Cell Structure, Tissue Types, and Mitosis: Foundations of Anatomy & Physiology

Cell Membrane

Structure and Function

The cell membrane (also called the plasma membrane) forms the outer boundary of the cell and is essential for maintaining cellular integrity and function.

• Made of two layers: Composed of a phospholipid bilayer with embedded proteins and molecules that perform specialized functions.

• Extracellular = outside the cell; Intracellular = inside the cell.

Selective Permeability

• Regulates what enters and leaves the cell, and in what amounts.

• Controls exchange of nutrients, waste, and signals.

• Protein channels allow specific substances to pass through.

Homeostasis

Definition and Role

Homeostasis is the process by which living organisms maintain a stable internal environment despite external changes.

• Cell membrane controls substance movement in/out of the cell and protects cells from harmful external conditions.

• Failure of homeostasis can lead to cell malfunction or death.

Examples of Homeostasis

• Sweating cools the body when hot.

• Heart rate & blood vessel diameter adjust to maintain blood pressure.

• Thermostat analogy: The body works like a heating/AC system to keep internal conditions stable.

Negative Feedback Loops

• Definition: The body reverses a change to bring conditions back to normal.

• If something goes up → body works to bring it down.

• If something goes down → body works to bring it up.

• Most homeostasis mechanisms use negative feedback.

Cytoplasm and Cytoskeleton

Cytoplasm

The cytoplasm is the region between the plasma membrane and the nucleus, containing all cellular contents except the nucleus.

• Organelles: Specialized structures with specific functions ("little organs").

• Cytosol: Liquid inside the cell, mostly water.

• Cytoskeleton: Framework made of protein filaments.

Cytoskeleton

• Structure: Protein filaments (thread-like proteins).

• Functions:

  • Provides structural support and maintains cell shape.

  • Enables cell movement.

  • Acts like train tracks for intracellular transport (moving materials within the cell).

Key Idea: The cytoplasm is the "workspace" of the cell, with the cytoskeleton providing support and transport, like bones and highways inside the cell.

Major Organelles

Nucleus

• Control center of the cell.

• Directs metabolic activity.

• Contains DNA – blueprint for cell structure and reproduction.

Mitochondria

• Powerhouse of the cell.

• Produces ATP energy for cellular activities.

Lysosomes

• The cell’s digestive system.

• Break down waste for elimination or recycling.

Ribosomes, Endoplasmic Reticulum (ER), & Golgi Apparatus

• Ribosomes – build proteins.

• Rough ER – produces & folds proteins (covered in ribosomes).

• Smooth ER – makes lipids, detoxifies, stores calcium.

• Golgi apparatus – modifies, packages, and ships proteins/lipids.

• Work together using DNA instructions from the nucleus.

Core Idea: The cell membrane protects and regulates exchange, crucial for homeostasis. The cytoplasm & cytoskeleton support and transport inside the cell.

Tissues

Definition

A tissue is a group of similar cells working together to perform a specific function.

4 Main Types of Tissues

• Epithelial Tissue: Covers body surfaces, lines body cavities, forms glands.

• Connective Tissue: Provides support, connects other tissues and organs.

• Muscle Tissue: Produces movement by contracting (shortening).

• Nervous Tissue: Transmits information via electrical & chemical signals.

Key Idea: All body structures and functions are built from these four primary tissue types.

Mitosis

Process and Importance

Mitosis is the process where one cell divides into two identical daughter cells, each with the same number of chromosomes (DNA) as the parent cell.

Mitosis in Different Tissue Types

• Epithelial tissue: Mitosis occurs most rapidly.

• Connective tissue: Mitosis rate varies by type.

• Muscle & nervous tissue (in adults): Generally do NOT undergo mitosis (very limited repair capacity).

Key Idea: Tissue repair depends on cell division. Rapidly dividing tissues (like epithelium) heal quickly, while muscle and nerve tissues have very limited repair capacity.

Mitosis and Cancer

Cancer and Mitosis

• Cancer: Group of diseases characterized by uncontrolled cell proliferation and spread of abnormal cells.

• Key concept: Cancer is essentially unregulated mitosis.

Consequences of Accelerated Mitosis

• Uncontrolled cell division → tissue accumulates → forms a tumor (neoplasm).

