Overview
This lecture covers the main structures (organelles) of the cell and their functions, providing concise details on key cellular components and their roles in cell biology.
The Nucleus
- The nucleus is the control center of the cell, surrounded by a double-layered nuclear envelope.
- Outer nuclear membrane contains ribosomes for protein synthesis; inner membrane has lamins important for structure and cell division.
- Nuclear pores allow transport of molecules between the nucleus and cytoplasm.
- The nucleolus synthesizes ribosomal RNA (rRNA) and assembles ribosomes.
- Chromatin (DNA + histone proteins) can be euchromatin (loose, active) or heterochromatin (tight, inactive).
- Functions: DNA replication and transcription to produce mRNA, tRNA, and rRNA.
Endoplasmic Reticulum (Rough and Smooth)
- Rough ER has surface ribosomes; main site of synthesis and folding of proteins for lysosomes, membranes, or secretion.
- Performs N-type glycosylation (adds sugars), helps fold and package proteins for transport to the Golgi apparatus.
- Smooth ER lacks ribosomes; synthesizes lipids (fatty acids, phospholipids, cholesterol), steroid hormones, and detoxifies drugs/toxins (via CYP450 enzymes).
- Involved in glucose-6-phosphate metabolism and storage/release of calcium (especially in muscle as sarcoplasmic reticulum).
Golgi Apparatus
- Receives vesicles from ER at the cis face, modifies proteins/lipids (O-type glycosylation, phosphorylation), and packages them at the trans face.
- Directs modified products to lysosomes, membranes, or for secretion.
Cell Membrane
- Composed of a phospholipid bilayer (hydrophilic heads, hydrophobic tails), cholesterol (regulates fluidity), and proteins (integral and peripheral).
- Functions as a selectively permeable barrier enabling various forms of transport (simple/facilitated diffusion, vesicular transport).
Lysosomes
- Spherical organelles containing hydrolytic enzymes (proteases, nucleases, lipases, glucosidases) to break down macromolecules and worn-out organelles (autophagy).
- Involved in cell self-destruction via autolysis in response to severe damage.
Peroxisomes
- Contain catalase and oxidase enzymes to detoxify free radicals (especially hydrogen peroxide) and participate in fatty acid oxidation.
- Synthesize specific lipids (e.g., plasmalogen for myelin) and contribute to ethanol metabolism.
Mitochondria
- Double-membraned organelle; site of ATP synthesis (mainly via oxidative phosphorylation/electron transport chain).
- Houses mitochondrial DNA (maternally inherited), mediates metabolic processes: Krebs cycle, heme synthesis, urea cycle, gluconeogenesis, and ketogenesis.
Ribosomes
- Consist of large (60S) and small (40S) subunits, made from rRNA and proteins.
- Can be membrane-bound (on rough ER) for secretory/lysosomal/membrane proteins, or free in cytosol for intracellular proteins.
- Site of translation: synthesizes proteins from mRNA templates.
Cytoskeleton
- Includes microfilaments (actin: cell shape, contraction, cytokinesis, diapedesis, phagocytosis), intermediate filaments (structural support and anchoring cells/organelles), and microtubules (intracellular transport, cell division, forming cilia/flagella).
- Microtubules use motor proteins (dynein, kinesin) for ATP-dependent transport and chromosome separation during mitosis.
Key Terms & Definitions
- Nucleus — organelle housing DNA and coordinating genetic activities.
- Rough ER — endoplasmic reticulum with ribosomes; protein synthesis site.
- Smooth ER — endoplasmic reticulum without ribosomes; lipid synthesis and detoxification.
- Golgi apparatus — organelle modifying, packaging, and sorting cellular products.
- Lysosome — organelle with enzymes to digest macromolecules.
- Peroxisome — organelle for detoxification and lipid metabolism.
- Mitochondria — “powerhouse” organelle, primary site of ATP production.
- Ribosome — molecular machine for protein synthesis.
- Cytoskeleton — network of fibers (microfilaments, intermediate filaments, microtubules) for cell structure and transport.
Action Items / Next Steps
- Review and memorize the structure and function of each organelle.
- Study the processes of protein synthesis, lipid metabolism, and cellular transport.
- Prepare for future detailed lectures on nucleus transport proteins and membrane transport mechanisms.