Plants are multicellular organisms with specialised organs, just like animals.
The main plant organs are:
LEAF — the main site of photosynthesis and gas exchange.
STEM — provides support and transports substances between roots and leaves.
ROOT — anchors the plant and absorbs water and mineral ions from the soil.
FLOWER — involved in reproduction.
Each organ is made of different types of TISSUE, each adapted to its specific function.
Tissues in the Leaf
The leaf is the most important organ for photosynthesis. It contains several specialised tissues:
EPIDERMIS (upper and lower):
A thin, transparent layer covering the leaf surface.
Produces a WAXY CUTICLE — a waterproof coating that reduces water loss by evaporation.
PALISADE MESOPHYLL:
Found in the upper part of the leaf — closest to sunlight.
Cells are tall and column-shaped, packed with many CHLOROPLASTS.
This is the main site of photosynthesis — receives maximum light.
SPONGY MESOPHYLL:
Below the palisade layer.
Cells are loosely arranged with large AIR SPACES between them.
Air spaces allow CO₂ to diffuse easily to photosynthesising cells.
Also allows O₂ produced by photosynthesis to diffuse out.
GUARD CELLS and STOMATA:
Guard cells are pairs of cells surrounding tiny pores called STOMATA (singular: stoma) on the lower leaf surface.
Stomata open to allow CO₂ in for photosynthesis and O₂ out — also allow water vapour to escape (transpiration).
Guard cells control stomata opening: in the light, guard cells absorb water and become turgid → stomata OPEN. In the dark or when dehydrated, guard cells lose water → stomata CLOSE.
XYLEM and PHLOEM:
Run through the leaf as vascular bundles (veins).
Xylem brings water to the leaf. Phloem takes away sugars made by photosynthesis.
Xylem Tissue
Xylem is a tissue that transports WATER and dissolved MINERAL IONS from the roots upwards to the leaves.
Key features of xylem cells:
DEAD cells — no cytoplasm or nucleus, forming a completely hollow tube.
No end walls — cells are stacked end-to-end to form one continuous open column.
Walls strengthened with LIGNIN — a hard, waterproof material that makes xylem very strong and prevents the tubes from collapsing.
Water moves through xylem in one direction only — upwards.
The movement of water through xylem is driven by the TRANSPIRATION STREAM — water evaporating from leaves creates a pulling force that draws water up from the roots.
Phloem Tissue
Phloem is a tissue that transports dissolved SUGARS (mainly sucrose) from leaves to other parts of the plant. This process is called TRANSLOCATION.
Key features of phloem cells:
LIVING cells — unlike xylem, phloem cells are alive.
SIEVE TUBES — cells with perforated end walls (sieve plates) that allow sugar solution to flow through.
COMPANION CELLS — next to each sieve tube cell, providing energy (ATP) for the active loading of sugars into the phloem.
Sugars travel in BOTH DIRECTIONS in phloem — from leaves (source) to roots, growing tips, fruits and other areas where sugar is needed (sinks).
⚠️ Common Mistake
Xylem transports WATER and MINERALS. Phloem transports SUGARS. Students regularly mix these up. A good memory trick: Xylem = water (X for H2O is a stretch, but think: Xylem goes up like water). Phloem = food (sugars). Also: xylem cells are DEAD; phloem cells are LIVING.
📌 Key Note
Xylem: water + minerals, upwards only, dead cells, lignified walls. Phloem: sugars (translocation), both directions, living cells. Stomata: CO₂ in, O₂ and water vapour out. Controlled by guard cells.
🎯 Matching Activity — Match the Plant Tissue to its Function
Match each tissue to its role in the plant. — drag the symbols on the right to match the component names on the left.
Xylem
Drop here
Phloem
Drop here
Palisade mesophyll
Drop here
Spongy mesophyll
Drop here
Guard cells
Drop here
Waxy cuticle
Drop here
Transports water and mineral ions from roots to leaves — dead, hollow, lignified
Control opening and closing of stomata — regulate gas exchange and water loss
Waterproof covering on epidermis — reduces water loss from the leaf
Air spaces between cells — allows CO₂ and O₂ to diffuse easily
Packed with chloroplasts near leaf surface — main site of photosynthesis
Transports dissolved sugars from leaves to the rest of the plant — translocation
🎯 Test Yourself
Question 1 of 3
1. What is transported in xylem?
2. Why are xylem cells dead?
3. What is the role of stomata in leaves?
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