Chapter 3: Cells – The Living Units (Comprehensive Study Notes)

Cells: The Living Units

Cell Theory and Cell Diversity

The cell is the fundamental structural and functional unit of life. The activities of an organism depend on the combined actions of its cells, whose structure and function are closely related. Over 200 types of human cells exist, differing in size, shape, and subcellular components, which determine their specialized functions.

• Cell Theory: All living things are composed of cells; cells arise only from preexisting cells.

• Generalized Cell Structure: All human cells share three basic parts:

  • Plasma membrane: Flexible boundary separating intracellular and extracellular environments.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: Control center containing DNA.

• Extracellular Materials: Substances outside cells include body fluids (interstitial fluid, blood plasma, cerebrospinal fluid), cellular secretions (saliva, mucus), and the extracellular matrix (a jelly-like substance holding cells together).

Structure of the Plasma Membrane

The plasma membrane acts as a dynamic barrier, controlling the movement of substances into and out of the cell. Its structure is described by the fluid mosaic model.

• Lipid Bilayer: Composed of 75% phospholipids (hydrophilic heads, hydrophobic tails), 5% glycolipids (lipids with sugar groups), and 20% cholesterol (provides stability).

• Membrane Proteins: Integral proteins (embedded, often transmembrane) and peripheral proteins (loosely attached). Functions include transport, enzymatic activity, receptors, cell-to-cell connections, and support.

• Glycocalyx: Surface carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins), serving as biological markers for cell recognition and immune response.

Cell Junctions

Cells are often bound together to form tissues and organs via specialized junctions.

• Tight Junctions: Integral proteins fuse to form impermeable barriers, preventing passage of substances between cells (e.g., in intestinal lining).

• Desmosomes: Rivet-like junctions with linker proteins and plaques, providing mechanical strength and flexibility (e.g., in skin and heart tissue).

• Gap Junctions: Connexons form channels allowing passage of ions and small molecules, facilitating communication (e.g., in cardiac muscle).

Membrane Transport

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

• Diffusion: Movement of molecules from high to low concentration. Types include:

  • Simple Diffusion: Nonpolar, lipid-soluble substances (e.g., O2, CO2) pass directly through the bilayer.

  • Facilitated Diffusion: Polar or charged molecules (e.g., glucose, ions) use carrier or channel proteins.

    • Carrier-mediated: Specific molecules bind to protein carriers.

    • Channel-mediated: Molecules move through water-filled channels (aquaporins for water).

  • Osmosis: Movement of water across a selectively permeable membrane, from low solute (high water) to high solute (low water) concentration.

• Filtration: Movement of substances across capillary walls due to pressure gradients.

Tonicity

• Isotonic: Equal osmolarity; cell volume unchanged.

• Hypertonic: Higher osmolarity outside; cell shrinks (crenation).

• Hypotonic: Lower osmolarity outside; cell swells and may burst (lysis).

Active Transport

Active transport requires ATP and moves substances against their concentration gradients.

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

• Secondary Active Transport: Indirect use of ATP via ion gradients established by primary transport.

• Carrier Proteins: Solute pumps (antiporters and symporters) move substances in/out or in the same direction.

Sodium-Potassium Pump Equation

The Na+-K+ ATPase exchanges 3 Na+ out for 2 K+ in per ATP hydrolyzed:

$3\mathrm{Na}^+_{\text{in}} + 2\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\mathrm{Na}^+_{\text{out}} + 2\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P}_i$

Vesicular Transport

Transport of large particles and macromolecules via vesicles.

• Endocytosis: Transport into cell; includes phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis.

• Exocytosis: Ejection of substances from cell (e.g., hormones, neurotransmitters).

• Transcytosis: Movement into, across, and out of cell.

• Vesicular Trafficking: Transport between cell regions or organelles.

Membrane Potential

Resting Membrane Potential (RMP)

RMP is the electrical potential across the plasma membrane, typically –50 to –100 mV, due to separation of charged particles.

• K+ Diffusion: Outward movement through leakage channels creates negative interior.

• Na+ Influence: Inward movement can raise RMP to –70 mV.

• Na+-K+ Pump: Maintains electrochemical gradients essential for nerve and muscle function.

Cell-Environment Interactions

Cell Adhesion Molecules (CAMs) and Receptors

Cells interact with their environment via CAMs and plasma membrane receptors.

• CAMs: Glycoproteins that anchor cells, assist movement, attract immune cells, and transmit signals.

• Receptors: Bind chemical signals (ligands) for contact and chemical signaling, activating enzymes or ion channels.

• G Protein–Linked Receptors: Indirectly cause cellular changes via second messengers (e.g., cyclic AMP).

Cytoplasm and Organelles

Cytoplasmic Components

The cytoplasm includes cytosol (fluid), inclusions (insoluble molecules), and organelles (metabolic machinery).

• Membranous Organelles: Mitochondria, endoplasmic reticulum (ER), Golgi apparatus, peroxisomes, lysosomes.

• Nonmembranous Organelles: Ribosomes, cytoskeleton, centrioles.

Key Organelles

• Mitochondria: Double-membraned "power plants" producing ATP via aerobic respiration; contain their own DNA and replicate by fission.

• Ribosomes: Sites of protein synthesis; free ribosomes produce cytosolic proteins, membrane-bound ribosomes synthesize proteins for membranes or export.

