Cell Structure, Function, and Transport – Vocabulary Flashcards

Levels of Biological Organization

• Atom → Molecule → Cell → Tissue → Organ → Organ System → Organism
  • Emphasizes hierarchical integration; malfunction at one level can cascade to higher levels (clinical relevance in pathology).

What is a Cell?

• Coined by Robert Hooke (1635–1703) while observing cork; resemblance to a honey-comb → term “cell.”
• Fundamental structural & functional unit of all life.
• Two basic categories:
  • Prokaryotic
  • Eukaryotic

Prokaryotic vs. Eukaryotic Cells

• Shared core components
  • Plasma membrane, cytosol, DNA, ribosomes.
• Distinctions:
  • Nucleus: absent (prokaryote) vs. present (eukaryote).
  • Membrane-bound organelles: absent vs. present.
  • DNA form: circular vs. linear, chromatin within nucleus.
  • Size: typically 1\text{–}5\ \mu m vs. 10\text{–}100\ \mu m.
• Additional features
  • Capsule (some prokaryotes) – polysaccharide layer aiding pathogenicity & desiccation resistance.
  • Cell wall: universal in prokaryotes (peptidoglycan) and plants/fungi/algae; optional in some eukaryotes.
  • Flagella: present in certain members of both groups; structure differs (prokaryotic flagellin vs. eukaryotic 9+2 tubulin).

Plant vs. Animal Cells (and Prokaryotes as reference)

• Form:
  • Plant – generally fixed, rectangular due to rigid wall.
  • Animal – variable/irregular (no wall).
  • Prokaryote – typically fixed but small.
• Cell wall:
  • Plant – cellulose, hemicellulose, pectins, lignin.
  • Animal – none.
  • Prokaryote – peptidoglycan lipoprotein.
• Plastids / Chloroplasts:
  • Present in green plant cells; absent in animals & prokaryotes.
• Large central vacuole: present in mature plants (storage, hydrolysis, turgor).
• Centrioles / Centrosome:
  • Present in animals; absent in plants; prokaryotes lack microtubule-based centrosome.
• Reserve food:
  • Plant – starch/carbohydrates.
  • Animal – glycogen & lipids.
• Size: plant cells usually larger than animal; both dwarf prokaryotes.

Organelle Structure & Function

Nucleus

• Largest organelle; double membrane = nuclear envelope (continuous with rough ER).
• Nuclear pores control traffic (mRNA out, proteins in).
• DNA arranged as chromatin → condenses into chromosomes during division.
• Nucleolus: non-membranous body; synthesizes rRNA & ribosomal subunits.
• Functional importance: houses genetic instructions controlling metabolism, growth, reproduction.

Ribosomes

• Non-membranous complexes of rRNA + protein.
• Two subunits (large + small) assemble during translation.
• Size: 70S (prokaryotes, mitochondria, chloroplasts) vs. 80S (cytosolic eukaryotic).
• Free ribosomes: synthesize cytosolic proteins.
• Bound (rough ER/nuclear envelope): synthesize secretory, membrane, lysosomal proteins.

Endomembrane System

• Coordinates protein/lipid traffic & metabolism. Components: nuclear envelope (outer layer), ER, Golgi apparatus, lysosomes, transport vesicles, vacuoles, plasma membrane.

Endoplasmic Reticulum (ER)

1. Rough ER (RER)
   • Ribosome-studded; synthesizes polypeptides → lumen for folding, glycosylation.
   • Packages proteins into transport vesicles for Golgi.
2. Smooth ER (SER)
   • No ribosomes; roles: lipid/steroid synthesis, carbohydrate metabolism (e.g., glycogen breakdown), detoxification (liver hepatocytes), Ca^{2+} storage (sarcoplasmic reticulum in muscle).
   • Example: Leydig cells → testosterone; ovarian follicle cells → estrogen, progesterone.

