4.5: Cytoplasm
The cytoplasm consists of organelles and a framework of protein scaffolds called the cytoskeleton suspended in an aqueous solution, the cytosol. The cytosol is a rich broth of water, ions, salts, and various organic molecules.
Protein Folding and Misfolding
The cytoplasm is the location for several cellular processes, including protein synthesis and folding. The aqueous nature of the cytosol promotes protein folding such that the hydrophobic amino acid side chains are buried in the protein core while the hydrophilic amino acids face the cytosol. However, cellular stresses such as aging and changes in pH, temperature, or osmolarity can cause protein misfolding. Misfolded proteins may amass together in the cytoplasm to form insoluble protein aggregates. Such aggregates are implicated in neurodegenerative disorders, such as Alzheimer's and Parkinson's disease.
Cytoskeleton Composition and Function
The eukaryotic cytoskeleton consists of three types of filamentous proteins: microtubules, microfilaments, and intermediate filaments.
Microtubules, the largest type of filaments, are made up of the protein tubulin. They are dynamic structures that can grow or shrink by adding or removing tubulin molecules from the ends of their strands. They provide structural stability to the cell and act as tracks for transporting proteins, vesicles, and certain organelles within the cell. In addition, microtubules play a crucial role in cell division by providing a framework that guides chromosomes to opposite ends of the cell.
Microfilaments, or actin filaments, are smaller cytoskeletal filaments made up of a protein called actin. Actin proteins can assemble and disassemble rapidly to form filaments. Therefore, they enable motility in unicellular organisms like amoeba or the migration of white blood cells to sites of infection. In skeletal muscle cells, actin filaments slide along myosin filaments to mediate muscle contraction.
Intermediate filaments are not as dynamic as microtubules or actin filaments. However, like the other two types of filaments, they also provide structural support to the cell. Intermediate filaments are composed of different types of proteins based on the specific cell type. For instance, intermediate filaments in hair and nails contain keratin, whereas, in muscle cells, these filaments are made of desmins.
