The cytoplasm is the entire contents of a cell contained within the cell membrane. In eukaryotes (cells with a nucleus), it includes everything outside the nucleus. In prokaryotes (cells without a nucleus), the cytoplasm encompasses the cell’s entire interior. This dense, semi-viscous material acts as the background matrix where all other cellular components are suspended and fundamental life processes occur.
Providing the Cellular Environment
The majority of the cytoplasm consists of the cytosol, the gel-like fluid component that excludes membrane-bound organelles. This intracellular fluid is primarily composed of water, often making up 70 to 80 percent of the cell’s volume. Dissolved within this aqueous base are essential ions (such as sodium and potassium), small organic molecules, proteins, and carbohydrates. The cytosol functions as a universal solvent, ensuring that reactants and signaling molecules remain dissolved and accessible for chemical reactions.
The cytosol actively works to maintain the precise internal chemical conditions required for life, a state known as homeostasis. This involves regulating the cell’s pH level, which must be kept within a narrow range for enzymes to function correctly. By controlling the concentration of dissolved salts and molecules, the cytoplasm creates a stable, balanced environment, allowing the cell’s machinery to operate efficiently.
The Site of Chemical Reactions
A primary function of the cytoplasm is to serve as the location for numerous metabolic pathways that do not require specialized organelle membranes. One significant pathway is glycolysis, the first step in breaking down glucose to generate cellular energy. This process involves a sequence of ten enzymatic reactions occurring freely in the cytosol, converting glucose into two molecules of pyruvate. Glycolysis generates a small net yield of adenosine triphosphate (ATP), the cell’s energy currency, and reduced nicotinamide adenine dinucleotide (NADH).
Because the necessary enzymes for glycolysis are dissolved directly in the cytosol, the reaction occurs rapidly and is universally used by nearly all organisms. The cytoplasm is also the location for the initial stages of protein synthesis, a process called translation. Ribosomes, the molecular machines that read genetic instructions, are either suspended in the cytoplasm or attached to the endoplasmic reticulum. Ribosomes floating freely in the cytosol produce proteins used internally by the cell, such as the enzymes required for glycolysis.
Organizing the Cell’s Structure
Beyond its role as a reaction medium, the cytoplasm provides physical support and organization for the entire cell structure. The cytoskeleton, a dynamic network of protein filaments embedded within the cytoplasm, gives the cell its characteristic shape and mechanical strength. This network acts like an internal scaffolding, anchoring organelles and preventing the cell from collapsing.
The cytoplasm is also responsible for active movement within larger cells through a process called cytoplasmic streaming (cyclosis). This continuous, directed flow of fluid is powered by motor proteins moving along the cytoskeleton’s tracks. Cytoplasmic streaming is important in large cells where passive diffusion alone would be too slow to deliver materials. This movement ensures the rapid distribution of nutrients, organelles, and waste products, enhancing metabolic efficiency.