Golgi Apparatus

• Flattened sacs = cisternae (cis-face receives, trans-face ships).
• Functions: modify glycoproteins (glycosylation trimming/addition), synthesize polysaccharides, sort & package into vesicles for secretion, lysosomes, or plasma membrane.
• Cell type variability: pancreatic acinar cells may hold hundreds of Golgi stacks due to high secretory load.

Lysosomes

• Acidic lumen (≈pH\ 5) containing hydrolytic enzymes.
• Functions:
  • Heterophagy – digest engulfed pathogens (phagocytosis).
  • Autophagy – recycle damaged organelles/macromolecules.
• Clinical tie-in: lysosomal storage diseases (e.g., Tay-Sachs) from enzyme deficiency.

Vacuoles

• Derived from Golgi or ER vesicles. Types:
  • Food vacuole (phagocytosis).
  • Contractile vacuole – osmoregulation in protists.
  • Central vacuole (plants) – storage of ions, pigments, waste; maintains turgor; may contain hydrolytic enzymes (functional analog of lysosome).
• Membrane = tonoplast; selectively permeable.

Energy-Converting Organelles

Mitochondria

• Present in nearly all eukaryotes.
• Double membrane; inner membrane folds (cristae) ↑surface area for oxidative phosphorylation.
• Matrix hosts Krebs cycle; intermembrane space critical for H^+ gradient.
• Own circular DNA & 70S ribosomes (endosymbiotic evidence).
• Number correlates with metabolic demand (e.g., muscle, sperm midpiece).

Chloroplasts

• Found in plants & algae.
• Double membrane + internal thylakoid system (grana stacks).
• Stroma = fluid containing enzymes of Calvin cycle, DNA, ribosomes.
• Capture light → chemical energy (photosynthesis) → ultimately biomass & oxygen for biosphere.

Peroxisomes / Glyoxysomes

• Single membrane.
• Contain oxidases & catalase to convert H2O2\rightarrow H2O + O2.
• Roles: fatty-acid β-oxidation, detoxification (hepatocytes), seed germination (glyoxysomes convert lipids → sugars powering embryo until photosynthesis begins).

Cytoskeleton

• Dynamic protein network underpinning shape, motility, intracellular traffic.

Microtubules (≈25\ nm)

• Polymers of \alpha- & \beta-tubulin dimers → hollow tubes.
• Functions: cell shape, organelle movement, chromosome segregation (mitotic spindle).
• Specialized structures:
  • Centrosome (pair of centrioles) – microtubule-organizing center (MTOC).
  • Cilia/Flagella (9+2 arrangement) – locomotive/feeding/cleaning roles; dynein motors power beating.

Microfilaments (Actin Filaments, ≈7\ nm)

• Double helical polymers of globular actin.
• Roles: muscle contraction (with myosin), cytoplasmic streaming, amoeboid movement, cleavage furrow during cytokinesis, microvilli core stabilization.

Intermediate Filaments (≈8\text{–}12\ nm)

• Diverse fibrous proteins (e.g., keratins, vimentin).
• Provide mechanical strength; persist post-mortem (e.g., cornified skin layer).
• Form nuclear lamina; phosphorylation → nuclear envelope disassembly during mitosis.

Extracellular Components

Plant Cell Wall

• Matrix of cellulose microfibrils, hemicellulose, pectin.
• Protects against mechanical damage, osmotic lysis; imparts turgidity.

Extracellular Matrix (ECM) of Animals

• Composed of glycoproteins: collagen (≈40 % ECM protein mass), proteoglycans, fibronectin.
• Linked to integrin receptors → transduces mechanical signals, guides migration (embryogenesis, wound healing), influences gene expression (cancer metastasis relevance).

Intercellular Junctions

• Plasmodesmata (plants): cytoplasmic channels across walls; share water, ions, small molecules.
• Tight junctions (animals): seal adjacent membranes → prevent paracellular leakage (intestinal epithelium).
• Desmosomes: rivet-like anchoring via cadherins linked to intermediate filaments; resist shear (cardiac muscle).
• Gap junctions: connexon channels allow ions/small molecules (<1\ kDa) to pass; electrical coupling (heart, neurons).

