A eukaryotic cell contains membrane-bounded organelles that create chemically distinct internal compartments. Compartmentalization lets different reactions occur under different local conditions and allows materials to be processed and routed between specialized regions.

Major organelles include:

• the nucleus, which contains most of the cell's DNA;
• the endoplasmic reticulum (ER), a membrane network involved in synthesis and processing of many proteins and lipids;
• the Golgi apparatus, which receives, modifies, sorts and redistributes material arriving from the ER;
• lysosomes, acidic compartments containing enzymes that degrade many macromolecules and cellular components;
• peroxisomes, compartments that isolate particular breakdown and detoxification reactions;
• mitochondria, double-membrane organelles central to cellular energy conversion;
• in plants and algae, chloroplasts, organelles that use light energy to support chemical synthesis;
• vesicles and vacuoles, membrane-bounded compartments used for transport, storage and specialized chemistry.

The fluid outside these membrane-bounded organelles but inside the plasma membrane is the cytosol.

Compartments can maintain different conditions

A membrane can support differences in ion concentration, acidity, molecular composition and electrical potential between two regions. Lysosomes, for example, maintain a more acidic interior than the cytosol. Mitochondria maintain ion gradients across their inner membrane.

These differences matter because the same molecule can behave differently under different chemical conditions, and enzymes can be localized near the substrates and partners with which they must interact.

The endomembrane system

The ER, Golgi apparatus, lysosomes, transport vesicles and plasma membrane exchange membrane and cargo through budding and fusion events. Together they form a connected trafficking system even though the individual compartments remain compositionally distinct.

Mitochondria and chloroplasts are not simply stations in this endomembrane traffic. They have their own specialized internal membranes and a partly independent evolutionary history.

The important organizing principle is not memorizing a list of organelles. It is recognizing that internal membranes allow one cell to maintain several controlled chemical environments at once.