Chapter 2: Foundations: The Cell

Cellular boundaries are defined by the plasmalemma (cell membrane), a selectively permeable lipid bilayer composed of phospholipids, cholesterol, and various integral and peripheral proteins, which facilitates physical isolation and regulates exchange with the extracellular fluid. The movement of materials across this membrane occurs through passive processes such as diffusion, osmosis (the diffusion of water based on solute concentration), and facilitated diffusion (using carrier proteins); or through active processes that require energy reserves (ATP), including active transport (often via exchange pumps like the sodium-potassium pump) and vesicular transport mechanisms like endocytosis (which involves pinocytosis or "cell drinking," phagocytosis or "cell eating," and receptor-mediated endocytosis) and exocytosis (the release of substances). Within the cell, the cytoplasm is divided into the cytosol (intracellular fluid, rich in potassium and proteins) and organelles. Organelles are classified as nonmembranous, which includes the structural cytoskeleton (made of microfilaments, intermediate filaments, and microtubules) and ribosomes (protein synthesis sites); and membranous structures. The mitochondria are double-membraned organelles that produce the majority (95 percent) of cellular ATP through metabolic enzymes organized on cristae. The nucleus, surrounded by a nuclear envelope containing nuclear pores, is the central control hub, storing DNA in the form of chromatin or tightly coiled chromosomes, and housing the nucleolus for ribosome synthesis. The endoplasmic reticulum (ER) serves as a synthetic, storage, and transport network, differentiating into rough ER (RER) with attached ribosomes for protein modification, and smooth ER (SER) for lipid synthesis and toxin neutralization. Products are then processed and packaged by the Golgi apparatus for secretion or for incorporation into lysosomes (digestive enzyme vesicles, critical in autolysis) and peroxisomes (toxin neutralizing vesicles). The continuous exchange between these membranous structures is referred to as membrane flow. Cells also form stable tissues via intercellular attachments, such as communicating junctions (gap junctions) and adhering junctions (including tight junctions and anchoring junctions like desmosomes and hemidesmosomes). The chapter concludes with the cell life cycle, where somatic cells undergo interphase (G1, S phase for DNA replication, G2) before dividing via mitosis, a process comprising prophase, metaphase, anaphase, and telophase, followed by cytokinesis to produce two daughter cells, a tightly regulated cycle whose failure can result in abnormal growth, leading to tumors or cancer and its potential for metastasis.