Cells: The Living Units

Introduction to Cytology

Cytology is the study of cells, the fundamental units of life. Human cells are microscopic, requiring specialized instruments for visualization, and exhibit a wide variety of shapes and sizes that reflect their diverse functions.

• Cell Size: Measured in micrometers (µm), most cells are only visible under a microscope.

• Microscopy: Two main types are used:

  • Light Microscope: Uses colored stains and light to produce 2D images; limited resolution.

  • Electron Microscope: Uses heavy-metal stains and electron beams for higher resolution.

    • Scanning Electron Microscope (SEM): Produces 3D images of surfaces.

    • Transmission Electron Microscope (TEM): Produces 2D images of thin sections.

• Cell Shape and Function: Cell morphology (spherical, cuboidal, columnar, etc.) is closely related to function.

Common Features and General Functions of Cells

All human cells share certain structural features and perform essential functions necessary for life.

• Plasma Membrane: Outer boundary that maintains cell integrity and mediates interactions with the environment.

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

• Cytoplasm: Includes cytosol (intracellular fluid), organelles, and inclusions (e.g., pigments, nutrients).

• General Functions:

  • Maintaining cell shape and integrity

  • Acquiring nutrients and synthesizing molecules

  • Eliminating cellular waste

Structure and Function of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Phospholipid Bilayer: Amphipathic molecules with hydrophilic heads facing outward and hydrophobic tails inward.

• Cholesterol: Stabilizes membrane, especially during temperature changes.

• Membrane Proteins:

  • Integral Proteins: Span the bilayer; amphipathic.

  • Peripheral Proteins: Attach to integral proteins on the membrane surface.

  • Functional Types:

    • Transport proteins (channels, carriers, pumps)

    • Cell surface receptors (bind ligands such as hormones)

    • Identity markers (self-antigens)

    • Enzymes (catalyze reactions)

    • Anchoring sites (attach cytoskeleton and extracellular matrix)

    • Cell-adhesion proteins (connect cells)

Membrane Transport Mechanisms

Cells regulate the movement of substances across the plasma membrane through passive and active processes.

• Passive Transport: Does not require cellular energy; relies on concentration gradients.

  • Simple Diffusion: Movement from high to low concentration until equilibrium is reached.

  • Facilitated Diffusion: Requires membrane proteins for polar or charged solutes.

    • Channel-mediated (ion channels: leak or gated)

    • Carrier-mediated (for small polar molecules like sugars, amino acids)

  • Osmosis: Diffusion of water across a selectively permeable membrane.

    • Water moves through aquaporins (water channels).

    • Osmotic Pressure: The force driving water movement.

    • Hydrostatic Pressure: Pressure exerted by a fluid within a container (e.g., blood pressure).

    • Tonicity: Effect of solution on cell volume:

      • Isotonic: Equal solute concentration inside and outside; no net water movement.

      • Hypotonic: Lower solute concentration outside; water enters cell (may swell/lyse).

      • Hypertonic: Higher solute concentration outside; water leaves cell (crenation/shrinkage).

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradients.

  • Primary Active Transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary Active Transport: Uses energy from movement of another substance down its gradient.

    • Symport: Two substances move in the same direction.

    • Antiport: Two substances move in opposite directions.

  • Bulk (Vesicular) Transport: Movement of large substances via vesicles.

    • Exocytosis: Vesicles fuse with membrane to release contents outside cell.

    • Endocytosis: Cell engulfs substances by forming vesicles.

      • Phagocytosis: "Cell eating" of large particles.

      • Pinocytosis: "Cell drinking" of extracellular fluid.

      • Receptor-mediated Endocytosis: Specific uptake via ligand-receptor complexes.

Table: Comparison of Membrane Transport Mechanisms

Electrochemical Gradients and Resting Membrane Potential (RMP)

The plasma membrane maintains an electrochemical gradient essential for nerve and muscle function.

• Electrochemical Gradient: Combination of concentration (chemical) and electrical (charge) differences across the membrane.

• Resting Membrane Potential (RMP): The voltage difference across the membrane at rest, typically −70 mV in neurons.

• Key Factors:

  • Unequal distribution of Na+ (higher outside) and K+ (higher inside)

  • Large negatively charged proteins inside the cell

  • Na+/K+ pump maintains gradients

  • K+ diffusion is most important in establishing RMP

Equation for Nernst Potential (for a single ion):

Additional info: R = gas constant, T = temperature, z = charge, F = Faraday's constant.

