A Cell Is Like a Factory

Analogies are ubiquitous in molecular biology. Traditionally, two analogies have been used to describe the cell: the cell as a factory and the cell as a city. Some will say these analogies are overused. I disagree. They are heavily used because they are effective. We’ll use the factory analogy. I like it a bit better because, like a factory, cells perform specific jobs—they have a specific output or function. Cities don’t have a primary function or output.

So, let’s think about a factory. Big picture, the typical factory is a building that brings in raw materials and uses human workers and energy to produce some kind of finished product. Factories need workers. Factories need a source of power. They need an assembly line. They need instructions on how to build the product and on how to keep all the company’s internal processes running smoothly. Factories need facilities to store the finished product. And of course they need something like a trash and recycling center.

Cells have analogs for all these factory features and functions. Let’s get into them.

Physical Facility

The cell’s physical facility can be thought of as its cell membrane (outer walls, also called the plasma membrane) and its cytoskeleton (inner walls, including structures called microtubules). The cell membrane encloses the cell: its gelatinous cytoplasm and the various organelles, or "small organs." In other words, the cell membrane creates a compartment for that emergent property called “life” to happen. The cytoskeleton is a network of fibers inside the cell that provides additional support for the cell and its contents.

In addition to walls, factories also have systems for controlling the entrance and exit of both people and materials. A receptionist greets outsiders. Employees use card keys to enter the building. And the shipping and receiving department makes sure only the right materials enter, and the right products exit the building. The cell membrane performs this gatekeeper function, too. It has membrane-embedded proteins, membrane pores and ion pumps for controlling exactly what enters and exits the cell.   

Factory Workers

Factories can’t produce anything without employees (or robots). Likewise, the cell needs workers to build product and to keep the cell running, generally. The cell’s equivalents of factory workers are its proteins, and specifically its functional proteins such as enzymes (less so those proteins that play a structural role in the cell).

Proteins perform almost all the jobs necessary for the cell to live, grow, and divide. They perform thousands of different chemical reactions, they unwind double-stranded DNA, they break apart old proteins and organelles for recycling, they transport materials to specific destinations in the cell… the list goes on. Proteins are so central to the cell’s functioning and to our story about genome replication that I’ll be covering them in depth in a future post.

One other "worker" in our cellular factory is called a ribosome. Ribosomes are complexes of specific proteins and specific RNA molecules. And they have a specific job: ribosomes literally build proteins. Ribosomes can be free-floating in the cytoplasm or they can be attached to one of the organelles that I'll introduce shortly, the endoplasmic reticulum (ER)

Energy

Real-life factories need a source of power, or energy, to both build their product and to run the other aspects of its operation. Typically, with a factory, power lines deliver electricity generated by a utility company. Cells need energy to run their operations as well. But in a cell, energy is generated internally by organelles called mitochondria. So, for the sake of the health of our analogy, we’ll assume our factory uses internal power generators for its energy rather than current supplied by a utility company!

“The mitochondrion is the powerhouse of the cell” might be the oldest biology analogy around. Almost everyone seems to remember this exact phrasing from high school biology class. But… it’s true! A mitochondrion takes in large molecules and breaks them down, borrowing the energy held in their chemical bonds to build small molecules—most importantly adenosine triphosphate (ATP), which is the main energy currency used by the cell’s proteins to fuel their activities. There are typically a few hundred to a couple thousand mitochondria per cell.

Assembly Line

The cell’s assembly line consists of two organelles that both take the form of membrane-bound passageways. They are the endoplasmic reticulum (ER) and the Golgi apparatus. The ER, in fact, accounts for over 50% of the total membrane in eukaryotic cells. Many macromolecules get built in the ER and Golgi apparatus. For example, lipid synthesis (including the lipids that make up the cell’s membranes) occurs in the ER. Also, proteins—especially those that tend to be secreted by the cell—are synthesized by the ER. The ER is referred to as "rough" or "smooth" depending on whether there are ribosomes attached to the outer membrane surface.

Some proteins synthesized in the ER travel to the Golgi apparatus. Think of the Golgi as a facility for receiving, modifying (through chemical addition), sorting and shipping proteins to other destinations in the cell. The Golgi apparatus has two faces. One receives proteins. Once received, they travel through the Golgi’s passageways (being chemically modified along the way if necessary) toward the other face, which sends proteins to other sites in the cell. The Golgi apparatus can place molecular identification tags on proteins that act like zip codes on mailing labels. It can also package proteins in membrane-bound transport vesicles that it marks—again, chemically—for delivery to specific sites in the cell.   

Document Control

How does a factory “know” how to build its product(s) or how to run its many departments? Companies typically have formal, version-controlled standard operating procedures (SOPs) that are maintained by a Document Control department.

In the cell, the Document Control department lives in the nucleus and the equivalent of the factory’s SOPs are the cell’s genes: its genomic DNA. The cell’s “SOPs” tell it how to build its products, when to grow and divide, when to replicate its genome, how to replicate its genome, and how to produce every protein in the cell, again, to name a few. The genes tell the cell everything it needs to do and provides plans to build all the required proteins and other cellular structures.

Storage, Trash and Recycling

Both the ER and Golgi apparatus take the form of membrane-bound passageways. Other membrane-bound structures can be found in the cell as well. These membrane-bound sacs include vacuoles (also known as vesicles) that store nutrients, ions, and other small molecules. Some vacuoles specialize in isolating, breaking down and expelling waste products.

Lysosomes represent a specific kind of vacuoles that maintains a high internal pH conducive to the action of hydrolytic enzymes that break macromolecules down into smaller components that are then released into the cytosol where they serve as nutrients. Some lysosomes, complete with their internal digestive enzymes, are believed to arise from budding of the trans face of the Golgi apparatus.

So when you think of a cell, think of a small factory. The cell takes in raw materials and processes them, using energy and people, to create some kind of finished product or activity. An example of a finished product would be the insulin secreted from cells of the pancreas. And example of an activity would be the contractions of muscle cells that result in gross muscle contraction. Regardless, every cell has a productive raison d'etre!