Summary
Highlights
The lecture begins by introducing Chapter 3, which focuses on cell components. It highlights that much of the information, especially regarding eukaryotic cells, may be familiar from previous biology classes. The primary focus of this introductory microbiology course will be on prokaryotic cells, specifically bacteria. The instructor outlines the objectives for the chapter, providing a roadmap for the topics to be covered, including the characteristics of life and the distinctions between prokaryotic and eukaryotic cells, and viruses.
The lecture discusses the characteristics of life: growth, replication, responsiveness, metabolism, and cellular structure. It compares how these characteristics manifest in bacteria, archaea, eukaryotes, and viruses. Viruses are noted to be unique as they often rely on host cells for replication and metabolism, lacking independent growth and a cytoplasmic membrane, thereby being characterized as acellular.
Prokaryotic cells, which are the main subject of this chapter, lack a nucleus (pro means 'before' and karyote means 'nut' or 'nucleus'). Their genetic material (DNA) is found in a nucleoid area. They can perform transcription and translation simultaneously. Prokaryotes are simpler than eukaryotes, lacking internal membrane-bound organelles like mitochondria and Golgi apparatus, though they do have ribosomes. They are typically small, about one micrometer in diameter or smaller. Bacteria and archaea are the primary types of prokaryotes discussed.
Eukaryotic cells are presented as largely opposite to prokaryotes. They possess a nucleus and membrane-bound organelles, are much larger (10 to 100 micrometers in diameter), and have more complex structures. This category includes algae, protozoa, fungi, animals, and plants. A diagram of a stereotypical eukaryotic cell is shown, prompting students to recall the functions of its organelles.
The lecture visually compares the approximate sizes of various cells, reinforcing that eukaryotic cells are generally much larger than prokaryotic cells. Viruses are the smallest, followed by bacteria, and then larger eukaryotic cells like Giardia. This highlights why the field is called 'microbiology,' as these organisms are typically invisible to the naked eye.
The discussion moves to the external structures of bacteria, starting with the glycocalyx. It exists in two forms: a capsule (tightly organized and attached, aiding in evading host immune recognition by covering antigens) and a slime layer (looser, water-soluble, and sticky, facilitating attachment to surfaces). The capsule provides protection from the host's immune system, while the slime layer helps with adhesion.
Flagella, present in some bacteria, are responsible for movement. Unlike eukaryotic flagella, which move in a whip-like fashion, bacterial flagella rotate like a rudder. They consist of a filament, hook, and basal body, with the basal body anchoring the flagellum to the cell wall. The complexity of the basal body varies between Gram-positive (simpler attachment) and Gram-negative bacteria (thicker attachment due to an outer membrane).
Bacterial flagella rotate to propel the bacterium, allowing for movement towards (positive chemotaxis) or away from (negative chemotaxis) stimuli like food or antibiotics. This movement involves a series of 'runs' and 'tumbles.' Different flagellar arrangements include: monotrichous (one flagellum), amphitrichous (flagella at both ends), lophotrichous (a tuft of flagella at one end), and peritrichous (flagella distributed all over the surface). Some bacteria, like spirochetes (e.g., syphilis), have endoflagella embedded in the periplasmic space, enabling corkscrew-like movement through viscous environments.
Fimbriae are short, bristle-like extensions that help bacteria adhere to each other and to surfaces, playing a role in biofilm formation. Pili, also known as sex pili, are special types of fimbriae that are longer, hollow, and used for DNA transfer between bacterial cells, a process called conjugation. This genetic exchange is a 'bacterial equivalent of sex.'
The cell wall is a crucial structure that prevents osmotic lysis, provides structural strength, and aids in attaching to other cells or resisting antimicrobial drugs. It also gives bacteria their shape. The primary component of bacterial cell walls is peptidoglycan, a combination of proteins (peptid) and sugars (glycan). Specifically, these sugars are N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked together by amino acid cross-bridges.
Gram-positive bacteria have a thick peptidoglycan layer and contain unique teichoic acids (and lipoteichoic acids, if extending to the plasma membrane) embedded within their cell wall. They stain purple during the Gram staining process. Some Gram-positive bacteria, like acid-fast bacteria, also have a high mycotic acid content, which enhances their survival against desiccation.
Gram-negative bacteria have a relatively thinner peptidoglycan layer compared to Gram-positives. A defining feature is an outer membrane composed of phospholipids, proteins, and lipopolysaccharides (LPS). This outer layer provides increased resistance to antibiotics. The LPS layer can contain lipid A, which acts as an endotoxin, causing fever, vasodilation, inflammation, and shock. Gram-negative bacteria stain red or pink because the alcohol used in Gram staining removes the outer membrane, allowing the safranin counterstain to bind.
