Chapter 3: Cells – The Living Units (A&P Study Guide)

Cells: The Living Units

Cell Theory and Generalized Cell Structure

Cells are the fundamental units of life, forming the basis for all structure and function in living organisms. Understanding their structure and function is essential for the study of anatomy and physiology.

• Cell Theory:

  • All living things are composed of one or more cells.

  • The cell is the basic structural and functional unit of life.

  • Cells arise only from pre-existing cells.

• Major Regions of a Generalized Cell:

  • Plasma Membrane: Selectively permeable outer barrier.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: Organelle that controls cellular activity.

• Extracellular Materials:

  • Extracellular Fluid: Includes interstitial fluid, blood plasma, and cerebrospinal fluid; contains nutrients, regulatory substances, and wastes.

  • Cellular Secretions: Digestive secretions, lubricating fluids.

  • Extracellular Matrix: Protein and polysaccharide mesh that binds cells together.

Plasma Membrane Structure and Function

The plasma membrane is a dynamic structure that controls the movement of substances into and out of the cell, and facilitates communication with the environment.

• Chemical Composition:

  • Double layer of phospholipids with embedded proteins, glycolipids, and cholesterol.

  • Phospholipid tails are hydrophobic (face inward); heads are hydrophilic (face outward).

• Membrane Proteins:

  • Integral Proteins: Span the membrane; function as channels or carriers.

  • Peripheral Proteins: Attached to integral proteins or phospholipids; function as enzymes or in mechanical support.

• Glycocalyx: Carbohydrate-rich area on the cell surface; acts as a biological marker for cell recognition.

• Cell Junctions:

  • Tight Junctions: Impermeable junctions preventing passage of molecules between cells.

  • Desmosomes: Mechanical couplings that prevent cell separation under stress.

  • Gap Junctions: Allow passage of small molecules and electrical signals between cells.

Membrane Transport Mechanisms

Cells regulate their internal environment by controlling the movement of substances across the plasma membrane, using both passive and active processes.

Passive Transport

• Diffusion: Movement of molecules from high to low concentration, driven by kinetic energy.

  • Rate depends on gradient steepness, molecule size, and temperature.

• Simple Diffusion: Small or lipid-soluble molecules cross the membrane unassisted.

• Facilitated Diffusion: Movement of larger or charged molecules via protein carriers or channels.

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

  • Water moves toward higher osmolarity (solute concentration).

  • Tonicity: Describes a solution's effect on cell volume:

    • Isotonic: No net water movement.

    • Hypertonic: Water moves out; cell shrinks.

    • Hypotonic: Water moves in; cell swells.

Active Transport

• Primary Active Transport: Direct use of ATP to move substances against their concentration gradient (e.g., Na+/K+ pump).

  • Equation: $\mathrm{ATP} + \mathrm{Na}^+_{\text{in}} + \mathrm{K}^+_{\text{out}} \rightarrow \mathrm{ADP} + \mathrm{P_i} + \mathrm{Na}^+_{\text{out}} + \mathrm{K}^+_{\text{in}}$

• Secondary Active Transport: Uses energy from ionic gradients created by primary active transport.

• Vesicular Transport: Movement of large particles via vesicles.

  • Endocytosis: Brings substances into the cell.

    • Phagocytosis: Engulfs large particles (e.g., pathogens).

    • Pinocytosis: Engulfs extracellular fluid and solutes.

    • Receptor-Mediated Endocytosis: Selective uptake of specific molecules.

  • Exocytosis: Expels substances from the cell (e.g., secretion, waste removal).

Membrane Potential

The membrane potential is the voltage difference across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential: Typically –70 mV; inside of the cell is more negative than outside.

• Established mainly by K+ diffusion out of the cell and maintained by active transport pumps.

• Equation (Nernst Equation for K+):

  • $E_K = \frac{RT}{zF} \ln \left( \frac{[K^+]_{\text{out}}}{[K^+]_{\text{in}}} \right)$

Cell-Environment Interactions

Cells interact with their environment through adhesion molecules and membrane receptors, enabling communication and response to external signals.

• Cell Adhesion Molecules (CAMs): Glycoproteins that mediate cell attachment, migration, and signaling.

• Membrane Receptors:

  • Contact Signaling: Direct cell-to-cell recognition.

  • Chemical Signaling: Ligand binding triggers cellular responses.

  • G Protein-Coupled Receptors: Activate second messenger systems (e.g., cAMP pathway).

The Cytoplasm and Organelles

Cytoplasmic Components

The cytoplasm is the site of most cellular activities, containing organelles that perform specialized functions.

• Cytosol: Viscous fluid where organelles are suspended.

