Ch.2

Introduction to the Cell

Universal Cell Features

All human cells contain a plasma membrane, cytoplasm, nucleus, and genetic material.


Three Main Constituents

  • Plasma Membrane: Outer limiting barrier that holds cell contents in place, separates internal contents from the external environment, allows cells to take in nutrients, build molecules, enables cell communication, and controls movement in and out of the cell.

  • Cytoplasm: Internal contents located between the plasma membrane and nucleus; contains viscous cytosol fluid, stored inclusions, and organelles.

  • Nucleus: Double-membrane enclosed cell control center that houses DNA, directs protein synthesis, and regulates overall cell functions.

Plasma Membrane: Composition, Functions, and Transport

Composition and Structure

  • Plasmalemma: Flexible, selectively permeable outer barrier.

  • Glycocalyx: External carbohydrate (sugar) coat on the outer surface used for cell recognition and adhesion.

  • Membrane Lipids:

    • Phospholipids: Form the primary phospholipid bilayer.

      • Polar Head: Charged, hydrophilic ("water-loving"), facing the extracellular fluid (outside) and intracellular fluid (inside).

      • Nonpolar Tails: Uncharged, hydrophobic ("water-hating"), facing inward toward each other.

    • Cholesterol: Scattered within the hydrophobic interior; strengthens and stabilizes the membrane against temperature extremes.

    • Glycolipids: Lipids with attached carbohydrate chains on the outer layer; act in cell recognition.

  • Membrane Proteins:

    • Integral Proteins: Embedded within and span through the lipid bilayer (e.g., membrane channels, receptors).

    • Peripheral Proteins: Attached loosely to the outer or inner surface.

    • Glycoproteins: Proteins with attached carbohydrate groups exposed on the external surface.

Protein-Specific Functions

  • Transport: Move substances across the membrane via channel or carrier proteins.

  • Intercellular Connection: Form junctions to attach neighboring cells.

  • Anchorage for Cytoskeleton: Maintains cell shape by anchoring internal structural proteins.

  • Enzyme Activity: Catalyzes metabolic reactions on inner or outer membrane surfaces.

  • Cell-Cell Recognition: Carbohydrates act as identification tags.

  • Signal Transduction: Transmits external chemical signals to alter internal cell activity.

Membrane Transport Mechanisms

  • Passive Transport: Requires no cellular energy; solutes move down their concentration gradient.

    • Simple Diffusion: Unaided movement of small, nonpolar/fat-soluble molecules directly through the lipid bilayer down their gradient (O2 and CO2 gas exchange).

    • Osmosis: Passive diffusion of water across a selectively permeable lipid bilayer from high to low water concentration.

    • Facilitated Diffusion: Passive movement of larger or polar molecules using transport/carrier proteins (e.g., glucose).

    • Bulk Filtration: Hydrostatic pressure forces solvents and dissolved solutes across a membrane (e.g., fluid movement in capillary walls).

  • Active Transport: Requires ATP energy; moves solutes against their concentration gradient (low to high concentration).

    • Ion Pumps: Transport proteins that move ions across membranes (e.g., Sodium-Potassium Pump, which pumps 3 Na+ out and 2 K+ into the cell).

  • Vesicular / Bulk Transport: Transports macromolecules or large volumes via membrane-bound vesicles.

    • Exocytosis: Secretion of large molecules out of the cell by fusing internal vesicles with the plasma membrane.

    • Endocytosis: Ingestion of large particles or fluid droplets into the cell by forming new vesicles.

    • Phagocytosis: "Cell eating"; pseudopodia surround and engulf large solid particles or microbes into a vacuole.

    • Pinocytosis: "Cell drinking"; membrane invaginates to ingest tiny droplets of extracellular fluid.

    • Receptor-Mediated Endocytosis: External molecules bind to specific surface receptors, triggering localized vesicle formation.

Cytoplasm: Cytosol, Inclusions, Organelles, and Surface Structures

Cytosol and Inclusions

  • Cytosol: The viscous, syrup-like fluid (cytoplasmic matrix/intracellular fluid) containing water, dissolved ions, nutrients, proteins, carbs, lipids, and enzymes. Serves as a medium for diffusion and cellular chemical reactions, providing energy and building blocks.

  • Inclusions: Non-membrane-bound, temporary stores of cellular products.

    • Melanin: Pigment stored in skin, hair, and eye cells that absorbs UV light.

    • Glycogen: Stored carbohydrate polysaccharide primarily located in liver and skeletal muscle cells.

Membrane-Bound Organelles

  • Smooth Endoplasmic Reticulum (SER): Network of smooth membrane tubules lacking ribosomes (does not make proteins). Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/alcohol/poisons, and stores lipids.

  • Rough Endoplasmic Reticulum (RER): Parallel membrane sacs (cisternae) studded with bound ribosomes. Synthesizes, modifies, packages, and transports proteins destined for secretion, plasma membrane insertion, or lysosomes via transport vesicles sent to the Golgi apparatus.

  • Golgi Apparatus: Stacked series of flattened membrane sacs (cisternae) with structural polarity: receiving region (cis-face) and shipping region (trans-face). Modifies, packages, and sorts RER proteins into secretory vesicles (exocytosis), membrane vesicles, or lysosomes.

  • Lysosomes: Membrane-bound sacs formed by the Golgi apparatus containing digestive enzymes (acid hydrolases). Performs digestion of unneeded cellular waste/endocytosed materials, removal of damaged organelles (autophagy), and cell self-destruction (autolysis).

  • Peroxisomes: Small sacs formed from the Rough ER containing specific enzymes like oxidases/catalase. Neutralize free radicals (metabolic byproducts), detoxify harmful substances using oxygen, break down hydrogen peroxide (H2O2) into water, and break down fatty acids.

