Comprehensive Biology Review Guide: Cell Theory, Histology, and Cellular Function

Principles of Cell Theory

• Foundational Tenets (C150):     * The cell is the smallest structure capable of carrying out life processes.     * Every organism is made up of one or more cells.     * Cells can only form from pre-existing cells.

• Organization of Cells:     * Prokaryotic cells: Cells without an organized nucleus.     * Eukaryotic cells: Cells with an organized and well-defined nucleus.

• Structures Common to Most Cells:     * Plasma (cell) membrane: Separates internal and external environments.     * DNA: Genetic material that controls an organism’s composition and replicates the cell.     * Cytoplasm: Can be divided into two portions:         * Cytosol: The liquid portion, often referred to as intracellular fluid (ICF). It is the clear gel within the cell.         * Organelles: Tiny, organized structures that perform functions within the cell.     * Ribosomes: The site of protein synthesis.     * Cytoskeleton: Found in Eukaryotic Human Cells. Responsible for intracellular support, cell movement, and moving organelles.

• Cell Classification by Function:     * Gamete (sex) cells: Cells used for reproduction, specifically sperm and ovum. These replicate via meiosis.     * Somatic (body) cells: All other cells in the body. These replicate via mitosis.

Introduction to Cell Membranes and Function

• Functions (C151):     * Serves as a barrier protecting the cell’s inner content from the extracellular environment.     * Anchors the cytoskeleton and shapes the cell.     * Allows cell ‘communication’ through the passage of chemicals.     * Facilitates the movement of essential molecules into cells.     * Facilitates the removal of waste from cells.

• Phospholipid Bilayer Composition:     * Hydrophilic heads: Water ‘loving’ (lipophobic) and charged chemicals that attract water.     * Hydrophobic tails: Water ‘hating’ (lipophilic) and non-charged or neutral chemicals that repel water.

• Molecules Embedded in the Bilayer:     * Transmembrane proteins: Cross the entire width of the membrane, touching both internal and external surfaces.     * Peripheral proteins: Found only on one surface (inner or outer surfaces).     * Cholesterol: Creates the fluid mosaic model, providing structure, flexibility, and insulation from heat while decreasing overall membrane fluidity (‘stiffness’).     * Sugars: Includes Glycolipids and Glyocoproteins [sic]. They provide protection, display signals, and attach cells.

Plasma Membrane Proteins

• Integral Proteins (Transmembrane):     * Examples: Ion channels and proton pumps.     * Channel proteins: Allow ions to cross the membrane from one side to another.     * Carrier proteins: Transport specific substances from one side of the plasma membrane to the other.     * Receptor proteins: Recognize and bind to specific molecules called ligands.     * Cell Identity Markers: Allow the cell to be identified and distinguished by the immune system.     * $Na^+/K^+$ ATPase pump: Maintains an electrochemical gradient.     * Gap Junctions: Form a connection between two cells.     * Enzymes: Catalyze (speed up) chemical reactions inside or outside the cell.     * Cell Adhesion Molecules: Serve as structural support and stability for a cell.

• Peripheral Proteins:     * Cadherins: Proteins that transport extracellular ligand (chemical) signaling into cells.     * Cyochrome C [sic]: Transfer electrons from one protein to another.     * Spectrin: In charge of maintaining red-cell shape.         * Hereditary spherocytosis: A clinical condition caused by the absence of spectrin, resulting in red blood cells that lack the central pallor (absence of the pale center).

Mechanisms of Movement Across the Plasma Membrane

• Passive Transport:     * Does not use $ATP$ as an energy source.     * Occurs from high to low concentration (downhill).     * Simple Diffusion: Movement along the concentration gradient from higher to lower concentration.     * Facilitated Diffusion: Use of a protein channel to move chemicals passively across the membrane.     * Osmosis: Movement of water across a semipermeable membrane from low solute concentration to high solute concentration.         * Hypertonicity: Solutions with high ECF osmolarity; water moves out of the cell, causing it to shrink.         * Isotonicity: Solutions with the same osmolarity; no net movement of water.         * Hypotonicicity: Solutions with low ECF osmolarity; water moves into the cell, causing swelling and potential bursting.

• Active Transport:     * Utilizes energy to move substances against the concentration gradient (lower to higher concentration).     * Dependencies: $ATP$, Membrane proteins, Concentration of molecules, Membrane potentials.     * Bulk Transport: Uses vesicles for large molecules.         * Endocytosis: Transporting material into the cell (‘Entering’).             * Phagocytosis: Cell eating (solids).             * Pinocytosis: Cell drinking (liquids).         * Exocytosis: Transporting material out of the cell (‘Exiting’).

Membrane Potential and Electrochemical Dynamics

• Fluid Compartments:     * Intracellular fluid (ICF): Also known as cytosol; more gel-like than ECF.     * Extracellular fluid (ECF): Sometimes called ‘tissue fluid’; includes plasma.

• Resting Membrane Potential (RMP):     * Voltage: Any difference in electrical charge ($+‐$).     * RMP Value: $-70\,mV$.     * Uneven distribution of ions is controlled by protein pumps and leak channels, resulting in more positive ions outside and more negative ions inside.

• The Sodium-Potassium Pump:     * Moves $3$ sodium ions ($Na^+$) out and $2$ potassium ions ($K^+$) into the cell.     * Creates a more negative charge inside the cell.     * Critical for muscle and nervous system function.

• Voltage Changes:     * Depolarization: $Na^+$ channels open and $Na^+$ enters the cell, making it more positive.     * Repolarization: $Na^+$ channels close, $K^+$ channels open, and $K^+$ rushes out of the cell.     * The sodium-potassium pump helps the cell return to $RMP$ after these events.

