Organelle
An organelle is a specialized structure inside a cell, such as the nucleus, mitochondrion, or chloroplast, that performs a specific job.
Definition
An organelle is a tiny part inside a cell with a special job, like an organ in your body but much smaller; the word means "little organ." Each organelle is a specialized subunit that carries out a particular function, and most, including the nucleus, mitochondria, chloroplasts, endoplasmic reticulum, and Golgi apparatus, are enclosed by membranes, while ribosomes are not membrane-bound but are usually counted as organelles. Prokaryotic cells lack membrane-bound organelles. In eukaryotes these compartments spatially organize metabolism, keep distinct internal chemistries (the lumen of a lysosome sits at pH $4.5$ to $5.0$ while the cytosol is near pH $7.2$), and move materials by vesicle budding and fusion. Mitochondria and chloroplasts are semi-autonomous, with their own DNA and ribosomes, consistent with an endosymbiotic origin, and proteins reach the right organelle through signal sequences recognized by receptor and translocation machinery.
Example
The nucleus stores the instructions, the mitochondria release energy, and the ribosomes build proteins, each an organelle doing its own work. A liver cell may hold $1{,}000$ to $2{,}000$ mitochondria to meet its energy demand, while a single leaf mesophyll cell contains around $30$ to $100$ chloroplasts, differences that match each cell's job. A secreted enzyme shows the organelles working as a chain: it is made on ribosomes bound to the rough endoplasmic reticulum, folded and glycosylated in the ER lumen, carried in vesicles to the Golgi for further processing, and packaged into secretory vesicles that fuse with the plasma membrane, a route that takes roughly $30$ to $60$ minutes.
Key Insight
A cell is like a city, with organelles serving as the power plant, the library, the factories, and the recycling center. Structure fits function: a cell that burns lots of fuel has many mitochondria, and a cell that exports lots of protein has a large endoplasmic reticulum and Golgi. Compartmentalization also allows incompatible reactions, such as protein synthesis and protein digestion, to run at the same time in the same cell. Many diseases, including lysosomal storage disorders and mitochondrial myopathies, are failures of a single organelle.