Overview

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.

Procedure

In eukaryotic cells, the cytoplasm is the area between the plasma membrane and nuclear envelope. Since prokaryotic cells lack a nucleus, the cytoplasm refers to everything found within the plasma membrane. In both cell types, filling the cytoplasmic space is a gel-like matrix called the cytosol, an aqueous solution that contains many soluble ions, small molecules and macromolecules.

Exclusive to eukaryotic cells, the cytoplasm includes numerous membrane-bound organelles that are suspended in the cytosol, as well as the cytoskeleton, a network of fibers which gives cells their shape and also helps with cell motility and transport. The cytoplasm, though, is also where many non-organelle cellular functions take place such as protein synthesis from free ribosomes. Furthermore, the aqueous nature of the cytoplasm facilitates protein folding by repelling hydrophobic amino acid side-groups into the protein core.