Chapter 9: How Cells Communicate

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Chapter 9 How Cells Communicate
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This chapter discusses how cells receive and
respond to signals from other cells.

• Multicellular organisms require specialized cells
  to perform the various processes of life
  efficiently. Specialization requires
  communication between cells.
• Cells use a variety of different signaling
  molecules to communicate with one another,
  including proteins, fatty acid derivatives, and
  even gases.
• To receive and interpret signals, cells have
  specific receptor proteins that bind signaling
  molecules and trigger certain chemical reactions
  inside the cell.
• Some receptor proteins reside in the cytosol and
  bind to signaling molecules that pass through the
  plasma membrane others reside in the plasma
  membrane and bind only to external signaling
  molecules.
• One signaling molecule can induce many chemical
  reactions, amplifying the cells response to the
  signal.
• Different signaling molecules can affect some of
  the same chemical reactions in the cell,
  resulting in the combining of different signals.

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Specialization and Communication in the Community
of Cells

• The principle of cell specialization means that
  the cells found in a multicellular organism are
  not all the same, nor do they all have the same
  function.
• Specialized cells allow multicellular organisms
  to efficiently carry out processes necessary for
  life.
• Although cells are specialized, they do not
  operate in isolation.
• The lungs are a tissue that is made up of a
  number of specialized cells that work together to
  exchange carbon dioxide and oxygen (Figure 9.1).
• The second organizational principle that applies
  to all multicellular organisms is that of cell
  communication.
• Cells in a multicellular organism must
  communicate.
• The reflex of jerking away from a painful
  stimulus is an example of cell-to-cell
  communication between several different cell
  types (Figure 9.2).
• Communication between cells is accomplished with
  small proteins or other molecules that are
  released by one cell and received by another cell
  (target cell).
• Signaling molecules is a general term for the
  proteins and molecules used by cells for
  communication.

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The Roles of Signaling Molecules in Cell
Communication

• Signaling molecules that travel only between
  neighboring cells in a tissue are usually fragile
  and short-lived.
• Signaling molecules that travel through the
  bloodstream are usually less prone to degradation
  than signaling molecules that travel only short
  distances.
• Specific receptors (proteins) on or inside target
  cells bind signaling molecules and transfer their
  message to the cell (Figure 9.3).

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Nitric oxide is a short-range signaling molecule

• Many signaling molecules are proteins or fatty
  acid derivatives.
• Nitric oxide is a gas and is involved in lowering
  blood pressure.
• Endothelial cells in blood vessels release nitric
  oxide when induced by specific nerve signals.
• Nitric oxide causes muscle cells around blood
  vessels to relax, which results in lowered blood
  pressure (see Figure 9.4).
• The nitric oxide molecules last only a few
  seconds outside the cell that produced them.
• The process of relaxing muscles around blood
  vessels is called vasodilation.

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Hormones are long-range signaling molecules

• Long-distance communication in an organism is
  accomplished by signaling molecules called
  hormones.
• Hormones are produced by cells in one part of the
  body and transported to target cells in another
  part of the body.
• Hormones travel in the sap of plants and the
  blood of animals.

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Steroid hormones can cross cell membranes

• Steroid hormones are derived from a lipid called
  cholesterol and are hydrophobic.
• Because steroid hormones are hydrophobic, they
  can pass through cell membranes.
• Steroid hormones are transported in the
  bloodstream bound to proteins.
• Bound to a protein, a steroid hormone can remain
  in the bloodstream for days.
• Progesterone is a hormone produced in the ovaries
  of female mammals. It circulates through the
  bloodstream and ensures that the cells of the
  uterus are ready to support an embryo.
• To affect target cells, steroid hormones bind
  receptors inside their target cells and alter the
  production of specific proteins in the cells
  (Figure 9.5).

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Some hormones require the help of cell surface
receptors

• Adrenaline (epinephrine) is a hormone that binds
  to cell surface receptors.
• Adrenaline (epinephrine) has different effects on
  the liver and heart (Figure 9.6).
• Liver glycogenolysis, mobilization of free fatty
  acids and stimulation of metabolic rate
• Heart increase HR and force of contraction,
  glycogenolysis.

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Epinephrine Norepinephrine Act Via Receptors,
G-Proteins cAMP

• The end result differs depending on the tissue
• Heart beta/gamma subunit directly affects
  channels.
• Smooth muscle, liver, etc adenylate cyclase ? ?
  cAMP (? ? calcium).

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Growth factors induce cell division

• Growth factors also bind cell surface receptors.
• Platelets produce a growth factor called
  platelet-derived growth factor (PDGF).
• PDGF induces cell division at sites of tissue
  damage, which speeds wound healing.
• There are many speculative therapeutic uses of
  growth factors.

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How Cell Surface Receptors Initiate Changes
inside the Cell

• When a signaling molecule binds a cell surface
  receptor, it changes shape and induces a specific
  chemical reaction in the cell.
• The chemical reaction is called signal
  transduction.
• Proteins are activated in the cells during signal
  transduction in a signal cascade.
• Each type of signaling molecule binds to a
  specific type of receptor on the cell surface,
  causing a specific signal cascade that activates
  specific proteins.
• Adrenaline binds a G protein receptor and
  initiates a signal cascade that activates a
  number of proteins in the cell (see Figure 9.7).
• Phosphate transfer is a common way for cells to
  activate proteins when a cell surface receptor
  binds a signaling molecule (see Figures 9.8 and
  9.9).
• Phosphate transfer changes the three-dimensional
  shape of a protein, which can make it a more
  efficient enzyme or allow it to interact with
  other proteins.
• Phosphate transfer is very common in signal
  transduction.

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Different signals can be combined inside the cell

• Several different signaling molecules can share
  some of the components of a single signal cascade
  in a manner that results in cooperation between
  different signals.
• Signal integration is a process in which one
  signal can either enhance or inhibit the effects
  of another signal.
• Combining different signals gives cells an
  increased range of possible responses to their
  environment.

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