Chapter 3: The Cell

Module 3.1: Introduction to Cells

Cells are the basic structural and functional units of life. They are classified into sex cells (gametes) and somatic cells, each with distinct roles in reproduction and body maintenance.

• Sex Cells (Gametes): Include sperm (spermatocyte) and oocyte. Undergo both mitosis and meiosis for reproduction.

• Somatic Cells: All other body cells. Divide by mitosis for growth and repair.

• Plasma Membrane: A selectively permeable barrier that regulates exchange with the environment, responds to chemical signals, and provides structural support.

• Cytoplasm: Contains organelles and cytoskeleton suspended in cytosol.

• Cytoskeleton: Provides structural support, maintains cell shape, anchors organelles, and facilitates intracellular transport. Composed of microtubules (largest), intermediate filaments, and actin filaments (microfilaments).

• Nucleus: Surrounded by a phospholipid bilayer (nuclear envelope), contains DNA, and is the primary site for RNA synthesis.

• Cilia and Flagella: Cilia move substances along cell surfaces (e.g., respiratory tract), while flagella (e.g., sperm) provide motility.

Intracellular vs. Extracellular Fluid

• Intracellular Fluid (ICF): Fluid inside the cell; major cation is potassium (K+).

• Extracellular Fluid (ECF): Fluid outside the cell; major cation is sodium (Na+). Contains higher protein content.

Cytoplasm vs. Cytosol

• Cytoplasm: The entire contents within the plasma membrane, excluding the nucleus. Includes cytosol, organelles, and inclusions.

• Cytosol: The fluid portion of the cytoplasm, making up about half the cell's volume.

Organelles

• Definition: Specialized structures within the cell that perform specific functions.

• Nonmembranous Organelles: Lack a surrounding membrane (e.g., cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes).

• Membranous Organelles: Surrounded by a membrane (e.g., endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria).

Module 3.2: Structure of the Plasma Membrane

The plasma membrane is a dynamic structure composed of lipids, proteins, and carbohydrates, organized according to the fluid mosaic model.

• Phospholipids: Form a bilayer with hydrophilic (polar) heads facing outward and hydrophobic (nonpolar) tails facing inward.

• Cholesterol: Stabilizes membrane fluidity, especially during temperature changes.

• Glycolipids: Carbohydrate chains attached to lipids; function in cell recognition.

• Membrane Proteins: Integral proteins span the membrane; peripheral proteins are attached to one side. Functions include channels, carriers, receptors, enzymes, structural support, and cell linking.

• Glycoproteins: Carbohydrate chains attached to proteins; involved in cell recognition.

Fluid Mosaic Model

• The membrane is a mosaic of lipids, proteins, and carbohydrates, all capable of lateral movement, contributing to membrane fluidity and functionality.

Module 3.3: Transport Across the Plasma Membrane

Cells regulate the movement of substances across the plasma membrane through various transport mechanisms, classified as passive or active.

• Diffusion (Passive): Movement of molecules from high to low concentration without energy input.

• Facilitated Diffusion: Passive transport of polar or charged molecules via channels or carrier proteins.

• Carrier-Mediated Transport: Can be passive (facilitated diffusion) or active (requires ATP). Includes single molecule transport, cotransport (symport), and countertransport (antiport).

• Vesicular Transport (Active): Movement of large particles via vesicles (endocytosis and exocytosis).

Simple vs. Facilitated Diffusion

• Simple Diffusion: Nonpolar solutes (e.g., O2, CO2, alcohol, fatty acids, steroids) pass directly through the lipid bilayer.

• Facilitated Diffusion: Polar or charged solutes (e.g., ions, glucose) require membrane proteins (channels or carriers) for transport.

Factors Influencing Diffusion

• Molecule size (smaller = faster)

• Temperature (higher = faster)

• Concentration gradient (steeper = faster)

• Electrical forces (opposites attract)

Osmosis

• Passive movement of water from areas of low solute concentration to high solute concentration across a selectively permeable membrane.

• "Water follows salt"—water moves toward higher solute concentrations.

