The leaf as an organ
The leaf is a plant organ made of several tissues, each
adapted for its job in photosynthesis and gas exchange
Tissue
Structure
Function
Upper epidermis thin, transparent, covered by a waxy cuticle
lets light through; cuticle reduces water loss
Palisade mesophyll tightly packed column-shaped cells near the top, many chloroplasts
main site of photosynthesis
Spongy mesophyll loosely packed cells with air spaces
gases diffuse through the air spaces to and from the cells
Xylem hollow dead tubes
bring water and mineral ions from the roots
Phloem living tubes
carry dissolved sugars away from the leaf
Lower epidermis contains stomata surrounded by guard cells
gas exchange; guard cells open and close the stomata to control water loss
Meristem at growing tips of shoots and roots
cells that keep dividing, then differentiate into any tissue
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A cross-section through a leaf X P gap waxy cuticle — waterproof, cuts down evaporation upper epidermis — thin and transparent, lets light through palisade mesophyll — tall cells packed with chloroplasts, at the top where the light is spongy mesophyll — air spaces let gases diffuse to and from the cells lower epidermis vascular bundle X = xylem, brings water up P = phloem, carries sugars away stoma between two guard cells — they open it in the light to let CO₂ in, and close it to save water CO₂ in O₂ and water vapour out Every layer is arranged around one job: get light and carbon dioxide to the chloroplasts, and lose as little water as possible doing it. A cross-section of a leaf with every layer labelled from the waxy cuticle and upper epidermis, through palisade and spongy mesophyll with a vascular bundle of xylem and phloem, to the lower epidermis with a stoma and guard cells
The photosynthesising tissue near the top is the palisade mesophyll ,
not the upper epidermis. And meristem is a tissue of dividing cells;
stem cells are what it contains, not another name for it
A tissue answer must be a tissue: naming "stomata" or "chloroplasts" for a
tissue scores nothing
The rest of this topic is in the full Exam Guide The complete AQA GCSE Biology guide — every topic, exam checklist and common mistake — is coming soon. Until then, the first topic in every unit is free to read.
In the full topic
• The leaf as an organ• Xylem and phloemRead the free biology topics →