Chapter 6: Interactions Between Cells

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Chapter 6 Interactions Between Cells the
Extracellular Environment
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Extracellular Environment

• Includes all constituents of body outside cells
• 67 of total body H20 is inside cells
  (intracellular compartment) 33 is outside
  cells (extracellular compartment-ECF)
• 20 of ECF is blood plasma
• 80 of ECF is interstitial fluid contained in
  gel-like matrix

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Extracellular Matrix

• Is a meshwork of collagen elastin fibers linked
  to molecules of gel-like ground substance to
  plasma membrane integrins
• glycoprotein adhesion molecules that link
  intracellular extracellular compartments
• Interstitial fluid resides in hydrated gel of
  ground substance

Fig 6.1
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The Plasma Membrane
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• The plasma membrane regulates what enters and
  exits the cell.
• Inside the plasma membrane, the nucleus is
  surrounded by cytoplasm.
• Plant cells have a cell wall in addition to the
  plasma membrane.

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Plasma Membrane Structure and Function

• The plasma membrane separates the internal
  environment of the cell from its surroundings.
• The plasma membrane is a phospholipid bilayer
  with embedded proteins.
• The plasma membrane has a fluid consistency and a
  mosaic pattern of embedded proteins.

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Fluid-mosaic model of membrane structure
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• Plasma membrane proteins may be peripheral
  proteins or integral proteins.
• Aside from phospholipid, cholesterol is another
  lipid in animal plasma membranes related
  steroids are found in plants.
• Cholesterol strengthens the plasma membrane.

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• When phospholipids have carbohydrate chains
  attached, they are called glycolipids.
• When proteins have carbohydrate chains attached,
  they are called glycoproteins.
• Carbohydrate chains occur only on the exterior
  surface of the plasma membrane.
• The outside and inside surfaces of the plasma
  membrane are not identical.

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Functions of plasma proteins

• Plasma proteins have a variety of functions.
• Some help to transport materials across the
  membrane.
• Others receive specific molecules, such as
  hormones.
• Still other membrane proteins function as enzymes.

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• In animal cells, the carbohydrate chains of cell
  recognition proteins are collectively called the
  glycocalyx.
• The glycocalyx can function in cell-to-cell
  recognition, adhesion between cells, and
  reception of signal molecules.
• The diversity of carbohydrate chains is enormous,
  providing each individual with a unique cellular
  fingerprint.

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The Permeability of the Plasma Membrane

• The plasma membrane is differentially permeable.
• Macromolecules cannot pass through because of
  size, and tiny charged molecules do not pass
  through the nonpolar interior of the membrane.
• Small, uncharged molecules pass through the
  membrane, following their concentration gradient.

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How molecules cross the plasma membrane
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Transport Across Plasma Membrane

• Plasma membrane is selectively permeable--allows
  only certain kinds of molecules to pass
• Many important molecules have transporters
  channels
• Carrier-mediated transport involves specific
  protein transporters
• Non-carrier mediated transport occurs by diffusion

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• Movement of materials across a membrane may be
  passive or active.
• Passive transport does not use chemical energy
  diffusion and facilitated transport are both
  passive.
• Active transport requires chemical energy and
  usually a carrier protein.
• Exocytosis and endocytosis transport
  macromolecules across plasma membranes using
  vesicle formation, which requires energy.

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Diffusion

• Diffusion is the passive movement of molecules
  from a higher to a lower concentration until
  equilibrium is reached.
• Gases move through plasma membranes by diffusion.

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Process of diffusion
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Gas exchange in lungs by diffusion
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Osmosis

• The diffusion of water across a differentially
  permeable membrane due to concentration
  differences is called osmosis.
• Diffusion always occurs from higher to lower
  concentration.
• Water enters cells due to osmotic pressure within
  cells.

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Osmosis

• Is net diffusion of H20 across a selectively
  permeable membrane
• H20 diffuses down its concentration gradient
• H20 is less concentrated where there are more
  solutes
• Solutes have to be osmotically active
• i.e. cannot freely move across membrane

Fig 6.5
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Osmosis continued
Fig 6.6

• H20 diffuses down its concentration gradient
  until its concentration is equal on both sides of
  membrane
• Some cells have water channels (aquaporins) to
  facilitate osmosis

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Osmotic Pressure

• Is force that would have to be exerted to stop
  osmosis
• Indicates how strongly H20 wants to diffuse
• Is proportional to solute concentration

Fig 6.7
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Osmosis in cells

• A solution contains a solute (solid) and a
  solvent (liquid).
• Cells are normally isotonic to their
  surroundings, and the solute concentration is the
  same inside and out of the cell.
• Iso means the same as, and tonocity refers to
  the strength of the solution.

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Osmosis in plant and animal cells
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• Hypotonic solutions cause cells to swell and
  possibly burst.
• Hypo means less than.
• Animal cells undergo lysis in hypotonic solution.
• Increased turgor pressure occurs in plant cells
  in hypotonic solutions.
• Plant cells do not burst because they have a cell
  wall.

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• Hypertonic solutions cause cells to lose water.
• Hyper means more than hypertonic solutions
  contain more solute.
• Animal cells undergo crenation (shrivel) in
  hypertonic solutions.
• Plant cells undergo plasmolysis, the shrinking of
  the cytoplasm.

