Cytosol, Cytoplasmic Inclusions, and Cell Cycle
Cytosol and Cytoplasmic Inclusions
Cytosol: The aqueous environment inside the cell, encompassing cytoplasmic organelles, a large number of enzymes, and structures such as inclusions. It is the site of protein synthesis, degradation, cytoskeleton processes, and most intermediary metabolism reactions.
Cytoplasmic Inclusions: Materials stored in the cell cytoplasm that are not membrane-bound. The most common are:
- Fat: The most important fuel store; most cells contain small droplets of water-insoluble triacylglycerols, especially adipose tissue.
- Glycogen: A branched polymer; the main carbohydrate store in animal cells (liver and muscles), ensuring ATP levels.
Cytoskeleton
A network of different protein filaments extending throughout the cytoplasm and anchored to the plasma membrane in eukaryotic cells. It comprises:
Microtubules
Cylindrical structures (25 nm diameter) that radiate from the centrosome (the microtubule-organizing center). They are constituted by 13 protofilaments of tubulin and associated proteins (MAPs).
Properties: They are polar (a fast-growing (+) end and a slow-growing (-) end), labile (undergo cycles of growth and shortening), and crucial for various functions, including structural dynamics, cell shape maintenance, intracellular transport, and stable structures like cilia and flagella.
Centrioles: A pair of perpendicular cylindrical structures embedded in the centrosome of animal cells; each centriole has nine triplets of microtubules.
Microfilaments
Thin and flexible structures (7 nm diameter) extending throughout the cytoplasm. They are formed by two intertwined protofilaments of the protein actin.
Properties: Similar to microtubules. Functions: They interact with accessory proteins to form rigid structures (microvilli) and contractile structures (pseudopods, contractile rings).
Intermediate Filaments
Filaments (10 nm diameter) whose size is intermediate between actin filaments and microtubules. They extend throughout the cytoplasm and are anchored to the plasma membrane at desmosomes.
Functions: They are very resistant and provide mechanical stability to cells (epithelial cells, neurons, muscle cells).
Molecular Motors
Proteins that move unidirectionally along microtubules and microfilaments using ATP. Microtubule-associated motors (kinesin and dynein) have two globular heads and a tail. Actin-associated motors (myosins) also consist of a head and tail. Motor proteins move via saltatory movements.
Cell Cycle
A regular, repeating sequence of growth and division, comprising four successive phases: G1 (gene transcription and translation), S (DNA replication), G2 (transcription of genes needed for cell division), and M (mitosis).
Control of the Cell Cycle
Cells control the initiation of each phase (checkpoints) to ensure that each phase is completed successfully before the next begins.
- G1 Checkpoint: Determines whether or not the cell will divide. A negative signal leads to a resting state (G0); a positive signal initiates division.
- G2 Checkpoint: Determines whether or not to initiate mitosis. A positive signal is given if conditions are favorable and DNA replication is complete.
- M Checkpoint: Ensures that the mitotic spindle is correctly formed and chromosomes are properly aligned.
Cell Division in Animal Cells
Cells vary in their capacity for multiplication:
- G0 (indefinitely): Adult cells that do not divide (e.g., neurons).
- Temporary G0: Cells that divide only when stimulated (e.g., smooth muscle cells).
- Stem cells: Continuously divide to produce daughter cells that can differentiate into various cell types.
- Cancer cells: Divide indefinitely due to abnormal behavior and accumulate mutations; they are immortal.
Programmed Cell Death (Apoptosis)
A mechanism of cell death important in embryonic development (removal of unnecessary cells, such as interdigital tissue) and adulthood (replacement of damaged tissue and destruction of cells that pose a threat). Changes during cell death include cell shrinkage, water loss, blebbing, nuclear fragmentation, and engulfment by neighboring cells. Apoptosis is initiated by the production of lethal proteins such as endonucleases, hydrolases, and proteases.
The Interphase Nucleus
Contains the genetic material and directs all cell activity. In animal cells, it is centrally located; in plant cells, it is not always central. The nucleus is separated from the cytoplasm by a double membrane nuclear envelope, containing granular chromatin, a nucleolus, and a nuclear matrix.
Nuclear Components
Nuclear Envelope: Two membranes separated by the perinuclear space and perforated by numerous pores. The outer nuclear membrane may have ribosomes attached to its cytosolic face. The inner nuclear membrane organizes chromatin and regulates nuclear envelope growth after mitosis.