The cytoplasm, or cytosol of eukaryotic cells is the gel-like, water-based fluid that occupies the majority of the volume of the cell. Cytoplasm functions as the site of energy production, storage, and the manufacture of cellular components. The various organelles that are responsible for some of these functions in the eukaryotic cell are dispersed throughout the cytoplasm, as are the compounds that provide structural support for the cell.
The cytoplasm is the site of almost all of the chemical activity occurring in a eukaryotic cell. Indeed, the word cytoplasm means "cell substance."
Despite being comprised mainly of water (about 65% by volume), the cytoplasm has the consistency of gelatin. Unlike gelatin, however, the cytoplasm will flow. This enables eukaryotes such as the amoeba to adopt different shapes, and makes possible the formation of pseudopods that are used to engulf food particles. The consistency of the cytoplasm is the result of the other constituents of the cell that are floating in fluid. These constituents include salts, and organic molecules such as the many enzymes that catalyze the myriad of chemical reactions that occur in the cell.
When viewed using the transmission electron microscope , the cytoplasm appears as a three-dimensional lattice-work of strands. In the early days of electron microscopy there was doubt as to whether this appearance reflected the true nature of the cytoplasm, or was an artifact of the removal of water from the cytoplasm during the preparation steps prior to electron microscopic examination . However, development of techniques that do not perturb the natural structure biological specimens has confirmed that this latticework is real.
The lattice is made of various cytoplasmic proteins. They are scaffolding structures that assist in the process of cell division and in the shape of the cell. The shape-determinant is referred to as the cytoskeleton. It is a network of fibers composed of three types of proteins. The proteins form three filamentous structures known as microtubules, intermediate filaments, and microfilaments. The filaments are connected to most of organelles located in the cytoplasm and serve to hold together the organelles.
The microtubules are tubes that are formed by a spiral arrangement of the constituent protein. They function in the movement of the chromosomes to either pole of the cell during the cell division process. The microtubules are also known as the spindle apparatus. Microfilaments are a composed of two strands of protein that are twisted around one another. They function in the contraction of muscle in higher eukaryotic cells and in the change in cell shape that occurs in organisms such as the amoeba. Finally, the intermediate filaments act as more rigid scaffolding to maintain the cell shape.
The organelles of the cell are dispersed throughout the cytoplasm. The nucleus is bound by its own membrane to protect the genetic material from potentially damaging reactions that occur in the cytoplasm. Thus, the cytoplasm is not a part of the interior of the organelles.
The cytoplasm also contains ribosomes , which float around and allow protein to be synthesized all through the cell. Ribosomes are also associated with a structure called the endoplasmic reticulum. The golgi apparatus is also present, in association with the endoplasmic reticulum. Enzymes that degrade compounds are in the cytoplasm, in organelles called lysosomes. Also present throughout the cytoplasm are the mitochondria, which are the principal energy generating structures of the cell. If the eukaryotic cell is capable of photosynthetic activity, then chlorophyll containing organelles known as chloroplasts are also present.
The cytoplasm of eukaryotic cells also functions to transport dissolved nutrients around the cell and move waste material out of the cell. These functions are possible because of a process dubbed cytoplasmic streaming.
See also Eukaryotes