Plant Tissues and Organ Systems

Plants, like animals, are organised into cells, tissues, organs and organ systems. For AQA GCSE Biology, you need to understand the key plant tissues (epidermal, palisade mesophyll, spongy mesophyll, xylem, phloem and meristem), how they combine to form plant organs (roots, stems and leaves), and how the leaf in particular is adapted for photosynthesis. This lesson covers all of these areas in detail.

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Plant Organs

The three main organs in a plant are the root, the stem and the leaf. Each has a specific role:

Organ	Main Functions
Root	Absorbs water and mineral ions from the soil; anchors the plant in the ground; stores food (e.g. in carrots, potatoes)
Stem	Supports the plant and holds leaves up towards the light; transports water, minerals and sugars between roots and leaves via xylem and phloem
Leaf	Main site of photosynthesis; also the site of gas exchange and transpiration (water loss)

Other plant organs include flowers (reproduction) and fruits (seed dispersal).

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Plant Tissues

Plants have several specialised tissues. You need to know the following:

Epidermal Tissue

Epidermal tissue covers the outer surfaces of the plant. It is usually one cell thick and provides protection. The upper and lower surfaces of a leaf are covered by epidermal tissue. The epidermis of the leaf often has a waxy layer called the cuticle, which reduces water loss by evaporation.

Palisade Mesophyll Tissue

Palisade mesophyll is located near the upper surface of the leaf. The cells are:

• Tall and column-shaped, packed closely together
• Packed with chloroplasts (containing chlorophyll) — the green pigment that absorbs light energy for photosynthesis
• Positioned near the top of the leaf to absorb the maximum amount of light

This tissue is the main site of photosynthesis in the plant.

Spongy Mesophyll Tissue

Spongy mesophyll is located below the palisade mesophyll. The cells are:

• Loosely packed with large air spaces between them
• These air spaces allow gases (carbon dioxide and oxygen) to diffuse easily through the leaf
• Also contain chloroplasts, so some photosynthesis occurs here too

Xylem Tissue

Xylem is a transport tissue that carries water and dissolved mineral ions from the roots to the leaves and other parts of the plant. Key features:

Feature	Detail
Dead cells	The cells die and become hollow tubes
No end walls	This creates a continuous column of water
Lignified walls	The cell walls are strengthened with lignin, a waterproof substance that provides structural support
One-way transport	Water moves upward only (from roots to leaves)

Phloem Tissue

Phloem is a transport tissue that carries dissolved sugars (mainly sucrose) and other organic compounds from the leaves (where they are made by photosynthesis) to the rest of the plant. This process is called translocation. Key features:

Feature	Detail
Living cells	Unlike xylem, phloem cells are alive
Sieve tube elements	Elongated cells with perforated end walls called sieve plates, allowing dissolved sugars to flow through
Companion cells	Adjacent cells that provide energy (from respiration) to drive the movement of sugars through the sieve tubes
Two-way transport	Sugars can move both upward and downward (from source to sink)

graph TD
    A[Plant Transport Tissues] --> B[Xylem]
    A --> C[Phloem]
    B --> B1[Transports water and minerals]
    B --> B2[Dead, hollow tubes with lignified walls]
    B --> B3[One direction: roots to leaves — upward]
    C --> C1[Transports dissolved sugars]
    C --> C2[Living cells with sieve plates and companion cells]
    C --> C3[Both directions: source to sink — translocation]

Meristem Tissue

Meristem tissue is found at the growing points of a plant — the tips of roots and shoots (apical meristems) and in some species along the stem (lateral meristems). Meristem cells are undifferentiated (unspecialised) and can divide rapidly by mitosis to produce new cells that then differentiate into specialised cell types.

Exam Tip: Meristem tissue in plants is similar to stem cells in animals — both are undifferentiated and can develop into different cell types. Plants retain meristems throughout their lives, which is why plants can continue growing and regenerating unlike most animals.

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The Leaf as a Plant Organ

The leaf is an excellent example of how tissues combine to form an organ. It contains several different tissues, each contributing to the leaf's function as the main site of photosynthesis.

Structure of a Leaf — Cross Section

From top to bottom, a leaf cross-section shows:

Layer	Tissue Type	Function
Upper epidermis	Epidermal tissue	Transparent (allows light through); covered in a waxy cuticle to reduce water loss
Palisade mesophyll	Palisade mesophyll tissue	Main site of photosynthesis; cells packed with chloroplasts near the top of the leaf to absorb maximum light
Spongy mesophyll	Spongy mesophyll tissue	Some photosynthesis; air spaces allow gases to diffuse easily to and from the palisade layer and stomata
Lower epidermis	Epidermal tissue	Contains stomata (pores) and guard cells that control gas exchange and water loss
Vascular bundle	Xylem and phloem	Xylem brings water to the leaf; phloem takes away the sugars produced by photosynthesis

Stomata and Guard Cells

Stomata (singular: stoma) are tiny pores found mainly on the lower epidermis of the leaf. Each stoma is surrounded by a pair of guard cells.

Feature	Function
Open stomata	Allow carbon dioxide to diffuse in (for photosynthesis) and oxygen to diffuse out; also allow water vapour to escape (transpiration)
Closed stomata	Reduce water loss when the plant is at risk of dehydration (e.g. at night, in drought)
Guard cells	Control the opening and closing of stomata; when turgid (full of water), they swell and the stoma opens; when flaccid (low water), they become limp and the stoma closes

Exam Tip: Guard cells open stomata when they become turgid (absorb water by osmosis and swell). They close stomata when they become flaccid (lose water). This is a frequently tested mechanism. Most stomata are on the lower surface to reduce water loss from direct sunlight.

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Adaptations of the Leaf for Photosynthesis

Adaptation	How It Helps Photosynthesis
Broad, flat shape	Provides a large surface area to absorb light and carbon dioxide
Thin	Short diffusion distance for gases (CO2 in, O2 out)
Transparent upper epidermis	Allows light to pass through to the palisade layer
Palisade layer near the top	Receives the most light; cells are packed with chloroplasts
Air spaces in spongy mesophyll	Allow carbon dioxide to reach all photosynthesising cells by diffusion
Stomata on lower epidermis	Allow CO2 in and O2 out; being on the lower surface reduces water loss
Network of veins (vascular bundles)	Xylem supplies water for photosynthesis; phloem carries sugars away
Waxy cuticle	Reduces water loss from the upper surface without blocking light

Exam Tip: You may be shown a cross-section diagram of a leaf and asked to label it or explain how each layer is adapted for photosynthesis. Practise labelling and learn the function of each layer.

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Plant Organ Systems

Plant organs are organised into two main organ systems: