How many organelles does a cell have

Golgi Apparatus : Processes, packages and distributes proteins to other organelles for export. Cytoskeleton : Structural support of cells; facilitates the movement of the organelles. Smooth endoplasmic reticulum SER : Lipid synthesis; drug metabolism. Mitochondria : Produces energy for the cell called ATP. The genetic material DNA is localized to a region called the nucleoid which has no surrounding membrane.

The cell contains large numbers of ribosomes that are used for protein synthesis. At the periphery of the cell is the plasma membrane. In some prokaryotes the plasma membrane folds in to form structures called mesosomes, the function of which is not clearly understood. Outside the plasma membrane of most prokaryotes is a fairly rigid wall which gives the organism its shape. The walls of bacteria consist of peptidoglycans.

Sometimes there is also an outer capsule. Note that the cell wall of prokaryotes differs chemically from the eukaryotic cell wall of plant cells and of protists. Eukaryotic cells contain a membrane-bound nucleus and numerous membrane-enclosed organelles e. Animals, plants, fungi, and protists are all eukaryotes. Eukaryotic cells are more complex than prokaryotic cells and are found in a great many different forms.

The nucleus contains most of the genetic material DNA of the cell. Additional DNA is in the mitochondria and if present chloroplasts. The nuclear DNA is complexed with proteins to form chromatin, which is organized as a number of linear chromosomes.

Genetic control of the cell is carried out by the production of RNA in the nucleus the process of transcription and the subsequent transfer of this RNA to a ribosome in the cytoplasm, where protein synthesis the process of translation is directed. The resulting proteins carry out cell functions. Also located in the nucleus is the nucleolus or nucleoli, organelles in which ribosomes are assembled. The nucleus is bounded by a nuclear envelope, a double membrane perforated with pores and connected to the rough endoplasmic reticulum membrane system.

The cytoskeleton consists of microtubules, intermediate fibers, and microfilaments, which together maintain cell shape, anchor organelles, and cause cell movement. Rather, these structures are in constant motion, sometimes moving to a particular place within the cell, sometimes merging with other organelles, and sometimes growing larger or smaller.

These dynamic changes in cellular structures can be observed with video microscopic techniques, which provide lower-resolution movies of whole organelles as these structures move within cells. Of all eukaryotic organelles, the nucleus is perhaps the most critical. In fact, the mere presence of a nucleus is considered one of the defining features of a eukaryotic cell.

This structure is so important because it is the site at which the cell's DNA is housed and the process of interpreting it begins. Recall that DNA contains the information required to build cellular proteins. In eukaryotic cells, the membrane that surrounds the nucleus — commonly called the nuclear envelope — partitions this DNA from the cell's protein synthesis machinery, which is located in the cytoplasm. Tiny pores in the nuclear envelope, called nuclear pores, then selectively permit certain macromolecules to enter and leave the nucleus — including the RNA molecules that carry information from a cellular DNA to protein manufacturing centers in the cytoplasm.

This separation of the DNA from the protein synthesis machinery provides eukaryotic cells with more intricate regulatory control over the production of proteins and their RNA intermediates. In contrast, the DNA of prokaryotic cells is distributed loosely around the cytoplasm, along with the protein synthesis machinery. This closeness allows prokaryotic cells to rapidly respond to environmental change by quickly altering the types and amount of proteins they manufacture.

Note that eukaryotic cells likely evolved from a symbiotic relationship between two prokaryotic cells, whereby one set of prokaryotic DNA eventually became separated by a nuclear envelope and formed a nucleus. Over time, portions of the DNA from the other prokaryote remaining in the cytoplasmic part of the cell may or may not have been incoporated into the new eukaryotic nucleus Figure 3.

Figure 3: Origin of a eukaryotic cell. A prokaryotic host cell incorporates another prokaryotic cell. Each prokaryote has its own set of DNA molecules a genome. The genome of the incorporated cell remains separate curved blue line from the host cell genome curved purple line.

The incorporated cell may continue to replicate as it exists within the host cell. Over time, during errors of replication or perhaps when the incorporated cell lyses and loses its membrane separation from the host, genetic material becomes separated from the incorporated cell and merges with the host cell genome. Eventually, the host genome becomes a mixture of both genomes, and it ultimately becomes enclosed in an endomembrane, a membrane within the cell that creates a separate compartment.

This compartment eventually evolves into a nucleus. Figure Detail. Besides the nucleus, two other organelles — the mitochondrion and the chloroplast — play an especially important role in eukaryotic cells.

These specialized structures are enclosed by double membranes, and they are believed to have originated back when all living things on Earth were single-celled organisms. At that time, some larger eukaryotic cells with flexible membranes "ate" by engulfing molecules and smaller cells — and scientists believe that mitochondria and chloroplasts arose as a result of this process. In particular, researchers think that some of these "eater" eukaryotes engulfed smaller prokaryotes, and a symbiotic relationship subsequently developed.

Once kidnapped, the "eaten" prokaryotes continued to generate energy and carry out other necessary cellular functions, and the host eukaryotes came to rely on the contribution of the "eaten" cells. Over many generations, the descendants of the eukaryotes developed mechanisms to further support this system, and concurrently, the descendants of the engulfed prokaryotes lost the ability to survive on their own, evolving into present-day mitochondria and chloroplasts. This proposed origin of mitochondria and chloroplasts is known as the endosymbiotic hypothesis.

Figure 4: The origin of mitochondria and chloroplasts Mitochondria and chloroplasts likely evolved from engulfed bacteria that once lived as independent organisms. At some point, a eukaryotic cell engulfed an aerobic bacterium, which then formed an endosymbiotic relationship with the host eukaryote, gradually developing into a mitochondrion.

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