Cellular Biology Review Notes - Slide Deck #2

Properties of Life and Cellular Functions

• The cell is the functional unit of animal life.
• Core cellular properties include:
  • Homeostasis: tendency to maintain a relatively stable internal state.
  • Growth: increase in size or number of cells; adaptive changes in response to stimuli.
  • Reproduction: ability to produce more cells or organisms essentially like the original.
  • Absorption: uptake of dissolved materials or water through the cell membrane into the cell; can be passive or active.
  • Metabolism: sum of physical and biochemical reactions occurring in each cell and the organism.
  • Secretion: production and discharge of substances from a cell, gland, or organ for a specific function; also called exocytosis.
  • Irritability (excitability): ability to react to a stimulus.
  • Conductivity: transmission of electrical impulses within the cell.
  • Contractility: ability to shorten or lengthen a cell.
• Cellular functions can be injured or altered; adaptive changes include hypertrophy and hyperplasia (see details below).
• Microscopic scale of cells:
  • Typical animal cell diameter: $10\,\mu\text{m} \leq D \leq 100\,\mu\text{m}.$
  • Actively multiplying cells: about $20\,\mu\text{m} \leq D \leq 30\,\mu\text{m}$.
  • Demonstrative example: 1000 cells of $25\,\mu\text{m}$ diameter lined up would span $1000 \times 25\,\mu\text{m} = 25{,}000\,\mu\text{m} = 2.5\,\text{cm}.$
• Foundational context and relevance:
  • These properties underpin how tissues function, respond to injury, and regenerate.
  • Growth factors and signaling regulate hypertrophy and hyperplasia (see below).

The Cell Membrane: Structure and Composition

• The plasma (cell) membrane and intracellular membranes mainly consist of:
  • Phospholipids, proteins, and cholesterol.
• Phospholipid bilayer arrangement:
  • Polar (hydrophilic) heads face the aqueous surroundings and protein layers.
  • Nonpolar (hydrophobic) tails face each other toward the interior of the membrane.
• Membrane components and their roles:
  • Integral (transmembrane) proteins: span the membrane, often forming channels or transporters.
  • Peripheral proteins: associated with the surface.
  • Transport proteins (glcyoproteins/globular proteins): facilitate movement of substances.
  • Glycolipids and glycoproteins: involved in cell recognition and signaling.
  • Cholesterol: modulates membrane fluidity and stability.
• Structural elements shown in typical diagrams:
  • Extracellular fluid, cytoplasm, phospholipid bilayer, hydrophobic tails, hydrophilic heads, integral/transmembrane proteins, glycoproteins, glycolipids, cholesterol.
• Importance:
  • Membrane composition determines permeability, signaling, and interaction with the environment.

Transport Across Cell Membranes

• The membrane regulates entry and exit of substances for organelles and the cell.
• Modes of transport include:
  • Simple diffusion: movement down a concentration gradient without energy or transport proteins.
  • Facilitated diffusion: diffusion aided by carrier or channel proteins.
  • Osmosis: diffusion of water across semipermeable membranes; often via aquaporins.
  • Active transport: movement against gradient requiring energy (e.g., ATP).
  • Endocytosis: uptake of large molecules or particles via vesicle formation.
  • Exocytosis: secretion or disposal of materials via vesicle fusion with the plasma membrane.
• Carrier systems:
  • Transmembrane proteins bind the diffusing molecule on one side and release it on the opposite side.
• Osmosis and water movement:
  • Water moves to balance solute concentrations; aquaporins facilitate rapid water movement.
• Energy considerations:
  • Active transport uses ATP or another energy source to move substances up their gradients.
• Practical implications:
  • Selective permeability enables homeostasis, nutrient uptake, and waste removal.

Membrane Potentials and Excitable Cells

• Ion distributions:
  • Extracellular space: high Na+ concentration.
  • Intracellular space: high K+ concentration.
• Na+/K+ pump:
  • Maintains gradients by pumping Na+ out and K+ in (energy-dependent).
• Diffusion and diffusion-driven currents:
  • Ions diffuse along electrochemical gradients, contributing to membrane potential.
• Excitable cells:
  • Neurons and muscle cells are specialized for rapid electrical signaling.
• Propagation of signals:
  • Action potentials propagate along neurons; in myelinated vs unmyelinated axons, the node of Ranvier and myelin sheath influence speed of conduction.
• Conceptual summary:
  • Resting membrane potential is a baseline voltage; depolarization and repolarization constitute action potentials that transmit information or trigger contraction.

