Energy and Protein Organelles: Mitochondria, Ribosomes, ER, and Golgi

Mitochondria, ribosomes, the endoplasmic reticulum and the Golgi apparatus are organelles that supply energy and make, process and ship proteins, while lysosomes, vacuoles, chloroplasts and the cell wall add digestion, storage, photosynthesis and support. In brief: mitochondria release usable energy, ribosomes build proteins, the rough endoplasmic reticulum folds them, the Golgi apparatus packages them and vesicles deliver them. A cell that makes a lot of protein, such as a cell of the pancreas, has abundant ribosomes, endoplasmic reticulum and Golgi.

These organelles work as a production line inside a eukaryotic cell. This guide describes each organelle, shows the path of a secreted protein through the cell, compares them in a table, works through a matching question and corrects common mistakes.

What do mitochondria do?

Mitochondria (singular: mitochondrion) are the main sites of aerobic respiration in eukaryotic cells. They convert energy from glucose and other nutrients into ATP, the molecule that cells use as an immediate energy source. They are sometimes called the “powerhouses of the cell”, though this is a simplification.

A mitochondrion is a bean-shaped organelle, about 1 to 2 micrometers long, with two membranes. The outer membrane is smooth, and the inner membrane is folded into cristae, which increase the surface area for the reactions that produce ATP. The space inside the inner membrane is the matrix. Mitochondria have their own small circular DNA and ribosomes, and they can divide. Cells with high energy needs, such as muscle and liver cells, contain many mitochondria.

What do ribosomes do?

Ribosomes are the sites of protein synthesis. They read the sequence of messenger RNA and join amino acids in the matching order. Each ribosome has two subunits made of ribosomal RNA and protein, and they are not surrounded by a membrane.

Free ribosomes float in the cytoplasm and make proteins that are used inside the cell. Bound ribosomes are attached to the endoplasmic reticulum and make proteins that will be secreted or placed in membranes. A fast-growing cell may have millions of ribosomes.

What is the endoplasmic reticulum?

The endoplasmic reticulum (ER) is a network of flattened sacs and tubes that is connected to the nuclear envelope. It comes in two forms.

• Rough ER has ribosomes attached to its surface. Proteins made there enter the interior (lumen) of the ER, where they fold and are modified. It is abundant in cells that secrete proteins.
• Smooth ER has no ribosomes. It makes lipids, such as phospholipids and steroids, helps detoxify drugs and poisons in liver cells and stores calcium ions in muscle cells.

What does the Golgi apparatus do?

The Golgi apparatus is a stack of flattened membrane sacs. It receives vesicles from the ER on one side, modifies the contents, sorts them and packages them into new vesicles from the other side. Modifications include adding sugar groups to proteins.

The vesicles then go to different destinations: to the cell membrane for secretion, to lysosomes or to other parts of the cell. The Golgi apparatus is large in cells that secrete a lot, such as the cells that make mucus in the intestine. It is named after the Italian scientist Camillo Golgi, who described it in 1898.

What do lysosomes, vacuoles, chloroplasts and the cell wall do?

• Lysosomes: Small vesicles with digestive enzymes that break down worn-out organelles, food particles and bacteria. They are common in animal cells.
• Vacuoles: Fluid-filled sacs for storage. Plant cells usually have a large central vacuole that holds water, ions and sugars, and presses the cytoplasm against the wall to keep the cell firm.
• Chloroplasts: Organelles of plants and algae where photosynthesis takes place. They have a double membrane and stacks of membranes (grana) containing chlorophyll. Like mitochondria, they have their own DNA.
• Cell wall: A rigid layer outside the cell membrane in plants (cellulose), fungi (chitin) and bacteria (peptidoglycan). It gives strength and shape and prevents bursting when water enters.

Organelles compared

Worked example: the path of a secreted protein

The hormone insulin is a protein made by beta cells in the pancreas. Follow the route of the insulin molecule from instruction to secretion.

1. The gene for insulin in the nucleus is copied into messenger RNA, which leaves through a nuclear pore.
2. The messenger RNA binds to a ribosome on the rough ER. The ribosome builds the protein chain and passes it into the ER lumen.
3. In the ER, the protein folds and receives its first modifications. A vesicle carries it from the ER to the Golgi apparatus.
4. In the Golgi apparatus, the protein is modified further and sorted. It is packed into a secretory vesicle.
5. The vesicle moves to the cell membrane and fuses with it, releasing insulin outside the cell by exocytosis.

Energy for the whole process comes from ATP made in the mitochondria. A beta cell is therefore rich in rough ER, Golgi and mitochondria. The order nucleus, ribosome, rough ER, Golgi, vesicle and cell membrane is a standard exam answer for secreted proteins.

How were these organelles discovered?

Mitochondria were seen in the nineteenth century, and Carl Benda named them in 1898. The endoplasmic reticulum, ribosomes and the detailed structure of Golgi stacks were clearly observed only after electron microscopy in the 1940s and 1950s. George Palade described ribosomes and traced the path of secreted proteins through the cell, work for which he shared the 1974 Nobel Prize in Physiology or Medicine. Christian de Duve discovered lysosomes in the 1950s.

Which mistakes are common?

Mistake 1: Saying that mitochondria make energy

Energy cannot be created. Mitochondria convert chemical energy in food into ATP, which is a usable form.

Mistake 2: Thinking ribosomes have a membrane

Ribosomes are not membrane-bound. This is why prokaryotic cells also have them.

Mistake 3: Mixing up the roles of the ER and the Golgi apparatus

The rough ER makes and folds proteins, and the Golgi modifies, sorts and ships them.

Mistake 4: Saying that only plants have mitochondria

Plant cells have both chloroplasts and mitochondria. Both photosynthesis and respiration occur in plants.