Membrane
Supplement. There are two distinct types of cells: prokaryotic cells (e.g. bacterial cells) and eukaryotic cells (e.g. plant or animal cell). The main difference between the two is a well-defined nucleus surrounded by a membranous nuclear envelope present only in eukaryotic cells.  If the DNA within a cell is not separated from the cytoplasm, then that cell is a prokaryote. All known prokaryotes, such as bacteria and archaea, are single cells. In contrast, if the DNA is partitioned off in its own membrane-bound room called the nucleus, then that cell is a eukaryote. Some eukaryotes, like amoebae, are free-living, single-celled entities. That's because all cells are surrounded by a structure called the cell membrane — which, much like the walls of a house, serves as a clear boundary between the cell's internal and external environments. The cell membrane is sometimes also referred to as the plasma membrane. All known prokaryotes, such as bacteria and archaea, are single cells. In contrast, if the DNA is partitioned off in its own membrane-bound room called the nucleus, then that cell is a eukaryote. Some eukaryotes, like amoebae, are free-living, single-celled entities. Still, as different as these cells are, they all rely on the same basic strategies to keep the outside out, allow necessary substances in and permit others to leave, maintain their health, and replicate themselves. In fact, these traits are precisely what make a cell a cell. All known prokaryotes, such as bacteria and archaea, are single cells. In contrast, if the DNA is partitioned off in its own membrane-bound room called the nucleus, then that cell is a eukaryote. Some eukaryotes, like amoebae, are free-living, single-celled entities. Cells can vary between 1 micrometer (μm) and hundreds of micrometers in diameter. Within a cell, a DNA double helix is approximately 10 nanometers (nm) wide, whereas the cellular organelle called a nucleus that encloses this DNA can be approximately 1000 times bigger (about 10 μm). All known prokaryotes, such as bacteria and archaea, are single cells. In contrast, if the DNA is partitioned off in its own membrane-bound room called the nucleus, then that cell is a eukaryote. Some eukaryotes, like amoebae, are free-living, single-celled entities. Cytoskeleton. The cytoskeleton is a network of long fibers that make up the cell’s structural framework. The cytoskeleton has several critical functions, including determining cell shape, participating in cell division, and allowing cells to move. Mitochondria (illustration). Mitochondria are complex organelles that convert energy from food into a form that the cell can use. They have their own genetic material, separate from the DNA in the nucleus, and can make copies of themselves. Cells are the smallest unit of life that can replicate independently, and are often called the building blocks of life. The study of cells is called cell biology. Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Most important among these is a cell nucleus, an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means true kernel (nucleus). Other differences include: 1  The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. 1 Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. 2  It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. Most important among these is a cell nucleus, an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means true kernel (nucleus). Other differences include: 1  The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup.
http://www.biology-online.org/dictionary/Cell http://www.nature.com/scitable/topicpage/what-is-a-cell-14023083 http://www.nature.com/scitable/topicpage/what-is-a-cell-14023083 http://www.nature.com/scitable/topicpage/what-is-a-cell-14023083 http://www.nature.com/scitable/topicpage/what-is-a-cell-14023083 http://ghr.nlm.nih.gov/handbook/basics/cell https://en.wikipedia.org/wiki/Cell_(biology) https://en.wikipedia.org/wiki/Cell_(biology)
what is in a cell