Overview
This lecture explains the structure and function of major cellular organelles and components, including their roles in cell biology, synthesis, metabolism, and division.
Nucleus Structure & Function
- The nucleus contains a double-layered nuclear envelope (outer with ribosomes, inner with lamins for structure and division).
- Nuclear pores allow transport between the nucleus and cytoplasm.
- The nucleolus synthesizes rRNA and assembles ribosomes.
- Chromatin is made of DNA and histone proteins, existing as euchromatin (loose, active) or heterochromatin (tight, inactive).
- Functions: DNA replication (making DNA), transcription (making RNA: mRNA, tRNA, rRNA).
Endoplasmic Reticulum (ER)
- Rough ER has ribosomes and synthesizes, folds, and glycosylates proteins destined for lysosomes, membranes, or secretion.
- Smooth ER lacks ribosomes; synthesizes lipids (fatty acids, phospholipids, cholesterol), detoxifies (via CYP450), metabolizes glucose 6-phosphate, and stores Ca²⁺ (especially in muscles).
Golgi Apparatus
- Receives vesicles from ER at the cis face, modifies proteins/lipids (glycosylation—N and O types, phosphorylation), and packages them at the trans face.
- Directs vesicles to become lysosomes, membrane proteins, or secreted products.
Cell Membrane
- Made of a phospholipid bilayer (hydrophilic heads, hydrophobic tails), cholesterol (regulates fluidity), and proteins (integral/peripheral—for transport, enzymes, adhesion).
- Acts as a selectively permeable barrier allowing specific transport (diffusion, facilitated diffusion, vesicular transport).
Lysosomes
- Spherical organelles with hydrolytic enzymes (proteases, nucleases, lipases, glucosidases) to digest macromolecules.
- Involved in autophagy (recycling organelles) and autolysis (self-breakdown of damaged cells).
Peroxisomes
- Contain catalase, oxidase, and metabolic enzymes for detoxifying free radicals (hydrogen peroxide), fatty acid oxidation, plasmalogen synthesis (for myelin), and minor alcohol metabolism.
Mitochondria
- Double membrane organelle (outer smooth, inner folded/cristae); matrix contains mitochondrial DNA (maternal origin).
- Site of ATP synthesis via oxidative phosphorylation (electron transport chain) and metabolic cycles (Krebs, heme synthesis, urea cycle, gluconeogenesis, ketogenesis).
Ribosomes
- Composed of rRNA and proteins: large (60S) and small (40S) subunits.
- Can be membrane-bound (make exported or membrane proteins) or free in the cytosol (make cytosolic proteins).
- Function: protein synthesis via translation of mRNA.
Cytoskeleton
- Microfilaments (actin): muscle contraction (with myosin), cytokinesis, diapedesis, phagocytosis.
- Intermediate filaments: anchor cells to each other, to the extracellular matrix, and position organelles.
- Microtubules: made of α- and β-tubulin; enable intracellular transport (dynein, kinesin, ATP-dependent), chromosome separation, and form bases of cilia/flagella.
Key Terms & Definitions
- Nuclear Envelope — double membrane surrounding nucleus.
- Nucleolus — site of rRNA synthesis and ribosome assembly.
- Chromatin — DNA with histone proteins (genetic material).
- Rough ER — ER with ribosomes; synthesizes and processes proteins.
- Smooth ER — ER without ribosomes; lipid synthesis, detoxification.
- Golgi Apparatus — modifies, sorts, and ships cell products.
- Lysosome — organelle with digestive enzymes.
- Peroxisome — organelle for detoxification and lipid metabolism.
- Mitochondria — organelle for ATP production and metabolism.
- Ribosome — protein synthesis machinery.
- Cytoskeleton — cell’s structural network (actin, intermediate filaments, microtubules).
Action Items / Next Steps
- Review diagrams of cell organelles and their functions.
- Study types of transport through the cell membrane and protein targeting pathways.
- Prepare for follow-up lessons on detailed membrane transport and nuclear processes.