## Cell Wall The cell wall is a rigid, extracellular structure found outside the plasma membrane. It is not present in animal cells, likely due to their locomotor lifestyle. - **Presence:** Plant cells, prokaryotes, fungi, and some protists. - **Functions:** - Protects the cell. - Maintains the cell's shape. - Prevents excessive uptake of water (prevents lysis). - Provides structural support to the entire plant against gravity. - **Permeability:** The cell wall is porous and fully permeable, allowing free passage of water and dissolved substances. ### Chemical Composition The chemical makeup of the cell wall varies between different types of organisms. | Organism | Main Component | | :---------- | :---------------| | Plants | Cellulose | | Fungi | Chitin | | Prokaryotes | Peptidoglycan | ### Structure of the Plant Cell Wall The plant cell wall is secreted by the cell and consists of three main layers. 1. **Primary Cell Wall** - A true wall that develops in newly growing cells. - **Location:** Inner to the middle lamella. - **Properties:** Thin, flexible, and adapted for growth (stretches irreversibly). - **Composition:** Composed of cellulose microfibrils arranged in a crisscross pattern within a matrix of other polysaccharides like *hemicelluloses* and *pectin*. This arrangement provides great strength. 2. **Secondary Cell Wall** - Forms only after a cell has stopped growing. - **Location:** Between the primary cell wall and the plasma membrane. - **Found in:** Specialized cells like sclerenchyma, which are often dead at maturity. - **Properties:** Very thick and rigid; it does not allow for further growth. - **Composition:** Consists of cellulose, hemicelluloses, *lignin*, inorganic salts, and waxes. Lignin is a key component that cements cellulose fibers together, providing rigidity. - **Function:** Provides definite shape and strong mechanical support. 3. **Middle Lamella** - **Location:** A layer present between the primary walls of adjacent cells. - **Function:** Acts as a cement, holding the cells together to form tissues. - **Composition:** A sticky, gel-like substance made of pectin, calcium, and magnesium salts. ## Plasma Membrane Also called the *cell membrane*, it is the living boundary of the protoplasm and is found in all prokaryotic and eukaryotic cells. - **Function:** It is *selectively permeable*, controlling the passage of materials into and out of the cell to maintain a stable internal environment (homeostasis). ### Composition The plasma membrane is chemically composed of: - **Proteins:** 60-80% - **Lipids:** 20-40% (mainly phospholipids and cholesterol) - **Carbohydrates:** Small quantity (found on the outer surface) ### Structure: The Fluid Mosaic Model This model describes the plasma membrane as a dynamic and fluid structure. - **Phospholipid Bilayer:** The core of the membrane is a double layer of phospholipids. - *Hydrophilic (water-loving) heads* face the watery environments inside and outside the cell. - *Hydrophobic (water-fearing) tails* face inward, away from water. - **Proteins:** Protein molecules are embedded either partially or wholly within the bilayer, like "icebergs floating in a sea." These proteins can drift sideways. - **Cholesterol:** These steroid molecules are wedged between phospholipids, helping to stabilize the membrane and regulate its fluidity. - **Carbohydrates:** Chains of carbohydrates are attached to proteins (*glycoproteins*) or lipids (*glycolipids*) on the outer surface. They act as cell surface markers. - **Cytoskeleton:** Filaments of the cytoskeleton are attached to the inner surface, providing support. ### Functions of Membrane Components - **Proteins:** Determine most of the membrane's functions. - *Transport:* Act as channels or carriers to move substances across the membrane. - *Enzymatic Activity:* Catalyze metabolic reactions on the membrane surface. - *Signal Transduction (Receptors):* Bind to specific molecules (like hormones) to transmit signals into the cell. - *Cell Recognition (Antigens):* Allow cells to recognize each other. - **Glycoproteins and Glycolipids:** - Act as *cell surface markers* for cell-to-cell recognition (Glycocalyx). - Help cells stick together to form tissues. - **Lipids (Phospholipids and Cholesterol):** - Control the fluidity of the membrane. - Restrict the passage of polar molecules and ions.