🫧 Cell Membrane, Cell Wall & Cytoplasm

Cell Membrane (Plasma Membrane)

The cell membrane (also called the plasma membrane) is the outermost living boundary of every cell. It separates the internal environment of the cell from the external surroundings and controls what enters and leaves the cell.

• Composition: Lipids (20–40%), Proteins (60–70%), Carbohydrates (1–5%). The high protein content reflects the many functional roles the membrane performs, from transport to signaling.

• Fluid Mosaic Model — proposed by Singer and Nicolson (1972). This is the currently accepted model of membrane structure.

  • The membrane is a fluid bilayer of phospholipids with embedded proteins. The term "fluid" refers to the lateral movement of molecules within the plane of the membrane, and "mosaic" refers to the patchwork of different proteins scattered throughout.
  • Phospholipids are amphipathic — they have a hydrophilic head (polar, water-loving) and a hydrophobic tail (non-polar, water-fearing). This dual nature is what drives the spontaneous formation of the bilayer in an aqueous environment.
  • Integral proteins (intrinsic) — penetrate the lipid bilayer partially or completely (transmembrane proteins). These are firmly embedded and often serve as channels, carriers, or receptors.
  • Peripheral proteins (extrinsic) — loosely attached to the membrane surface. They can be easily detached and often function as enzymes or structural supports.
  • Glycoproteins and glycolipids — carbohydrates attached to proteins/lipids on the outer surface; involved in cell recognition, immune response, and cell-to-cell communication. Together, they form the glycocalyx.

Functions of Cell Membrane

1. Selectively permeable barrier — allows only certain molecules to pass through, maintaining the internal environment of the cell.
2. Cell recognition and signaling — surface receptors detect chemical signals from other cells and the environment.
3. Transport of molecules (passive and active) — passive transport requires no energy (diffusion, osmosis), while active transport uses ATP to move substances against their concentration gradient.
4. Endocytosis and exocytosis — bulk transport mechanisms for large molecules. Endocytosis brings materials in; exocytosis pushes them out.
5. Maintains cell shape and integrity — the membrane provides a flexible boundary that defines the cell.

Cell Wall

The cell wall is a rigid, non-living outer covering found outside the cell membrane in certain organisms. It provides structural support and protection.