Lipid bilayer

• Chapters


  • Cells: The Fundamental Units of Life
  • UNITY AND DIVERSITY OF CELLS
  • CELLS UNDER THE MICROSCOPE
  • THE PROKARYOTIC CELL
  • THE EUKARYOTIC CELL
  • MODEL ORGANISMS
  • What does it mean to be living?
• All living things (or organisms) are built from cells: small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two
• Higher organisms, including ourselves, are communities of cells derived by growth and division from a single founder cell
• Every animal or plant is a vast colony of individual cells, each of which performs a specialized function that is integrated by intricate systems of cell-to-cell communication
• Cells are the fundamental units of life
• Cell biology is the study of cells and their structure, function, and behavior
• Cell biology provides answers to questions about life on Earth, its origins, diversity, and adaptation to different habitats, as well as questions about human development, health, and aging
• Biologists estimate that there may be up to 100 million distinct species of living things on our planet
• Cells vary enormously in appearance, function, size, shape, and chemical requirements
• Despite the diversity, all cells share a fundamental chemistry and common features
• All present-day cells appear to have evolved from a common ancestor
• Living cells all have a similar basic chemistry
• Genetic instructions of the organism to the next generation

  Delegated to specialists—the egg and the sperm
• Despite the extraordinary diversity of plants and animals, organisms have something in common entitling them to be called living things
• Textbooks define life in abstract general terms related to growth, reproduction, and an ability to actively alter behavior in response to the environment
• Discoveries of biochemists and molecular biologists show that cells of all living things are fundamentally similar inside
• Cells resemble one another to an astonishing degree in the details of their chemistry
• Genetic information in all organisms is carried in DNA molecules
• Information in DNA molecules is written in the same chemical code, constructed out of the same chemical building blocks, interpreted by essentially the same chemical machinery, and replicated in the same way
• Cells come in a variety of shapes and sizes
• Life is easy to recognize but difficult to define
• Unity and Diversity of Cells
• In every cell, long polymer chains of DNA are made from the same set of four monomers, called nucleotides, strung together in different sequences like the letters of an alphabet
• Information encoded in DNA molecules is read out, or transcribed, into a related set of polynucleotides called RNA
• RNA molecules are translated into a different type of polymer called a protein
• Flow of information from DNA to RNA to protein is referred to as the central dogma
• Appearance and behavior of a cell are dictated largely by its protein molecules
• Proteins serve as structural supports, chemical catalysts, molecular motors, and much more
• Proteins are built from amino acids, and all organisms use the same set of 20 amino acids to make their proteins
• Amino acids are linked in different sequences, giving each type of protein molecule a different three-dimensional shape, or conformation
• Living Cells Are Self-Replicating Collections of Catalysts
• Cells reproduce by duplicating their genetic material and other components and then dividing in two, producing a pair of daughter cells capable of undergoing the same cycle of replication
• Special relationship between DNA, RNA, and proteins makes self-replication possible
• Feedback loop between proteins and polynucleotides underlies the self-reproducing behavior of living things
• Proteins also catalyze many other chemical reactions that keep the self-replicating system running
• Genetic information flow


  1. From DNA to RNA (transcription)

  2. From RNA to protein (translation)
• All living organisms are constructed from cells
• Components of living cells

  • Polynucleotides
  • Proteins
  • Other cell constituents
• Living cells can perform self-replication
• Viruses contain information in the form of DNA or RNA but cannot reproduce by their own efforts