Cell Wall
The cell wall is a rigid or semi-rigid layer located outside the plasma membrane in plant cells, fungi, bacteria, and some algae, providing structural support and protection against mechanical stress and osmotic lysis. In plants, the primary cell wall is composed mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin, while the secondary cell wall may also contain lignin for added rigidity. The cell wall is absent in animal cells, which instead rely on the extracellular matrix for structural support.
Cell Wall Composition Across Different Organisms
| Organism | Main Component | Additional Components | Rigidity | Thickness |
|---|---|---|---|---|
| Plant (primary) | Cellulose | Hemicellulose, pectin | Flexible | 0.1–1 µm |
| Plant (secondary) | Cellulose | Lignin, suberin | Rigid | 1–10 µm |
| Fungi | Chitin | Glucans, mannoproteins | Semi-rigid | 100–200 nm |
| Gram-positive bacteria | Peptidoglycan | Teichoic acids | Thick, rigid | 20–80 nm |
| Gram-negative bacteria | Peptidoglycan | Outer membrane (LPS) | Thin | 5–10 nm |
| Algae | Cellulose / Agar | Varies by species | Variable | Variable |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Osmosis (Biology)
Osmosis is the passive movement of water molecules across a selectively permeable membrane from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration). This net movement continues until equilibrium is reached or until an opposing pressure prevents further flow. Osmosis is fundamental to cell function, driving turgor pressure in plant cells, reabsorption of water in kidney tubules, and the movement of water into roots from soil.
Vacuole
A vacuole is a membrane-bound organelle found in the cytoplasm of cells that stores water, nutrients, waste products, and other materials. In plant cells, the large central vacuole can occupy up to 90% of the cell's volume and plays a critical role in maintaining turgor pressure. Vacuoles help regulate cell volume, pH balance, and the disposal of cellular waste through a process called autophagy.
Diffusion (Biology)
Diffusion is the passive net movement of molecules or ions from a region of higher concentration to a region of lower concentration, driven by the concentration gradient until equilibrium is reached. In biology, simple diffusion across cell membranes occurs for small, nonpolar molecules such as oxygen, carbon dioxide, and ethanol, which can pass directly through the phospholipid bilayer. Facilitated diffusion involves channel or carrier proteins to transport polar or charged molecules down their concentration gradient without requiring energy.
From Old English "cell" (Latin "cella", small room) and "wall" (Latin "vallum", rampart). Robert Hooke first described cells by observing cork cell walls in 1665, publishing his findings in "Micrographia".