Summary
Highlights
This section introduces prokaryotic cells and begins by discussing structures found in all cells: a plasma or cell membrane, chromosomes (DNA), ribosomes for protein synthesis, and cytosol. It highlights that not all cells have a cell wall.
This part contrasts prokaryotic and eukaryotic cells, focusing on the absence of a membrane-bound nucleus and histones in prokaryotes, their circular and unpaired chromosomes, and their asexual reproduction via binary fission. Eukaryotic cells, in contrast, have a true nucleus, paired linear chromosomes, histones, and membrane-bound organelles, replicating using mitotic spindles.
Discusses the average size of bacteria (around one micrometer) and introduces monomorphic (single shape) and pleomorphic (many shapes) bacteria. Common shapes include bacillus (rod), coccus (sphere), and spiral forms like vibrio, spirillum, and spirochete. It also covers bacterial arrangements such as diplo (paired), staphylo (clusters), and strepto (chains).
Exploration of the glycocalyx, a gelatinous outer layer that can be a loosely organized slime or a tightly attached capsule. Functions include preventing dehydration and nutrient loss, aiding adherence to surfaces, forming protective biofilms, and acting as a virulence factor against phagocytosis. Biofilms are complex microbial communities that increase antibiotic resistance and nutrient access.
Details the S-layer, a protective protein layer produced in hostile environments. It then delves into flagella, the primary structures for motility in prokaryotes, consisting of a filament, hook, and basal body. Different arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous. Bacterial movement involves 'runs' (directed movement) and 'tumbles' (changing direction), guided by chemotaxis or phototaxis. The H antigen on flagella is used for strain identification.
Discusses endoflagella (axial filaments) in spirochetes, which enable corkscrew-like movement, such as in the syphilis-causing Treponema pallidum. Fimbriae are hair-like bristles for attachment to host tissues, essential for colonization. Pili are rigid, tubular structures in Gram-negative cells primarily used for conjugation (transfer of genetic material), but can also assist in attachment and motility.
Covers the cytoplasm, a semi-fluid substance vital for metabolism and cellular synthesis. The nucleoid region houses the bacterial chromosome, typically a single, circular, double-stranded DNA molecule containing essential genes. Plasmids are extra-chromosomal genetic elements carrying non-crucial genes (e.g., antibiotic resistance) that replicate independently. Prokaryotic ribosomes (70S) are responsible for protein synthesis and are distinct from eukaryotic ribosomes (80S).
Describes various intracellular inclusions for temporary storage, including metachromatic granules (phosphate reserves), polysaccharide granules (energy), lipid inclusions, sulfur granules, carboxysomes (rubisco enzyme for photosynthesis), gas vacuoles (buoyancy), and magnetosomes (destroy hydrogen peroxide). Endospores, mainly found in Bacillus and Clostridium, are dormant structures for survival in harsh conditions, undergoing sporulation and germination.
Focuses on the bacterial cell wall, primarily composed of peptidoglycan, which prevents osmotic lysis and protects the cell membrane. Peptidoglycan is a polymer of disaccharides (NAG and NAM) linked by amino acid cross-bridges, with alternating D and L amino acids for structural stability.
Compares Gram-positive and Gram-negative cell walls. Gram-positives have a thick peptidoglycan layer with teichoic acids (lipoteichoic and wall teichoic acids), which help regulate ion movement and serve as antigens. Gram-negatives have a thin peptidoglycan layer, lack teichoic acids, and possess an outer membrane containing lipopolysaccharides (LPS) with O polysaccharide (antigen), core polysaccharide (stability), and lipid A (endotoxin). The outer membrane protects against phagocytosis, complement, and antibiotics.
Reviews the four steps of the Gram stain, explaining how crystal violet, iodine, decolorizer, and safranin differentiate bacteria based on cell wall thickness. Gram-positives stain purple, while Gram-negatives stain reddish-pink. Also discusses atypical cell walls such as Mycobacterium (acid-fast with mycolic acid, resistant to chemicals and digestion) and Mycoplasma (lack cell walls, pleomorphic, sensitive to dehydration and hypotonic environments, contain sterols).
Introduces the fluid mosaic model of the cell membrane, describing it as a fluid phospholipid bilayer with embedded proteins (integral and peripheral). The amphipathic nature of phospholipids, with hydrophilic heads and hydrophobic tails, creates the bilayer structure. Prokaryotic cell membranes often lack sterols, unlike eukaryotic cells which use cholesterol (or ergosterol in fungi) to maintain fluidity.
Details the four main components of the cell membrane: phospholipid bilayer, cholesterol (for fluidity), proteins (transport, enzymatic activity, signal transduction, cell-to-cell recognition, intercellular joining, attachment to cytoskeleton), and glycocalyx (sugars for binding, lubrication, and adhesion). The hydrophobic core of the membrane determines what molecules can cross without aid.
Explains diffusion as the movement of molecules from high to low concentration (down the concentration gradient), a spontaneous process dependent on molecular thermal motion. Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from high water concentration (low solute) to low water concentration (high solute).
Defines tonicity as a solution's ability to cause a cell to gain or lose water. In a hypertonic solution (high solute outside), water leaves the cell, causing animal cells to shrivel and plant cells to undergo plasmolysis. In a hypotonic solution (low solute outside), water enters the cell; animal cells may lyse, while plant cells become turgid (desired state). An isotonic solution (equal solute) results in no net water movement, leading to normal animal cells but flaccid plant cells.
Explains passive transport, which requires no energy and moves molecules down the concentration gradient. Simple diffusion allows small, hydrophobic, nonpolar molecules (e.g., steroid hormones, O2, CO2) to cross the membrane directly. Facilitated diffusion uses transport proteins (channel or carrier proteins) to help ions and polar molecules (e.g., water via aquaporins) cross the membrane.
Describes active transport, which uses energy (usually ATP) and transport proteins to move molecules against their concentration gradient (from low to high). Examples include ion pumps (e.g., proton pump, sodium-potassium pump), which are electrogenic (create a charge difference across the membrane). Co-transporters couple the uphill movement of one molecule with the downhill movement of another, like sucrose-proton co-transporters.
Covers bulk transport for large molecules. Exocytosis (exit) uses vesicles that fuse with the plasma membrane to release contents outside the cell (e.g., proteins, hormones, neurotransmitters), also increasing membrane surface area. Endocytosis (into cell) brings large particles in: - Phagocytosis ('cellular eating') involves pseudopodia engulfing particles like bacteria or food. - Pinocytosis ('cellular drinking') involves the membrane pinching inward to take in fluid and non-specific solutes. - Receptor-mediated endocytosis uses specific receptors to bind ligands, forming coated pits to internalize large amounts of specific molecules (e.g., LDLs carrying cholesterol).