Prokaryotes and Eukaryotes

<h1>Text Preview</h1> <p>Once the cell theory became widely accepted, the field of cell biology took off! If cells are the smallest units of life, it follows that they must somehow be separated from each other. The plasma membrane is the barrier between cells. It's made of a bilayer, or double layer, of phospholipid molecules. </p> <p>All cells have a plasma membrane, but not all cells have the same internal structure. Prokaryotic cells don't have membrane-enclosed internal compartments. Organisms with this type of cell are called prokaryotes. Eukaryotic cells do contain membrane-enclosed compartments. Their DNA is enclosed in a membrane-covered compartment called the nucleus. Eukaryotic cells also contain other membrane-enclosed compartments that isolate certain molecules and chemical reactions from the rest of the cell. Organisms containing eukaryotic cells are called eukaryotes.</p> <p>All organisms are grouped into one of three broad categories called domains. Prokaryotes include members of the domain Archaea, called archaebacteria, and members of the domain Bacteria, called eubacteria.</p> <p>Prokaryotes are single-celled organisms, roughly one micrometer in diameter. The cell's internal fluid is called the cytosol. Its genetic material consists of a large DNA molecule compacted in an area called the nucleoid region. The plasma membrane encases the cytosol and its contents. Members of the domain Bacteria have a cell wall surrounding the plasma membrane. A flagellum gives the organism mobility. </p> <p>Eukaryotes include fungi, animals, and plants. Eukaryotic cells are typically 10 to 30 micrometers in diameter. Eukaryotes contain intracellular fluid called cytosol, just like prokaryotes. As mentioned earlier, one of the distinguishing features of eukaryotes is the compartmentalization of their DNA in the nucleus. A pair of membranes called the nuclear envelope surrounds the nucleus. Most of the other functions of the cell are carried out by organelles. An organelle is a distinct structure or compartment within a eukaryotic cell that performs specific tasks. </p> <p>Eukaryotic cells have a number of organelles besides the nucleus. The endoplasmic reticulum, or ER, is a network of membranes extending away from the nucleus. It performs complex tasks involved with the synthesis of RNA into proteins. Another membranous organelle, the Golgi apparatus, resides near the nucleus and ER. It sorts and modifies proteins produced in the ER, and prepares them for transport to specific parts of the cell. Mitochondria and chloroplasts are the main sites of energy transformation in eukaryotic cells. Chloroplasts are found only in plant cells, and we'll talk more about them soon. Lysosomes are membrane-enclosed sacs containing digestive enzymes that break down proteins, carbohydrates, and fats.</p> <p>Like organelles, the cytoskeleton is unique to eukaryotic cells. It's a dynamic three-dimensional structure that fills the cytosol and anchors many of the organelles in place. The cytoskeleton contains three primary types of fibers: microtubules, microfilaments, and intermediate filaments. These fibers aren't visible in this image because of the technique used for preparing the sample for viewing under the microscope. Here we see another image in which special substances have been added to change the color of the cytoskeleton's components. The microtubules are green and the microfilaments are blue. Intermediate filaments form most of the rest of the network.</p> <p>Let's consider the distinctions between plant cells and animal cells. Plant cells have three distinguishing features: a cell wall, chloroplasts, and large vacuoles. The cell wall protects a plant cell, helps maintain the cell's shape, and contributes to the overall structural support of the plant. Chloroplasts are found only in plants and certain algae. They convert the sun's energy into chemical energy in a complex process called photosynthesis. In plants, the vacuole is a specialized, membrane-enclosed region within a plant cell that serves as a toxic waste dump. Many compounds are isolated here, away from the cytosol. As the plant ages, vacuoles begin to leak, hastening the death of the cell and eventually the whole plant.</p> <p>Now that we've explored many of the differences between prokaryotes and eukaryotes, let's ask the question: "Where did eukaryotes come from?" Chemical processes on the early Earth probably led to the formation of the first cells, which were primitive prokaryotes. Scientists believe that eukaryotic cells evolved from prokaryotic cells in a stepwise process that includes endosymbiosis, where one organism lives inside the body or cells of another organism. For example, ancestral prokaryotes that could use oxygen to convert food molecules to energy may have lived symbiotically inside primitive eukaryotes. These prokaryotes eventually became modern-day mitochondria. Other prokaryotes with photosynthetic properties may also have lived symbiotically in these early eukaryotes. These prokaryotes may have become the chloroplasts in modern photosynthetic eukaryotes. Today, biologists know more about cells than Schleiden and Schwann could have imagined when they proposed the cell theory! </p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>