Cell Biology: Structure and Function
Cell Theory
Postulates of the cell theory:
- All living organisms are composed of one or more cells.
- The cell is the basic structural, functional, and physiological unit of life.
- All cells arise from pre-existing cells through cell division.
- Hereditary material, containing the genetic characteristics of a cell, is passed from parent cell to daughter cell.
Prokaryotic Cells
Prokaryotic cells are typically small and possess a cell wall with a variably composed membrane. They lack a true nucleus, sometimes exhibiting a gelatinous sheath (glycocalyx) above the cell wall. The cytoplasm consists of two distinct regions: the nucleoid, containing the genetic material, and the remaining cytoplasm, housing ribosomes but lacking a cytoskeleton and endomembrane system. Prokaryotic cells may have flagella and reproduce through binary fission. Their key characteristic is morphological diversity.
Eukaryotic Cells
Eukaryotic cells are significantly more complex than prokaryotic cells. While they share features like a plasma membrane and ribosomes, they differ by possessing a nucleus, cytoplasmic organelles, and a cytoskeleton. These organelles enable compartmentalization, creating distinct spaces for specific metabolic activities, enhancing cellular function. Compartmentalization represents a territorial division within the cell, each area enclosed by a membrane. Polarity, on the other hand, refers to the specific arrangement of organelles in certain cells, crucial for neurons but unnecessary in blood cells.
Cell Membranes
Lipids: Primarily phospholipids, glycolipids, and sterols, all amphipathic, forming spherical micelles or lipid bilayers in aqueous environments. Their distribution in the membrane is asymmetrical and heterogeneous, allowing movement and fluidity, influenced by temperature, lipid composition, and cholesterol presence.
Proteins: Confer specific functions to the membrane, characteristic of each species. Globular in structure, they are classified as intrinsic (integrated within the bilayer, some spanning the entire membrane) and extrinsic (attached to lipids or transmembrane proteins, not crossing the bilayer). Their lateral diffusion contributes to membrane fluidity.
Carbohydrates: Oligosaccharides forming glycoproteins and glycolipids, distributed asymmetrically on the outer membrane surface. They constitute the glycocalyx, protecting the surface, conferring viscosity, aiding cell recognition, and facilitating substance uptake through phagocytosis or pinocytosis.
Features:
- Facilitates molecular transport.
- Acts as a semipermeable barrier.
- Enables ion, molecular, and macromolecular exchange.
- Recognizes extracellular signals and transmits information intracellularly.
- Mediates cell adhesion.
Plant Cell Wall
Acting as an exoskeleton, the plant cell wall is thick and rigid, present in bacteria, plants, algae, and fungi, external to the plasma membrane. Composed mainly of polysaccharides, it consists of chitin in fungi and cellulose in plants and algae. Structurally, it comprises a middle lamella, the primary wall, and the secondary wall (formed in non-growing cells, containing pectin, cellulose, and lignin). This wall is prominent in specialized plant tissues, providing mechanical support and transport, as in xylem.
Functions:
- Provides rigidity.
- Unites adjacent cells.
- Facilitates fluid exchange and cell communication.
- Allows plant cells to thrive in hypotonic environments.
- Waterproofs plant surfaces in some tissues.
- Acts as a barrier against pathogens.
Cytosol
Also known as hyaloplasm, the cytosol is the aqueous environment of the cytoplasm, housing cellular organelles. It constitutes about half the cell volume and contains high concentrations of proteins, mostly enzymes catalyzing metabolic reactions. Glycolysis and the biosynthesis of sugars, fatty acids, amino acids, and nucleotides occur in the cytosol. It also contains protein filaments forming the cytoskeleton. A significant portion of cellular proteins synthesized on ribosomes reside in the cytosol. Its high protein concentration gives it a viscous, gel-like consistency, organized by cytoskeletal fibers, believed to facilitate enzymatic reactions by localizing proteins to specific regions.
Functions:
- Regulates pH.
- Hosts most metabolic reactions.
- Facilitates biosynthesis of fatty acids and amino acids.
- Carries out glycogenesis and glycogenolysis.
Communication between the nucleoplasm and cytosol occurs through nuclear membrane pores.
Cytoskeleton
The cytoskeleton is a three-dimensional protein network providing internal support, anchoring internal structures, and participating in cell movement and division. In eukaryotes, it comprises microfilaments, intermediate filaments, and microtubules, while in prokaryotes, it mainly consists of structural proteins. It maintains cell shape, enables cellular motion (using cilia and flagella), and plays a crucial role in intracellular transport (vesicle and organelle movement) and cell division.
Components:
Microfilaments: Approximately 7 nm in diameter, composed of actin, existing in two forms: non-polymerized actin (G-actin) and polymerized actin (F-actin). F-actin forms a double helix and interacts with structural and regulatory proteins, notably myosin, enabling muscle contraction. Microfilaments are involved in muscle contraction, pseudopodia formation, cell morphology maintenance, and cytokinesis in animal cells.
Intermediate Filaments: Fibrous protein filaments (10-12 nm diameter), the most stable cytoskeletal components, supporting organelles and exhibiting tissue-specific composition (e.g., cytokeratin, vimentin, neurofilament). They organize the cell’s internal structure, participate in intercellular junctions (desmosomes).
Microtubules: Tubular structures (25 nm diameter), originating from microtubule organizing centers, composed of alpha and beta tubulin dimers forming 13 protofilaments. They participate in vesicle movement, organelle movement, intracellular transport, and cell division (mitosis and meiosis) by forming the mitotic spindle. They also constitute the internal structure of cilia and flagella.