As is often repeated, cells are the basic building blocks of all life. They are responsible for generating the energy that sustains life. They also eliminate waste and quickly replicate themselves to replace damaged tissues.
Cells are fascinatingly complex organisms, able to perform a wide variety of tasks. This is true from single-celled organisms up to multicellular organisms, such as humans.
Bacteria: In Sickness and Health
Some organisms consist of a single cell with only the most basic parts: genetic material (DNA), ribosomes, cytoplasm, and a cell membrane. Bacteria, for example, mainly consist of these basic cell parts, and may also have a cell wall.
Bacteria are capable of causing human illnesses, from mild food poisoning to deadly tuberculosis. They are also capable of promoting human health. For example, bacteria living in the human gut aid in digestion and absorbing nutrients, among other things.
Bacteria are able to form biofilm, a layer of microbes held together by certain molecules. The molecules are secreted by the bacterial cell membrane. The cell membrane also has surface structures, such as proteins and flagella, that help bacteria form biofilms.
Some biofilms are harmful and may grow on medical equipment and cause infection. They also form on industrial materials, such as oil and gas pipelines. Corrosion due to biofilm can cost billions of U.S. dollars to prevent and fix. This type of corrosion may even result in environmental damage when pipelines leak.
Cells have another ability called horizontal gene transfer. Genetic material can exist in mobile segments, or movable sections. Horizontal gene transfer is when bacteria exchange portions of this DNA.
In vertical gene transfer, parents pass on DNA to their offspring. Horizontal gene transfer is different because it involves genetic material moving from one living organism to another, regardless of relatedness. This capability allows many bacteria to quickly resist antibiotics, which is a medical tool humans use to fight bacterial infections. Whenever antibiotics are used, some bacteria may have genes that allow them to survive. When these genes are passed on to other bacteria, the whole group becomes more resistant.
Horizontal gene transfer is more common in prokaryotic, or single-celled, organisms, such as bacteria. This is because they lack a nuclear membrane that protects the organism's genetic material from foreign DNA.
Yeast and Fermentation
More complex single-celled organisms, such as yeast, are eukaryotes. Unlike prokaryotic cells, eukaryotic cells contain a nucleus as well as other organelles, which are specialized cell parts that perform specific functions. These organelles are like the cell's organs, or parts that perform specific tasks.
For example, the organelles in yeast allow it to perform processes like fermentation. Humans have long harnessed fermentation to make bread, wine, beer, and even biofuel. Fermentation is possible because of certain enzymes within yeast that allow it to convert sugars into alcohol. Like all proteins, enzymes are produced by ribosomes within a cell.
Cellular Slime Molds
Single-celled organisms can aggregate to form a multicellular structure. One example is the cellular slime mold, a type of single-cell amoeba. These social amoebae will function as individual organisms when soil nutrients are present. In times of low nutrients, however, they band together into a slug-like form. They migrate together in search of food.
The cellular communication between amoebas during aggregation involves many cell parts. These parts include the cytoskeleton and the nuclear membrane. The nuclear membrane controls the entry of key molecules from the cytoplasm into the nucleus. In the nucleus, these key molecules regulate gene transcription, or gene copying—the first stage of gene expression.
Ultimately, the aggregate amoeba typically splits into stalk cells and spore cells. A large vacuole, or space, forms within stalk cells as they undergo cell death and form a column. In this process, spore cells are lifted and then scattered to a new location. Many cell parts play a role in this complex behavior of social amoebas, including functions of the mitochondria. These are critical to cell movement, differentiation, and patterning of cells within the multicellular slug.
Specialized Cells in Plants And Animals
In true multicellular organisms, a variety of organelles allow equally incredible feats. Chloroplasts in plant cells allow the organism to capture the sun's energy and produce food. In a growing animal, the cytoskeleton sorts critical parts and molecules within the cell. It defines which end of the cell is which to enable specific functions as the tiny animal embryo grows bigger.
Following development, specialized cells within multicellular organisms perform specific functions to support the body. Meanwhile, organelles help cells accomplish various tasks. For example, mature red blood cells in mammals lack a nucleus. This helps them clear out as much cellular space as possible for a protein called hemoglobin. This protein allows the cell to carry oxygen from the lungs to the rest of the body.
White blood cells are part of the body's immune system. They use lysosomes to engulf and destroy bacteria, preventing infection.
Neurons in the Brain
Neurons in the human brain allow problem solving, memory, and emotion. A neuron's cell parts are critical to these functions. Neurons release neurotransmitters in response to environmental signals. The secretion of neurotransmitters is regulated by organelles called Golgi bodies. These are capable of making special vesicles, or sacs, to transport neurotransmitters outside the neuron. Once there, they aid in cell communication to help regulate things like mood. A long axon fiber extending from the cell releases these critical signaling chemicals. Then, a neighboring cell's many fingerlike dendrites receive the signals.