Cell Structure, Homeostasis, Tissues, and Mitosis: Study Notes for 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: The membrane consists of a phospholipid bilayer with embedded proteins and molecules that perform specialized functions.

• Terminology: Extra- refers to outside the cell (extracellular), Intra- refers to inside the cell (intracellular).

Selective Permeability

The cell membrane is selectively permeable, meaning it 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 things maintain a stable internal environment despite external changes.

• Role of Cell Membrane: Controls substance movement in/out of the cell and protects cells from harmful external conditions.

• Failure of homeostasis can result in 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

Most homeostasis mechanisms use negative feedback to maintain stability.

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

• If something goes up, the body works to bring it down; if something goes down, the body works to bring it up.

Cytoplasm and Cytoskeleton

Cytoplasm

The cytoplasm is the region between the plasma membrane and the nucleus.

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

• Cytosol: Liquid inside the cell, mostly water.

• Cytoskeleton: Framework made of protein filaments.

Cytoskeleton

The cytoskeleton consists of protein filaments that provide structural support and maintain 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 (e.g., top layer of skin, digestive tract).

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

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

• Nervous (Nerve) 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.

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

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.

Cancer and Mitosis

Definition and Causes

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

• Cause: DNA damage → mutations in daughter cells during mitosis; accumulation of mutations → abnormal cells and accelerated mitosis.

• 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.

• Malignant (Cancerous) Tumor: Cells invade the basement membrane, can metastasize (spread to other parts of the body), life-threatening.

Key Takeaways

• Normal mitosis = controlled, produces healthy cells.

• Accelerated/unregulated mitosis = abnormal cell growth → tumors.

• Tumors may be benign or malignant, depending on invasiveness and risk.

Tumor Characteristics: Benign vs Malignant

Feature	Benign Tumor	Malignant Tumor
Encapsulation	Usually encapsulated	Irregular borders
Invasion	Do not invade other tissues	Invade other tissues
Metastasis	No metastasis	Can metastasize
Growth Rate	Slow	Fast
Blood Vessels	No new blood vessels	Develop new blood vessels (angiogenesis)
Recurrence	Low chance	High chance

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: Absorbs nutrients 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: Synthesizes 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), basal surface (attached to basement membrane).

• Blood Supply and Innervation: Avascular (lacks direct blood supply), but nerve supply is present.

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

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 easy movement of substances in/out. Locations: Walls of tiny blood vessels (capillaries), alveoli in lungs.

  • Simple columnar (non-ciliated): Tall cells specialized for absorption. Locations: Lining of stomach and intestines.

  • Pseudostratified columnar: Appears layered but all cells touch the basement membrane; often ciliated. Locations: Respiratory tract.

• Stratified Epithelium: Multiple layers of cells.

  • Stratified squamous: Flat cells in layers; protects against friction and wear. Locations: Epidermis (outer layer of skin), esophagus.

  • Stratified cuboidal: Cube-shaped cells in layers. Locations: Some glands.

  • Stratified columnar: Tall cell layers. Locations: Large ducts.

  • Transitional epithelium: Specialized for stretching. Locations: Bladder.

Surface Specializations

• Microvilli: Finger-like projections from the apical surface; increase surface area for absorption. Example: Lining of the small intestine.

• Cilia: Hair-like projections from the apical surface; move substances through tubes. Example: Moving sperm in reproductive tract, preventing unwanted particles from entering airways.

Key Concept: The location and morphology of epithelial tissue reflect its function—protection, absorption, secretion, filtration, or stretching.