Overview
This lecture covers the main structures (organelles) of the cell, detailing their components, functions, and how they work together.
Nucleus
- The nucleus is the control center (brain) of the cell.
- Surrounded by a double-layered nuclear envelope with outer (has ribosomes) and inner (lined with lamins for structure/division) membranes.
- Nuclear pores in the envelope allow transport of molecules between nucleus and cytoplasm.
- Nucleolus inside nucleus synthesizes rRNA and assembles ribosomes.
- Chromatin consists of DNA and histone proteins; two forms: euchromatin (loose, active) and heterochromatin (tight, inactive).
- Functions: DNA replication, transcription (making RNA: mRNA, tRNA, rRNA).
Endoplasmic Reticulum (ER)
- Rough ER (RER): has ribosomes; synthesizes, folds, and glycosylates proteins that are secreted, sent to membranes, or lysosomes.
- Smooth ER (SER): no ribosomes; synthesizes lipids (fatty acids, phospholipids, cholesterol), detoxifies (CYP450 enzymes), metabolizes glucose-6-phosphate, and stores calcium.
Golgi Apparatus
- Receives vesicles from ER at the cis face and exports them from the trans face.
- Modifies proteins and lipids (glycosylation—N and O type, phosphorylation).
- Packages and sorts molecules for delivery to membranes, lysosomes, or secretion.
Cell Membrane
- Composed of a phospholipid bilayer (polar hydrophilic heads, non-polar hydrophobic tails), cholesterol (controls fluidity), and proteins (integral and peripheral).
- Acts as a selectively permeable barrier for cellular transport.
Lysosomes
- Spherical organelles containing hydrolytic enzymes (proteases, nucleases, lipases, glucosidases).
- Digest macromolecules, damaged organelles (autophagy), and entire damaged cells (autolysis).
Peroxisomes
- Contain catalase and oxidase enzymes for breaking down hydrogen peroxide (Hâ‚‚Oâ‚‚) and free radicals.
- Involved in fatty acid oxidation (breakdown), synthesis of plasmalogens (myelin component), and limited ethanol metabolism.
Mitochondria
- "Powerhouse" of the cell; site of ATP synthesis (mainly via oxidative phosphorylation/electron transport chain).
- Structure: outer membrane (permeable), inner membrane with cristae (less permeable), matrix with mitochondrial DNA (maternal origin).
- Host metabolic pathways: Krebs cycle, heme synthesis, urea cycle, gluconeogenesis, ketogenesis.
Ribosomes
- Made of rRNA and proteins; consist of small (40S) and large (60S) subunits.
- Free ribosomes synthesize cytosolic proteins; ER-bound ribosomes make secretory, membrane, and lysosomal proteins.
- Main function: protein synthesis (translation).
Cytoskeleton
- Microfilaments (actin): cell shape, muscle contraction, cytokinesis, diapedesis, phagocytosis.
- Intermediate filaments: anchor cells to each other, matrix, or organelles.
- Microtubules (tubulin): intracellular transport (ATP-dependent, via kinesin/dynein), mitosis (chromatid separation), cellular extensions (cilia/flagella).
Key Terms & Definitions
- Nucleus — Organelle housing genetic material (DNA).
- Nuclear Envelope — Double membrane around nucleus.
- Nucleolus — Site of rRNA synthesis.
- Chromatin — DNA-histone complex inside nucleus.
- Rough ER — ER with ribosomes; protein synthesis.
- Smooth ER — ER without ribosomes; lipid synthesis, detoxification.
- Golgi Apparatus — Organelle for protein/lipid modification and sorting.
- Lysosome — Organelle with enzymes for digestion.
- Peroxisome — Organelle for fatty acid oxidation, detoxification.
- Mitochondria — Organelle for ATP production.
- Ribosome — Site of protein synthesis.
- Cytoskeleton — Network of protein filaments for structure and transport.
Action Items / Next Steps
- Review diagrams of cell organelles and their functions.
- Study types of transport across the cell membrane.
- Prepare for future deeper dives into the nucleus and transport processes.