Chapter 5: Cellular Membranes

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Chapter 5 Cellular Membranes
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Membrane Composition and Structure

• Biological membranes consist of lipids, proteins,
  and carbohydrates. The fluid mosaic model
  describes a phospholipid bilayer in which
  membrane proteins move laterally within the
  membrane.

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5.2
figure 05-02.jpg
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Components

• Phospholipid bilayer
• Transmembrane proteins (integral membrane
  proteins)
• Interior protein network (peripheral membrane
  proteins)
• Cell surface markers

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Membrane Fuctions

• Transport
• Enzyme scaffold
• Cell Surface Receptors
• Identity
• Adhesion and Attachments

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Cell Recognition, Adhesion and Attachment

• Recognition In an organism or tissue, cells
  recognize and bind to each other by means of
  membrane proteins protruding from the cell
  surface. Glycoproteins and Glycolipids

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Cell Adhesion and Attachment

• Tight junctions--prevent passage of molecules
  through space around cells, create functional
  regions, restrict migration of membrane proteins
  over the cell surface.
• Anchoring Junctionsmechanically attach two
  cytoskeletons
• Communicating Junctionsdirect connections with
  other cells.

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5.6 Part 1
figure 05-06a.jpg

• Figure 5.6 Part 1

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Transport

• Passive Substances can diffuse passively across
  a membrane by unaided diffusion through the
  phospholipid bilayer, facilitated diffusion
  through protein channels, or by means of a
  carrier protein.

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Transport

• Diffusion Solutes diffuse across a membrane from
  a region with a greater solute concentration to a
  region of lesser. Equilibrium is reached when the
  concentrations are identical on both sides.

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Transport

• Osmosis water diffuses from regions of higher
  water concentration to regions of lower
  concentration.

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5.8
figure 05-08.jpg

• Figure 5.8

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5.9
figure 05-09.jpg
Channel proteins and carrier proteins function in
facilitated diffusion

• Figure 5.9

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5.10
figure 05-10.jpg

• Rate is maxed when the solute concentration
  saturates carrier protein

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Active Transport

• Active transport proteins may be uniports,
  symports, or antiports.

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• In primary active transport, energy from the
  hydrolysis of ATP is used to move ions into or
  out of cells against their concentration
  gradients.

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• Secondary active transport couples the passive
  movement of one solute with its concentration
  gradient to the movement of another solute
  against its concentration gradient. Energy from
  ATP is used indirectly to establish the
  concentration gradient resulting in movement of
  the first solute.

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Bulk Transport

• Endocytosis transports macromolecules, large
  particles, and small cells into eukaryotic cells
  by means of engulfment and by vesicle formation
  from the plasma membrane. Phagocytosis,
  pinocytosis, receptor-mediated endocytosis.
• In exocytosis, materials in vesicles are secreted
  from the cell when the vesicles fuse with the
  plasma membrane.

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Cell Signaling

• Receptor proteins have a three-dimensional shape
  that fits a specific signal molecule.
• When a signal molecule and receptor protein bind,
  a change in the receptor protein is induced and a
  response in the cell is generated.

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Intracellular Receptors

• Intracellular receptors are located within the
  cell, and trigger a variety of responses,
  depending on the receptor.
• acting as gene regulators
• acting as enzymes

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Cell Surface Receptors

• Extracellular signals converted to intracellular
• chemically-gated ion channels
• open or close when signal molecules bind to the
  channel
• enzyme receptors
• usually activate intracellular proteins by
  phosphorylation
• G-protein receptors
• activate intermediary protein

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Cell Surface Receptors
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Initiating the Intracellular Signal

• Second messengers relay messages from receptors
  to target proteins.
• cAMP
• calcium