The microtubule organizing center (MTOC) is the region where microtubules are assembled in vivo. The kinetochores, centrosomes, and basal bodies (cilia and flagella) of chromosomes all function as microtubule organizing centers. All microtubule organizing centers contain γ-tubulin.
(3) Intermediate fibers:
Process: ① Two monomers form two superhelical dimers (keratin is a heterodimer)
② Two dimers are antiparallel and staggered to assemble into a tetramer
③The tetramers then connect to form a fibril
④8 protofibrils form an intermediate filament (symmetrical, non-polar)
Assembly characteristics of intermediate filaments: 1. Since IF is composed of antiparallel α-helices, it has no polarity, unlike microfilaments and microtubules.
2. Most of the intermediate filament proteins in the cell are assembled into intermediate filaments, unlike microfilaments and microtubules, which have a protein pool (only about 30% are in an assembled state).
3. IF assembly is independent of temperature and protein concentration and does not require ATP or GTP.
24. The main biological functions of lysosomes:
The main function of lysosomes is digestion. They are digestive organs within cells. Cell autolysis, defense, and the utilization of certain substances are all related to the digestive function of lysosomes.
Intracellular digestion: For higher animals, the nutrients of cells mainly come from water molecules in the blood, and some large molecules enter the cells through endocytosis, such as endocytosis of low-density lipoprotein to obtain cholesterol. For some single-cell eukaryotes, the digestive function of lysosomes is even more important.
Apoptosis: The process of ontogeny often involves the transformation or reconstruction of tissues or organs, such as the metamorphosis of insects and frogs. This process is genetically controlled and is called programmed cell death. Cells destined for elimination bud off to form apoptotic bodies, which are then engulfed and digested by macrophages.
Autophagy: Clears out useless biological macromolecules and aging organelles in cells. For example, the half-life of many biological macromolecules is only a few hours to a few days, and the average lifespan of mitochondria in liver cells is about 10 days.
Defensive function: For example, macrophages can ingest pathogens, kill and degrade them in lysosomes.
Participates in the regulation of secretory processes: such as degrading thyroglobulin into active thyroxine.
Forming the acrosome of the sperm: The acrosome is equivalent to a chemical drill that can dissolve through the cortex of the egg, allowing the sperm to enter the egg
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