Foundations: The Cell

Introduction to Cells

All living organisms are composed of cells, which are the basic structural and functional units of life. The study of cells, known as cytology, is fundamental to understanding anatomy and physiology.

• Cell Theory:

  • Cells are the basic unit of structure in all living things.

  • New cells are produced from pre-existing cells through cell division.

  • Cells perform all vital functions necessary for life.

• Cellular Diversity: The human body contains trillions of cells, comprising about 200 different types that vary in size, shape, and function.

• Types of Cells:

  • Somatic Cells: All body cells except reproductive cells.

  • Sex Cells (Germ Cells): Reproductive cells (sperm and egg).

• Genetic Material: Most cells contain the same DNA, located primarily in the nucleus and, to a lesser extent, in mitochondria (inherited maternally).

Microscopy in Cytology

To study cells and their structures, scientists use various types of microscopy:

• Light Microscopy (LM): Uses visible light to magnify cellular structures up to 2000x their natural size.

• Electron Microscopy (EM): Uses electron beams to visualize cell ultrastructure at magnifications up to 2 million times.

Basic Structure of a Cell

Most human cells share three main structural components:

• Plasmalemma (Plasma Membrane): The outer boundary of the cell, also called the cell membrane.

• Cytoplasm: The internal fluid and organelles between the membrane and nucleus.

• Nucleus: The control center containing most of the cell's DNA.

Cell Membrane (Plasma Membrane or Plasmalemma)

Structure of the Plasma Membrane

The plasma membrane is a selectively permeable barrier that separates the cell's internal environment from the extracellular fluid. Its structure is best described by the Fluid Mosaic Model:

• The membrane is a phospholipid bilayer with proteins embedded within it.

• A thin layer of extracellular fluid surrounds the cell.

• The membrane is dynamic, with components able to move laterally within the layer.

Components of the Plasma Membrane

• Phospholipids: Form the basic structure of the bilayer, with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Integral Proteins: Embedded within the phospholipid bilayer; may span the membrane (transmembrane proteins) and are involved in transport and signaling.

• Peripheral Proteins: Attached to the membrane surface; can be removed without disrupting the bilayer and often function in signaling or as enzymes.

• Channels: Protein structures that allow water and ions to move across the membrane.

• Gated Channels: Specialized channels that can open or close in response to specific stimuli, regulating the passage of substances.

Example: Selective Permeability

The plasma membrane allows certain molecules (such as oxygen, carbon dioxide, and small nonpolar substances) to pass freely, while restricting others (such as ions and large polar molecules). This selective permeability is essential for maintaining cellular homeostasis.

Additional info:

• Functions of the Plasma Membrane: In addition to acting as a barrier, the membrane is involved in communication, cell recognition, and anchoring the cytoskeleton.

• Membrane Proteins: Can function as receptors, enzymes, transporters, or anchors.