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All living things are made of cells. They may have only one cell, as in bacteria, or they made be made up of trillions of cells, such as in humans. Each cell has structures within it that perform various functions to keep the organism alive. These can include: making energy for the organism, exchanging gases with the external environment, and synthesizing proteins to be used as structural components elsewhere in the body. Humans start as a single cell that grows and divides, eventually changing into all the structures found in the human body. Certain cells continue to divide throughout a person's life, while others only get larger in size. Since cells have their own metabolisms, they are able to transport water and nutrients back and forth across their outer membranes. This effort maintains the internal continuity of the cell and keeps it functioning normally.

At A Glance

  • Cells are the smallest units of life, and all organisms are composed of cells.
  • Although each cell in an organism has the same genetic information, the differential expression of genes allows cells to specialize.
  • Human cells represent a variety of shapes specific to their function within tissues and organs. As cells grow, the surface area to volume ratio decreases until there is not enough surface area to allow for nutrient transport across the cell membrane.
  • Cells have a variety of internal organelles, including endoplasmic reticulum, mitochondrion, and centriole, each with a specific function to help the cell maintain its metabolism.
  • Cells have a membrane composed of a lipid bilayer that regulates the flow of materials into and out of the cell.
  • Cells have a unique cell membrane that serves many functions, including controlling the passage of substances into and out of the cell and cell communication, and has protein channels and pumps to facilitate the movement of molecules across the membrane.
  • Passive transport is the movement of material across the cell membrane from an area of high concentration to an area of low concentration, through diffusion or facilitated diffusion, without the expenditure of cellular energy.
  • Osmosis is a type of passive transport that involves the diffusion of water from an area of low solute concentration to an area of high solute concentration across a semipermeable membrane.
  • The cell must expend its own energy (ATP) to drive active transport mechanisms, such as ion pumps, that move molecules against the concentration gradient.
  • Bulk transport is the movement of many molecules or large substances into the cell (endocytosis) or out of the cell (exocytosis) in membrane-bound vesicles.
  • Metabolism is the sum total of all anabolic (those that build molecules) and catabolic (those that break down molecules) reactions of the body; aerobic reactions require oxygen and anaerobic reactions do not.
  • Enzymes are proteins that catalyze chemical reactions.
  • Proteins are produced from the transcription of DNA and then translation to amino acid chains, the peptide bonds of which are catalyzed by rRNA; proteins are then modified and secreted from the cell or used within the cell.
  • Transcription transfers genetic information from the gene in DNA to RNA.
  • Translation converts the coded sequence of RNA into proteins.
  • The cell cycle is the span of a cell from one division to another. Mitosis is the process of cell division to produce two daughter cells from one, with DNA being synthesized and each daughter cell receiving a copy.