Cell Communication

Cells communicate via direct contact and chemical signaling to coordinate activities.

• Direct Contact: Immune cells recognize normal vs. abnormal cells by glycoprotein patterns (glycocalyx).

• Ligand-Receptor Signaling: Ligands (e.g., hormones, neurotransmitters) bind to cell receptors.

  • Channel-linked Receptors: Open/close ion channels.

  • Enzymatic Receptors: Activate enzymes (e.g., phosphorylation).

  • G Protein-Coupled Receptors: Activate intracellular signaling cascades via G proteins.

Cellular Structures: Organelles and Surface Extensions

Organelles are specialized structures within cells, each with distinct functions.

• Membrane-bound Organelles:

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

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

  • Lysosomes: Contain digestive enzymes for breakdown of materials and autophagy.

  • Peroxisomes: Detoxify harmful substances and perform beta oxidation of fatty acids.

  • Mitochondria: Site of ATP production via cellular respiration.

• Non-membrane-bound Organelles:

  • Ribosomes: Protein synthesis; free ribosomes (cytosol), bound ribosomes (ER).

  • Cytoskeleton: Structural support, movement, and division; includes microfilaments, intermediate filaments, and microtubules.

  • Centrosome: Organizes microtubules during cell division.

  • Proteasomes: Degrade damaged or unneeded proteins.

• Cell Surface Extensions:

  • Cilia: Move substances across cell surfaces.

  • Flagella: Propel cells (e.g., sperm).

  • Microvilli: Increase surface area for absorption.

Membrane Junctions

Cells are connected by specialized junctions that regulate movement and communication.

• Tight Junctions: Prevent leakage between cells.

• Desmosomes: Provide strong adhesion between cells; hemidesmosomes anchor cells to the basement membrane.

• Gap Junctions: Allow direct communication via fluid-filled channels.

Nucleus and Genetic Material

The nucleus is the control center of the cell, housing DNA and the nucleolus.

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleolus: Produces ribosomes.

• DNA: Composed of nucleotides (adenine, thymine, cytosine, guanine) linked by phosphodiester bonds; double helix structure.

• Chromatin and Chromosomes: DNA-protein complexes; chromosomes are condensed forms during cell division.

• Genes: Segments of DNA coding for proteins or functional RNA.

Protein Synthesis: Transcription and Translation

Cells synthesize proteins through a two-step process: transcription (DNA to RNA) and translation (RNA to protein).

• Transcription: DNA is copied into pre-mRNA by RNA polymerase.

  • Initiation: RNA polymerase binds to DNA template.

  • Elongation: RNA polymerase adds complementary nucleotides.

  • Termination: RNA polymerase releases mRNA at gene end.

• RNA Processing: Introns removed, exons spliced, 5' cap and poly-A tail added.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a polypeptide.

  • Initiation: Ribosome assembles at start codon (AUG).

  • Elongation: tRNAs bring amino acids; peptide bonds form.

  • Termination: Stop codon reached; polypeptide released.

Equation for Central Dogma:

Cell Division: The Cell Cycle

Cells reproduce by dividing, either by mitosis (somatic cells) or meiosis (gametes).

• Cell Cycle Phases:

  • Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

  • Mitotic (M) Phase: Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis.

• Key Events:

  • Prophase: Chromatin condenses, spindle forms.

  • Metaphase: Chromosomes align at cell equator.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear envelopes reform, chromosomes decondense.

  • Cytokinesis: Cytoplasm divides, forming two daughter cells.

Table: Phases of the Cell Cycle

Cell Aging and Death

Cells undergo aging, which affects their structure and function, and may die by necrosis (injury) or apoptosis (programmed cell death).

• Aging: Alters organelle number, chromatin structure, and cell function.

• Apoptosis: Programmed cell death; essential for development and removal of damaged cells.

Clinical Views

• Familial Hypercholesterolemia: Genetic disorder affecting LDL receptor-mediated endocytosis, leading to high blood cholesterol and risk of atherosclerosis.

• Lysosome Storage Diseases: Genetic disorders (e.g., Tay-Sachs disease) due to defective lysosomal enzymes, causing accumulation of undigested substances.

• Tumors: Uncontrolled cell division due to mutations; cancerous tumors can invade and metastasize.