Cytoplasm
In cell biology, the cytoplasm is all of the material within a cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The main components of the cytoplasm are cytosol – a gel-like substance, the organelles – the cell's internal sub-structures, and various cytoplasmic inclusions. The cytoplasm is about 80% water and usually colorless.
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About half the total area of membrane in an eukaryotic cell encloses the space of the endoplasmic reticulum. The ER is an extremely dynamic membrane organelle which consists of the nuclear envelope and the peripheral ER. It is organized into a netlike labyrinth of branching tubules and flattened sheets that extends throughout the cytosol. These tubules and sheets are interconnected and their membrane is continuous with the outer nuclear membrane. The ER and nuclear membranes form a continuous layer enclosing a single internal space, called the ER lumen, which often occupies the 10% of the total cell volume. However, this unique space has several morphologically distinct areas. Thus, the peripheral ER is composed of a polygonal network of tubules and interdispersed sheets that spreads from the nuclear envelope to the cell cortex.