The Jobs of the Parts Inside a Cell

Let me show you how a cell is like a city — and every city needs workers with different jobs. A cell is not just a blob. The structures inside a cell that do those jobs are called **organelles** (or-gah-NELL-z), which means 'little organs.' **The nucleus** is the control center. It holds DNA — the instruction manual for the entire cell. When the cell needs to make a protein or divide, the nucleus sends out the directions. Think of it as the city hall that issues every official order. **Mitochondria** (my-toh-KON-dree-ah; one = mitochondrion) are the power plants. They take in oxygen and sugar and convert them into a usable form of energy called ATP. Almost every **eukaryotic** cell — plant, animal, fungus — has mitochondria. (Bacterial cells, called prokaryotes, have no mitochondria at all.) The more energy a cell needs, like a muscle cell, the more mitochondria it contains. **Chloroplasts** are found only in the cells of plants and some algae. They contain a green pigment called chlorophyll that captures sunlight. Using that light energy, water, and carbon dioxide, chloroplasts make sugar through **photosynthesis**. That sugar then fuels the rest of the cell — and eventually us, when we eat plants. Other important organelles include the **cell membrane** (controls what enters and leaves), the **cell wall** in plants (gives rigid shape), the **endoplasmic reticulum** or ER (a folded highway that moves materials and helps build proteins and lipids), the **Golgi apparatus** (packages and ships proteins, like the post office), **ribosomes** (tiny structures that read the nucleus's instructions and build proteins; found on the rough ER and floating freely in the cytoplasm), and **vacuoles** (storage tanks — plant cells have one large central vacuole; animal cells may have small, temporary vacuoles for tasks like digestion or waste removal, but they lack a large central vacuole). Here is the key insight: no organelle works alone. The nucleus writes the instructions, the ER and ribosomes build the products, the Golgi ships them, and the mitochondria keep the electricity on the whole time. Remove any one part, and the city starts to fail.