IV. Read the text below using a dictionary where necessary: The Cell

The basic structural and functional unit of every organism is one of two types of cells – prokaryotic or eukaryotic. Only organisms of the domains Bacteria and Archaea consist of prokaryotic cells. Protists, fungi, animals, and plants all consist of eukaryotic cells. Indeed, many forms of life exist as single-celled or unicellular organisms. More complex organisms, including plants and animals, are multicellular.

All cells have several basic features in common: they are all bounded by a selective barrier, called the plasma membrane. It is a semifluid, jellylike substance called cytosol which consists of organelles and other components. All cells contain chromosomes that carry genes in the form of DNA. And all cells have ribosomes, tiny complexes which make proteins according to instructions from the genes.

A major difference between prokaryotic and eukaryotic cells is the location of their DNA. In a eukaryotic cell, most of the DNA is in an organelle called the nucleus which is bounded by a double membrane. In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid.

The interior of a prokaryotic cell is called the cytoplasm; this term is also used for the region between the nucleus and the plasma membrane of a eukaryotic cell. Within the cytoplasm of a eukaryotic cell, there is a variety of organelles of specialized form and function. These membrane-bounded structures are absent in prokaryotic cells. Thus, the presence or absence of a true nucleus is just one example of the disparity in structural complexity between the two types of cells.

Membranes are fundamental to the organization of the cell. In general, biological membranes consist of a double layer of phospholipids and other lipids. However, each type of membrane has a unique composition of lipids and proteins according to that membrane's specific functions.

Ribosomes, which are complexes made of ribosomal RNA and protein, are the cellular components that carry out protein synthesis. Cells that have high rates of protein synthesis have particularly large numbers of ribosomes. For instance, a human pancreas cell has a few million ribosomes. Not surprisingly, cells active in protein synthesis also have prominent nucleoli.

Many of the different membranes of the eukaryotic cell are part of an endomembrane system, which carries out a variety of tasks in the cell. These tasks include synthesis of proteins and their transport into membranes and organelles or out of the cell, metabolism and movement of lipids, and detoxification of poisons. The membranes of this system are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles (sacs made of membrane).

In eukaryotic cells, mitochondria and chloroplasts are the organelles that convert energy to forms that cells can use for work. Mitochondria are the sites of cellular respiration, the metabolic process that generates ATP by extracting energy from sugars, fats, and other fuels with the help of oxygen. Chloroplasts, found in plants and algae, are the sites of photosynthesis. They convert solar energy to chemical energy by absorbing sunlight and using it to drive the synthesis of organic compounds such as sugars from carbon dioxide and water.