The Plasma Membrane and Transport across it

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The Plasma Membrane and Transport across it
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Cell Membrane

• Controls what enters and leaves the cell
• Like water, nutrients and waste
• The membrane is Selectively Permeable
• Membrane selects which molecules can permeate
  (pass through) into the cell.
• Like a window screen allows what you want in
  and keeps others out.

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Structure of Plasma Membrane

• Lipid Bilayer (2 layers of phospholipids)
• Lipids with phosphate heads attached
• Head of phospholipid is polar
• Hydrophilic (points outward, likes watery
  environment)
• Fatty acids (tails) are nonpolar
• Hydrophobic (points inward, avoids water)

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Other Membrane Molecules

• Membrane Proteins - (transport)
• Allow substances/waste to move in and out of the
  cell.
• Creates a tunnel that can be closed or open.
• Carbohydrates/ Other Proteins-
• Stick out of cell surface (id card for other
  cells to see)

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Cell Membrane Structures
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How stuff gets into the cell

• 2 ways cell gets nutrients it needs and gets rid
  of waste
• Passive Transport
• Requires no energy
• Active Transport
• Requires an input of energy

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PassiveTransport

• Occurs due to concentration gradient
• Molecules naturally move from areas of higher
  concentration to lower concentration (no energy
  required)
• Diffusion natural movement of molecules
• Facilitated Diffusion through protein channels
• Osmosis diffusion of water molecules

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Diffusion

• In nature, molecules ALWAYS move from areas of
  higher concentration to areas of lower
  concentration, as is shown in this picture.
• They move to reach equilibrium
• This also happens across the cell membrane!

These odor molecules are diffusing from the
bottle (area of high concentration) to the
surrounding air (lower concentration)
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Facilitated Diffusion

• Diffusion of molecules through protein channels
  in the cell membrane
• Used for substances that the cell needs but
  cannot pass through the membrane
• Large molecules
• Charged (ions)

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Osmosis

• The diffusion of water molecules is called
  osmosis
• Just like diffusion
• Water molecules move from areas of higher conc.
  to areas of lower water conc.
• They will move until there are equal
  concentrations of water (and solute) molecules
  everywhere
• Called equilibrium
• Molecules still diffuse, but at equal rates

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Osmosis

• The movement of water molecules across a membrane
• Water moves from areas of high water conc. to
  areas of lower water conc.
• This is how cells maintain a stable internal
  environment
• Homeostasis reacting to environment

Blue water molecules, easily pass through
membrane Red large (or charged) molecules,
cant pass through membrane
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Passive Transport Video
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Isotonic Solutions - Iso equal

• Cells that are in equilibrium with the
  surrounding solution do not experience osmosis.

• These cells have the same concentration of solute
  (and water) as the surrounding solution

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Hypotonic Solutions hypo lower

• solute (solute concentration) is lower outside
  the cell

• More water outside, so water moves into the cell,
  to try to reach equilibrium
• The cell swells with the extra water.
• Animal cells may swell so much, they may burst
  (especially in pure water)
• Plant cells cell swells and membrane pushes
  against the cell wall
• Like celery in water crisp/ rigid from pressure
  on cell walls

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Hypertonic Solutions - hyper higher

• solute is higher outside of the cell.

• Less water outside the cell, more inside. Water
  moves out of the cell.
• Animal cells shrivel because of water loss.
• Plant cell membranes move away from cell wall as
  water moves out.
• Why plants wilt.

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Other Forms of Transport

• Active Transport (needs energy)
• Large particle transport
• Endocytosis cell surrounds molecule(s), which
  are then engulfed by cell
• doesnt pass through membrane
• Exocytosis - expulsion/secretion of materials.
  Opposite of endocytosis

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Active Transport (cont)

• Movement of molecules against (up) the
  concentration gradient
• From areas of lower concentration to areas of
  higher concentration
• For instance, if a cell needs a high
  concentration of something (that would normally
  diffuse OUT of cell)

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Summary!