for active loading.)
transpiration: The loss of water from a plant by evaporation, mostly from the leaves through the stomata. (Examiners want 'evaporation from leaves' AND 'through stomata'. Just 'water loss' isn't enough.)
transpiration stream: The continuous flow of water up through the xylem from roots to leaves, driven by evaporation at the leaves. (Linking word here is 'pulled' — water is pulled up by the cohesive force of water molecules sticking together (hydrogen bonds).)
translocation: The movement of sugars (sucrose) and other organic substances through the phloem from source (where they're made/stored) to sink (where they're used/stored). (Examiners distinguish 'source' (e.g. leaves in summer) from 'sink' (e.g. growing roots or fruits). Both terms are required.)
stoma: A pore in the leaf surface (mostly on the underside) through which gases enter and leave. Surrounded by two guard cells that open and close the pore.
guard cell: One of a pair of cells flanking a stoma. When turgid, they curve outward and open the stoma; when flaccid, they close it. (The marking chain: turgid guard cells (full of water) → curve outward → stoma opens. Reverse for closing.)
epidermal tissue: The outer covering of a plant. The upper leaf epidermis has a waxy cuticle to reduce water loss; the lower epidermis contains stomata.
waxy cuticle: A waterproof, transparent layer secreted by the upper epidermis. Reduces water loss from the leaf surface. (Must say 'waterproof' or 'reduces water loss' — examiners reject 'protects the leaf' as too vague.)
source and sink: Source = a region where sugars are made or stored (e.g. photosynthesising leaves, or roots in spring). Sink = a region where sugars are used or stored (e.g. growing tips, fruits, roots in winter). (Translocation can run in EITHER direction — source/sink depends on the season + plant stage.)

Notes

The plant tissues you need to know

Epidermal tissue — covers the surface of the plant. The upper leaf epidermis has a waxy cuticle that reduces water loss + lets light through. The lower epidermis has stomata (pores) for gas exchange.
Palisade mesophyll — column-shaped cells packed with chloroplasts, located just below the upper leaf surface. The main site of photosynthesis.
Spongy mesophyll — loosely packed cells with air spaces between them. Allows CO₂ to diffuse in and O₂ + water vapour to diffuse out.
Xylem — dead, hollow, lignified tubes. Carry water + dissolved mineral ions from roots to leaves. One direction only — UP.
Phloem — living sieve tubes + companion cells. Carry sugars + amino acids from leaves (source) to growing tips, roots, fruits (sinks). Either direction.
Meristem tissue — undifferentiated dividing cells found at the tips of roots and shoots. The plant equivalent of stem cells (covered in 4.1.2).

The leaf — a layered organ

From top to bottom in a typical dicot leaf:

Waxy cuticle — waterproof + transparent.
Upper epidermis — thin, transparent (lets light through), no chloroplasts.
Palisade mesophyll — tightly packed, full of chloroplasts. Most photosynthesis happens here, near the top where light is brightest.
Spongy mesophyll — loose with air spaces. Gas exchange happens here; some chloroplasts too.
Lower epidermis — contains the stomata.
Stomata — pores controlled by guard cells. Open in light → CO₂ in, O₂ + water vapour out. Close in dark or drought → reduce water loss.
Xylem + phloem — run as vascular bundles through the leaf, branching into smaller and smaller veins.

Adaptations of the leaf for photosynthesis + gas exchange:

Broad and flat — large surface area to absorb light.
Thin — short diffusion path for gases.
Air spaces in spongy mesophyll — maximise surface area for gas exchange.
Stomata + guard cells — open/close to balance CO₂ uptake against water loss.
Vascular bundles — bring water from roots, take sugars away.

Xylem vs phloem — the canonical comparison

| | Xylem | Phloem |

|---|---|---|

| Cells | DEAD, hollow, lignified | LIVING (sieve elements + companion cells) |

| Carries | water + dissolved mineral ions | sugars (sucrose) + amino acids |

| Direction | one way: roots → leaves | either way (translocation source → sink) |

| Driving force | transpiration pull | active transport into sieve tubes (energy from companion cells) |

Key marking phrases:

'Xylem cells are dead and hollow / lignified so they form long continuous tubes that water flows through without obstruction.'
'Phloem is living because moving sugars (translocation) requires active transport / energy.'

Transpiration — water up the xylem

Transpiration is the loss of water from a plant by evaporation, mostly from the leaves through the stomata. Despite sounding wasteful, it's the engine that drives water up the xylem:

Water evaporates from spongy mesophyll cells into the air spaces inside the leaf.
Water vapour diffuses out through the stomata down the concentration gradient.
As water leaves the mesophyll, water is pulled from the xylem up into the mesophyll (cohesion — water molecules stick to each other via hydrogen bonds).
The water column in the xylem moves up — like sucking water up a straw.
Water is replaced at the roots by uptake from root hair cells.

This is called the transpiration stream. Factors that increase the rate: higher temperature, drier air (lower humidity), more air movement (wind), brighter light (stomata open more).