Organelles and Cellular Structure (C152)

• Membranous Organelles (covered with lipid membrane):     * Nucleus, Mitochondrion, Lysosomes, Peroxisomes.     * Endoplasmic reticulum (Smooth & Rough).     * Golgi complex.

• Non-membranous Organelles (made of proteins, not covered by lipids):     * Nucleolus: Dark region at the center of the nucleus made of chromatin (thread-like strands of $DNA$).     * Inclusion Bodies: An abnormal structure floating within the cytoplasm.     * Cytoskeleton: Facilitates intracellular transport and provides structure.

Cell Functions and Metabolism (C153)

• The Central Dogma:     * 1. Transcription: Double-stranded ($DS$) $DNA$ template creates an $mRNA$ copy using $RNA$ bases.     * 2. Translation: Ribosome interprets $mRNA$ code to construct protein.

• RNA Types:     * Messenger RNA (mRNA): The genetic code.     * Transfer RNA (tRNA): Carries amino acids to the ribosome.     * Ribosomal RNA (rRNA): Forms the ribosome.

• Steps of Translation:     * 1. Initiation: Using a start codon such as $AUG$.     * 2. Elongation.     * 3. Termination: Using stop codons such as $UGA$, $UAA$, or $UAG$.

• Genetic Coding and Base Pairing:     * DNA: $A-T$ (Apple – Tree), $G-C$ (Car – Garage).     * RNA: $A-U$ (Apple – Under), $G-C$.

• Cellular Respiration and ATP:     * ATP (Adenosine Triphosphate): The cell's energy transfer molecule; energy is stored in phosphate-phosphate bonds.     * Chemiosmosis: Use of the electrochemical $H^+$ gradient to create $ATP$.     * Process Equation: $C_6H_{12}O_6 + 6O_2 + ADP + Pi \rightarrow 6CO_2 + 6H_2O + ATP$.     * Steps: 1. Glycolysis, 2. Krebs cycle, 3. Electron transport.

Cell Cycle and Division

• Interphase:     * S phase: $DNA$ division (Semi-conservative replication results in $2$ new identical strands from one original).     * G1 & G2: Gap phases.     * Chromatin: $DNA$ wrapped around histones.     * Chromosomes: Further wrapping of chromatin occurring in prophase.

• Mitosis (Division of Nucleus):     * Prophase: Nuclear membrane breaks down; chromatin condenses; spindle fibers form from centrioles; centrioles migrate; Cross-over occurs between non-sister chromatids for genetic variability.     * Metaphase: Chromosomes line up in the middle.     * Anaphase: Chromosomes separate and pull to opposite poles.     * Telophase: New nuclear membranes form.     * Cytokinesis: The cell itself divides into two identical daughter cells.

• Ploidy levels:     * Diploid: $2n$.     * Haploid: $n$.

Histology: The Study of Tissues

• Major Tissue Types (C-MEN):     * Epithelial: Lines cavities, covers surfaces, forms glands.     * Connective: Support and protection; the most diverse type.     * Muscle: Body movement and internal substance movement.     * Nervous: Conducts electrical impulses and processes information.

• Epithelial Tissue Traits:     * Avascular (no blood vessels); depends on connective tissue for $O_2$ and nutrients.     * Attached via basement membrane; has an apical (top) surface.     * Classification by Layers: Simple (single), Stratified (multiple), Pseudostratified (fake multilayers), Transitional (switches).     * Classification by Shape: Squamous (flat), Cuboidal (square), Columnar (column).     * Glands:         * Merocrine: Released via exocytosis (salivary, sweat).         * Apocrine: Released in apical cytoplasm (mammary, ceruminous ear wax).         * Holocrine: Secretions via loss of the entire cell (oil glands).

• Connective Tissue Traits:     * Sparse cells with an extracellular matrix (ground substance + protein fibers like elastin, collagen, reticular).     * Cell types: Fibroblasts, Mast cells, Macrophages, Mesenchymal cells, Adipocytes.     * Types: Loose, Dense, Blood, Cartilage, Bone.

• Muscle Tissue Subtypes:     * Skeletal: Voluntary, striated, multinuclear, attaches to bone/skin.     * Cardiac: Involuntary, striated, uninuclear, intercalated discs/specialized junctions.     * Smooth: Involuntary, uninuclear, found in hollow organs.

• Nervous Tissue Anatomy:     * Soma: Cell body with nucleus.     * Dendrites: Receive signals.     * Axon: Carries electrical signal.     * Axon Terminals: Convert electrical signals to chemical neurotransmitters.     * Synapse: Space between neuron and effector cell.

Body Membranes and Intercellular Junctions

• Membrane Types:     * Cutaneous: Epithelium attached to connective tissue; first line of defense; thermoregulation.     * Mucous: Epithelium attached to lamina propria (areolar CT); lines cavities exposed to external environment (Respiratory, Digestive, Urogenital).     * Serous: Found in closed ventral cavities; has Parietal layer (lines walls), Visceral layer (covers organs), and Serous fluid.         * Pleura: Lungs.         * Pericardium: Heart.         * Peritoneum: Enclosed organs.     * Synovial: Loose connective tissue ONLY (no epithelium); produces synovial fluid for joint/tendon lubrication.

• Intercellular Junctions:     * Tight junctions: Impermeable barrier; ‘seals off’ regions.     * Desmosome: Strong structural network; resists twisting/stretching.     * Hemidesmosomes: Use integrins to anchor epithelial cells to the basal lamina/basement membrane.     * Gap junctions: Allow ions and molecules to pass directly between two cells.