Cell Theory and Cellular Organization

Principles of Cell Theory

The cell theory is a foundational concept in biology, stating that all living organisms are composed of cells, which are the basic units of life. This theory underpins our understanding of anatomy and physiology at the cellular level.

• All living organisms are composed of cells. Cells are the fundamental units of living organisms.

• Cellular activity: The activity of an organism depends on both the individual and collective activities of its cells.

• Biochemical activities: The biochemical activities of cells are determined by their subcellular structures.

• Continuity of life: Cells arise only from pre-existing cells.

Cells vary greatly in size and shape, but all share three major parts: plasma membrane, cytoplasm, and nucleus.

Plasma Membrane: Structure and Function

Chemical Composition and Fluid Mosaic Model

The plasma membrane defines the boundary of the cell and acts as a selective barrier. It is primarily composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. The fluid mosaic model describes the membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Phospholipid bilayer: Two layers of phospholipids with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Proteins: Integral (span the membrane) and peripheral (attached to the surface) proteins serve as channels, carriers, receptors, and enzymes.

• Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids), important for cell recognition.

• Cholesterol: Stabilizes membrane fluidity.

The plasma membrane is selectively permeable, allowing some substances to pass while restricting others.

Membrane Junctions

Cells are often connected by specialized junctions:

• Tight junctions: Prevent passage of molecules between adjacent cells (e.g., in the digestive tract).

• Desmosomes: Provide mechanical strength by anchoring cells together (e.g., in skin and heart tissue).

• Gap junctions: Allow direct communication between cells via channels (e.g., in cardiac muscle).

Membrane Transport

Types of Membrane Transport

The plasma membrane controls the movement of substances into and out of the cell. Transport mechanisms are classified as passive or active.

Passive Processes

• Simple diffusion: Movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) down their concentration gradient.

• Facilitated diffusion: Movement of larger or polar molecules via protein channels or carriers.

• Osmosis: Diffusion of water through a selectively permeable membrane via aquaporins.

Fick's Law of Diffusion:

where is the flux, is the diffusion coefficient, and is the concentration gradient.

Active Processes

• Active transport: Uses ATP to move substances against their concentration gradient via carrier proteins (e.g., sodium-potassium pump).

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

Sodium-Potassium Pump Equation:

Summary Table: Membrane Transport Mechanisms

Cytoplasmic Organelles

Major Organelles and Their Functions

• Cytoplasm: The cellular material inside the plasma membrane, excluding the nucleus. Contains cytosol (fluid) and organelles.

• Ribosomes: Sites of protein synthesis, composed of rRNA and protein. Can be free or bound to rough ER.

• Endoplasmic Reticulum (ER): Network of membranes; rough ER has ribosomes and synthesizes proteins, smooth ER synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules and worn-out organelles.

• Peroxisomes: Contain enzymes for detoxification and breakdown of fatty acids; produce hydrogen peroxide.

Cytoskeleton, Cilia, and Flagella

Cytoskeletal Components

• Microfilaments: Thin filaments of actin; involved in cell movement and shape.

• Intermediate filaments: Tough, rope-like fibers; provide tensile strength and resist mechanical stress.

• Microtubules: Hollow tubes of tubulin; maintain cell shape, enable intracellular transport, and form mitotic spindle.

Cilia and Flagella

• Cilia: Short, numerous projections that move substances across cell surfaces (e.g., respiratory tract).

• Flagella: Longer, usually singular; propel cells (e.g., sperm cell).

Nucleus: Structure and Function

Nuclear Components

• Nucleus: Control center of the cell; contains genetic material (DNA).

• Nuclear envelope: Double membrane with nuclear pores for transport.

• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

The nucleus directs cellular activities by regulating gene expression and mediating the replication of DNA during the cell cycle.

Example: The nucleolus is prominent in cells with high rates of protein synthesis, such as liver cells.