• Tumor: Swelling or mass of tissue.

• Neo = new, plasm = tissue/cells.

Types of Tumors

• Benign Tumor:

  • Cells do not invade other tissues.

  • Remains localized.

  • Usually not life-threatening.

  • Cannot spread.

• Malignant (Cancerous) Tumor:

  • Cells invade the basement membrane.

  • Can metastasize (spread to other parts of the body).

  • Life-threatening.

Tumor Characteristics: Benign vs Malignant

• Benign Tumors: Encapsulated, localized, low risk, low chance of recurrence.

• Malignant Tumors: Irregular borders, invasive, fast-growing, can metastasize, higher likelihood of recurrence after removal.

Key Concept: Benign tumors are localized and low-risk, while malignant tumors are invasive, fast-growing, and potentially life-threatening.

Epithelial Tissue

Functions

• Protection: Shields underlying tissues from damage, pathogens, or friction.

  • Example: Esophagus epithelium protects against friction from food.

• Secretion: Produces and releases substances like sweat, enzymes, or mucus.

  • Example: Sweat glands secrete sweat.

• Absorption: Takes in nutrients, water, and other substances.

  • Example: Digestive tract lining absorbs nutrients and vitamins.

• Filtration: Filters substances from the blood or other fluids.

  • Example: Kidney epithelium filters blood.

• Synthesis: Produces chemical compounds like hormones.

  • Example: Adrenal gland epithelium synthesizes cortisol and adrenaline.

Characteristics

• Cell Arrangement: Cells are tightly packed in sheets or layers.

• Polarity:

  • Apical surface: Free, not attached to anything (faces lumen or outside).

  • Basal surface: Attached to the basement membrane (anchors cells to underlying tissue).

• Blood Supply and Innervation:

  • Avascular: Lacks direct blood supply.

  • Nerve supply present.

• Regeneration: High rate of mitosis due to wear and tear.

Epithelial Tissue – Morphological Classification

Basis of Classification

• Shape of cells: Squamous (flat), cuboidal (cube-like), columnar (tall).

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

• Surface modifications: Presence or absence of cilia on the apical surface.

Types of Epithelium

• Simple Epithelium (one layer of cells):

  • Simple squamous: Flat cells; allows diffusion (e.g., capillaries, alveoli).

  • Simple cuboidal: Cube-shaped cells; secretion and absorption (e.g., kidney tubules).

  • Simple columnar: Tall cells; absorption and secretion (e.g., digestive tract lining).

  • Pseudostratified columnar: Appears layered, but all cells touch the basement membrane; often ciliated (e.g., respiratory tract).

• Stratified Epithelium (multiple layers):

  • Stratified squamous: Top layer flat, lower layers cuboidal; protection (e.g., skin, esophagus).

  • Stratified cuboidal: Rare; protection and secretion (e.g., some gland ducts).

  • Stratified columnar: Rare; protection (e.g., large ducts).

  • Transitional epithelium: Specialized for stretching (e.g., bladder).

Key Concept: The location of epithelial tissue is determined by its function.

Surface Specializations of Epithelial Tissue

• Microvilli: Finger-like projections from the apical surface; increase surface area for absorption (e.g., lining of the small intestine).

• Cilia: Hair-like projections from the apical surface; move substances through tubes or prevent unwanted particles from entering (e.g., respiratory tract).

Summary Table: Types of Epithelial Tissue

Type	Structure	Main Function	Location Example
Simple Squamous	Single layer, flat cells	Diffusion, filtration	Alveoli, capillaries
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, glands
Simple Columnar	Single layer, tall cells	Absorption, secretion	Digestive tract lining
Pseudostratified Columnar	Appears layered, all cells touch basement membrane	Secretion, movement (cilia)	Respiratory tract
Stratified Squamous	Multiple layers, top flat	Protection	Skin, esophagus
Stratified Cuboidal	Multiple layers, cube-shaped	Protection, secretion	Some gland ducts
Stratified Columnar	Multiple layers, tall cells	Protection	Large ducts
Transitional	Multiple layers, variable shape	Stretching	Bladder

*Additional info: Some explanations and examples were expanded for clarity and completeness. Equations are not directly relevant to this section but would be included for processes like diffusion or osmosis if required in later chapters.*