Cell Structure and Function

Cytology Terms

Understanding the structure and function of cellular components is fundamental in Anatomy & Physiology. Each organelle plays a specific role in maintaining cellular homeostasis and supporting life processes.

• Plasma Membrane: The outer boundary of the cell, regulating the movement of substances in and out. Composed of a phospholipid bilayer with embedded proteins.

• Exocytic Transport Vesicle: Membrane-bound sacs that transport materials out of the cell via exocytosis.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: The "powerhouse" of the cell, responsible for producing ATP through cellular respiration.

• Ribosomes: Sites of protein synthesis; can be free in the cytoplasm or attached to the rough endoplasmic reticulum.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and detoxifies certain chemicals.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; involved in protein synthesis and processing.

• Chromatin: DNA and associated proteins found in the nucleus; condenses to form chromosomes during cell division.

• Nuclear Envelope: Double membrane surrounding the nucleus, containing nuclear pores for transport.

• Nuclear Pore: Openings in the nuclear envelope allowing exchange of materials between nucleus and cytoplasm.

• Nucleolus: Dense region within the nucleus where ribosomal RNA is synthesized.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Peroxisome: Organelle involved in lipid metabolism and detoxification of harmful substances.

• Endocytic Transport Vesicle: Vesicles that bring substances into the cell via endocytosis.

• Centrosome: Region containing centrioles; important for organizing microtubules during cell division.

• Centrioles: Cylindrical structures involved in cell division and formation of cilia/flagella.

• Cytosol: The fluid portion of the cytoplasm where organelles are suspended.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

Cell Cycle Phases

The cell cycle is the series of events that cells go through as they grow and divide. It consists of several distinct phases:

• Interphase: Period of cell growth and DNA replication; includes G1, S, and G2 phases.

• Prophase: Chromatin condenses into visible chromosomes; nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equatorial plate.

• Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

• Telophase/Cytokinesis: Chromosomes decondense, nuclear envelopes reform, and the cell divides into two daughter cells.

Example: During mitosis, epithelial cells in the skin rapidly divide to replace lost or damaged cells.

Epithelial Tissue

Overview of Epithelial Tissues

Epithelial tissues cover body surfaces, line cavities, and form glands. They serve protective, absorptive, secretory, and sensory functions. Classification is based on cell shape and number of layers.

• Simple Squamous Epithelium: Single layer of flat cells; found in alveoli of lungs and lining blood vessels. Function: Facilitates diffusion and filtration.

• Simple Cuboidal Epithelium: Single layer of cube-shaped cells; found in kidney tubules and glandular ducts. Function: Secretion and absorption.

• Simple Columnar Epithelium: Single layer of tall, column-like cells; lines digestive tract (stomach, intestines). Function: Absorption and secretion of mucus and enzymes.

• Pseudostratified Columnar Epithelium: Appears layered due to varying cell heights, but all cells touch the basement membrane; found in trachea and upper respiratory tract. Function: Secretion and movement of mucus by cilia.

• Stratified Squamous Epithelium: Multiple layers of flat cells; found in skin, mouth, and esophagus. Function: Protection against abrasion.

• Transitional Epithelium: Multiple layers with cells that change shape; found in urinary bladder. Function: Allows stretching and recoil.

Example: Stratified squamous epithelium forms the outer layer of skin, providing a barrier against pathogens and physical injury.

Epithelial Tissue Structures

Specialized structures within epithelial tissues contribute to their function and organization.

• Basement Membrane: Thin, fibrous layer anchoring epithelium to underlying connective tissue; provides support and regulates exchange.

• Goblet Cells: Unicellular glands found in columnar epithelium; secrete mucus for lubrication and protection.

• Microvilli: Finger-like projections on apical surface of epithelial cells; increase surface area for absorption.

• Cilia: Hair-like structures that move substances across the epithelial surface, especially in respiratory tract.

Example: Microvilli in the small intestine enhance nutrient absorption, while cilia in the trachea help clear mucus and debris.

Classification of Epithelial Tissues

Epithelial tissues are classified by cell shape and number of layers. The following table summarizes key features:

Key Equations

While cell biology and histology do not rely heavily on equations, understanding diffusion is important for epithelial tissue function:

• Fick's Law of Diffusion:

Where: = diffusion flux = diffusion coefficient = concentration gradient

Additional info: Expanded explanations and table added for clarity and completeness.