Leaf Structure

1. Waxy cuticle.

• A thin, waterproof layer made of wax.
• Reduces water loss by EVAPORATION.
• TRANSPARENT — light passes straight through.
• Thicker on the upper surface (more sun, more potential water loss).

2. Upper epidermis.

• A single layer of close-packed cells.
• Few or no chloroplasts (passes light through).
• PROTECTS the inner tissues.

3. Palisade mesophyll.

• Tall, column-shaped cells just below the upper epidermis.
• Tightly packed, no big air gaps.
• Each cell PACKED with CHLOROPLASTS.
• THE MAIN SITE OF PHOTOSYNTHESIS.

4. Spongy mesophyll.

• Loose, irregularly-shaped cells with LARGE air spaces between them.
• FEWER chloroplasts than palisade.
• Air spaces let CO₂ DIFFUSE TO ALL CELLS.
• Some photosynthesis, but main role is gas exchange.

5. Lower epidermis.

• Single layer of cells (similar to upper).
• Contains the STOMATA — pores for gas exchange.
• Cuticle here is THINNER (less direct sunlight, less water loss).

6. Stomata + guard cells.

• See dedicated section below.

7. Vein (vascular bundle).

• XYLEM (top side of vein): brings water + dissolved minerals UP from roots.
• PHLOEM (bottom): takes sucrose AWAY to growing parts / storage.

Worked qualitative. Why are palisade cells at the top, not the bottom?

• Light comes from above.
• Top of the leaf gets more light.
• More chloroplasts at the top = more light absorbed = more photosynthesis.

Cambridge tip. When labelling a leaf diagram, ALWAYS distinguish palisade (column-like, tightly packed) from spongy (irregular, gaps). Confusing them is a common mark loss.

Every adaptation makes ONE input easier to absorb or ONE output easier to remove.

LIGHT-related.

• Broad and flat → large SURFACE AREA for sunlight.
• Thin → light reaches palisade quickly.
• Cuticle and epidermis transparent → light passes through to chloroplasts.
• Chlorophyll in chloroplasts → ABSORBS light.

CO₂ / gas exchange-related.

• Stomata → pores let CO₂ enter, O₂ leave.
• Spongy mesophyll air spaces → CO₂ diffuses to all photosynthesising cells.
• Thin → short diffusion path.
• Broad → large surface for diffusion through stomata.

WATER-related.

• Xylem in veins → carries water to all cells.
• Cuticle → reduces water loss.
• Stomata mostly on lower side → reduces water loss (cooler, shadier).
• Guard cells → can close stomata to conserve water.

Worked qualitative. A leaf is held vertically (e.g. eucalyptus in hot countries). What advantage might this give?

• Reduces direct sun on the broad face → less overheating.
• Slows water loss in hot dry conditions.
• Trade-off: less light absorbed at noon, but enough is captured at sunrise/sunset when the leaf face matches the angle.

Cambridge tip. Two-mark questions on leaf adaptation expect FEATURE + EXPLANATION. "Broad to give a large surface area for light absorption" earns 2; "broad" earns 0.

Where they are. Mostly on the LOWER epidermis. Each stoma (singular) is a tiny pore.

Why mostly underneath?

• Reduces water loss (lower side is cooler, less sunlit, less wind exposure).
• Still allows gas exchange because CO₂ can reach the spongy mesophyll easily.

Guard cells.

• Pair of KIDNEY-shaped cells flanking each stoma.
• They have CHLOROPLASTS (unlike most epidermis cells).
• Inner wall (facing the pore) is THICKER than the outer wall.

How opening works (day).

1. Light → guard cells photosynthesise → make sugars (or stimulate ion uptake).
2. Water moves IN by OSMOSIS → guard cells become TURGID.
3. The thinner OUTER wall stretches more than the thicker INNER wall.
4. Each cell BENDS outward.
5. Stoma OPENS → gas exchange + transpiration occur.

How closing works (night, drought).

1. No light (or low water) → solutes leave guard cells.
2. Water moves OUT by osmosis → guard cells become FLACCID.
3. Walls relax → guard cells straighten.
4. Stoma CLOSES → gas exchange stops; water conserved.

Worked qualitative. Why don't plants leave stomata open all the time (more photosynthesis, surely)?

• Water loss would be too high → plant wilts and dies.
• At night, no photosynthesis → no need for CO₂ → save water.
• In drought, stomata close to prioritise survival over growth.

Cambridge tip. Always describe the OUTWARD bend (because of the asymmetric wall). Not just "swell up".

Xylem.

• DEAD, hollow tubes made of lignified cells.
• Carries water + dissolved minerals (nitrate, phosphate, etc.) UP from the roots.
• One-way flow.

Phloem.

• LIVING cells (sieve tubes + companion cells).
• Carries SUCROSE + amino acids AWAY from the leaf, to wherever they're needed (growing tips, fruits, storage organs like potatoes).
• Two-way flow (translocation).

Position in the vein.

• XYLEM on the upper side (close to the palisade).
• PHLOEM on the lower side.
• Surrounded by sclerenchyma fibres for support (especially in the midrib).

Worked qualitative. When a leaf wilts, has the plant lost xylem function or phloem function?

• Wilting = lack of WATER in cells.
• Water transport is XYLEM's job.
• So either xylem is blocked, or water uptake at roots has stopped (drought), or transpiration loss is too fast.
• Phloem failure would show up as poor growth in fruits/roots, not wilting.

Cambridge tip. When asked "what is in the vein?", say BOTH xylem and phloem and their functions. "The vein contains xylem and phloem" alone is incomplete.