Plant tissues, organs and systemsOxford AQA IGCSE Biology: Revision notes

Like animals, plants are built from tissues — groups of cells with a similar structure and function. The specification identifies four plant tissues:

Each tissue is specialised for a different job, and several of them come together inside plant organs such as the leaf.

Plant organs include the stem, roots and leaves. Each is made of several plant tissues working together:

• The leaf contains epidermal tissue (top and bottom surfaces), palisade mesophyll and spongy mesophyll (the photosynthetic layers), and xylem and phloem (running through the veins)
• The stem contains epidermal tissue on the outside, and xylem and phloem running through it to connect roots to leaves
• The root absorbs water and mineral ions from the soil, and also contains xylem and phloem

The specification limits the internal structure you need to know to the leaf, and to the position of the xylem and phloem in a dicotyledonous primary root and primary stem.

The leaf is organised in layers, each suited to its job:

1. A transparent epidermal tissue layer on top lets light through without absorbing it
2. Palisade mesophyll sits just beneath the upper epidermis; its cells are packed with chloroplasts and arranged in a tall, tightly packed layer, so they are positioned to absorb as much light as possible for photosynthesis
3. Spongy mesophyll lies below the palisade layer; it has large air spaces between its cells, which allow carbon dioxide and oxygen to diffuse easily to and from the photosynthesising cells
4. Xylem and phloem, bundled together as veins, run through the leaf to bring water in and carry the products of photosynthesis away
5. A lower epidermal tissue layer, containing stomata, completes the leaf

Each layer's position matches its function: light-absorbing cells are placed where the light arrives first, and gas-exchange cells are positioned where diffusion pathways can be short.

In a dicotyledonous plant, xylem and phloem are arranged differently in the root compared with the stem:

This arrangement is consistent throughout the plant: the xylem transport pathway and the phloem transport pathway both run continuously from root to stem to leaf, even though their exact position within the organ changes.

Plant organisation follows the same hierarchy as animal organisation:

1. Cells — differentiate to become specialised, e.g. a palisade cell packed with chloroplasts
2. Tissues — groups of similar cells, e.g. palisade mesophyll
3. Organs — different tissues working together, e.g. the leaf
4. Organism — the whole plant, built from its organs

Unlike animals, the specification does not require plants to be organised into named organ systems for this sub-topic — the level of organisation you need is tissues and organs, built from cells, and understood in terms of size and scale.