1.3 Part of a Cell
Fundamentals of Cell Biology
Universal Components: All cells, whether they are prokaryotic or eukaryotic, share basic components and principles referred to as the building blocks and the dogma of biology.
Cell Biology Context: Understanding cell parts is critical for clinicians and scientists because many microbes and viruses function by disrupting these components.
Clinical Application: Knowledge of cellular substructures allows for:
Understanding the consequences for patients suffering from viral or bacterial infections.
Designing targeted medical treatments based on specific cellular parts.
The Plasma Membrane: Structure and Chemical Dynamics
Overview: The plasma membrane serves as the outer covering of the cell, functioning similarly to human skin. It provides a barrier that separates the internal environment of the cell from the external environment.
Composition: It is defined as a semipermeable membrane composed of a phospholipid bilayer.
Structural Differences by Cell Type:
Prokaryotic Cells: Lack internal membrane-bound organelles; the plasma membrane is the primary separator of internal and external environments.
Eukaryotic Cells: Possess internal membranes that create intracellular compartments, allowing for higher levels of organization and separation of molecules from the external environment.
Phospholipid Chemistry:
Phosphate Head Group: Negatively charged and hydrophilic (attracted to water).
Lipid Tails: Hydrophobic (repel water). They are attracted to one another, much like the separation seen in an oil-based salad dressing where oil and water do not mix.
Arrangement: This attraction/repulsion dynamic results in the bilayer arrangement where heads face the water (inside and outside the cell) and tails face inward.
Permeability and Transport:
Selective Passage: Small molecules such as , small nutrients, and wastes can pass through the membrane selectively.
Protein Channels: Larger molecules (e.g., sugar) or charged particles require specific protein channels, embedded in the membrane, to move across.
Mechanical and Functional Properties:
Movement of Particles: Membrane-embedded proteins facilitate the import of nutrients and the export of wastes or signaling molecules.
Sensory Proteins: Protein sensors receive environmental information, allowing the cell to adjust its behavior to survive extracellular changes.
Expansion and Repair: The membrane can grow and expand by adding more membrane material as the cell enlarges. Additionally, the membrane can self-reseal if it is torn or punctured.
The Cell Wall: Structure and Diversity
Definition: A rigid outer membrane located outside the plasma membrane that provides support, tensile strength, structure, and turgor pressure.
Osmotic Protection: Since water moves freely across both the plasma membrane and cell wall, cells are at risk of osmotic imbalance. The cell wall prevents osmotic lysis (bursting) when a cell moves between different environments.
Bacterial Cell Walls:
Prevalence: of bacterial cells possess a wall.
Exceptions: Bacteria known as Mycoplasma lack a cell wall.
Composition: Composed of Peptidoglycan, a molecule consisting of polysaccharides (sugars) and proteins. This specific type of peptidoglycan is unique to bacteria and is not found anywhere else on Earth.
Structure: Polysaccharides act like "bricks" that stack together, while protein chains (amino acids) act as the cross-linking "mortar" to stabilize the structure.
Antibiotic Target: Because eukaryotic cells lack peptidoglycan, it is a primary target for antibiotics designed to disrupt bacterial infections.
Bacterial Classification (Gram Staining):
Gram-Positive: Bacteria with a very thick peptidoglycan wall that take up specific stains.
Gram-Negative: Bacteria with a very thin peptidoglycan wall.
Archaea Cell Walls: Do not contain peptidoglycan. Instead, they use surface layer proteins (S-layer proteins) which vary by organism. Some Archaea lack a wall entirely.
Plant Cell Walls:
Cellulose: A long linear polymer of sugar molecules; it is one of the most abundant macromolecules on Earth.
Pectin: A carbohydrate and soluble protein. Ratios of cellulose to pectin vary by plant type.
Fungal Cell Walls: Composed of a combination of cellulose and Chitin.
Chitin: A derivative of glucose associated with high strength. It is also found in the exoskeletons of crustaceans (lobsters, crabs) and arthropods (spiders).
Yeast: A type of fungus that contains varying amounts of chitin and mannoproteins.
Diatoms: A group of microalgae found in soil and water. Their cell walls are unique in that they are made of silica.
Internal Environments: Cytoplasm, Cytosol, and the Nucleus
Distinguishing Cytoplasm and Cytosol:
Cytoplasm: The entire compartment contained by the plasma membrane, including macromolecules and organelles.
Cytosol: The watery gel that suspends organelles and provides the site for many chemical reactions.
The Nucleus: The defining feature of eukaryotic cells and the "command center."
Primary Function: To house and protect DNA (hereditary information) while preventing contact with chemicals that could cause mutations.
Nuclear Envelope: A double membrane that creates the nuclear compartment.
Nuclear Pores: Channels or holes in the envelope allowing large molecules to enter the nucleus and RNA to exit into the cytoplasm.
Nucleolus: A darkened area within the nucleus where ribosomal RNA () is transcribed and synthesized.
The Endomembrane System and Protein Synthesis
Ribosomes: These are not organelles because they are not membrane-bound. They are made of and are responsible for protein synthesis.
Endoplasmic Reticulum (ER): A series of interconnected membrane-enclosed sacs and tubes continuous with the nuclear membrane.
Rough ER: Studded with ribosomes, giving it a "rough" appearance. It is the site of translation (RNA into proteins).
Smooth ER: Lacks ribosomes. It is responsible for:
Production of membrane components: lipids, phospholipids, and steroids.
Storage of calcium ions ().
Metabolism of carbohydrates.
Golgi Complex (Apparatus): A series of flattened membranous sacs called cisternae. It functions as the cell's "distribution center" or "post office."
Cis-face: The receiving side for vesicles/macromolecules coming from the ER.
Trans-face: The shipping side where modified molecules are sent to their destination or exocytosed (exported).
Lysosomes: The waste disposal system of the cell.
Mechanism: Contain hydrolytic enzymes that function only at an acidic pH. If they escape the lysosome, they become inactive in the less acidic environment of the rest of the cell.
Phagocytosis: Fusing with food vacuoles to digest particles for cell use.
Autophagy: A process meaning "eat itself" where the cell disposes of its own worn-out organelles (e.g., mitochondria) or macromolecules by merging them with lysosomes.
Energy-Converting Organelles
Mitochondria: Sites of cellular respiration.
Function: Use oxygen to generate (Adenosine Triphosphate), the fuel for the cell's chemical reactions.
Unique Traits: Double membrane structure, contain their own ribosomes, and possess their own DNA. They can grow and reproduce independently of the cell.
Structure:
Cristae: Inner folded membranes that increase surface area for energy production reactions.
Spaces: Divided into the intermembrane space and the mitochondrial matrix.
Endosymbiont Theory: The theory that an ancestral eukaryotic predecessor cell engulfed a prokaryotic cell (bacterium), creating a symbiotic relationship that eventually led to the development of mitochondria.
Chloroplasts: Found in green plants and algae; part of the plastid group.
Function: Photosynthesis—using sunlight to produce glucose (chemical energy), which is then shuttled to mitochondria to generate .
Pigment: Contain Chlorophyll, which provides the green color.
Structure: Contain their own DNA. Features include membranous sacs called thylakoids, which stack together to form structures called grana (singular: granum).