Chapter 3: part 3

Cytosol: Gel-like intracellular fluid composed of water and dissolved solutes including proteins, salts, and sugars.
Inclusions: Insoluble storage substances that vary by cell type (e.g., glycogen granules, pigments, lipid droplets, vacuoles, and crystals).
Organelles: Specialized metabolic structures categorized into membranous and nonmembranous types.

Mitochondria:
- Primary site of ATP synthesis via aerobic cellular respiration.
- Enclosed by a double membrane; inner membrane forms folds called cristae enclosing the internal matrix.
- Possess independent DNA, RNA, and ribosomes; multiply independently via fission.

Endoplasmic Reticulum (ER):
- Network of interconnected membranous tubes (cisterns) continuous with the outer nuclear membrane.
- Rough ER: Studded with ribosomes; synthesizes proteins destined for secretion, plasma membranes, or lysosomes, as well as membrane phospholipids.
- Smooth ER: Lacks ribosomes; houses enzymes for lipid and steroid synthesis, lipid transport, drug detoxification, glycogen breakdown, and calcium storage/release (sarcoplasmic reticulum in muscle).
Golgi Apparatus:
- Stacked membranous sacs that modify, package, and direct proteins and lipids delivered from the rough ER.
- Receives vesicles at the cis (receiving) face and dispatches packaged products from the trans (shipping) face along three main pathways:
- Pathway A: Secretory vesicles for exocytosis.
- Pathway B: Vesicles fusing with the plasma membrane or organelle membranes.
- Pathway C: Digestive enzymes retained inside lysosomes.

Peroxisomes:
- Sacs containing oxidases and catalase to detoxify free radicals, converting harmful substances to hydrogen peroxide ( $H_2O_2$ ) and then into water ( $H_2O$ ).
- Participate in fatty acid synthesis and degradation.
Lysosomes:
- Spherical vesicles containing acid hydrolases for intracellular digestion of foreign substances, cell debris, and old organelles.
- Responsible for autolysis upon cellular injury; enzyme deficiencies cause lysosomal storage diseases such as Tay-Sachs disease.

Ribosomes:
- Sites of protein synthesis composed of protein and ribosomal RNA (rRNA).
- Free Ribosomes: Synthesize soluble proteins operating in the cytosol or organelles.
- Membrane-Bound Ribosomes: Attached to the ER; synthesize membrane, lysosomal, or secreted proteins.
Cytoskeleton:
- Network of structural rods providing internal support and facilitating movement:
- Microfilaments: Thinnest strands ( $7\,nm$ ) made of actin; anchor to the terminal web to reinforce cell shape and support cell motility.
- Intermediate Filaments: Tough, insoluble tetramer fibers ( $10\,nm$ ) that resist mechanical tension (e.g., neurofilaments, keratin).
- Microtubules: Hollow tubes ( $25\,nm$ ) composed of tubulin subunits radiating from the centrosome; serve as cellular tracks for motor proteins.
Centrosome and Centrioles:
- Centrosome acts as the microtubule organizing center near the nucleus.
- Contains a pair of barrel-shaped centrioles aligned at right angles, which organize spindle fibers and form the bases of cilia and flagella.

Cilia and Flagella:
- Motile surface projections built from a $9+2$ pattern of microtubule doublets, anchored to a basal body possessing a $9+0$ pattern.
- Cilia move substances across cell surfaces using alternating power and recovery strokes; flagella propel entire cells.

Microvilli:
- Non-motile, fingerlike extensions of the plasma membrane with a core of actin filaments, structured to maximize surface area for absorption.

Nuclear Envelope:
- Double-membrane barrier penetrated by nuclear pore complexes; outer membrane connects to the rough ER, while the inner surface is lined by the proteinaceous nuclear lamina.

Nucleoli:
- Dark-staining nuclear sites responsible for rRNA synthesis and ribosome assembly.
Chromatin and Chromosomes:
- Composed of $30\%$ DNA, $60\%$ histone proteins, and $10\%$ RNA.
- Form nucleosome structures ( $10\,nm$ diameter) consisting of DNA coiled around histone cores, which condense into $30\,nm$ fibers, $700\,nm$ chromatids, and metaphase chromosomes during division.