  • Mitochondria: Double-membrane organelles ("powerhouses of the cell") with inner membrane folds (cristae) and an internal fluid space (matrix). Convert nutrients into ATP energy via aerobic cellular respiration. They contain their own circular DNA (1% of cellular DNA) and are capable of self-replication.

Non-Membrane-Bound Organelles

  • Ribosomes: Dense granules made of small and large subunits composed of protein and RNA. They read mRNA to synthesize proteins.

    • Free Ribosomes: Float freely in the cytosol; synthesize proteins for internal cellular use.

    • Bound Ribosomes: Attached to the Rough ER; synthesize proteins for secretion, membrane insertion, or lysosomes.

  • Cytoskeleton: Structural network of protein filaments and tubules that provides support, cell division aid, and organelle/cell movement.

    • Microfilaments: Smallest components (7 nm); actin proteins that assist in cell shape, muscle contraction, and division.

    • Intermediate Filaments: Rigid structural support (8–12 nm); insoluble fibers that stabilize cell junctions.

    • Microtubules: Hollow tubulin protein cylinders (25 nm); move organelles, direct chromosomes in division, and form the base for cilia/flagella.

  • Centrosome and Centrioles: Spherical region near the nucleus containing a pair of perpendicular centrioles (barrel-shaped microtubule triplets). Organizes microtubules and directs spindle fiber formation during mitosis.

Surface Structures

  • Cilia: Short hair-like surface projections that move fluid, mucus, and materials across the external cell surface.

  • Flagella: Long, singular projections that propel an entire cell forward (e.g., human sperm cell).

  • Microvilli: Thin, non-motile membrane folds that increase surface area for absorption and secretion.

Nucleus, DNA, and Genetics

Nuclear Structure

  • Nuclear Envelope: Double-membrane structure enclosing the nucleus. Its outer layer is continuous with the RER and studded with ribosomes.

  • Nuclear Pores: Open passageways penetrating the envelope that selectively permit passage of water-soluble molecules, ions, and RNA between the nucleus and cytoplasm.

  • Nucleoli: Dark-staining, non-membrane-bound spherical bodies that synthesize ribosomal RNA (rRNA) and assemble small and large ribosomal subunits. Active in protein-synthesizing cells; absent in sperm cells.

DNA, Chromatin, and Chromosomes

  • Deoxyribonucleic Acid (DNA): Double-helix macromolecule containing genetic instructions for cell activities and protein synthesis.

  • Nucleotides: Structural units consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base.

  • Complementary Base-Pairing: Adenine pairs with Thymine (A-T); Cytosine pairs with Guanine (C-G).

  • Double Helix Functions: DNA replication (duplication of genetic material) and protein synthesis (directing RNA production).

  • Chromatin: Unwound, finely filamentous mass of DNA strands wrapped around histone proteins (forming nucleosomes) present during interphase.

  • Chromosomes: Highly coiled, tightly condensed strands of DNA visible during cell division.

  • Somatic Cells: All non-sex body cells are genetically identical. They are diploid, containing 46 total chromosomes (23 pairs).

  • Gametes: Sex cells (sperm and egg) contain a haploid chromosome number (23 single chromosomes).

Life Cycle of the Cell & Cell Division

Cell Life Cycle & Interphase

  • Cell Life Cycle: Sequence of events from cell formation to cell division.

  • Interphase: Non-dividing growth phase where the cell carries out normal functions and prepares for division.

    • G1 Phase (First Gap): Cell grows, produces new organelles and proteins; centrioles begin replicating near the end. Non-dividing cells enter a resting state called G0 phase.

    • S Phase (Synthesis): DNA replication occurs; each DNA molecule creates an exact copy of itself so that all resulting somatic cells remain genetically identical.

    • G2 Phase (Second Gap): Brief growth phase; centriole replication finishes, organelle production continues, and enzymes required for division are synthesized.

Mitotic (M) Phase

  • Purpose of Mitosis: Cell division in somatic cells used for growth, tissue repair, and replacement of old/damaged cells. (Note: Mitosis does not typically occur in mature nervous tissue, skeletal muscle, or cardiac muscle cells).

  • Stages of Mitosis:

    • Prophase: Chromatin condenses into visible, replicated chromosomes (each consisting of two identical sister chromatids joined at a centromere). The nucleolus breaks down, the nuclear envelope dissolves, centrioles move to opposite poles, and spindle fibers form.

    • Metaphase: Replicated chromosomes align along the middle (equatorial plate) of the cell; spindle fibers attach to centromeres.

    • Anaphase: Spindle fibers shorten and pull sister chromatids apart at the centromeres, moving single-stranded chromosomes toward opposite cell poles. Cytokinesis begins in late anaphase.

    • Telophase: Single chromosomes arrive at opposite poles and uncoil back into chromatin. Nuclear envelopes re-form, nucleoli reappear, and spindle fibers dissolve.

    • Cytokinesis: Division of the cytoplasm via a cleavage furrow (a ring of protein filaments) that pinches the cell, yielding two genetically identical daughter cells.

Aging and Cell Death

  • Cellular Aging: A natural process involving decreased metabolic function, reduced ability to maintain homeostasis, and organelle degradation. Uncontrolled cell division leads to cancer.

  • Cell Death Mechanisms:

    • Necrosis: Irreversible cell death caused by severe physical damage, toxins, or mechanical injury; results in cell swelling, rupture, and inflammatory tissue response.

    • Apoptosis: Programmed cell death ("cell suicide") essential for normal embryonic development (e.g., unwebbing fingers and toes) and the safe removal of infected, old, or damaged cells.