Plant Tissues and Organs

Plants, like animals, are organised into cells, tissues, organs and organ systems. This lesson covers the structure and function of plant tissues, the leaf as a plant organ, and the plant organ system, as required by the AQA GCSE Combined Science Trilogy specification (8464).

---

Plant Organ System

The main plant organ system is responsible for transporting substances around the plant. It consists of three main organs:

Plant Organ	Function
Roots	Absorb water and mineral ions from the soil; anchor the plant
Stem	Supports the plant and transports water, minerals and sugars between roots and leaves
Leaves	Main site of photosynthesis; gas exchange

These organs work together as a transport system:

graph TD
    R["Roots"] -->|"Water and minerals absorbed"| S["Stem"]
    S -->|"Water and minerals transported upward (via xylem)"| L["Leaves"]
    L -->|"Sugars produced by photosynthesis"| S2["Stem"]
    S2 -->|"Sugars transported to all parts (via phloem)"| R2["Roots, flowers, fruits, storage organs"]

---

Plant Tissues

Plants contain several specialised tissue types:

Epidermal Tissue

• A single layer of cells that covers the entire outer surface of the plant
• Function: protection — acts as a barrier against pathogens, water loss and physical damage
• On leaves, the upper epidermis is often covered with a waxy cuticle to reduce water loss
• The lower epidermis contains stomata (pores) for gas exchange, each surrounded by a pair of guard cells

Palisade Mesophyll Tissue

• Found near the upper surface of the leaf
• Cells are tall, column-shaped and packed closely together
• Contain many chloroplasts — this is the main site of photosynthesis
• Positioned near the top of the leaf to be close to the light source

Spongy Mesophyll Tissue

• Found in the lower half of the leaf, below the palisade mesophyll
• Cells are loosely packed with large air spaces between them
• Function: the air spaces allow gases (CO₂ and O₂) to diffuse easily through the leaf and reach the palisade cells
• Also contains chloroplasts and carries out some photosynthesis

Xylem Tissue

• Made of dead, hollow cells joined end to end with no end walls, forming continuous tubes
• Walls are strengthened with lignin (a waterproof, rigid substance)
• Function: transports water and dissolved mineral ions from the roots to the leaves in a one-way flow
• Also provides structural support to the plant

Phloem Tissue

• Made of living cells called sieve tube elements, joined end to end with sieve plates (perforated end walls)
• Each sieve tube element is accompanied by a companion cell, which provides metabolic energy for transport
• Function: transports dissolved sugars (mainly sucrose) and amino acids from the leaves (where they are produced by photosynthesis) to the rest of the plant — this process is called translocation
• Transport in phloem is bidirectional (up and down)

Meristem Tissue

• Found at the growing tips of roots and shoots (root tip, shoot tip)
• Contains undifferentiated cells that are capable of rapid cell division by mitosis
• These cells can differentiate into any type of plant cell
• Responsible for growth in the plant throughout its life

Exam Tip: Unlike most animal cells, plant cells retain the ability to differentiate throughout the plant's life, thanks to meristem tissue. This is why a cutting from a plant can grow into a whole new plant.

---

The Leaf as a Plant Organ

The leaf is the main organ of photosynthesis. It contains all the tissue types described above, working together.

graph TD
    LEAF["Leaf (Organ)"] --> UE["Upper epidermis + waxy cuticle"]
    LEAF --> PM["Palisade mesophyll"]
    LEAF --> SM["Spongy mesophyll"]
    LEAF --> LE["Lower epidermis + stomata + guard cells"]
    LEAF --> XV["Xylem vessels (in veins)"]
    LEAF --> PH["Phloem tubes (in veins)"]
    UE --> UE1["Transparent — lets light through; waxy cuticle reduces water loss"]
    PM --> PM1["Main site of photosynthesis — packed with chloroplasts"]
    SM --> SM1["Air spaces allow gas diffusion; some photosynthesis"]
    LE --> LE1["Stomata allow gas exchange; guard cells control opening/closing"]
    XV --> XV1["Brings water and minerals to leaf cells"]
    PH --> PH1["Carries sugars away from the leaf"]

Adaptations of Leaves for Photosynthesis

Adaptation	How It Helps Photosynthesis
Broad and flat shape	Large surface area to absorb maximum light
Thin	Short distance for gases to diffuse and light to penetrate
Transparent upper epidermis	Allows light to pass through to the palisade mesophyll
Waxy cuticle	Reduces water loss from the upper surface
Palisade mesophyll near the top	Cells with most chloroplasts are closest to the light
Spongy mesophyll with air spaces	Allows efficient diffusion of CO₂ to photosynthesising cells
Stomata on the lower surface	Allow CO₂ in and O₂ out; positioned on the lower surface to reduce water loss
Network of veins (vascular bundles)	Xylem brings water; phloem removes sugars

---

Guard Cells and Stomata

Stomata (singular: stoma) are small pores on the surface of leaves (mainly the lower epidermis). Each stoma is surrounded by a pair of guard cells that control its opening and closing.

Condition	Guard Cell State	Stoma	Effect
Daytime / light	Guard cells absorb water by osmosis, become turgid and swell, curving apart	Open	CO₂ enters for photosynthesis; O₂ exits; water vapour exits (transpiration)
Night-time / dark	Guard cells lose water, become flaccid and straighten	Closed	Reduces water loss; gas exchange slows
Very dry / hot conditions	Guard cells lose water, become flaccid	Closed	Conserves water to prevent wilting

Exam Tip: Guard cells have an unevenly thickened cell wall — the inner wall (facing the stoma) is thicker than the outer wall. When the guard cell swells with water, the thinner outer wall stretches more, causing the cell to curve and open the stoma. This is a popular exam question.

---

Comparison: Xylem and Phloem

Feature	Xylem	Phloem
Cell type	Dead, hollow tubes	Living sieve tube elements + companion cells
End walls	No end walls (open tubes)	Sieve plates (perforated end walls)
Wall structure	Strengthened with lignin	Thin cellulose walls
What is transported	Water and dissolved mineral ions	Dissolved sugars (sucrose) and amino acids
Direction of flow	One-way: roots → leaves (upward)	Bidirectional: source → sink (up or down)
Mechanism	Transpiration pull (passive)	Translocation (requires energy — active)
Additional function	Structural support	None

---

Common Mistakes