The Cell and Epithelial Tissues: Structure, Function, and Identification

The Cell: Structure and Function

Overview of the Composite Cell

The cell is the fundamental structural and functional unit of the human body. Cells vary in size, shape, and function, but share many common features. Understanding the structure and function of cellular components is essential for studying tissues and organs.

• Cell Model and Cell Structure: Laboratory models and posters are used to identify organelles and cell parts. Key organelles include the nucleus, nucleolus, nuclear membrane, plasma membrane, cytoplasm, endoplasmic reticulum, ribosomes, Golgi complex, lysosomes, mitochondria, centrioles, cytoskeleton, and various inclusions.

• Organelle Identification: Students should be able to identify organelles on both models and cell structure posters, as summarized in the table below.

Organelle or Part of the Cell	Cell Model	Cell Structure
Nucleus	x	x
Nucleolus	x	x
Nuclear membrane	x	x
Plasma membrane (cell membrane)	x	x
Cytoplasm (cytosol)	x	x
Rough endoplasmic reticulum	x	x
Smooth endoplasmic reticulum	x	x
Golgi complex	x	x
Lysosome	x	x
Mitochondrion	x	x
Centrioles	x	x
Cytoskeleton (microfilaments & microtubules)		x

Cell Organelles: Structure and Function

Each organelle has a specific structure and function, contributing to the overall activity of the cell.

Organelle or Part of the Cell	Structure	Function(s)
Nucleus	Spherical/ovoid organelle; surrounded by a nuclear membrane; contains nucleoplasm, nucleolus, and chromatin; may have one or more nucleoli	Acts as the cell's control center; houses DNA; provides "directions" for protein synthesis
Nucleolus	Small, roundish body within a nucleus; consists of proteins and RNA	Acts in the synthesis of ribosomes
Plasma membrane (Cell membrane)	Phospholipid bilayer; integral proteins; peripheral proteins; cholesterol; lipid rafts; glycolipids; glycoproteins	Provides a physical barrier between the internal environment of the cell and the surrounding environment; controls entry and exit of substances
Cytoplasm	Cellular region between the plasma membrane and nucleus; consists of cytosol (a thick, semitransparent semi-elastic fluid), dissolved solutes, organelles, and inclusions	Provides the site where many cellular functions occur (cytosol – suspends organelles and molecules; organelles – perform specific cellular metabolic activities)
Ribosome	Composed of ribosomal RNA and proteins; has 2 subunits; globular size; not membrane bound; free or attached to organelles	Site of protein synthesis
Endoplasmic reticulum (ER)	Membranous network of tubular channels; has parallel membranes; may or may not have ribosomes attached	Rough ER: Has ribosomes attached; involved in protein synthesis and modification Smooth ER: No ribosomes attached; involved in lipid synthesis and detoxification
Golgi complex (apparatus)	Stacks of flattened membranous sacs with expanded vesicles near or at their ends	Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles
Lysosome	Membrane-bound sphere; contains digestive enzymes (e.g., acid hydrolases)	Breaks down waste materials and cellular debris
Peroxisome	Small membrane-bound spheres; contains oxidative enzymes and catalase	Breaks down fatty acids and detoxifies harmful substances
Mitochondrion	Double-membrane bound spherical or rod-like organelle; smooth outer membrane with folded inner membrane (cristae)	Site of ATP (energy) production via cellular respiration
Centrioles	Paired cylindrical structures, each composed of nine triplets of microtubules; not membrane bound; located in the centrosome	Organizes microtubules during cell division
Microfilament	Small thread-like structures; not membrane bound; part of cytoskeleton; actin and myosin	Provides structural support and aids in cell movement
Microtubule	Straight, slender cylinders; not membrane bound; part of cytoskeleton; tubulin	Maintains cell shape, enables intracellular transport, and forms mitotic spindle
Flagellum	Long cytoplasmic projection containing microtubules (9 doublets + 2 in center arrangement); covered by plasma membrane	Propels a cell (e.g., sperm)
Cilium	Short cytoplasmic projection containing microtubules (9 doublets + 2 in center arrangement); covered by plasma membrane	Moves substances over the cell surface
Microvillus	Small, finger-like cytoplasmic projection; has a core of microfilaments; covered by plasma membrane	Increases the surface area of the plasma membrane
Inclusion	Chemical substance contained in the cell; usually produced by the cell; variable appearance; stores substances/materials (e.g., lipids, glycogen granules)	Acts as a storage site (lipids: source of energy; pigments: provide tissue color; glycogen: source of energy; UV light protection)

Key Terms and Definitions

• Organelle: Specialized subunit within a cell that has a specific function.