• Organelles: Specialized structures with distinct functions.

• Inclusions: Stored nutrients, secretory products, and pigment granules.

Major Organelles and Their Functions

• Mitochondria: Powerhouse of the cell; site of ATP production via aerobic respiration.

• Ribosomes: Sites of protein synthesis; can be free or bound to rough ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes secreted and membrane proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid and glycogen metabolism, detoxification.

• Golgi Apparatus: Modifies, concentrates, and packages proteins and lipids for export or use within the cell.

• Peroxisomes: Contain oxidases and catalases; detoxify harmful substances.

• Lysosomes: Contain digestive enzymes; degrade ingested particles, worn-out organelles, and non-useful tissues.

• Endomembrane System: Includes ER, Golgi, lysosomes, vesicles, and plasma membrane; coordinates synthesis, storage, and transport of molecules.

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

• Centrosome and Centrioles: Organize microtubules and the mitotic spindle; centrioles form bases of cilia and flagella.

Cellular Extensions

• Cilia: Motile, whip-like extensions that move substances across cell surfaces.

• Flagella: Longer projections that propel the cell (e.g., sperm cell).

• Microvilli: Finger-like projections that increase surface area for absorption.

The Nucleus

Nuclear Structure and Function

The nucleus is the control center of the cell, housing genetic material and coordinating cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores; regulates entry and exit of materials.

• Nucleolus: Site of ribosomal subunit assembly; prominent in active cells.

• Chromatin: DNA (30%), histone proteins (60%), and RNA (10%); condenses to form chromosomes during cell division.

• Nucleosomes: Fundamental units of chromatin; DNA wrapped around histone protein clusters.

Cell Cycle and Division

Phases of the Cell Cycle

• Interphase: Period of cell growth and DNA replication.

  • G1 Phase: Cell growth and protein synthesis.

  • S Phase: DNA replication.

  • G2 Phase: Preparation for division; synthesis of enzymes and proteins.

• Cell Division: Necessary for growth and repair.

  • Mitosis: Nuclear division in four phases (prophase, metaphase, anaphase, telophase).

  • Cytokinesis: Division of the cytoplasm.

• Control Mechanisms: Surface-to-volume ratio, chemical signals, and contact inhibition regulate division.

DNA Replication

• DNA helix uncoils; hydrogen bonds between base pairs break.

• Each strand serves as a template for a new complementary strand.

• Enzymes (e.g., DNA polymerase) facilitate the process.

Protein Synthesis

Genetic Code and Gene Expression

• Gene: Segment of DNA coding for a polypeptide.

• Triplet: Sequence of three DNA bases specifying an amino acid.

• Codon: Complementary three-base sequence on mRNA.

• Anticodon: Three-base sequence on tRNA complementary to mRNA codon.

Phases of Protein Synthesis

• Transcription: DNA information is copied to mRNA in the nucleus.

  • RNA polymerase synthesizes mRNA from DNA template.

  • Primary transcript contains introns (non-coding) and exons (coding); introns are removed.

• Translation: mRNA is decoded to build a polypeptide chain at the ribosome.

  • tRNA brings specific amino acids to the ribosome.

  • Ribosome moves along mRNA, adding amino acids in sequence.

  • Process continues until a stop codon is reached.

• Other Roles of DNA: Codes for regulatory RNAs (e.g., miRNA, siRNA) that modulate gene expression.

Cellular Cleanup and Death

Autophagy, Proteasomes, and Apoptosis

• Autophagy: Cell digests its own components via autophagosomes and lysosomes; important for removing damaged organelles and during stress or development.

• Ubiquitin-Proteasome Pathway: Tags unneeded proteins with ubiquitin for degradation by proteasomes.

• Apoptosis: Programmed cell death; involves mitochondrial membrane permeabilization, release of cytochrome c, activation of caspases, and phagocytosis of cell remnants.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Examples
Simple Diffusion	No	High to Low	O2, CO2
Facilitated Diffusion	No	High to Low	Glucose, Ions
Osmosis	No	High to Low (water)	Water
Primary Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Secondary Active Transport	Indirect (uses gradient)	Low to High	Glucose-Na+ cotransport
Endocytosis	Yes (ATP)	Into cell	Phagocytosis, Pinocytosis
Exocytosis	Yes (ATP)	Out of cell	Secretion of hormones

Additional info:

• For more on membrane potentials, see the Nernst and Goldman equations in neurophysiology (Ch. 11).

• For details on the genetic code, see Appendix C for mRNA codons and their corresponding amino acids.

• Cross-references to other chapters highlight the integration of cell structure and function with other body systems (e.g., nervous, muscular, reproductive).