The cell is the fundamental unit of living organisms. Although cells come in many different forms, each cell consist of many organelles, such as cell membrane, cytoskeleton, nucleus, ribosome, etc. These organelle structures play a crucial role in cell function. The cell membrane, also called plasma membrane, separates the inside and outside of a cell and determines materials enter or exit the cell. The cytoskeleton maintains the shape of the cell and provides the mechanical support that enables the cell to perform necessary functions such as division and movement. Cytoskeletal elements including microtubules, actin filaments, intermediate filaments, and myosin filaments are dynamic, allowing the cell to change shape and to transport materials along cytoskeletal tracks. The nucleus is the largest single organelle, enclosed by a double membrane with numerous nuclear pores. The functions of the nucleus are involved in regulating gene expression, initiating cellular reproduction, and storing genetic materials. The ribosome is the site of protein synthesis in a cell and consists of special nucleic acids and proteins. The ribosome translates the encoded message of messenger RNA (mRNA) from the nucleus and link together amino acids to synthesize proteins by transferring the RNA (tRNA). Ribosomes are widely distributed in cells, including free in the cytoplasm or attached to the nuclear membrane and endoplasmic reticulum (ER). The ER is a network of membranous tubules in a cell, which can be classified as smooth or rough type according to the presence or absence of ribosomal attachment. The roles of ER include calcium storage, protein synthesis and transport, and lipid metabolism. The rough ER transfers proteins and lipids to the Golgi apparatus, which functions as a shipping and packaging factory of the cell. In Golgi apparatus, proteins are further processed and sorted for transport to their eventual destinations such as lysosomes, the plasma membrane, or secretion. All these cellular activities require energy supplied as ATP, which is synthesized by mitochondria through the tricarboxylic acid cycle and oxidative phosphorylation. A mitochondrion has an outer and an inner membrane, both of which are made of phospholipid layers. A cell also contains a variety of other organelles, such as lysosomes, peroxisomes, and proteasomes. These structures are responsible for degrading material that enter cells through endocytosis, damaged organelles, and cellular proteins. In addition, cells form various attachments with other cells and extracellular matrix, including zonula occludens and adherens junctions, gap junctions, and desmosomes.
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