• Endoplasmic Reticulum (ER):

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

  • Smooth ER: Lipid metabolism, detoxification, calcium storage; specialized as sarcoplasmic reticulum in muscle cells.

• Golgi Apparatus: Modifies, sorts, and packages proteins/lipids from ER; directs vesicles to plasma membrane, organelles, or lysosomes.

• Peroxisomes: Detoxify harmful substances; contain oxidase and catalase enzymes.

• Lysosomes: Digestive enzymes for breakdown of macromolecules, pathogens, and old organelles; involved in autolysis.

Clinical Example: Tay-Sachs Disease

Tay-Sachs is a lysosomal storage disease caused by a missing enzyme for glycolipid breakdown in brain cells, leading to neurological impairment and early death.

Cytoskeleton

• Microfilaments: Actin filaments; support cell shape, involved in movement.

• Intermediate Filaments: Ropelike fibers; resist mechanical stress, anchor organelles.

• Microtubules: Tubulin-based hollow tubes; determine cell shape, serve as tracks for organelle movement.

Centrosome and Centrioles

• Centrosome: Microtubule organizing center near nucleus.

• Centrioles: Barrel-shaped structures forming basis of cilia and flagella.

Cellular Extensions

• Cilia: Motile extensions moving substances across cell surfaces.

• Flagella: Longer extensions propelling cells (e.g., sperm).

• Microvilli: Fingerlike projections increasing surface area for absorption.

Nucleus and Genetic Material

Nuclear Structure

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

• Nucleoli: Sites of rRNA synthesis and ribosome assembly.

• Chromatin: DNA, histone proteins, and RNA arranged in nucleosomes; condenses into chromosomes during cell division.

Cell Cycle and Division

Interphase

Cell grows, performs normal functions, and prepares for division. Subphases include:

• G1: Growth and metabolism.

• S: DNA replication.

• G2: Preparation for division.

DNA Replication

• DNA helices unwind and separate at replication fork.

• RNA primer initiates synthesis; DNA polymerase adds nucleotides.

• Leading strand synthesized continuously; lagging strand in segments (joined by DNA ligase).

• Result: Two identical daughter DNA molecules (semiconservative replication).

Mitotic Phase

Division of nucleus (mitosis) and cytoplasm (cytokinesis).

• Mitosis Stages:

  • Prophase: Chromatin condenses, spindle forms.

  • Metaphase: Chromosomes align at metaphase plate.

  • Anaphase: Chromatids separate and move to poles.

  • Telophase: Chromosomes uncoil, nuclear envelope reforms.

• Cytokinesis: Actin ring forms cleavage furrow, dividing cell into two.

Cell Cycle Regulation

• "Go" signals: Surface-to-volume ratio, growth factors, hormones.

• "Stop" signals: Contact inhibition, space availability.

• Cyclins and Cdks: Regulatory proteins controlling cell cycle progression.

• Checkpoints: Ensure proper division; G1 checkpoint is most important.

Protein Synthesis

Genetic Code and RNA Types

• DNA: Master blueprint; sequence of bases (A, G, T, C) codes for proteins.

• Gene: Segment of DNA coding for a polypeptide; composed of exons (coding) and introns (noncoding).

• RNA Types:

  • mRNA: Messenger RNA; carries code from DNA to ribosome.

  • rRNA: Ribosomal RNA; structural component of ribosomes.

  • tRNA: Transfer RNA; brings amino acids to ribosome, matches codon with anticodon.

Transcription

• DNA code is transcribed to mRNA in the nucleus.

• Transcription factors bind promoter, RNA polymerase synthesizes mRNA.

• mRNA is processed (introns removed, exons spliced).

Translation

• mRNA is decoded at ribosome; tRNA brings amino acids.

• Ribosome has A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA.

• Three phases: Initiation (start codon), Elongation (peptide bond formation, translocation), Termination (stop codon, release factor).

• Polyribosomes allow multiple proteins to be synthesized from one mRNA.

Genetic Code Table (Sample)

DNA Triplet	mRNA Codon	Amino Acid
GGC	GCC	Arginine
GGC	GGC	Proline
UAA	UAA	Stop
UAU	AUA	Methionine
Additional info: ...	...	...

Other Roles of DNA

• MicroRNA (miRNA): Silences mRNA from certain exons.

• Riboswitches: Folded RNAs acting as switches for protein synthesis.

• Small interfering RNA (siRNA): Silences pathogenic mRNA.

Cell Death, Differentiation, and Aging

Apoptosis, Autophagy, and Proteasomes

• Autophagy: Disposal of nonfunctional organelles via autophagosomes and lysosomes.

• Proteasomes: Degrade ubiquitin-tagged proteins, recycling amino acids.

• Apoptosis: Programmed cell death via caspase activation, DNA and cytoskeleton degradation, and phagocytosis.

Cell Differentiation and Aging

• Cells differentiate via gene regulation during embryonic development.

• Cell division continues for growth and repair; hyperplasia (increased cell number), atrophy (decreased size).

• Aging theories include wear and tear, mitochondrial dysfunction, immune system decline, and genetic programming (telomere shortening).

• Telomerase: Enzyme lengthening telomeres, present in germ cells and cancer cells.