The specific components of the cell work together to effect growth, energy usage, cell division, and many other tasks. In this unit, we focus on three components of the eukaryotic cell, although many other components exist. The structures we will discuss are the plasma membrane, the cytoplasm, and the ribosomes.



The plasma membrane of a eukaryotic cell consists of a lipid bilayer composed of numerous different phospholipids, both integral and peripheral membrane proteins, and various sugar modifications to both the phospholipids and the proteins on exterior of the cell. The phospholipids bilayer is arranged as a double sheet such that the hydrophobic moieties of the molecules face each other within the sheet, while the hydrophilic head groups face the cellular interior and extracellular medium, both of which are water-based. The plasma membrane confers selective permeability to the cell, such that certain substances are capable of transporting into and out of the cell, but this constant flux is highly regulated, which gives rise to cellular homeostasis. Integral and peripheral membrane proteins can be involved in cell-cell interactions, can form pores for transport of large molecules into and out of the cell, and can be modified via sugar moieties for interactions with other cells or with foreign substances, such as viruses. Without the plasma membrane, the cell would have no spherical shape and its contents would spill into the exterior environment, preventing any sort of concerted regulation. Indeed, without the plasma membrane, there would be no cell.

The cytoplasm composes the entire contents of the cell and is bounded by the plasma membrane. It consists of the nucleus, endoplasmic reticulum, mitochondria, and a number of other membrane-bounded organelles, as well as other small molecules and the associated fluid, called the cytosol. Numerous cellular mechanisms contribute to maintain a specific pH, salt concentration, and charge balance within the cytoplasm .

Ribosomes are the molecular machines that are used to translate messenger RNA (mRNA)—the molecules copied from the cell’s DNA genome—into proteins, which are used for all the processes in the cell. The process of translation is essential to the cell, and it is via ribosomes that the cell’s DNA is expressed in the form of proteins. Ribosomes function in the cytoplasm, translating mRNA that was initially transcribed in the nucleus. From there, the resulting proteins may remain in the cytoplasm, may enter the nucleus, or may be secreted from the cell.

These components and many others comprise the cell, and each component is important for its own independent activity, but the components also need to constantly communicate with each other. So, for instance, the cytoplasm would not be functional without the plasma membrane or ribosomes, and likewise, the plasma membrane’s composition would be incorrect if proteins were not synthesized for insertion into the membrane. As you can see, the interactions of proteins, lipids, and sugars that effect cellular function are extremely complex, and this discussion only scratched the surface of the immense number of interactions and reactions that are required for cellular activity.