Cells and Tissues: Structure and Function in Human Anatomy & Physiology

Cells

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. In human anatomy and physiology, understanding cell structure and function is essential for comprehending how the body operates at the microscopic level.

• Definition: The smallest living units in the body.

• Functions: Obtain nutrients, make molecules needed for survival, dispose of wastes, maintain shape, and replicate.

Main Components of a Cell

• Plasma Membrane: The outer boundary of the cell, selectively permeable to regulate entry and exit of substances.

• Nucleus: Controls cellular activities and contains genetic material (DNA).

• Cytoplasm: Contains most organelles and the cytosol (fluid component).

Plasma Membrane

The plasma membrane separates the internal environment of the cell from the external environment and regulates the movement of substances.

• Structure: Composed of a phospholipid bilayer with embedded proteins.

• Functions: Selectively permeable, supports cell communication, and maintains homeostasis.

Types of Membrane Proteins

• Integral Proteins: Firmly embedded in or attached to the lipid bilayer; often function as channels or transporters.

• Peripheral Proteins: Loosely attached to the membrane surface; often function as enzymes or in cell signaling.

Nucleus

• Approximately 5 μm in diameter.

• Contains genetic material (DNA) organized as chromatin.

• Surrounded by a nuclear envelope with nuclear pores for material exchange.

• Contains nucleolus (site of ribosome synthesis).

Cytoplasm

• Region between the plasma membrane and nucleus.

• Includes cytosol (fluid), organelles, and inclusions (stored nutrients, secretory products).

Organelles

Organelles are specialized subunits within a cell that perform specific functions necessary for cell survival.

• Ribosomes: Sites of protein synthesis; composed of protein and rRNA.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

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

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

• Mitochondria: Double-membraned organelles; site of ATP (energy) production via cellular respiration.

• Peroxisomes: Contain enzymes for detoxification and breakdown of fatty acids.

• Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) that provide structural support and facilitate movement.

Membrane Transport

Cells regulate the movement of substances across the plasma membrane through various transport mechanisms.

• Passive Transport: Movement of substances down their concentration gradient without energy input (e.g., diffusion, osmosis, facilitated diffusion).

• Active Transport: Movement of substances against their concentration gradient using energy (ATP).

• Endocytosis: Process by which cells engulf external substances, forming vesicles.

• Exocytosis: Process by which cells expel materials in vesicles to the extracellular environment.

Cellular Diversity

Cells vary in size, shape, and function, reflecting their specialized roles in the body.

• Some cells move body parts (e.g., muscle cells), others cover and line organs (e.g., epithelial cells), or store nutrients (e.g., fat cells).

• Cell structure is closely related to function.

Tissues

Introduction to Tissues

Tissues are groups of similar cells that perform a common function. The study of tissues is called histology.

• Four Basic Types: Epithelial, connective, muscle, and nervous tissue.

Epithelial Tissue

• Functions: Covers body surfaces, lines cavities, forms glands, provides protection, absorption, secretion, and sensation.

• Characteristics: High cellularity, specialized contacts, polarity (apical and basal surfaces), supported by connective tissue, avascular but innervated, high regenerative capacity.

Classification of Epithelia

• By Number of Layers: Simple (one layer), stratified (multiple layers).

• By Cell Shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

Glands

• Exocrine Glands: Secrete products onto epithelial surfaces via ducts (e.g., sweat, salivary glands).

• Endocrine Glands: Ductless; secrete hormones directly into the bloodstream (e.g., thyroid, pituitary glands).

• Unicellular Exocrine Glands: Single cells (e.g., goblet cells) that produce mucus.

• Multicellular Exocrine Glands: Composed of multiple cells with distinct duct and secretory units.

Cell Junctions

• Tight Junctions: Prevent passage of molecules between adjacent cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between neighboring cells via channels.

Summary Table: Types of Epithelial Tissue

Type	Number of Layers	Cell Shape	Main Function	Example Location
Simple Squamous	1	Flat	Diffusion, filtration	Alveoli of lungs
Simple Cuboidal	1	Cube-shaped	Secretion, absorption	Kidney tubules
Simple Columnar	1	Tall, column-like	Absorption, secretion	Digestive tract lining
Stratified Squamous	Multiple	Flat	Protection	Skin epidermis
Stratified Cuboidal	Multiple	Cube-shaped	Protection	Large ducts of glands
Stratified Columnar	Multiple	Tall, column-like	Protection, secretion	Male urethra

Example: Goblet Cell

• Unicellular exocrine gland found in the lining of the intestines and respiratory tract.

• Secretes mucus to lubricate and protect epithelial surfaces.

Additional info:

• Some details, such as the specific mechanisms of membrane transport and the full classification of connective, muscle, and nervous tissues, are not included in the slides but are essential for a complete understanding of tissue types.