Biological Membranes

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Biological Membranes
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• Biological membranes
• Complex, dynamic structures made of lipid and
  protein molecules
• Perform many functions
• Define cell as a compartment
• Regulate passage of materials
• Participate in chemical reactions
• Transmit signals between cell interior and the
  environment
• Act as part of energy transfer and storage

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• Biological membranes
• Physically separate cell interior from
  extracellular environment
• Form compartments within eukaryotic cells
• Plasma membrane
• Regulates passage of materials
• Participates in biochemical reactions
• Receives information about environment
• Communicates with other cells

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• Fluid mosaic model
• Membranes consist of fluid phospholipid bilayer
  with a mosaic pattern of associated proteins
• Phospholipid molecules are amphipathic and
  contain
• Hydrophobic regions which are repelled by water
• Hydrophilic regions which are attracted to water

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Phospholipid molecules
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• Since the cell is a very watery environment, the
  heads of the phospholipid molecules face toward
  the cell.
• The fatty acid tails of the molecules face
  inward toward the center of the bilayer.

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• Phospholipids form bilayers in water

Phospholipids in water
Detergent in water
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• Plasma membrane of mammalian red blood cell

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• Membrane properties
• Orderly arrangement of phospholipid molecules
  make the cell membrane a liquid crystal
• Allow molecules to move rapidly
• Proteins move within membrane
• Lipid bilayers are
• Flexible
• Self-sealing
• Can fuse with other membranes

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• Detailed structure of the plasma membrane

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Proteins are embedded in the bilayer

• Integral membrane proteins
• Embedded in the bilayer
• Transmembrane proteins
• Integral proteins that extend completely through
  the membrane
• Peripheral member proteins
• Associated with the surface of the bilayer

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• Membrane proteins, lipids, and carbohydrates
• Asymmetrically positioned to bilayer
• Sides have different composition and structure
• Function of member proteins
• Transport of materials
• Acting as enzymes or receptors
• Cell recognition
• Structurally linking cells

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• Asymmetry of the plasma membrane

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• Functions of membrane proteins

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• Membranes are selectively permeable (only some
  materials are allowed in and out)
• Physical processes
• Osmosis
• Diffusion
• Carrier-mediated processes
• Channel proteins
• Carrier proteins

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Diffusion the movement of molecules from a
region of higher concentration to one of lower
concentration.

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Rate of diffusion depends on

• Temperature (higher temperature, more movement of
  molecules)
• Size of molecules (smaller molecules tend to
  diffuse faster)
• Electrical charge (like charges repel and
  opposites attract)
• Difference in concentration (concentration
  gradient)

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Some molecules easily diffuse through the membrane

• Water
• Gases
• Small polar molecules
• Large, lipid soluble molecules

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Osmosis

• The diffusion of water across a selectively
  permeable membrane from a region where there is a
  greater concentration of water to a region where
  there is less water.
• Osmotic pressure the tendency of water to move
  into a solution.

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• Osmotic pressure
• Concentration of dissolved substances in a
  solution
• Isotonic equal solute concentration
• Hypertonic solution has a greater concentration
  of solutes than the cell, loses water in
  plasmolysis
• Plasmolysis (collapse of a cell due to loss of
  water.
• Hypotonic solution has a lesser concentration of
  solutes than the cell, gains water and swells

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Turgor pressure

• The internal hydrostatic pressure usually present
  in walled cells.
• Turgor pressure provides structural support in
  plants which do not have wood.

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Movement of particles through proteins

• If substances cannot pass through the lipid
  bilayer, they may still move through the membrane
  via protein channels.
• This may or may not require energy from the cell.
  
• Passive transport no energy expenditure
• Active transport energy expended by the cell.

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• Facilitated diffusion does not require energy
• Occurs down a concentration gradient
• Active transport requires energy
• Moves ions or molecules against a concentration
  gradient
• Cotransport requires energy
• ATP-powered pump maintains a concentration
  gradient

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Sodium-Potassium Pump
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Sucrose proton cotransport
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• Cells expend metabolic energy to carry on
  physiological processes
• Exocytosis large molecules leave the cell
  through fusion with the membrane
• Endocytosis large molecules enter the cell
  through fusion with the membrane
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis

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• Exocytosis

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• Phagocytosis

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Phagocytosis is used by macrophages to help
defend the body from bacteria
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Amoebae feed using phagocytosis
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• Pinocytosis

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• Receptor-mediated endocytosis

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• Cells communicate by cell signaling
• Signaling molecules include
• Neurotransmitters
• Hormones
• Regulatory molecules

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• Cell signaling involves
• Synthesis and release of signaling molecule
• Transport to target cells
• Reception by target cells involving special
  surface receptors where the signaling protein
  docks.
• Signal transduction cells convert an
  extracellular into an intracellular one.
• Response by the cell interior of the membrane
  protein undergoes a change in conformation, which
  activates proteins in the cytoplasmoften in a
  chain reaction.
• Termination of signal

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• Signaltransduction

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• Cells in close contact often develop
  intercellular junctions
• Anchoring junctions connect epithelial cells
• Desmosomes composed of filaments which hold cells
  subject to mechanical stresses together (in
  animal cells)
• Adhering junctions cement cells together with
  proteins
• Tight junctions seal off intercellular spaces in
  some animal cells. Seals are protein links.

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Gap junctions permit transfer of small molecules
and ions. These contain pores that connect cells.
Allow rapid chemical and electrical communication
between cells. Plasmodesmata allow movement of
certain molecules and ions between plant cells.
Plasma membranes are continuous though the gaps.
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• Desmosomes

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• Tight
• junctions

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• Gap junctions

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• Plasmodesmata