Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen. The organization of these molecules into cells is one feature that distinguishes living things from all other matter. The cell is the smallest unit of matter that can carry on all the processes of life .
Progestins , the most important of which is progesterone , are the other type of female sex hormone and are named for their role in maintaining pregnancy (pro- gestation ). Estrogens and progestins are secreted cyclically during menstruation . During the menstrual cycle , the ruptured ovarian follicle (the corpus luteum ) of the ovary produces progesterone, which renders the uterine lining receptive to the implantation of a fertilized ovum . Should this occur, the placenta becomes the main source of progesterone, without which the pregnancy would terminate. As pregnancy progresses, placental production of progesterone increases, and these high doses suppress ovulation , preventing a second conception . The contraceptive quality of progesterone led to the development of structurally modified progestins and estrogens—the oral contraceptives known as birth-control pills, used by women to prevent unwanted pregnancy.
Ribosomes are particles that synthesize proteins from amino acids. They are composed of four RNA molecules and between 40 and 80 proteins assembled into a large and a small subunit. Ribosomes are either free (., not bound to membranes) in the cytoplasm of the cell or bound to the RER. Lysosomal enzymes, proteins destined for the ER, Golgi, and cell membranes, and proteins to be secreted from the cell are among those synthesized on membrane-bound ribosomes. Fabricated on free ribosomes are proteins remaining in the cytosol and those bound to the internal surface of the outer membrane, as well as those to be incorporated into the nucleus , mitochondria, chloroplasts, peroxisomes, and other organelles. Special features of proteins label them for transport to specific destinations inside or outside of the cell. In 1971 German-born cellular and molecular biologist Günter Blobel and Argentinian-born cellular biologist David Sabatini suggested that the amino-terminal portion of the protein (the first part of the molecule to be made) could act as a “signal sequence.” They proposed that such a signal sequence would facilitate the attachment of the growing protein to the ER membrane and lead the protein either into the membrane or through the membrane into the ER lumen (interior).