Introduction
Outer Morphology of Eukaryotic Cells:
- Cell Membrane: This is the outermost layer that surrounds the cell, regulating the passage of substances into and out of the cell. It is made up of phospholipids, cholesterol, proteins, and oligosaccharide chains. The phospholipids form a bilayer, with their hydrophilic heads facing outwards and their hydrophobic tails facing inwards. Cholesterol molecules are embedded in the bilayer, helping to regulate its fluidity. Proteins are also embedded in the membrane, and they carry out a variety of functions, such as transporting molecules across the membrane, acting as receptors for signals, and providing structural support. The oligosaccharide chains are attached to some of the proteins and lipids, and they form a layer called the glycocalyx on the outer surface of the membrane. The glycocalyx plays a role in cell recognition and communication.
- Cell Wall (in plants and fungi): A rigid outer layer that provides structural support and protection. Animal cells lack a cell wall.
- Cilia and Flagella: Hair-like projections (cilia) or whip-like tails (flagella) that protrude from the cell surface, aiding in movement. Cilia are shorter and more numerous than flagella. Both cilia and flagella are made up of microtubules arranged in a 9 + 2 pattern. Cilia move in a coordinated, back-and-forth motion to propel fluids or particles past the cell. Flagella move in a whip-like motion to propel the cell itself. The tail of a spermatozoon is a basic flagellum.
- Glycocalyx (in some cells): A layer of carbohydrates coating the cell surface, playing a role in cell recognition and communication.