Plant Cell Specific Structures
In addition to the organelles shared with animal cells, plant cells have features that reflect their sessile, autotrophic lifestyle: the cell wall, the large permanent vacuole, plasmodesmata, and chloroplasts (covered in detail in the previous lesson). This lesson focuses on plant-specific structures and systematically compares plant cells with animal cells, as required by OCR 2.1.1 (e).
The Plant Cell Wall
The plant cell wall is a rigid layer found outside the plasma membrane. It is secreted by the cell itself, principally by the Golgi apparatus. The primary wall of a growing plant cell is largely composed of cellulose, a polymer of β-glucose, along with hemicelluloses, pectins, and some structural proteins. In tissues that have finished expanding, a secondary cell wall may be laid down inside the primary wall, often containing lignin, which provides additional rigidity and waterproofing.
Composition
- Cellulose microfibrils — bundles of up to 10,000 β-glucose polymer chains hydrogen-bonded together to form long, strong microfibrils about 10 nm wide. Microfibrils are laid down in crisscross patterns to resist mechanical stress in multiple directions.
- Hemicelluloses — shorter, branched polysaccharides that cross-link cellulose microfibrils into a mesh.
- Pectins — gel-like polysaccharides rich in galacturonic acid that lie between adjacent cells, especially in the middle lamella, binding cells together.
- Lignin (in secondary walls) — a complex phenolic polymer that impregnates cell walls in woody tissues, providing compressive strength and water resistance (e.g., in xylem vessels and sclerenchyma fibres).
- Glycoproteins — structural proteins such as extensin and arabinogalactan proteins.
Functions of the Cell Wall
- Structural support — provides mechanical strength, allowing plants to grow upright against gravity. In woody plants, lignified cell walls support very large structures.
- Prevents bursting (lysis) — when plant cells take up water by osmosis, the cell wall exerts an inward pressure that prevents the protoplast from expanding indefinitely. The turgor pressure inside a fully inflated cell keeps herbaceous plants upright.
- Full permeability — the cell wall is freely permeable to water and solutes (unlike the plasma membrane). Substances move through the wall via the apoplast pathway.
- Defence — provides a physical barrier against pathogens and herbivores. Some walls contain toxic or hard-to-digest compounds.
- Cell-to-cell adhesion — the middle lamella glues neighbouring cells together. Dissolving the middle lamella with enzymes (e.g., pectinase) separates cells for slide preparation.
- Water transport — in xylem, the lignified cell walls of vessels and tracheids form the pipework along which water moves up the plant.
The Middle Lamella
The middle lamella is a thin layer of calcium and magnesium pectates that cements adjacent plant cells together. It is the first layer deposited between daughter cells after cytokinesis and lies between the primary walls of neighbouring cells.
The Large Permanent Vacuole and Tonoplast
While animal cells have small, temporary vacuoles, plant cells have a single, large permanent vacuole that can occupy up to 90% of the cell volume in mature cells. The vacuole is bounded by a specialised single membrane called the tonoplast.
Structure
- Enclosed by the tonoplast — a selectively permeable phospholipid bilayer with its own transport proteins.
- Contains cell sap, a watery solution of ions, sugars, amino acids, organic acids, pigments, and sometimes toxins.
Functions of the Vacuole
- Maintains turgor pressure — by accumulating solutes, the vacuole draws in water by osmosis, pressing the protoplast against the cell wall. This turgor makes non-woody tissues rigid. Wilting occurs when turgor is lost.
- Stores substances — including sugars, amino acids, and mineral ions (used as reserves) and also waste products or toxic metabolites (isolated from the cytoplasm).
- Contains pigments — flower petals and fruits often owe their colour to anthocyanins in vacuolar sap.
- Contains hydrolytic enzymes — in some plant cells, vacuoles act analogously to lysosomes, breaking down damaged organelles or toxins.
- Isolates harmful substances — plants cannot excrete, so secondary metabolites such as alkaloids and tannins (many of which deter herbivores) are stored in vacuoles.
- pH regulation — the tonoplast pumps H⁺ into the vacuole, helping regulate cytoplasmic pH.
Exam Tip: A common question asks why a plant cell bursts in very hypotonic solutions only to a limited extent. Answer: the cell wall resists expansion, so although the cell becomes fully turgid, it does not burst as an animal cell would. The wall provides the pressure that balances the osmotic pull.
Plasmodesmata
Plant cells are connected to their neighbours by cytoplasmic channels called plasmodesmata (singular: plasmodesma). They pass through the cell wall at specific pit fields and link the cytoplasm of adjacent cells.
Structure