Cell Biology and Histology: Core Concepts and Processes

Cell Biology and Histology

Characteristics of Cell Biology

Cell biology is the study of cells, the fundamental units of life. Understanding cell structure and function is essential for comprehending all biological processes.

• Cell: the smallest unit of life, capable of performing all vital physiological functions.

• Cytoplasm vs. Cytosol: Cytoplasm includes all cellular contents between the plasma membrane and the nucleus, including organelles and cytosol. Cytosol is the fluid portion of the cytoplasm, containing dissolved ions, nutrients, and proteins.

• Plasma Membrane: Serves as a selective barrier, controlling entry and exit of substances; also involved in cell communication.

• Nucleus: Contains genetic material (DNA) and directs cellular activities.

• Cell Shapes: Cells vary in shape (e.g.,squamous, cuboidal, columnar, spherical, spindle-shaped to suit their functions.

• Surface-to-Volume Ratio: As a cell increases in size, its volume grows faster than its surface area, affecting nutrient exchange. Example: Microvilli increase surface area for absorption in intestinal cells.

• Apoptosis: Programmed cell death, a normal process for removing unneeded or damaged cells.

Cell Membrane

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates molecular traffic.

• Chemical Composition: Composed of lipids (mainly phospholipids), proteins, and carbohydrates.

• Fluid Mosaic Model: Describes the membrane as a flexible layer with proteins embedded or attached, allowing lateral movement.

• Membrane Permeability: Determined by the distribution of lipids and proteins; lipid-soluble molecules pass more easily.

• Membrane Proteins: Include structural proteins (support), receptor proteins (signal reception), and channels (transport).

• Intracellular vs. Extracellular Fluid: Intracellular fluid is inside cells, rich in potassium and proteins; extracellular fluid is outside, rich in sodium and chloride.

• Selective Permeability: Essential for homeostasis; allows some substances to cross while restricting others.

• Permeable Molecules: Small, nonpolar molecules (O2, CO2); Impermeable: Large, polar molecules (glucose), ions.

Membrane Transport

Cells use various mechanisms to move substances across membranes, maintaining internal balance and communication.

• Types of Gradients: Concentration (difference in solute), osmotic (water), pressure, and electrical (charge) gradients.

• Fick's Law of Diffusion: The rate of diffusion increases with greater concentration gradient, surface area, and solubility, and decreases with larger molecular size. Equation: $J = -D \frac{dC}{dx}$ Where: J = flux, D = diffusion coefficient, dC/dx = concentration gradient.

• Passive Transport: No energy required; includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (ATP); moves substances against gradients (e.g., Na+/K+ pump).

• Types of Membrane Transport:

  • Simple Diffusion: Movement of small, nonpolar molecules.

  • Facilitated Diffusion: Uses carrier or channel proteins for larger or polar molecules.

  • Osmosis: Diffusion of water across a semipermeable membrane.

  • Primary Active Transport: Direct use of ATP (e.g., Na+/K+ ATPase).

  • Secondary Active Transport: Uses energy from another gradient (e.g., glucose-sodium cotransport).

• Leak and Gated Channels: Leak channels are always open; gated channels open in response to ligands, voltage, or mechanical stimuli.

• Tonicity: Hypotonic solutions cause cells to swell, isotonic solutions maintain cell size, hypertonic solutions cause cells to shrink.

• Vesicular Transport: Exocytosis (out of cell), endocytosis (into cell), phagocytosis (engulfing large particles).

Membrane Potential

Membrane potential is the voltage difference across the plasma membrane, crucial for nerve and muscle function.

• Resting Membrane Potential (RMP): The steady-state voltage (typically -70 mV in neurons) across the membrane when the cell is at rest.

• Ion Gradients and Permeability: RMP is established by differences in ion concentrations (mainly Na+ and K+) and selective permeability.

• Electrochemical Gradient: The combined effect of concentration and electrical gradients on ion movement.

• Sodium-Potassium ATPase: Maintains RMP by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Cell Signaling

Cells communicate with their environment and each other through signaling molecules and membrane proteins.

• Cell Adhesion Molecules (CAMs): Enable cells to adhere to each other and the extracellular matrix, important for tissue structure and signaling.

• Membrane Receptors: Detect external signals; G protein–coupled receptors activate intracellular pathways in response to ligand binding.

Cytoplasmic Organelles

Organelles are specialized structures within cells, each performing distinct functions necessary for cell survival.

• Organelle: A membrane-bound structure with a specific function (e.g., mitochondria, endoplasmic reticulum).