A potometer is a device used to measure water uptake (and indirectly transpiration rate) by a cut shoot.

Translocation — sugars in phloem

Translocation is the movement of sugars (made by photosynthesis) and other organic substances through the phloem from source (where they're made or stored — usually leaves) to sink (where they're used or stored — growing tips, fruits, roots in winter).

Key features:

Requires energy (ATP from respiration in companion cells) — sugars are actively loaded into the sieve tubes.
Either direction — in summer, source = leaf, sink = root. In spring, the previous winter's stored sugars in roots become source, and growing buds become sink.
Sieve plates separate sieve elements but allow sap to flow.

Stomata and guard cells

Stomata are tiny pores in the leaf surface (mostly on the underside). Each is bounded by a pair of guard cells whose shape changes the size of the pore:

Light + water available: guard cells take up water by osmosis → become turgid → bend outward → stoma opens → CO₂ in for photosynthesis (water vapour also escapes).
Dark or drought: guard cells lose water → become flaccid → straighten → stoma closes → reduces water loss (but CO₂ uptake stops too).

This is the plant's compromise between photosynthesis (needs open stomata) and water conservation (needs closed stomata).

Exam tips

Xylem: dead, hollow, lignified, water + minerals, ONE DIRECTION UP. Phloem: living, sieve tubes, sugars + amino acids, EITHER DIRECTION. Don't mix these up.
On 'how is water pulled up the xylem' questions, the chain is: water evaporates from spongy mesophyll → water vapour exits stomata → cohesion between water molecules → water pulled up xylem → roots absorb more.
Palisade mesophyll is the UPPER layer (where light hits first). Spongy mesophyll is the LOWER layer (where the stomata are).
Translocation: 'source to sink' is the marking phrase. Don't just say 'leaves to roots' — it can go either way depending on season.
Stomata open in light + close in dark. Guard cells become TURGID (full of water) to bend outward and open the pore.
Leaf adaptations for photosynthesis + gas exchange: broad + thin + air spaces + stomata + vascular bundles. Name at least three for full marks.
Don't write 'transpiration is bad'. Write 'transpiration drives water uptake' — it's the engine that pulls water up the xylem.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What's the difference between xylem and phloem?

Both are plant transport tissues, but with totally different structure and cargo. XYLEM: DEAD, hollow, lignified tubes that carry WATER and DISSOLVED MINERAL IONS in one direction (roots upward). PHLOEM: LIVING sieve tubes (with companion cells) that carry SUGARS and AMINO ACIDS from source (e.g. leaves) to sink (e.g. growing tips) in either direction. Mnemonic: Xylem = X (water); Phloem = food (P for produce).

Why does water move UP through the xylem if it doesn't have a pump?

Water is pulled up by transpiration. Water evaporates from the spongy mesophyll cells into the leaf's air spaces, then the water vapour diffuses out through the stomata. As water leaves the mesophyll cells, more water is pulled into them from the xylem. Because water molecules stick to each other (cohesion — hydrogen bonds between H₂O molecules), the water column in the xylem doesn't break — it's all pulled up as one continuous stream. Water lost at the leaves is replaced by uptake at the root hair cells. The whole system is the 'transpiration stream'.

Why are most stomata on the underside of the leaf?

To reduce water loss. The upper surface is in direct sunlight and gets hotter — so putting stomata there would lose more water by evaporation. The lower surface is shaded and cooler, so stomata there can do gas exchange with less water lost. Some plants in dry environments take this further — closing stomata during the day entirely (and opening them at night, like cacti).

What does 'source to sink' mean in translocation?

Translocation is the movement of sugars in the phloem. SOURCE = wherever sugars are being PRODUCED or RELEASED from storage. In summer, source = photosynthesising leaves. In spring, source = roots releasing stored starch (now broken down to sugar). SINK = wherever sugars are USED or BEING STORED. Summer: sink = growing tips, fruits, roots laying down winter stores. So the same phloem tissue can carry sugars UP from roots in spring and DOWN from leaves in summer — depending on where source and sink are.

What's the role of the waxy cuticle on a leaf?

The waxy cuticle has two jobs: (1) WATERPROOFING — it's hydrophobic and reduces water loss from the leaf surface by evaporation. Without it, the leaf would dry out rapidly. (2) TRANSPARENCY — it's clear, so it lets sunlight pass through to the palisade mesophyll cells below. It's usually thicker on the upper leaf surface (where the sun is) than the lower.

Why is the palisade mesophyll on the TOP of the leaf?

Because the palisade cells are full of chloroplasts and do most of the photosynthesis — and photosynthesis needs light. Putting these cells just below the upper epidermis (which is thin and transparent) means light reaches them first, before being absorbed by any other layers. Below them, the spongy mesophyll handles gas exchange (CO₂ in, O₂ out) — that doesn't need as much direct light, so it can be lower down. The leaf is precisely layered for its job.