Ligand-Receptor Signaling and Membrane Receptors

• Ligand: a molecule that binds to a receptor to elicit a cellular response.
• L-R relationship terminology:
  • Reversible binding: association and dissociation occur dynamically.
  • Specificity: receptor responds to particular ligands or a subset.
  • Affinity: strength of the ligand-receptor interaction.
  • Saturation: proportion of receptors bound by ligand at a given concentration.
• Significance:
  • Ligand-receptor interactions regulate many cellular processes, including metabolism, growth, and signal transduction.

The Nucleus: DNA, RNA, and Gene Expression

• The nucleus contains the cell’s genetic material encoded in DNA.
• Nucleotides:
  • Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
  • DNA strands are composed of these nucleotides in specific sequences.
  • Representation of complementary base pairs and replication processes underlies genetic copying.
• RNA roles and forms:
  • Messenger RNA (mRNA): carries genetic information from DNA to ribosomes for protein synthesis.
  • Transfer RNA (tRNA): brings amino acids to ribosomes during translation.
  • Ribosomal RNA (rRNA): core component of ribosomes.
• Relationship to biotechnology:
  • DNA and RNA underpin genetic engineering and biotechnology applications.

Cell Division: Mitosis and Meiosis

• Mitosis (somatic cell division):
  • Purpose: generate two genetically identical diploid daughter cells from a diploid parent.
  • Key feature: replication and equal distribution of the genome; supports growth and tissue maintenance.
  • Concept: DNA replication occurs during interphase before mitotic division.
• Meiosis (reduction division):
  • Purpose: produce gametes (ova/oogenesis and spermatozoa/spermatogenesis).
  • Outcome: four haploid cells that are genetically distinct from each other and from the parent.
  • Key differences from mitosis: two rounds of division (Meiosis I and Meiosis II), homologous chromosomes separate in Meiosis I, leading to halved chromosome number.
• Visual summary from slides:
  • Mitosis: two diploid daughter nuclei produced.
  • Meiosis: a specialized process reducing chromosome number by half, resulting in four haploid, genetically diverse cells; includes stages Meiosis I and Meiosis II.
• Core definitions:
  • Diploid: 2n chromosome set; Haploid: n chromosome set. In meiosis, $2n \to n$.

Regulation of Growth, Replication, and Cell Fate

• Growth and replication are not uniform across tissues:
  • Some cells can grow by hypertrophy (increase in cell size).
  • Other cells can replicate and divide (mitotic activity).
• Examples of continuous replication:
  • Epithelium of the small intestine.
  • Certain blood cells.
• Examples of limited replication:
  • Cardiac and skeletal muscle cells typically do not divide in the mature organism.
• Signals controlling growth:
  • Growth factors stimulate cell growth and sometimes division.
• Apoptosis:
  • Programmed cell death; a controlled process removing damaged or unnecessary cells.

Microscopy and Size Considerations

• Microscopic context:
  • Typical animal cells range from about $10\,\mu\text{m}$ to $100\,\mu\text{m}$ in diameter.
  • Actively multiplying cells are usually around $20\,\mu\text{m}$ to $30\,\mu\text{m}$.
• Educational resource reference:
  • Histology guide: https://histologyguide.com/slidebox/slidebox.html
• Measurement scales included in lecture materials (for reference):
  • From 1 nm to 1 m, with examples illustrating how microscopes resolve structures from atoms to whole cells.

Connections, Implications, and Real-World Relevance

• Foundational principles:
  • The cell theory overarching all topics: cells are the basic units of life, with membranes creating essential compartmentalization and control.
  • Energy use and transport across membranes underpin homeostasis and signal transduction.
• Ethical and practical implications:
  • Understanding gene expression (DNA/RNA) informs biotechnology, cloning, and medical therapies.
  • Regulation of cell division and apoptosis is central to cancer biology and therapies.
• Cross-topic synthesis:
  • Membrane transport and membrane potential underlie excitability of neurons and muscles, which relates to signaling and organismal function.
  • Growth factors connect cellular signaling to tissue regeneration and disease progression.
• Mathematical and conceptual notes:
  • Diploid to haploid transition in meiosis can be summarized as $2n \rightarrow n$ during gametogenesis.
  • Cellular size ranges and cumulative lengths (e.g., 1000 cells of 25 μm yield 2.5 cm) demonstrate the scale of tissue architecture, which has practical implications for histology and pathology.

Review of Learning Objectives (aligned topics)

• Cell properties: ability to explain and discuss unique cellular properties and their relevance to function.
• Membrane importance: explain the chemical composition and why membranes are essential for cellular homeostasis and signaling.
• Transport across membranes: describe how diffusion (simple and facilitated), osmosis, active transport, endocytosis, and exocytosis accomplish transport.
• Ligands and ligand-receptor interactions: define ligands, receptors, and key interaction properties (reversibility, specificity, affinity, saturation).
• Mitosis and meiosis basics: describe, at a foundational level, the roles and differences between mitosis and meiosis in growth, maintenance, and reproduction.