What the organelles do

A pancreas cell that makes digestive enzymes is packed with ribosomes, because every enzyme is a protein built on one.

Following one protein

The easiest way to learn the organelles is to follow a single product through the cell. Say a cell in your pancreas is making a digestive enzyme that will be sent out into the gut. The instructions start in the nucleus, the largest organelle, where the DNA is kept. A copy of one gene is made as messenger RNA, and it leaves the nucleus through pores in the nuclear envelope, a double membrane that wraps the nucleus.

Inside the nucleus there is a darker patch, the nucleolus. It does not make proteins itself. It puts together ribosomal RNA and proteins into the two halves of ribosomes, which are then sent out through the same pores. Ribosomes read the messenger RNA and join amino acids in the order it spells out. Some ribosomes float free in the cytosol and make proteins that stay there. Others sit on the outside of the rough endoplasmic reticulum.

The rough ER and the Golgi

The rough ER is a set of folded membrane sheets joined to the nuclear envelope. The ribosomes on its surface give it a studded look in an electron microscope, which is how it got its name. A protein meant for export is fed into the inside of the ER while it is being made. There it is folded and may get side chains such as sugars. Cells that send out a lot of protein, like liver cells, have a lot of rough ER. The smooth ER joins the rough ER but has few or no ribosomes. It makes lipids and steroid hormones, breaks down some drugs and poisons, and stores calcium ions. In muscle cells, a special smooth ER releases the calcium that starts each contraction.

Next the protein leaves the ER in a small bubble of membrane, a transport vesicle, and travels to the Golgi apparatus, a stack of flattened sacs. Vesicles arrive at the side facing the ER, the cis face. As the protein passes through the stack, more sugars are added and it is tagged with small chemical labels that act like a shipping address. At the far side, the trans face, it is packed into a secretory vesicle. That vesicle merges with the cell membrane and releases the enzyme outside. Salivary gland cells and antibody-making immune cells, which secrete a lot, have a large Golgi. In plant cells the Golgi also makes some of the polysaccharides for the cell wall. Press Play in the model to watch vesicles move from the Golgi to the membrane.

Recycling and power

Lysosomes are small sacs full of digestive enzymes. The enzymes work best in acid, and the inside of a lysosome is more acidic than the cytoplasm, so the enzymes cannot do damage elsewhere. Lysosomes break down proteins, sugars, fats, nucleic acids and worn-out organelles. In a white blood cell called a macrophage, the membrane folds around a bacterium, pinches off into a vesicle, and a lysosome merges with it and digests the germ.

All this work needs energy. Mitochondria make most of the cell’s ATP by cellular respiration, which uses oxygen and gives off carbon dioxide. The carbon dioxide you breathe out comes from these reactions. Each mitochondrion has two membranes; the inner one folds into cristae, which give more surface for making ATP. Cut a mitochondrion in the model and you can see the folds. Muscle cells, which use a lot of energy, are packed with mitochondria. Mitochondria and chloroplasts also carry their own DNA and ribosomes, a clue to their history that comes back in the next unit.

Biologists often sum this up as “form follows function”: a cell’s parts match its job. A pancreas cell is full of ribosomes and rough ER, a muscle cell is full of mitochondria, and a cell lining the small intestine has its membrane folded into tiny fingers called microvilli that add surface for absorbing food.