• Cytoskeleton: Network of protein filaments and tubules that provide structural support and shape to the cell.

• Inclusion: Non-living material within the cytoplasm, such as lipid droplets or pigment granules.

Examples and Applications

• Example: Mitochondria are abundant in muscle cells due to their high energy requirements.

• Application: Lysosomal storage diseases result from malfunctioning lysosomes, leading to accumulation of undigested substances.

Differences and Similarities in Cell Structure

Cellular Diversity

Cells vary in size, shape, and the presence of specific organelles. These differences are related to the specialized functions of each cell type.

• Plasma Membrane: All mature cells have a plasma membrane, which is essential for maintaining the internal environment.

• Nucleus: Most mature cells have a nucleus, but some (e.g., red blood cells) do not.

• Cytoplasm: The amount of cytoplasm varies; some cells have abundant cytoplasm (e.g., muscle cells), while others have little (e.g., mature red blood cells).

• Organelles: The presence and abundance of organelles depend on the cell's function (e.g., secretory cells have prominent Golgi complexes).

• Lipid Droplets: Lipid inclusions appear as clear areas in the cytoplasm and are common in adipocytes (fat cells).

• Cell Shape: Cells can be flat, cuboidal, columnar, or irregular, depending on their function and location.

Tissues: Classification and Epithelial Tissue Types

Overview of Tissue Types

A tissue is a group of similar cells and their extracellular substance united to perform a specialized function. The four basic tissue types are:

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nervous tissue

This section focuses on epithelial tissue, its classification, and identification under the microscope.

Epithelial Tissue: Structure and Classification

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is classified based on the number of cell layers and the shape of the cells at the free surface.

• Simple epithelium: Single layer of cells

• Stratified epithelium: Multiple layers of cells

• Cell shapes: Squamous (flat), cuboidal (cube-shaped), columnar (tall and column-like)

Types of Epithelial Tissue

Type	Description	Example Location
Simple squamous epithelium	Single layer of flat cells; all cells touch the basal lamina and the free surface	Alveoli of lungs, lining of blood vessels
Simple cuboidal epithelium	Single layer of cube-shaped cells; all cells touch the basal lamina and the free surface	Kidney tubules, glands
Simple columnar epithelium	Single layer of tall, column-like cells; all cells touch the basal lamina and the free surface	Lining of digestive tract
Pseudostratified columnar epithelium (with cilia)	Single layer of cells of varying heights; all cells touch the basal lamina, but not all reach the free surface; often ciliated	Lining of trachea
Stratified squamous epithelium	Many layers of cells; cells at the free surface are flat	Epidermis of skin, lining of mouth
Stratified cuboidal epithelium	Many layers of cube-shaped cells; cells at the free surface are as tall as they are wide	Ducts of sweat glands
Stratified columnar epithelium	Many layers of columnar cells; cells at the free surface are tall and column-like	Male urethra, some glandular ducts
Stratified transitional epithelium	Many layers; cells at the free surface change shape (relaxed: dome-shaped; stretched: flat)	Urinary bladder
Glandular epithelium	Specialized for secretion; cells are closely packed and similar in structure	Glands (e.g., salivary glands, pancreas)

Key Points for Epithelial Tissue Identification

• Identify the number of cell layers (simple vs. stratified)

• Determine the shape of the cells at the free surface

• Locate the basal lamina (basement membrane)

• Note the presence of specializations (e.g., cilia, microvilli)

Additional info:

• Students are expected to draw and label epithelial tissues as seen under the microscope, including the specific epithelium, specific cell, and location of the basal lamina.

• Understanding the structure-function relationship in epithelial tissues is essential for recognizing their roles in protection, absorption, secretion, and filtration.