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

Cell Membrane

Structure and Function

The cell membrane forms the outer boundary of the cell, playing a crucial role in maintaining cellular integrity and regulating interactions with the environment.

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

• Extra- vs. Intra-: Extracellular refers to outside the cell; Intracellular refers to inside the cell.

Selective Permeability

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

• Regulates exchange of nutrients, waste, and signals.

• Protein channels allow specific substances to pass through the membrane.

Homeostasis

Definition and Role

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

• Role of Cell Membrane:

  • Controls substance movement in/out of the cell.

  • 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, serving as the site for most cellular activities.

• 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 depending on 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.

Key Takeaways

• Normal mitosis is controlled, produces healthy cells.

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

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

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, allow diffusion (e.g., capillaries, alveoli).

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

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

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

• Stratified Epithelium (multiple layers):

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

  • Stratified cuboidal: Cube-shaped cells in layers, protection/secretion (rare, e.g., some glands).

  • Stratified columnar: Tall cells in layers, protection (rare, e.g., large ducts).

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

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