The nucleus and the cell's factory line

Think of the cell as a factory. Each organelle has a job, and the products move from one to the next.

The nucleus: the head office. It has a double membrane with pores (small openings). Inside is the nucleolus, which makes the two halves (subunits) of ribosomes. These leave through the pores, and a large and a small one join to make a ribosome.

The nucleus also holds the chromosomes, made of DNA and proteins. They carry the instructions passed from parents to children. A working piece of DNA is a gene. When the cell is not dividing, the DNA is a loose tangle called chromatin, like a tangled charger cable. When the cell gets ready to divide, the same cable winds up tightly into chromosomes.

Red blood cells break the rule. A mature human red blood cell has no nucleus. That leaves more room for haemoglobin, so it carries more oxygen. The cost: it cannot repair itself or divide, so it lives only about 120 days.

The production line:

• Ribosomes make proteins. Some float free in the cytoplasm; some sit on the ER.
• Rough ER (endoplasmic reticulum) is covered with ribosomes. It makes proteins and sends them out, for example in cells of the pancreas.
• Smooth ER has no ribosomes. It makes and stores fats and hormones.

The Golgi apparatus: the post office. It is a stack of flat sacs. It changes, sorts and packs proteins and fats into little bubbles (vesicles) and sends them where they are needed. Camillo Golgi first saw it in 1898, in the nerve cells of a barn owl. Electron microscopes proved it was real decades later.

Lysosomes: the cleaning team. They are small sacs full of enzymes that break down unwanted proteins, carbohydrates, fats and worn-out parts, so the pieces can be used again. In a sperm cell, lysosome enzymes help break through the egg's outer layer at fertilisation.