Cell Structure, Function, and Organellar Compartmentalization

Cell Fundamentals and Types

• Universal Cell Characteristics: All cells possess a plasma membrane, cytosol, chromosomes, and ribosomes.

• Prokaryotes:

  • Belong to domains Bacteria and Archaea.

  • DNA is located in an unbound nucleoid region.

  • Lack internal membrane-bound organelles; generally smaller than eukaryotes.

  • May possess localized internal regions for specialized functions such as motility and cell division.

• Eukaryotes:

  • Include protists, fungi, plants, and animals.

  • DNA is enclosed within a membrane-bound nucleus.

  • Contain membrane-bound organelles that compartmentalize cellular tasks.

• Unique Organellar Differences:

  • Plant Cells: Possess chloroplasts, a central vacuole, a cell wall, and plasmodesmata.

  • Animal Cells: Possess lysosomes, centrosomes, and flagella.

Organelles and Compartmentalization

• Compartmentalization:

  • Separates distinct enzymatic reactions and metabolic processes into specific locations.

  • Increases total surface area available for reactions.

  • Prevents incompatible or interfering reactions from occurring concurrently in the same space.

• Primary Organelle Classifications:

  • Endomembrane System: Subcellular components that interact directly or via vesicles to modify, package, and transport proteins, lipids, and polysaccharides.

  • Energy Organelles: Mitochondria and chloroplasts, which convert energy into usable cellular forms.

Endomembrane System Components

• Nucleus and Nucleolus:

  • Surrounded by a double-membrane nuclear envelope perforated with pores regulating material entry and exit; continuous with the endoplasmic reticulum membrane.

  • Nucleolus: Dense region where ribosomal RNA (rRNA) is synthesized and combined with proteins to produce large and small ribosomal subunits.

• Ribosomes:

  • Protein-rRNA complexes responsible for translating messenger RNA (mRNA) into polypeptide primary structure.

  • Free Ribosomes: Located in the cytosol; synthesize proteins functioning predominantly within the cytosol.

  • Bound Ribosomes: Attached to the endoplasmic reticulum or nuclear envelope; synthesize proteins destined for insertion into membranes, packaging into organelles, or secretion via transport vesicles.

• Endoplasmic Reticulum (ER):

  • Membranous network of sacs and tubules (cisternae) providing mechanical support and intracellular transport.

  • Rough ER: Studded with bound ribosomes; compartmentalizes protein synthesis.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies poisons/drugs.

• Golgi Complex:

  • Flattened membranous sacs (cisternae) exhibiting distinct polarity:

  • Cis face: Receives transport vesicles from the ER.

  • Trans face: Dispatches sorted, modified, and tagged cargo to other cellular sites or the plasma membrane for secretion via exocytosis.

  • Chemically modifies and folds newly synthesized products.

• Lysosomes:

  • Hydrolytic enzyme sacs in animal cells that degrade macromolecules.

  • Execute autophagy by recycling the cell's own damaged organelles and organic compounds.

  • Participate in apoptosis (programmed cell death) by increasing membrane permeability to release catabolic enzymes into the cytoplasm.

• Vacuoles:

  • Animal Vacuoles: Small, numerous vesicles assisting in endocytosis, exocytosis, and general storage.

  • Plant Central Vacuole: Large compartment storing nutrients and water, maintaining turgor pressure, and occasionally functioning similarly to lysosomes.

• Peroxisomes:

  • Non-endomembrane metabolic compartments that break down fatty acids, synthesize specific phospholipids, and detoxify compounds.

  • Produce hydrogen peroxide ($H_2O_2$), which catalase subsequently degrades into water and oxygen.

Energy Organelles and Endosymbiont Theory

• Mitochondria:

  • Primary site of cellular respiration; higher metabolic activity correlates with greater mitochondrial abundance.

  • Outer Membrane: Smooth exterior barrier.

  • Inner Membrane: Folded into convolutions called cristae to maximize surface area for ATP synthesis.

  • Internal Compartments: The intermembrane space and the inner mitochondrial matrix, which contains the Krebs cycle enzymes, mitochondrial DNA (mtDNA), and autonomous ribosomes.

• Chloroplasts:

  • Photosynthetic organelles found in plants and algae containing the pigment chlorophyll.

  • Thylakoids: Membranous sacs grouped into stacks called grana, where light-dependent reactions take place.

  • Stroma: Fluid surrounding the grana; location of the Calvin cycle, containing chloroplast DNA (cpDNA), ribosomes, and metabolic enzymes.

• Endosymbiont Theory:

  • Explains that mitochondria and chloroplasts originated from ancestral, free-living prokaryotes engulfed by ancestral eukaryotic cells that formed an endosymbiotic relationship.

  • Supporting Evidence: Double-membrane structure, independent circular DNA, autonomous prokaryotic-like ribosomes, and self-replication capability.

The Cytoskeleton

• Function: Network of fibers providing structural/mechanical support, anchoring organelles, and facilitating cellular and vesicle motility via motor proteins.

• Cytoskeletal Fiber Types:

  • Microtubules: Thickest hollow cylinders; originate from centrosomes in animal cells; function as tracks for organelle movement, guide chromosome separation during cell division, and form motile structures (cilia and flagella).

  • Microfilaments: Thin solid rods composed of actin; bear tension, support cell shape, drive muscle contraction alongside myosin, and assemble the contractile ring in animal cleavage furrows.

  • Intermediate Filaments: Intermediate diameter fibers made of diverse fibrous proteins; permanent structural elements anchoring the nucleus and organelles, and constructing the nuclear lamina.

Practice Free-Response Questions

• Practice Mini FRQ 1 (Albumin Synthesis and Liver Disease):

  • Context: Hepatitis C causes liver disease linked to a $10\text{--}30\%$ decrease in blood-albumin levels (a key protein maintaining oncotic pressure and fluid balance).

  • Prompt (a): Identify the organelle in liver cells most likely affected by hepatitis C.

  • Prompt (b): Justify the organelle identification in part (a).

• Practice Mini FRQ 2 (Insulin Secretory Pathway):

  • Context: Normal pancreatic cells synthesize insulin protein in the ER prior to release into the bloodstream.

  • Prompt (a): Describe the route of insulin production and trafficking in a normal pancreatic cell, beginning from protein synthesis in the ER to extracellular secretion.

• Practice Mini FRQ 3 (Alcohol Detoxification and Organelle Clearance):

  • Context: Liver tissue samples exposed to varying alcohol concentrations ($0.25\,mg/mL$, $0.35\,mg/mL$, and $0.45\,mg/mL$) for $20\text{ minutes}$ exhibit full detoxification, but organelle damage occurs at $0.45\,mg/mL$.

  • Prompt (a): Identify one liver organelle responsible for detoxifying alcohol across all samples.

  • Prompt (b): Identify the organelle responsible for clearing damaged organelles in the $0.45\,mg/mL$ condition.

  • Prompt (c): Describe the cellular mechanisms by which the organelle from part (b) removes the damaged structures.