Tonicity Effects on Cells

Active Transport

• Primary Active Transport: Uses ATP to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses the energy from the movement of one substance down its gradient to move another substance against its gradient.

Example Equation (Sodium-Potassium Pump):

Vesicular Transport

• Endocytosis: Uptake of materials into the cell via vesicles (e.g., phagocytosis by white blood cells, receptor-mediated endocytosis).

• Exocytosis: Export of materials from the cell (e.g., secretion of hormones, removal of waste).

Chapter 4: Histology

Module 4.1: Introduction to Tissues

Histology is the study of tissues, which are groups of similar cells performing a common function. Tissues are a key level of organization in the human body, situated between cells and organs.

• Epithelial Tissue: Covers surfaces and forms glands.

• Connective Tissue: Supports, binds, and protects other tissues.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Initiates and transmits electrical impulses.

Extracellular Matrix (ECM)

• Non-cellular component present within all tissues and organs.

• Composed of protein fibers (collagen, elastin) and ground substance.

• Provides structural and biochemical support to surrounding cells.

Cell Junctions

• Tight Junctions: Seal adjacent cells to prevent passage of molecules.

• Desmosomes: Provide mechanical strength by anchoring cells together.

• Gap Junctions: Allow communication between cells via channels.

Module 4.2: Epithelial Tissues

Epithelial tissues are classified by cell shape and number of layers. They serve as protective barriers and are involved in absorption, secretion, and sensation.

• Simple Epithelium: Single cell layer (e.g., simple squamous, cuboidal, columnar).

• Stratified Epithelium: Multiple layers (e.g., stratified squamous, cuboidal, columnar).

• Pseudostratified Epithelium: Appears layered but is a single layer.

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

Glandular Epithelium

• Exocrine Glands: Secrete products into ducts (e.g., sweat, salivary glands).

• Endocrine Glands: Secrete hormones directly into the bloodstream.

Module 4.3: Connective Tissues

Connective tissues provide support, protection, and insulation. They are characterized by abundant extracellular matrix and diverse cell types.

• Cell Types: Fibroblasts, adipocytes, macrophages, mast cells, leukocytes.

• Fiber Types: Collagen (strength), elastic (flexibility), reticular (support).

Types of Connective Tissue

• Loose Connective Tissue: Areolar, adipose, reticular.

• Dense Connective Tissue: Dense regular, dense irregular, elastic.

• Specialized Connective Tissue: Cartilage, bone, blood.

Module 4.4: Muscle Tissue

Muscle tissue is specialized for contraction and movement. It is classified based on structure and function.

• Skeletal Muscle: Voluntary, striated, attached to bones.

• Cardiac Muscle: Involuntary, striated, found in the heart.

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs.

Module 4.5: Nervous Tissue

Nervous tissue is responsible for transmitting electrical signals throughout the body. It consists of neurons and supporting cells (neuroglia).

• Neurons: Conduct electrical impulses; composed of cell body, dendrites, and axon.

• Neuroglia: Support, protect, and nourish neurons.

Module 4.7: Membranes

Membranes are sheets of tissue that cover or line body surfaces and cavities.

• Serous Membranes: Line body cavities not open to the exterior; secrete serous fluid (e.g., pericardium, pleura).

• Synovial Membranes: Line joint cavities; secrete synovial fluid.

• Mucous Membranes: Line cavities open to the exterior; secrete mucus (e.g., digestive, respiratory tracts).

• Cutaneous Membrane: The skin; protects the body surface.

Module 4.8: Tissue Repair

Tissue repair involves restoring structure and function after injury. The process varies by tissue type.

• Regeneration: Replacement of damaged tissue with the same cell type; restores normal function.

• Fibrosis: Replacement with scar tissue (dense connective tissue); may impair function.

• Epithelial and Connective Tissues: Generally have high regenerative capacity.

• Muscle and Nervous Tissues: Limited regeneration; more likely to undergo fibrosis.

Example: After a skin injury, epithelial cells proliferate to cover the wound (regeneration), while deeper injuries may result in scar formation (fibrosis).

Additional info: Some details, such as the full classification of connective tissues and the mechanisms of tissue repair, have been expanded for academic completeness.