The cytoplasm consists of organelles, an aqueous solution called the cytosol, and a framework of protein scaffolds called the cytoskeleton. The cytosol is a rich broth of ions, small organic molecules such as glucose, and macromolecules such as proteins. Several cellular processes including protein synthesis occur in the cytoplasm.
The composition of the cytosol promotes protein folding such that hydrophobic amino acid side chains are oriented away from the aqueous solution and towards the protein core. However, cellular stressors such as aging and changes in pH, temperature, or osmolarity cause protein misfolding. Misfolded proteins may aggregate to form insoluble deposits in the cytoplasm. Insoluble protein aggregates are implicated in neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis.
Cytoskeleton Composition and Function
The eukaryotic cytoskeleton consists of three types of filamentous proteins: microtubules, microfilaments, and intermediate filaments.
Microtubules–the largest type of filament–are made up of the protein tubulin. Microtubules are dynamic structures that can grow or shrink by adding or removing tubulin molecules from the ends of their strands. They provide structural stability and provide tracks for the transport of proteins and vesicles 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 are the smallest type of cytoskeletal filaments and are made of a protein called actin. Actin filaments can assemble and disassemble rapidly. Therefore, they enable movement in unicellular organisms like amoeba or 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. They provide structural support and are comprised of different types of proteins based on the specific cell type. For instance, intermediate filaments in hair and nails contain keratin whereas muscle cells contain desmins.