7.Cell Communication and Multicellularity

7.1

• Cells receive several types of signals

  • The environment is full of signals.
  • In multicellular organisms, chemical signals are often made in one part of the body and arrive at target cells by local diffusion or by circulation in the blood or the plant vascular system.
  • Chemical cell signals are usually present in tiny concentrations
    • Autocrine signals diffuse to and affect the cells that make them.
    • Juxtacrine signals affect only cells right next to and in contact with the cell producing the signal.
    • Paracrine signals diffuse to and affect nearby cells.
    • Hormones signals that travel through the circulatory systems of animals or the vascular systems of plants

• A signal transduction pathway involves a signal, a receptor, and responses:

  •  the elements of a signal transduction pathway are a signal, a receptor, and a response
  • For the information from a signal to be transmitted to a cell, the target cell must be able to receive the signal and respond to it. This is the job of receptors.
  • ll cells may be exposed to a chemical signal, but most cells in the organism may not be able to respond to it. 
  • Only cells with the appropriate receptors can respond.
  • The response can involve enzymes, which catalyze biochemical reactions, and transcription factors, which are proteins that turn the expression of particular genes on and off.
  • An important feature of signal transduction is that the activities of specific enzymes and transcription factors are regulated: they are either activated or inactivated to bring about cellular changes
  • he activity of a protein can also be regulated by mechanisms that control its location in the cell.

7.2

• Receptors that recognize chemical signals have specific binding sites

  • A ligand is a specific chemical signal molecule that fits into a three-dimensional site on its protein receptor
  • Binding of the signaling ligand causes the receptor protein to change its three-dimensional shape, and that conformational change initiates a cellular response.
  • The ligand does not contribute further to this response.
  • he sensitivity of a cell to a signal is determined in part by the affinity of the cell’s receptors for the signal ligand—the likelihood that the receptor will bind to the ligand at any given ligand concentration
  • Receptors (R) bind to their ligands (L) according to chemistry’s law of mass action. This means that the…