In some respects, the cell is like a factory. The first time you look at a microscope image of a cell, the cell seems impossibly complex. Look closely at a cell, however, and patterns begin to emerge. To see those patterns more clearly, we'll look at some structures that are common to cells.
In the same way that the main building controls a gigantic factory, the nucleus is the control center of the cell. This organelle holds the cell's DNA and the directions for producing proteins and other vital things. The nucleus is bordered by a nuclear envelope that is made up of two membranes. This envelope is dotted with thousands of nuclear pores, which permit material to go through the nucleus. Like messages, instructions and blueprints moving in and out of a main office, a river of proteins go through the nuclear pores to and from the rest of the cell.
Creating proteins is one of the most crucial jobs in a cell. They are made on ribosomes. Ribosomes are puny pieces of protein found all over the cell. They create proteins through obeying the nucleus’ hinted orders. Each ribosome, in its own way, is like a small machine in a factory, spiraling out proteins on instructions that arrive from its nucleus.
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Cells also contain the endoplasmic reticulum (en-doh-PLAZ-mik rih-TIK-yuh-lum), or ER. The endoplasmic reticulum is the site where the apparatus of the membrane are assembled, along with proteins and other materials that are exported from the cell. The rough ER is given this name because of the ribosomes found on its surface. Newly made proteins leave these ribosomes and are put in the rough endoplasmic reticulum, where they will be modified chemically.
Proteins made in the rough ER move next into an organelle called the Golgi apparatus. This organelle was discovered by the Italian scientist Camillo Golgi. The function of this organelle is to deliver proteins and other materials for the cell’s cargo space. In the Golgi apparatus, proteins are put all over the cell
Even the neatest, cleanest factory needs a cleanup crew, and that's what lysosomes (LY-suh-sohmz) are. Lysosomes are small organelles filled with enzymes. One function of lysosomes is the digestion of lipids into molecules that can be used by the rest of the cell. Lysosomes are also involved in breaking down organelles that have outlived their usefulness. Lysosomes perform the vital function of removing “junk” that might otherwise accumulate and clutter up the cell. A number of serious human diseases can be traced to lysosomes.
Every factory needs a place to store things, and cells contain places for storage as well. Some cells have compositions called vacuoles (VAK-yoo-ohlz) that stock up resources such as proteins and salts. Vacuoles are also found in paramecium. The paramecium contains a vacuole called a contractile vacuole. By contracting rhythmically, this specialized vacuole pumps excess water out of the cell. The control of water content within the cell is just one example of an important process known as homeostasis. Homeostasis is the maintenance of a controlled internal environment.
All living things require a source of energy. Factories are linked up to the local power system, but what about cells? Most cells get energy in one of two ways—from food molecules or from the sun. Mitochondria are organelles that exchange the energy placed in food into energy that is very convenient for cell usage. Mitochondria are put up with two membranes. The inner membrane is folded up inside the organelle