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Effects of tonicity on RBCs
Fig 6.11
crenated
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Transport by Carrier Proteins

• Some biologically useful molecules pass through
  the plasma membrane because of channel proteins
  and carrier proteins that span the membrane.
• Carrier proteins are specific and combine with
  only a certain type of molecule.
• Facilitated transport and active transport both
  require carrier proteins.

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Facilitated transport

• During facilitated transport, substances pass
  through a carrier protein following their
  concentration gradients.
• Facilitated transport does not require energy.
• The carrier protein for glucose has two
  conformations and switches back and forth between
  the two, carrying glucose across the membrane.

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Facilitated diffusion
Fig 6.14
Fig 6.15
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Active transport

• During active transport, ions or molecules are
  moved across the membrane against the
  concentration gradient from an area of lower to
  higher concentration.
• Energy in the form of ATP is required for the
  carrier protein to combine with the transported
  molecule.

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Active transport
Fig 6.16
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• Carrier proteins involved in active transport are
  called pumps.
• The sodium-potassium pump is active in all animal
  cells, and moves sodium ions to the outside of
  the cell and potassium ions to the inside.
• The sodium-potassium pump carrier protein exists
  in two conformations one that moves sodium to
  the inside, and the other that moves potassium
  out of the cell.

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The sodium-potassium pump
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Secondary Active Transport

• Uses energy from downhill transport of Na to
  drive uphill movement of another molecule
• Also called coupled transport
• ATP required to maintain Na gradient

Fig 6.18
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Secondary Active Transport continued

• Cotransport (symport) is secondary transport in
  same direction as Na
• Countertransport (antiport) moves molecule in
  opposite direction of Na

Fig 6.18
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Transport Across Epithelial Membranes

• Absorption is transport of digestion products
  across intestinal epithelium into blood
• Reabsorption transports compounds out of urinary
  filtrate back into blood

Fig 6.19
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Transport Across Epithelial Membranes continued

• Transcellular transport moves material from 1
  side to other of epithelial cells
• Paracellular transport moves material through
  tiny spaces between epithelial cells

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Bulk Transport

• Is way cells move large molecules particles
  across plasma membrane
• Occurs by endocytosis exocytosis (Ch 3)

Fig 6.21
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Exocytosis and Endocytosis

• During exocytosis, vesicles fuse with the plasma
  membrane for secretion.
• Some cells are specialized to produce and release
  specific molecules.
• Examples include release of digestive enzymes
  from cells of the pancreas, or secretion of the
  hormone insulin in response to rising blood
  glucose levels.

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Exocytosis
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Endocytosis

• During endocytosis, cells take in substances by
  invaginating a portion of the plasma membrane,
  and forming a vesicle around the substance.
• Endocytosis occurs as
• Phagocytosis large particles
• Pinocytosis small particles
• Receptor-mediated endocytosis specific
  particles

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Phagocytosis
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Pinocytosis
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Receptor-mediated endocytosis
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Membrane Potential

• Is difference in charge across membrane
• Results in part from presence of large anions
  being trapped inside cell
• Diffusable cations such as K are attracted into
  cell by anions
• Na is not permeable is pumped out

Fig 6.22
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Equilibrium Potential

• Describes voltage across cell membrane if only 1
  ion could diffuse

• If membrane permeable only to K, it would
  diffuse until reaches its equilibrium potential
  (Ek)
• K is attracted inside by trapped anions but also
  driven out by its gradient
• At K equilibrium, electrical diffusion forces
  are opposite
• Inside of cell has a negative charge of about
  -90mV

Fig 6.23
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Nernst Equation (Ex)

• Gives membrane voltage needed to counteract
  concentration forces acting on an ion
• Value of Ex depends on ratio of ion inside
  outside cell membrane
• Ex 61 log Xout z valence of ion X
• z Xin

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Nernst Equation (Ex) continued

• Ex 61 log Xout
• z Xin
• For concentrations shown at right
• Calculate EK
• Calculate ENa

Fig 6.24
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Nernst Equation (Ex) continued

• EK 61 log 5 1
  150
• -90mV
• ENa 61 log 145 1
  12
• 60mV

Fig 6.24
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Resting Membrane Potential (RMP)

• Is membrane voltage of cell in unstimulated state
• RMP of most cells is -65 to 85 mV
• RMP depends on concentrations of ions inside
  out
• on permeability of each ion
• Affected most by K because it is most permeable

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Resting Membrane Potential (RMP) continued

• Some Na diffuses in so RMP is less negative than
  EK

Fig 6.25
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Role of Na/K Pumps in RMP

• Because 3 Na are pumped out for every 2 K taken
  in, pump is electrogenic
• It adds about -3mV to RMP

Fig 6.26
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Cell Signaling

• Is how cells communicate with each other
• Some use gap junctions thru which signals pass
  directly from 1 cell to next

Fig 7.20
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Cell Signaling continued

• In paracrine signaling, cells secrete regulatory
  molecules that diffuse to nearby target cells
• In synaptic signaling, 1 neuron sends messages to
  another cell via synapses
• In endocrine signaling, cells secrete chemical
  regulators that move thru blood stream to distant
  target cells
• To respond to a chemical signal, a target cell
  must have a receptor protein for it