Structure of the Cytoplasm
Microtubules radiate towards the periphery of the cell from microtubule organizing centers (MTOCs) located close to the nucleus, and provide structure and shape to the cell.
This image shows the nucleus in blue, the actin filaments on the cell periphery are labeled red and the extensive microtubule network is marked green. The cytoplasm undergoes rapid reorganization during cell division with microtubules forming the spindle, which binds to chromosomes and segregates them into two daughter cells.
Similar to the previous image, chromosomes are stained blue and microtubules are green. Tiny red dots are kinetochores.
Microtubules are involved in cytoplasmic transport, chromosome segregation and in forming structures such as cilia and flagella for cellular movement.
Intermediate filaments are larger than microfilaments but smaller than microtubules and are formed by a group of proteins that share structural features. Though they are not involved in cell motility, they are important for cells to come together as tissues and to remain anchored to the extracellular matrix.
Organelles and Multi-protein Complexes
Most eukaryotic cells have a number of organelles that provide compartments within the cytoplasm for specialized microenvironments. For instance, lysosomes contain a number of hydrolases in an acidic environment that is ideal for their enzymatic activity. These hydrolases are actively transported into the lysosome after being synthesized in the cytoplasm. Mitochondria, while containing their own genome, also need many enzymes synthesized in the cytosol, which are then selectively moved into the organelle. These organelles are placed in specific locations due to the physical gel-like nature of the cytoplasm and by anchoring to the cytoskeleton.
In addition, the cytoplasm also plays host to multi-protein complexes like the proteasome and ribosomes. Ribosomes are large complexes of RNA and protein that are important for the translation of mRNA code into amino acid sequences of proteins. Proteasomes are giant molecular structures about 20, 000 kilodaltons in mass and 15 nm in diameter. Proteasomes are important for targeted destruction of proteins that are no longer needed by the cell.