What molecules cross the cell membrane? How do these molecules cross the cell membrane?

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What molecules cross the cell membrane?How do
these molecules cross the cell membrane?

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The phospholipd bilayer is permeable to gases,
water and lipids.
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Gases, water and lipids can pass through the
cell mb.These diffuse through the mb.

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Characteristics of diffusionmolecules are
always moving

• Down a concentration gradient
• Diffusion rate depends on the steepness of the
  concentration gradient
• Kinetic energy from molecules that are moving
• Eventually the molecules distribute themselves
  evenly (equilibrium).
• Molecules diffuse short distances quickly.

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Characteristics of diffusion

• Diffusion is directly related to temperature
• Diffusion is inversely related to molecular size
• Diffusion can occur in an open system
• Diffusion can occur across a barrier if the
  molecule can cross the barrier

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Diffusion rate of lipophilic molecules across the
cell mb (phospholipid bilayer)

• Diffusion rate depends of solubility in lipid
  layer
• Diffusion rate depends on the surface area of the
  cell mb.
• Faster diffusion with thinner mb (short distance
  to cross)

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Ions diffuse through channels down their
concentration gradient.
Ca
Ca
Ca
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How do the rules of diffusion apply to movement
by carrier proteins?
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Carrier mediated transport into cells- net
movement as long as there is a concentration
gradient across the mb ? Facilitated Diffusion
Diffusion and carrier proteins
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Carrier mediated transport into cellsSpecificity

• Eg. Glucose transporters move 6 carbon sugars
  (hexoses).

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Carrier mediated transport into cellsSpecificity

• Glucose transporters will not move maltose.

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Carrier mediated transport into cellsCompetition

• 6 carbon sugars bind to the glucose carrier
• The carrier has a higher affinity for glucose
  than for fructose or galactose
• Glucose moves into the cell faster than the
  others

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Carrier mediated transport into cellsCompetition
Only galactose
Glucose and galactose
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Carrier mediated transport into cellsCompetition

• A competing molecule can block the transport

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Carrier mediated transport into cellsSaturation

• Carriers transporting a substrate reach their
  maximal rate.

Diffusion and carrier proteins
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Carrier mediated transport into cellsSaturation
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Cells need glucose as a source of ATP for energy.
How can I get my glucose?
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Carrier mediated transport into cells.