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5.5Summary

  • A biological membrane consists of a lipid bilayer with membrane proteins attached.
  • Organisms are divided into the categories of eukaryotes, which have membrane-enclosed organelles in their cells (e.g., the nucleus), and prokaryotes, which have no organelles in their cells.
  • The biological membranes of eukaryotic cells include the plasma membrane, endoplasmic reticula, Golgi apparatuses and lysosomes (which have a single lipid bilayer) and the nuclear membrane, mitochondria and chloroplasts (which have two lipid bilayers, i.e., inner and outer membranes).
  • The nucleus contains genetic information, and is host to the occurrence of DNA replication and RNA transcription.
  • Endoplasmic reticula and Golgi apparatuses are involved in the synthesis, processing and selective transport of lipids, membrane proteins and secretory proteins.
  • Endosomes and lysosomes are involved in the uptake and degradation of extracellular materials, respectively.
  • Mitochondria are involved in ATP synthesis, and chloroplasts are involved in plant photosynthesis. These organelles have DNA that is unique only to them, giving rise to the possibility that they were originally primitive organisms that become incorporated into cells.
  • Biological membranes function as a barrier with limited permeability, and selectively allow the passage of metabolites and informational molecules.
  • Biological membranes have channel and transporter proteins that regulate the transport of ions and molecules.
  • The plasma membrane has receptor proteins that transfer extracellular information to the inside of the cell by detecting the information and changing the structure of intracellular elements.
  • Cells contain a transport system in which membrane structures known as transport vesicles perform the movement of materials. Transport vesicles bud before being cut off from an organelle with a membrane structure, and transport proteins and other materials by fusing with the membrane structure of target organelles.
  • Proteins to be transported to a particular organelle have a unique signal sequence, and membrane proteins on the surface of the organelle incorporate proteins with this sequence.

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