Anatomy of a Generic Prokaryote

• Mandatory: plasma membrane, peptidoglycan wall, cytosol, nucleoid (circular DNA), 70S ribosomes.
• Optional extras: capsule (virulence factor), flagella (rotary motor), pili/fimbriae (adhesion, conjugation), mesosomes (membrane infoldings), plasmids (accessory genes, antibiotic resistance).

Plasma Membrane – Fluid Mosaic Model (Singer & Nicolson, 1972)

• Dynamic two-dimensional fluid of amphipathic phospholipids with proteins, cholesterol, carbohydrates.

Phospholipids

• Hydrophilic head (phosphate + glycerol); hydrophobic fatty-acid tails (saturated vs. unsaturated affects fluidity).
• Lateral diffusion rapid; flip-flop rare (requires flippase).
• Non-polar gases O2, CO2 permeate easily.

Proteins

• Integral (intrinsic) vs. peripheral (extrinsic).
• Transport proteins:
  • Channels – hydrophilic pores (aquaporin, Na^+ channels).
  • Carriers – alternate conformations (GLUT1 for glucose).
• Structural, enzymatic, receptor, cell-adhesion roles.

Cholesterol

• Intercalates between phospholipids; modulates membrane fluidity (buffer: restrains at high T, prevents packing at low T).
• Abundant in animals, scarce in plants, absent in prokaryotes.

Glycolipids & Glycoproteins

• Carbohydrate chains project extracellularly; mediate cell-cell recognition (e.g., ABO blood antigens).
• Hydrogen-bonding with water stabilizes membrane surface.

Membrane Transport Mechanisms

Passive Transport (No ATP)

1. Simple diffusion – down concentration gradient (e.g., O_2 into mitochondria).
2. Facilitated diffusion – via channels/carriers; still down gradient (glucose uptake in erythrocytes).
3. Osmosis – water movement from high \Psiw to low \Psiw (potentials).
   • Animal cells:
   – Hypotonic medium → lysis (hemolysis).
   – Hypertonic → crenation.
   – Isotonic → homeostasis (IV saline 0.9\% NaCl).
   • Plant cells:
   – Hypotonic preferred (turgid); cell wall prevents bursting.
   – Hypertonic → plasmolysis; wilting.

Active Transport (ATP or electrochemical coupling)

• Moves solutes against gradient.
• Na⁺/K⁺‐ATPase cycle:
  1. 3\ Na^+_{in} bind → phosphorylation by ATP → conformational change.
  2. 3\ Na^+ released outside; high affinity now for 2\ K^+.
  3. Dephosphorylation → returns to original shape; 2\ K^+ released into cytosol; repeat.
     – Maintains [Na^+]{out} high & [K^+]{in} high, generating \approx -70\ mV resting membrane potential.
• Proton Pump (plants, fungi, bacteria): ATP → exports H^+ creating electrochemical gradient.
• Cotransport (secondary active): symport/antiport harness stored gradient energy (e.g., H^+‐sucrose symporter loads phloem).

Bulk (Vesicular) Transport – Energy Dependent

• Exocytosis: vesicle fusion → secretion (neurotransmitters, hormones, cell wall polysaccharides).
• Endocytosis:
  • Phagocytosis – “cell eating”; pseudopodia engulf large particles → food vacuole fuses with lysosome.
  • Pinocytosis – nonspecific “cell drinking” of extracellular fluid.
  • Receptor-mediated endocytosis – clathrin-coated pits capture specific ligands (LDL cholesterol uptake).
• Medical link: failure of LDL receptor → familial hypercholesterolemia.

Integration & Significance

• Cellular compartmentalization (endomembrane, energy organelles) optimizes metabolic efficiency, allowing eukaryotic complexity.
• Cytoskeleton-membrane synergy underlies cell morphogenesis, motility, and intracellular logistics (targets in cancer metastasis, neurodegeneration therapy).
• Membrane transport principles inform pharmacology (drug design exploiting carriers/channels), biotechnology (liposome delivery), and clinical interventions (osmotic therapy, dialysis).