Cells are the most fundamental unit in living organisms and are responsible for carrying out a variety of specialized functions. Evolution has led to cells becoming specialized, enabling them to perform a vast variety of tasks that depend upon their local environment. Despite this, all living cells have a number of common features, including the presence of a phospholipid membrane, DNA, cytoplasm and ribosomes.
The cell membrane is a protective layer that surrounds the cell and protects each of its organelles. The membrane is made of a bi-layer of phospholipids. Phospholipid molecules have a water-loving (hydrophilic) side and a water-hating (hydrophobic) side. The phospholipid bi-layer of the cell membrane have its molecules aligned such that the hydrophilic sides are pointing outwards, where water is encountered and the hydrophobic sides are pointing inwards. The cell membrane also also contains a number of proteins that facilitate the transport of molecules into and out of the cell.
Cytoplasm is the thick fluid that is encased within the cell membrane and holds the cell organelles. The cytoplasm is where the majority of cell metabolic processes take place. For example, the breakdown of glucose to create energy (glycolysis), takes place within the cytoplasm. Finally, the parts of the cytoplasm that are not encased by the cell organelles is known as cytosol. Cytosol is a a complex mixture consisting of protein filaments, dissolved molecules and water.
Ribosomes consist of a protein complex as well as chains of ribonucleic acid (RNA). In Eukaryotic organisms, the majority of ribosomes are attached to structures called the rough endoplasmic reticulem, which is associated with the nucleus of the cell. In prokaryotic cells, ribsomes are free-floating structures, since there is no nucleus. The function of ribosomes is to read messenger RNA and use this in conjunction with transfer RNA in order to carry out protein synthesis. Therefore the ribosomes within cells are often seen to be analogous to factories since they are the organelle responsible for protein manufacture.
Deoxyribonucleic Acid (DNA) is the genetic material held within the nucleus of eukaryote cells and in the nucleoid region of prokaryote cells. The structure of DNA consists of a chain of sugars (deoxyribose) that makes up its backbone. Each sugar molecule is connected to phosphoric acid and a base molecule. The four types of base molecules are adenine, cytosine, thymine and guanine. The genetic code is stored through the arrangement of bases within DNA, which encodes a particular protein.