Cell Structure and Function
Introduction to Cells Cells are the fundamental units of life, forming the structural and functional
foundation of all organisms. The study of cell structure and function provides the basis for
understanding biology, as every physiological process in organisms depends on cellular activity. Cell
theory, one of the central tenets of biology, states that all living organisms are composed of one or
more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
Prokaryotic vs. Eukaryotic Cells Cells can be divided into two categories: prokaryotic and eukaryotic.
Prokaryotic cells, such as bacteria and archaea, lack a nucleus and membrane-bound organelles. Their
DNA is free-floating in the nucleoid region. Eukaryotic cells, on the other hand, include plant, animal,
fungal, and protist cells, and they have a nucleus enclosed by a nuclear envelope as well as other
organelles that compartmentalize functions. This division of labor allows eukaryotes to achieve higher
complexity.
The Plasma Membrane The plasma membrane surrounds all cells and acts as a selectively permeable
barrier, controlling the entry and exit of substances. Its fluid mosaic model describes a bilayer of
phospholipids with proteins embedded throughout. Cholesterol molecules add stability, while
carbohydrates attached to proteins and lipids contribute to cell recognition. Transport across the
membrane occurs by passive diffusion, facilitated diffusion, osmosis, and active transport using ATP.
Specialized mechanisms like endocytosis and exocytosis enable bulk transport.
The Nucleus and Genetic Control The nucleus is the command center of the eukaryotic cell, containing
the genetic blueprint in the form of DNA. It is enclosed by the nuclear envelope, perforated with pores
that regulate transport of RNA and proteins. Inside, chromatin organizes DNA with histone proteins,
condensing into chromosomes during cell division. The nucleolus, a dense subregion, is the site of
ribosomal RNA synthesis and ribosome assembly.
Protein Factories: Ribosomes and the Endoplasmic Reticulum Ribosomes are the cellular machines
that synthesize proteins. They can be free in the cytoplasm, producing proteins for internal use, or
bound to the rough endoplasmic reticulum (RER), synthesizing proteins for secretion or membrane
insertion. The smooth endoplasmic reticulum (SER) lacks ribosomes and specializes in lipid synthesis,
calcium storage, and detoxification of harmful substances. Together, the ER systems contribute to the
production and processing of macromolecules.
Golgi Apparatus and Protein Trafficking The Golgi apparatus acts like a cellular post office, modifying,
sorting, and packaging proteins and lipids for delivery. Vesicles carry materials from the ER to the
Golgi, where they undergo modifications such as glycosylation. Finally, vesicles transport finished
products to the plasma membrane, lysosomes, or other organelles.
Lysosomes and Intracellular Digestion Lysosomes are membrane-bound organelles filled with
hydrolytic enzymes that break down macromolecules, old organelles, and pathogens. They maintain an
acidic environment, optimal for enzyme activity. In plant cells, the central vacuole fulfills some similar
functions while also maintaining turgor pressure.
Mitochondria and Energy Conversion Mitochondria are the “powerhouses” of the cell, converting
energy stored in glucose into ATP through cellular respiration. They contain their own DNA and