• Major Organelles:

  • Mitochondria: ATP production via cellular respiration.

  • Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies.

  • Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

  • Lysosomes: Digestive enzymes for breakdown of waste.

  • Peroxisomes: Detoxify harmful substances.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structure and movement.

• Microvilli vs. Cilia: Microvilli increase surface area for absorption; cilia move substances across cell surfaces.

Nucleus and Protein Synthesis

The nucleus stores genetic information and directs protein synthesis through transcription and translation.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Nucleoli: Sites of ribosome assembly.

• Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

• Genetic Code: The sequence of nucleotides in DNA that determines amino acid sequence in proteins.

• Gene: A segment of DNA coding for a protein.

• Transcription: Synthesis of mRNA from DNA template.

• Translation: Synthesis of protein from mRNA template at the ribosome.

• Central Dogma: DNA → RNA → Protein.

• RNA Synthesis: Involves RNA polymerase, producing mRNA, tRNA, and rRNA.

• Roles of RNA: mRNA carries code, tRNA brings amino acids, rRNA forms ribosomes.

Cellular Energy and Respiration

Cells obtain energy through the breakdown of glucose in a series of metabolic pathways.

• Cellular Respiration: The process of converting glucose and oxygen into ATP, CO2, and water.

• Pathways:

  • Glycolysis: Occurs in cytosol; glucose → 2 pyruvate + 2 ATP + 2 NADH.

  • Krebs Cycle: In mitochondria; pyruvate → CO2 + NADH + FADH2 + 2 ATP (per glucose).

  • Electron Transport Chain: In mitochondria; NADH/FADH2 → ATP (about 32-34 per glucose).

• Oxygen: Final electron acceptor in aerobic respiration.

• Anaerobic Respiration: Occurs without oxygen; yields less ATP (2 per glucose).

• Fermentation: Converts pyruvate to lactate or ethanol; regenerates NAD+ for glycolysis.

• Aerobic Respiration: Requires oxygen; yields up to 36-38 ATP per glucose.

Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• Phases: G1 (growth), S (DNA synthesis), G2 (preparation), M (mitosis), C (cytokinesis).

• Daughter Cell: Resulting cell after division; Parent Cell: Original cell.

• Cytokinesis: Division of cytoplasm.

• DNA Replication: Semi-conservative process involving helicase (unwinds), DNA polymerase (synthesizes), ligase (joins fragments).

• DNA Molecule vs. Chromosome vs. Sister Chromatids: DNA molecule is the double helix; chromosome is condensed DNA; sister chromatids are identical copies joined at the centromere.

• Mitosis vs. Meiosis: Mitosis produces 2 identical cells; meiosis produces 4 genetically unique gametes.

Characteristics of Histology

Histology is the study of tissues, groups of similar cells performing specific functions.

• Four Major Tissue Types: Epithelial (covering), Connective (support), Muscle (movement), Nervous (control).

Epithelial Tissue

Epithelia cover body surfaces and line cavities, forming protective barriers and specialized structures.

• Common Characteristics: Closely packed cells, polarity (apical/basal surfaces), avascular, high regeneration.

• Glands: Exocrine (secrete via ducts), endocrine (secrete hormones into blood).

Connective Tissue

Connective tissues support, bind, and protect other tissues and organs.

• Mesenchyme: Embryonic tissue giving rise to all connective tissues.

• Common Features: Cells scattered in extracellular matrix (ECM), presence of fibers (collagen, elastic, reticular).

• Comparison to Epithelial Tissue: Connective tissue has fewer cells, more ECM, and provides support; epithelial tissue has more cells, less ECM, and covers surfaces.

Membranes

Body membranes are sheets of tissue that cover or line body surfaces.

• Cutaneous Membrane: The skin; protects body surface.

• Mucous Membrane: Lines cavities open to exterior (e.g., digestive tract); secretes mucus.

• Serous Membrane: Lines closed cavities (e.g., peritoneum); secretes serous fluid.

Tissue Repair

Tissue repair restores structure and function after injury.

• Regeneration: Replacement of destroyed tissue with the same kind of cells.

• Fibrosis: Replacement with scar tissue (dense connective tissue).

• Healing Process: Involves inflammation, organization (granulation tissue), and regeneration/fibrosis.

Feature	Epithelial Tissue	Connective Tissue
Cell-ECM Ratio	High cell, low ECM	Low cell, high ECM
Fibers Present	Few or none	Collagen, elastic, reticular
General Function	Covering, lining, secretion	Support, binding, protection