Prokaryotic and Eukaryotic cell, Cell Function

Organisms whose cells do not have a nucleus are called prokaryotes (from pro, meaning “before”). The terms “bacterium” and “prokaryote” are often used interchangeably, although we shall see that the category of prokaryotes also includes another class of cells, the archaea (singular archaeon), which are so remotely related to bacteria that they are given a separate name. Prokaryotes are typically spherical, rod-like, or corkscrew-shaped, and small just a few micrometers long, although, there are some giant species as much as 100 times longer than this. They often have a tough protective coat, called a cell wall, surrounding the plasma membrane, which encloses a single compartment containing the cytoplasm and the DNA.

Prokaryotic and Eukaryotic cell, cell function

Eukaryotic cells, in general, are bigger and more elaborate than bacteria and archaea. Some live independent lives as single-celled organisms, such as amoebae and yeasts, others live in multicellular assemblies. All of the more complex multicellular organisms including plants, animals, and fungi are formed from eukaryotic cells. By definition, all eukaryotic cells have a nucleus. But possession of a nucleus goes hand-in-hand with possession of a variety of other organelles, subcellular structures that perform specialized functions. Most of these are likewise common to all eukaryotic organisms. 

Cell Function:
1. Cells are the fundamental units of life. All present-day cells are believed to have evolved from an ancestral cell that existed more than 3 billion years ago.

2. All cells, and hence all living things, grow, convert energy from one form to another, sense and respond to their environment, and reproduce themselves.

3. All cells are enclosed by a plasma membrane that separates the inside of the cell from 
the environment.

4. All cells contain DNA as a store of genetic information and use it to guide the synthesis of RNA molecules and of proteins.

5. Cells in a multicellular organism, though they all contain the same DNA, can be very different. They turn on different sets of genes according to their developmental history and to cues, they receive from their environment.

6. Cells of animal and plant tissues are typically 5–20 mm in diameter and can be seen with a light microscope, which also reveals some of their internal components or organelles.

7. The electron microscope permits the smaller organelles and even individual large molecules to be seen, but specimens require elaborate preparation and cannot be viewed alive.

8. The simplest of present-day living cells are prokaryotes: although they contain DNA, they lack a nucleus and other organelles and probably resemble most closely the ancestral cell.

9. Different species of prokaryotes are diverse in their chemical capabilities and inhabit an amazingly wide range of habitats. Two fundamental evolutionary subdivisions are recognized: bacteria and archaea.

10. Eukaryotic cells possess a nucleus and other organelles not found in prokaryotes. They probably evolved in a series of stages. An important step appears to have been the acquisition of mitochondria, which are thought to have originated from bacteria engulfed by an ancestral eukaryotic cell.

11. The nucleus is the most prominent organelle in most plant and animal cells. It contains the genetic information of the organism, stored in DNA molecules. The rest of the cell’s contents, apart from the nucleus, constitute the cytoplasm.

12. The cytoplasm includes all of the cell’s contents outside the nucleus. It contains a variety of membrane-enclosed organelles with specialized chemical functions. Mitochondria carry out the oxidation of food. In plant cells, chloroplasts perform photosynthesis. The endoplasmic reticulum, the Golgi apparatus, and lysosomes permit cells to synthesize complex molecules for export from the cell and for insertion in cell membranes and to import and digest large molecules.

13. Outside the membrane-enclosed organelles in the cytoplasm is the cytosol, a concentrated mixture of large and small molecules that carry out many essential biochemical processes.

14. The cytoskeleton extends throughout the cytoplasm. This system of protein filaments is responsible for cell shape and movement and for the transport of organelles and molecules from one location to another in the cytoplasm.

15. Free-living, single-celled eukaryotic microorganisms include some of the most complex eukaryotic cells known, and they are able to swim, mate, hunt, and devour food.

16. An animal, plant, or fungus consists of diverse eukaryotic cell types all derived from a single fertilized egg cell; the number of such cells cooperating to form a large multicellular organism such as a human runs into thousands of billions.

17. Biologists have chosen a small number of model organisms to study closely. These include the bacterium E. coli, brewer’s yeast, a nematode worm, a fly, a small plant, a fish, a mouse, and the human species itself.

18. Although the minimum number of genes needed for a viable cell is less than 400, most cells contain significantly more. Yet even such a complex organism as a human has only about 24,000 protein-coding genes twice as many as a fly and seven times as many as E. coli.

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