Transport of Substances Across a Cell Membrane

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Transport of Substances Across a Cell Membrane
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Diffusion

• 2nd Law of Thermodynamics governs biological
  systems
• universe tends towards disorder (entropy)

• Diffusion
• movement from high ? low concentration

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Diffusion

• Move from HIGH to LOW concentration
• passive transport
• no energy needed

movement of water
diffusion
osmosis
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Diffusion across cell membrane

• Cell membrane is the boundary between inside
  outside
• separates cell from its environment

NO!
Can it be an impenetrable boundary?
OUT waste ammonia salts CO2 H2O products
IN food carbohydrates sugars, proteins amino
acids lipids salts, O2, H2O
OUT
IN
cell needs materials in products or waste out
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Diffusion through phospholipid bilayer

• What molecules can get through directly?
• fats other lipids

• What molecules can NOT get through directly?
• polar molecules
• H2O
• ions
• salts, ammonia
• large molecules
• starches, proteins

lipid
salt
NH3
aa
H2O
sugar
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• Small nonpolar molecules such as fats, O2 and CO2
• Diffuse easily across the phospholipid bilayer of
  a membrane (nonpolar molecules)

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Osmosis is diffusion of water

• Water is very important to life, so we talk
  about water separately
• Diffusion of water from high concentration of
  water to low concentration of water
• across a semi-permeable membrane

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Concentration of water

• Direction of osmosis is determined by comparing
  total solute concentrations
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water

water
net movement of water
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Managing water balance

• Cell survival depends on balancing water uptake
  loss

freshwater
balanced
saltwater
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Managing water balance

• Isotonic
• animal cell immersed in mild salt solution
• example blood cells in blood plasma
• problem none
• no net movement of water
• flows across membrane equally, in both directions
• volume of cell is stable

balanced
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Managing water balance

• Hypotonic
• a cell in fresh water
• example Paramecium
• problem gains water, swells can burst
• water continually enters Paramecium cell
• solution contractile vacuole
• pumps water out of cell
• ATP
• plant cells
• turgid

ATP
freshwater
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Water regulation

• Contractile vacuole in Paramecium

ATP
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Managing water balance

• Hypertonic
• a cell in salt water
• example shellfish
• problem lose water die
• solution take up water or pump out salt
• plant cells
• plasmolysis wilt

saltwater
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Osmosis
.05 M
.03 M
Cell (compared to beaker) ? hypertonic or
hypotonic Beaker (compared to cell) ? hypertonic
or hypotonic Which way does the water flow? ? in
or out of cell
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• Transport proteins may facilitate diffusion
  across membranes
• Many kinds of molecules do not diffuse freely
  across membranes (size, charge, polarity)
• For these molecules, transport proteins
• Provide passage across membranes through a
  process called facilitated diffusion

Figure 5.15
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Channels through cell membrane

• Membrane becomes semi-permeable with protein
  channels
• specific channels allow specific material across
  cell membrane

inside cell
sugar
aa
H2O
salt
outside cell
NH3
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Facilitated Diffusion

• Diffusion through protein channels
• channels move specific molecules across cell
  membrane
• no energy needed

facilitated with help
open channel fast transport
The Bouncer
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Facilitated Diffusion

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Ion Channels

• allow specific ions to pass through the protein
  channel.
• regulated by the cell and are either open or
  closed to control the passage of substances into
  the cell

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Carrier Proteins

• bind to specific molecules, change shape and then
  deposit the molecules across the membrane.
• Once the transaction is complete the proteins
  return to their original position.

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Active Transport

• Cells may need to move molecules against
  concentration gradient
• shape change transports solute from one side of
  membrane to other
• protein pump
• costs energy ATP

conformational change
ATP
The Doorman
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Active transport

• Many models mechanisms

ATP
ATP
symport
antiport
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Sodium Potassium Pump

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• Cells expend energy for active transport
• Transport proteins can move solutes against a
  concentration gradient
• Through active transport, which requires ATP

Figure 5.18
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Getting through cell membrane

• Passive Transport
• Simple diffusion
• diffusion of nonpolar, hydrophobic molecules
• lipids
• high ? low concentration gradient
• Facilitated transport
• diffusion of polar, hydrophilic molecules
• through a protein channel
• high ? low concentration gradient
• Active transport
• diffusion against concentration gradient
• low ? high
• uses a protein pump
• requires ATP

ATP
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Transport summary
simplediffusion
facilitateddiffusion
ATP
activetransport
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How about large molecules?

• Moving large molecules into out of cell
• through vesicles vacuoles
• endocytosis
• phagocytosis cellular eating
• pinocytosis cellular drinking
• exocytosis

exocytosis
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• Exocytosis and endocytosis transport large
  molecules
• To move large molecules or particles through a
  membrane
• A vesicle may fuse with the membrane and expel
  its contents (exocytosis)

Insulin, Crying
Figure 5.19A
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• Membranes may fold inward
• Enclosing material from the outside (endocytosis)

Figure 5.19B
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Endocytosis
fuse with lysosome for digestion
phagocytosis
non-specificprocess
pinocytosis
triggered bymolecular signal
receptor-mediated endocytosis