Intracellular

Intracellular refers to anything inside a cell. The term contrasts with extracellular or intercellular. It encompasses the cytosol, organelles, and the nucleus, as well as intracellular signaling and transport processes that maintain cell function.

The cytoplasm consists of the cytosol, the fluid in which organelles reside, and the organelles themselves. The cytosol is a crowded, viscous solution with ions, metabolites, and proteins. Organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, and, in plant cells, plastids and large vacuoles. Some structures, like ribosomes and elements of the cytoskeleton, are not membrane-bound but are essential for intracellular work.

Key intracellular processes include metabolism (glycolysis in the cytosol, mitochondrial respiration), gene expression, protein synthesis at ribosomes, and vesicular trafficking through the endomembrane system. Intracellular signaling relies on messengers such as calcium ions and cyclic nucleotides to coordinate responses. The intracellular environment is maintained by ion pumps and channels, pH control, and selective transport across organelle and plasma membranes. Intracellular transport moves cargo via motor proteins along cytoskeletal tracks.

Understanding intracellular processes is essential in health and disease. Disruptions can lead to metabolic disorders, impaired signaling, or vulnerability to intracellular pathogens that exploit the cellular interior. Autophagy, mitochondria quality control, and lysosomal function are central to intracellular homeostasis. Techniques to study intracellular components include microscopy, cell fractionation, and molecular biology methods.