Modern biotechnology emerges from an understanding of molecular biology. The following sections serve to introduce the primary molecular parts and interactions that exist within cells.
MEMBRANES AND OVERALL CELLULAR STRUCTURE
All cells are physically distinguished from their surroundings by phospholipid bilayer membranes. The densely packed, yet fluid space within a cell is called the cytoplasm. Membranes are semi-permeable, meaning they only allow certain things to pass through. Proteins embedded within membranes help to transport molecules in and out of cells in a regulated fashion.
Prokaryotic cells—that is those of the bacterial and archaeal domains—are relatively simple. They contain one or more cell membranes that separate them from the rest of the world, but lack all intracellular compartments. Bacterial cells that contain two parallel cellular membranes are Gram negative (G-) bacteria.1 This name comes from a famous differentiating stain used for light microscopy, the Gram stain.2 The two cellular membranes found in G- cells are aptly called the outer and inner (or cytoplasmic) membranes. The space that exists between the outer and inner membrane is called the periplasm. It contains a thin layer of peptidoglycan—a complex matrix of interlocking proteins and sug- ars—that helps to maintain the integrity of the cell wall. Lipopolysac- charide (LPS) decorates the outer surface of G- outer membranes and is highly toxic to humans. Because of this, it also bears the name endotoxin. When G- bacteria are used to produce proteins for medicinal purposes, strict care must be taken to ensure that no endotoxin has contaminated the biomanufactured product. Harmless strains of G- Escherichia coli, such as E. coli K12, are the primary bacterium used for gene cloning in biotechnology. Bacterial cells that contain just one cellular membrane are called Gram positive (G+) bacteria.3 To compensate for their single-membrane cell walls, G+ cells contain an extensive outer layer of peptidoglycan to help maintain cellular integrity and inhibit membrane permeability. Since archaeal cells have been traditionally less widely studied, understood, and utilized in biotechnology, they will not be discussed at any length. That said, some archaea play important roles in the cleanup of toxic dumpsites.
Eukaryotic cells, like prokaryotic cells, contain a cellular membrane that separates them from the extracellular milieu. What differentiates a eukaryotic from a prokaryotic cell is the presence of an endomembrane system—an extensive set of intracellular membrane structures that create physical compartments within cells. Such membranes include those that separate the chromosomal DNA from the cytoplasm (i.e., the nuclear membrane), are involved in protein production/processing/degradation (e.g., membranes of the endoplasmic reticula, golgi apparati, degrada- tive pathways, plant vacuoles, and transport vesicles), and distinguish the intracellular compartments associated with primary metabolic processes (e.g., choloroplast and mitochondrion). All animal, plant, and fungal cells are eukaryotic.4
Regardless of type, all cells:
- 1. reproduce autonomously,
- 2. adapt/evolve and modify their behavior to maintain homeostasis based on environmental pressures and cues,
- 3. contain nucleic acids (deoxyribonucleic acid, DNA and ribonucleic acid, RNA), which serve as the instructions and internal messaging system required for the manufacture of cellular components, and
- 4. harness energy to do cellular work.5
There are universal, yet slightly specialized, processes for information flow within prokaryotic versus eukaryotic cells. The mechanistic understanding and manipulation of these processes is critical for biotechnology.