Some bacteria lack cell walls entirely (e.g., mycoplasmas) or have atypical cell walls. These wall-less bacteria can be mistaken for viruses due to their small size and absence of a cell wall. The cytoplasmic membrane, or plasma membrane, found in all bacteria, is a selectively permeable phospholipid bilayer with embedded proteins. It functions in controlling substance movement, energy storage, and for photosynthesizing bacteria, harvesting light energy. This membrane is organized according to the fluid mosaic model and contains components for cellular respiration and photosynthesis, compensating for the lack of mitochondria and chloroplasts.
The lecture explains how substances move across the cytoplasmic membrane through passive or active transport. Passive transport (no energy expenditure) includes: simple diffusion (e.g., oxygen, CO2), facilitated diffusion (via protein channels, nonspecific or specific for molecules like glucose and amino acids), and osmosis (diffusion of water). Cells can be in isotonic, hypertonic, or hypotonic solutions, affecting water movement and cell integrity, especially for wall-less cells.
Active transport requires cellular energy (ATP) and allows molecules to move against their concentration gradient. Types include uniport (single molecule movement), antiport (two molecules moving in opposite directions), and symport/coupled translocation (two molecules moving in the same direction). Inside the bacteria, the cytoplasm is primarily water, containing the nucleoid region for DNA. Inclusions are reservoirs for various chemicals and energy reserves. Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) during unfavorable conditions, preserving genetic material until conditions improve. This process involves DNA replication, forespore formation, peptidoglycan cortex deposition, and spore coat formation, making them resistant to heat, radiation, and chemicals, killable only by autoclaves.
Ribosomes, responsible for protein synthesis, are 70S in bacteria. A cytoskeleton helps maintain cell shape and aids in DNA segregation and movement. The lecture then transitions to Archaea, which also possess a glycocalyx for adherence, flagella (with structural differences from bacteria), fimbriae, and unique grappling-hook-like structures called hami for attachment. Most archaea lack the peptidoglycan layer, using specialized polysaccharides and proteins for their cell walls, but all have cytoplasmic membranes to maintain gradients and control substance import/export.
Archaea cytoplasm contains 70S ribosomes (like bacteria but distinct differences), a cytoskeleton, and circular DNA. Despite being prokaryotic, archaea's genetic code and metabolic enzymes are genetically more similar to eukaryotic cells than to bacteria. This genetic distinction is why Archaea constitute their own domain of life, alongside Bacteria and Eukarya. A comparative table summarizes structural differences between Archaea and Bacteria.
Eukaryotic cells are reviewed, beginning with their glycocalyx, which is less organized than bacterial capsules but aids in cell adhesion, strengthening the cell surface, and cell-to-cell recognition. Some eukaryotic cells (fungi, plants, some protozoa) have cell walls, but animal cells do not. Eukaryotic cell walls are made of various polysaccharides (e.g., cellulose in plants, chitin in fungi) and lack peptidoglycan, explaining why antibiotics targeting peptidoglycan don't harm eukaryotic cells. All eukaryotic cells have a selectively permeable plasma membrane that exhibits the fluid mosaic model, containing steroid lipids for fluidity and stability, and membrane rafts for communication.
Eukaryotic flagella differ from prokaryotic flagella; they are contained within the cytoplasmic membrane, may be single or multiple, and undulate in a whip-like motion (e.g., sperm). Cilia, exclusive to eukaryotic cells, are shorter, more numerous, and beat in a coordinated fashion for movement or moving substances past the cell (e.g., in the trachea). Eukaryotic ribosomes are larger (80S) than prokaryotic ribosomes (70S). A cytoskeleton of microtubules helps maintain cell shape.
The lecture covers key membrane-bound organelles: the nucleus (containing genetic material and nucleoli for RNA synthesis), endoplasmic reticulum (rough ER with ribosomes for protein synthesis, smooth ER for lipid synthesis, both forming a transport system), Golgi apparatus (packaging and exporting materials via vesicles), lysosomes and peroxisomes (transfer and store chemicals; peroxisomes degrade poisonous waste like hydrogen peroxide, explaining why it kills bacteria but not eukaryotic cells). Mitochondria, the 'powerhouses of the cell,' produce most cellular ATP and are notable for having their own 70S ribosomes, two phospholipid bilayer membranes, and circular genetic material.
Chloroplasts, exclusive to photosynthetic eukaryotes, harvest light energy and produce ATP. Like mitochondria, they have 70S ribosomes, two membranes, and circular genetic material. A comprehensive table compares structures in prokaryotes and eukaryotes. This leads to the Endosymbiotic Theory, which proposes that eukaryotes evolved from large anaerobic prokaryotes engulfing smaller aerobic prokaryotes (evolving into mitochondria) and photosynthetic prokaryotes (evolving into chloroplasts). Evidence supporting this theory includes the similar size and shape of mitochondria/chloroplasts to bacteria, their 70S ribosomes, and their circular genetic material, suggesting a past symbiotic relationship.