Overview
The cytoplasm is everything inside the plasma membrane except the nucleus: the cytosol with its organelles, inclusions and cytoskeleton. This topic describes the cytosol, an overview of the organelles, and the cytoskeleton (microtubules, actin and intermediate filaments).
Cytosol (hyaloplasm) and organelles
The cytosol (hyaloplasm) is the soluble, gel-like part of the cytoplasm, about 70–80 % water, with ions, small molecules, soluble proteins and enzymes. Many metabolic pathways take place in it, for example glycolysis and the synthesis of fatty acids. It is not homogeneous: it is organized by the cytoskeleton, and it contains ribosomes and inclusions.
Inclusions are non-living cytoplasmic deposits such as glycogen, lipid droplets and pigments (e.g. lipofuscin, melanin). They are not surrounded by a membrane and are not present in all cells.
The cytoskeleton
The cytoskeleton consists of the following components:
- The cytoskeleton is a complex network of filaments and tubules that extends through the cytoplasm from the nucleus to the plasma membrane.
- It is composed of protein polymers.
- Microtubules: These are one of the three types of polymers, measuring 25 nm in diameter.
- Microfilaments (Actin filaments): These are another type of polymer, with a diameter of 5-7 nm.
- Intermediate filaments: These constitute the third type of polymer, having a diameter of 8-10 nm.