Chapter 3: Cellular Form and Function

Cell Theory

The cell is the fundamental structural and functional unit of all living organisms. All physiological processes of an organism depend on the activities of its cells, and the specific biochemical activities of cells are determined by their unique subcellular structures.

• Smallest living unit: Cells are the basic units of life.

• Organismal function: The function of an organism is the sum of the functions of its cells.

• Structure dictates function: The activities of cells are determined by their subcellular structures.

Cell Diversity

Human bodies contain over 200 different types of cells, each varying in size, shape, organelle composition, and function. This diversity allows for specialization and efficient functioning of tissues and organs.

• Examples: Muscle cells (contractile), nerve cells (conduct impulses), epithelial cells (cover surfaces).

Generalized Cell Structure

Despite their diversity, all human cells share three basic structural components:

• Plasma membrane: Flexible outer boundary that separates the cell from its environment.

• Cytoplasm: Intracellular fluid containing organelles.

• Nucleus: Control center containing genetic material (DNA).

Plasma Membrane

Structure and Composition

The plasma membrane is a dynamic, selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid bilayer: Provides fluidity and forms the basic structure.

• Cholesterol: Stabilizes the membrane and modulates its fluidity (about 20% of membrane lipids).

• Proteins: Integral and peripheral proteins serve as channels, carriers, receptors, enzymes, and structural anchors.

• Carbohydrates: Attach to proteins (glycoproteins) or lipids (glycolipids) for cell recognition and signaling.

Functions of the Plasma Membrane

• Selective barrier: Regulates the movement of substances into and out of the cell.

• Communication: Contains receptors for signal transduction.

• Cell recognition: Glycoproteins and glycolipids allow cells to recognize each other.

• Structural support: Anchors the cytoskeleton and maintains cell shape.

Membrane Proteins

• Integral proteins: Firmly embedded in the membrane; function as channels, carriers, enzymes, or receptors.

• Peripheral proteins: Loosely attached to the membrane surface; function as enzymes, motor proteins, or provide structural support.

Membrane Junctions

• Tight junctions: Prevent passage of substances between cells (e.g., digestive tract lining).

• Desmosomes: Anchor cells together, providing mechanical stability (e.g., skin, heart muscle).

• Gap junctions: Allow passage of ions and small molecules for communication (e.g., cardiac and smooth muscle).

Membrane Transport

Overview

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. Transport mechanisms are classified as passive (no energy required) or active (energy required).

Passive Transport

• Simple diffusion: Movement of nonpolar and lipid-soluble substances directly through the lipid bilayer, down their concentration gradient (e.g., O2, CO2, fat-soluble vitamins).

• Facilitated diffusion: Movement of polar or charged substances via carrier or channel proteins, still down their concentration gradient (e.g., glucose, amino acids, ions).

• Osmosis: Diffusion of water across a selectively permeable membrane, either directly or through aquaporins.

Effects of Solutions of Varying Tonicity

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water moves out, causing cell shrinkage (crenation).

• Hypotonic: Lower solute concentration outside the cell; water moves in, causing cell swelling or lysis.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients, using carrier proteins known as pumps.

• Sodium-potassium pump (Na+/K+ ATPase): Moves 3 Na+ ions out and 2 K+ ions into the cell, maintaining electrochemical gradients essential for nerve and muscle function.

Vesicular Transport

Large particles and macromolecules are transported across membranes in vesicles. This includes exocytosis (out of the cell) and endocytosis (into the cell).

• Phagocytosis: "Cell eating"; engulfment of large particles by immune cells.

• Pinocytosis: "Cell drinking"; uptake of extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Specific uptake of substances via receptor binding.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., neurotransmitter release).

Cytoplasm and Organelles

Cytosol and Organelles

The cytoplasm consists of cytosol (fluid) and organelles (specialized structures performing specific functions).

• Cytosol: Water with dissolved proteins, salts, sugars, and other solutes.

• Organelles: "Little organs" that carry out metabolic activities.

Major Organelles and Their Functions

• Mitochondria: Site of ATP production via aerobic respiration; contains its own DNA and RNA.

• Golgi apparatus: Modifies, concentrates, and packages proteins and lipids for secretion or delivery to other organelles.

• Ribosomes: Sites of protein synthesis; free ribosomes produce cytosolic proteins, while membrane-bound ribosomes produce proteins for membranes or export.

• Rough endoplasmic reticulum (RER): Studded with ribosomes; synthesizes and modifies proteins for membranes or secretion.

• Smooth endoplasmic reticulum (SER): Lacks ribosomes; synthesizes lipids and steroid hormones, stores Ca2+, detoxifies chemicals.

• Peroxisomes: Contain enzymes for lipid metabolism and detoxification of harmful substances.

• Lysosomes: Digest cellular debris, pathogens, and worn-out organelles.

Cell Surface Structures

• Microvilli: Increase surface area for absorption (e.g., intestinal cells).

• Cilia: Motile extensions that move substances across cell surfaces (e.g., respiratory tract).

• Flagella: Long, whip-like structures for cell movement (e.g., sperm cell).

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, maintains cell shape, and facilitates movement.

• Microtubules: Hollow tubes that determine cell shape and organelle distribution.

• Microfilaments: Thin strands of actin that support the cell membrane and aid in movement.

• Intermediate filaments: Strong, rope-like fibers that resist mechanical stress and help form desmosomes.

Centrioles

• Centrioles: Organize the mitotic spindle during cell division and form the bases of cilia and flagella.

Nucleus and Genetic Material

Nucleus

The nucleus is the control center of the cell, containing the genetic library (DNA) and directing protein synthesis.

• Nuclear envelope: Double membrane with nuclear pores for molecular exchange.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA-protein complex that condenses to form chromosomes during cell division.

Chromatin Structure

• Nucleosomes: Fundamental units of chromatin, consisting of DNA wrapped around histone proteins.

• Chromosomes: Condensed chromatin visible during cell division.

Additional info: The cell's structure and function are foundational to understanding all physiological processes in the human body. Mastery of cellular anatomy and physiology is essential for further study in histology, organ systems, and disease mechanisms.