###### Compartments and functional organization
The cytoplasm consists of the cytosol, a gel-like material enclosed by
the cell or plasma membrane. The cytosol is made up of colloidal proteins such as enzymes, carbohydrates and small protein molecules,
together with ribosomes and ribonucleic acids. The cytoplasm contains
two cytomembrane systems, the endoplasmic reticulum and Golgi
apparatus, as well as membrane-bound organelles (lysosomes, peroxisomes and mitochondria), membrane-free inclusions (lipid droplets,
glycogen and pigments) and the cytoskeleton. The nuclear contents,
the nucleoplasm, are separated from the cytoplasm by the nuclear
envelope.
###### Endoplasmic reticulum
The endoplasmic reticulum is a system of interconnecting membranelined channels within the cytoplasm (Fig. 1.4). These channels take
various forms, including cisternae (flattened sacs), tubules and vesicles.
The membranes divide the cytoplasm into two major compartments.
The intramembranous compartment, or cisternal space, is where secretory products are stored or transported to the Golgi complex and cell