Take a look at your best friend, your dog — or even a snail using its muscular foot to move up the stalk of a flower. All of them look quite different. And that’s due to the highly organized cells from which they’re made. The human body has roughly 37 trillion cells.
Most living things, however, are not multicellular. They consist of a single cell. Such unicellular organisms generally are so small that we’d need a microscope to see them. Bacteria are among the simplest single-celled organisms. Protozoa, such as amoebas, are more complex types of one-celled life.
A cell is the smallest living unit. Inside every cell is a host of structures known as organelles. “Every cell has essential structures that are the same, like every house has a kitchen sink and a bed. But how big and complex they are, and how many of them there are, is going to vary from cell type to cell type,” says Katherine Thompson-Peer. She’s a cell biologist at the University of California, Irvine.
If cells were homes, the simplest ones — prokaryotes (Pro-KAER-ee-oats) — would be one-room studio apartments. The kitchen, bedroom and living room would all share one space, explains Thompson-Peer. With few organelles, and all of them next to each other, activities all take place in the middle of these cells.
Over time, some cells became more complex. Called eukaryotes (Yu-KAER-ee-oats), these now make up animals, plants and fungi. Some one-celled organisms, such as yeasts, also are eukaryotes. These cells are all like single-family houses — with walls and doors making up separate rooms. A membrane encloses each organelle in these cells. Those membranes “segregate different things that the cell does into different compartments,” explains Thompson-Peer.
The nucleus is the most important organelle in these cells. It houses a eukaryotic cell’s DNA. It’s also what distinguishes these cells from prokaryotes. Even one-celled eukaryotes, such as the amoeba, have a nucleus. But cellular complexity is most obvious in multi-celled organisms. If we follow the house analogy, a multi-celled organism would be a high-rise apartment building, says Thompson-Peer. It contains lots of homes — cells. “And they’re all a little bit different in terms of shape. But they all work together to be a building.”
Cells from organisms big and small include:
a cell membrane (also called a plasma membrane). This thin, protective outer layer surrounds a cell, like the outer walls of a house. It protects the structures inside and keeps their environment stable. This membrane also is somewhat permeable. That means it allows some things to move into and out of a cell. Think of windows in a house with screens. These let air flow in but keep unwanted critters out. In a cell, this membrane allows nutrients in and unwanted wastes to leave.
ribosomes. These are little factories that make proteins. Proteins are important to every function of life. We need proteins to grow, to repair an injury and to transport nutrients and oxygen in our bodies. To build proteins, a ribosome binds to a specific part of a cell’s genetic material known as messenger RNA. This allows it to read the instructions telling this factory which building blocks — called amino acids — to assemble in making a protein.
DNA. Every organism has a genetic code called DNA. That’s short for deoxyribonucleic (Dee-OX-ee-ry-boh new-KLAY-ick) acid. It’s like a huge instruction manual, telling cells what to do, how and when. All that information is stored in nucleotides (NU-klee-uh-tides). These are chemical building blocks made of nitrogen, sugar and phosphate. When new cells develop, they make an exact copy of the old cells’ DNA so the new ones know what tasks they’ll be expected to do.
Every cell in an organism’s body has the same DNA. Yet those cells can look and function quite differently. And here’s why: Different cell types access and use different parts of the DNA instruction book. For example, an eye cell is translating the parts of its DNA that tell it how to make eye-specific proteins. Similarly, a liver cell translates the sections of DNA that tell it how to make liver-specific proteins, explains Thompson-Peer.
You might think of DNA as the script for a play, she says. All the actors in Shakespeare’s Romeo and Juliet have the same script. Yet Romeo reads only his lines, Thompson-Peer says, before going off to do Romeo things. Juliet reads only her lines and then goes off and does Juliet things.
Key features of cells from multi-celled organisms include:
a nucleus. The nucleus is a protective membrane surrounding a cell’s DNA. It keeps this genetic “instruction manual” safe from molecules that could damage it. The presence of a nucleus is what makes a eukaryotic cell different from a prokaryotic one.
endoplasmic reticulum (En-doh-PLAZ-mik Reh-TIK-yoo-lum). This place, where a cell makes proteins and fats, has a long name. But you can call it “ER” for short. It’s a flat sheet that gets folded tightly back and forth. Those known as rough ERs makes proteins. The ribosomes that attach to this ER give it that “rough” appearance. Smooth ERs make not only lipids (fatty compounds such as oils, waxes, hormones and most parts of the cell membrane) but also cholesterol (a waxy material in plants and animals). Those proteins and other materials become packaged into tiny sacs that pinch off from the edge of the ER. These important products of cells are then transported to the Golgi (GOAL-jee) apparatus.
Golgi apparatus. This organelle modifies proteins and lipids in much the same way auto parts are added to the body of a car in the factory’s assembly line. For example, some proteins need carbohydrates attached to them. After these additions are made, the Golgi apparatus packages up the modified proteins and lipids, then ships them in sacs known as vesicles to where they will be needed in the body. It’s like a post office that receives lots of mail for different people. The Golgi apparatus sorts the cellular “mail” and delivers it to the proper body address.
cytoskeleton. This network of tiny fibers and filaments provides structure to a cell. It’s like the frame of a house. Different cells have different shapes and structures based on their function. For example, a muscle cell has a long, cylindrical structure so that it can contract.
mitochondria. These power generators of the cell break down sugars to release their energy. Then the mitochondria (My-toh-KON-dree-uh) package that energy into a molecule called ATP. It’s the form of energy that cells use to power their activities.
lysosomes. These organelles are the cell’s recycling centers. They break down and digest nutrients, waste or old parts of the cell that are no longer needed. If a cell is too damaged to repair, lysosomes help the cell destroy itself by breaking down and digesting all the structural supports as well. That type of cell suicide is known as apoptosis.
vacuoles. In animal cells, several of these small sac-like structures work a bit like lysosomes, helping to recycle wastes. In plant cells, there is one large vacuole. It mainly stores water and keeps a cell hydrated, which helps give a plant its rigid structure.
cell wall. This rigid layer jackets the outside of a plant’s cell membrane. It’s made of a network of proteins and sugars. It gives plants their stiff structure and provides some protection from pathogens and from stress, such as water loss.
chloroplasts. These plant organelles use energy from the sun, along with water and carbon dioxide in the air, to make food for plants through the process known as photosynthesis. Chloroplasts (KLOR-oh-plasts) have a green pigment inside them called chlorophyll. This pigment is what makes plants green.