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

Part I: Cells

Introduction to Cells

Cells are the fundamental building blocks of all living organisms. They vary in size, shape, and function, and are primarily composed of four elements: carbon, hydrogen, oxygen, and nitrogen. Approximately 60% of a cell's mass is water, and cells are surrounded by interstitial fluid, a dilute saltwater solution.

• Cell size: Ranges from microscopic (1/12,000th of an inch) to several feet.

• Cell shapes: Disk-shaped, cubelike, elongated, etc.

General Cell Anatomy

Most cells have three main regions:

• Nucleus – Control center, contains genetic material (DNA).

• Cytoplasm – Semifluid, surrounds nucleus.

• Plasma membrane – Surrounds cytoplasm, separates cell from environment.

Nucleus

• Nuclear membrane (envelope): Double membrane with fluid-filled space; contains nuclear pores for exchange of material.

• Nucleolus: Dense region; produces ribosomes.

• Chromatin: Threads of DNA; condenses to form chromosomes during cell division.

Plasma Membrane (Cell Membrane)

• Function: Separates cell contents from the environment; regulates entry/exit of substances.

• Structure: Composed of phospholipids, cholesterol, proteins; forms a bilayer.

Component	Role
Phospholipids	Form bilayer; polar heads (hydrophilic) face water, nonpolar tails (hydrophobic) avoid water.
Cholesterol	Stabilizes membrane.
Proteins	Specialized functions (transport, enzymes, receptors).

Membrane Specializations

• Sugars: Attach to proteins (glycoproteins); act as receptors, blood type markers.

• Microvilli: Fingerlike projections; increase surface area for absorption.

• Membrane Junctions:

  • Tight junctions: Bind cells into sheets; prevent leakage.

  • Desmosomes: Anchor cells together; resist pulling forces.

  • Gap junctions: Allow molecules to pass between cells; communication.

Cytoplasm

• Cytosol: Fluid; mostly water, contains nutrients and solutes.

• Organelles: Specialized structures with specific functions.

• Inclusions: Non-functioning units; stored nutrients or cell products.

Organelles

• Mitochondria: "Powerhouse"; site of ATP production. Double membrane; inner membrane has enzymes for respiration.

• Ribosomes: Made of RNA and protein; site of protein synthesis.

• Endoplasmic Reticulum (ER): Tubular network; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids and detoxifies drugs.

• Golgi Apparatus: Stack of sacs; modifies, packages, and ships proteins.

• Lysosomes: Contain digestive enzymes; break down waste.

• Peroxisomes: Use oxidase enzymes to detoxify harmful substances.

• Cytoskeleton: Protein framework; supports cell shape and organelles.

• Centrioles: Paired structures; organize cell division.

• Cilia: Hairlike projections; move substances along cell surface.

Part II: Body Tissues

Introduction to Tissues

Tissues are groups of cells with similar structure and function, organized into organs. There are four main types: epithelial, connective, nervous, and muscle tissue.

• Epithelial tissue: Covers and lines surfaces; forms glands.

• Connective tissue: Supports, binds, and protects.

• Nervous tissue: Controls and communicates.

• Muscle tissue: Produces movement.

Epithelial Tissue

• Functions: Protection, absorption, filtration, secretion.

• Characteristics:

  • Cells fit closely together; form continuous sheets.

  • Avascular (no blood supply); rely on diffusion.

  • Regenerate easily.

  • Apical surface: Exposed to body exterior or cavity.

  • Basement membrane: Lower surface; anchors epithelium.

Classification of Epithelium

Shape	Description
Squamous	Flattened cells
Cuboidal	Cube-shaped cells
Columnar	Column-shaped cells

Arrangement	Description
Simple epithelium	One layer of cells
Stratified epithelium	More than one cell layer

Types of Epithelial Tissue

• Simple squamous: Single layer; allows diffusion/filtration (lungs, capillaries).

• Simple cuboidal: Single layer; glands, ducts.

• Simple columnar: Single layer; digestive tract; includes goblet cells (mucus secretion).

• Pseudostratified columnar: Appears layered; absorption/secretion (respiratory tract).

• Stratified squamous: Several layers; protection (skin, mouth).

• Stratified cuboidal/columnar: Rare; ducts of large glands.

• Transitional epithelium: Modified stratified; stretches (bladder, ureters).

Glands

• Endocrine glands: Ductless; secrete hormones into blood (thyroid, adrenals).

• Exocrine glands: Have ducts; secrete onto surfaces (sweat, oil, liver, pancreas).

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type. It supports, protects, and binds other tissues.

• Characteristics:

  • Variations in blood supply (tendons/ligaments: poor; cartilage: avascular).

  • Extracellular matrix: Nonliving substance; includes ground substance and fibers.

Fiber Type	Function
Collagen	High tensile strength
Elastic	Stretch and recoil
Reticular	Forms soft organ skeletons

Types of Connective Tissue

• Bone (Osseous tissue): Hard matrix; protects/supports organs.

• Cartilage: Flexible; types include hyaline (most common), fibrocartilage (discs), elastic (external ear).

• Dense connective tissue: Collagen fibers; tendons, ligaments, dermis.

• Loose connective tissue: Softer, more cells/fewer fibers; includes areolar, adipose (fat), reticular.

• Blood: Vascular tissue; transports substances.

Loose Connective Tissue Types

• Areolar: Cushions/protects; holds organs in place; can cause edema (swelling).

• Adipose: Fat; insulates, protects, stores energy.

• Reticular: Forms framework for lymphoid organs.

Muscle Tissue

Muscle tissue contracts to produce movement. There are three types:

• Skeletal muscle: Attached to bones; voluntary; striated.

• Cardiac muscle: Heart; involuntary; striated; branched; intercalated discs.

• Smooth muscle: Walls of hollow organs; involuntary; spindle-shaped; non-striated.

Nervous Tissue

Nervous tissue is found in the brain, spinal cord, and nerves. It consists of neurons (transmit impulses) and supporting cells (insulate and protect neurons).

Tissue Repair (Wound Healing)

• Tissue injury: Stimulates inflammatory and immune responses.

• Inflammation: Prevents further injury.

• Immune response: Attacks invaders (bacteria, viruses, toxins).

• Tissue repair: Occurs by regeneration (replacement by same cell type) or fibrosis (scar tissue formation).

Events in Tissue Repair

• Capillaries become permeable; clot forms.

• Granulation tissue forms; contains new capillaries, phagocytes, and collagen fibers.

• Surface epithelium regenerates under scab.

Abnormal Tissue Growth

• Neoplasm: Abnormal mass; may be benign or malignant.

• Hyperplasia: Enlargement due to stimulation.

• Atrophy: Decrease in size due to inactivity.

Key Terms and Concepts

• Cell: Basic unit of life.

• Tissue: Group of similar cells performing a function.

• Organ: Structure composed of two or more tissue types.

• Matrix: Nonliving material in connective tissue.

• Edema: Swelling due to fluid accumulation.

• Regeneration: Replacement of destroyed tissue by same kind of cells.

• Fibrosis: Repair by dense connective (scar) tissue.

Important Equations

• ATP Production (Mitochondria):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Osmosis (Water Movement):

$\text{Osmosis}: \text{Movement of water from an area of low solute concentration to high solute concentration across a semipermeable membrane}$

Additional info: Some explanations and examples have been expanded for clarity and completeness.