Cells and cell organelles are separated from the rest of the world by a membrane

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• Cells and cell organelles are separated from the
  rest of the world by a membrane
• A membrane controls what goes in and out of the
  cell
• Like an animals skin
• http//www.youtube.com/watch?vowEgqrq51zY

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Membrane structure

• Cell membranes are composed of
• Phospholipids
• Transmembrane proteins (imbedded inside the lipid
  layer)
• External proteins (attached to transmembrane
  proteins)

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Diffusion

• Certain molecules can diffuse freely in or out of
  the cell
• Other molecules cannot get through the
  phospholipid layer
• Main determinants of diffusion
• Size ( small molecules diffuse better than big
  ones)
• Solubility in lipids (only molecules that are
  soluble in lipids can diffuse through the
  membrane
• Charge (molecules with a charge cannot diffuse
  through the membrane_

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Direction of diffusion

• Without any force or energy, molecules
  automatically diffuse from high concentration to
  low concentration
• Diffusion occurs down a concentration gradient
• Concentration gradient different concentrations
  of an object in space

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Gradient examples

• A color gradient
• A light gradient
• A molecular gradient

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• Movement of molecules DOWN a concentration
  gradient requires no energy
• Molecules move from high to low concentration
• Like a ball rolling downhill - natural
• Movement of molecules UP a concentration gradient
  requires energy
• Moving molecules from low to high concentration
• Pushing a ball uphill unnatural, requires extra
  force

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Going Down the gradient

• Some molecules move freely across the membrane
  down the gradient (O2, H2O, N2)
• Most molecules cant get through the lipid
  bilayer of the membrane (proteins, sugars, ions)
• Passive transport aka facilitated diffusion
  Proteins within the membrane act as channels for
  transporting molecules down the concentration
  gradient
• http//www.youtube.com/watch?vs0p1ztrbXPYNR1

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Going UP the gradient

• Sometimes cells need to maintain an uneven
  concentration of a molecule across the membrane
• But moving molecules UP their concentration
  gradient requires ENERGY

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Active transport

• Some proteins in the membrane act like molecule
  pumps
• Use ATP for energy
• Move molecules against concentration gradient (in
  or out of the cell)
• http//www.youtube.com/watch?vSTzOiRqzzL4feature
  related

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Active Transport

• Example
• Na / K pump animal cells need to have more
  sodium on the inside, and more potassium on the
  outside. This pump maintains an uneven gradient
  of each ion.
• http//www.youtube.com/watch?vLSm2jPgxcwk

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Summary

• A higher concentration of a molecule inside or
  outside of the cell creates a concentration
  gradient of that molecule across the membrane
• No energy is required for a molecule to move down
  the concentration gradient (high to low)
• Some molecules can diffuse right across the
  membrane down the gradient
• Others require protein channels to diffuse into
  the cell (passive transport aka facilitated
  diffusion)
• Moving molecules up the concentration gradient
  (low to high) requires energy, and must be done
  with special protein pumps in the membrane
• ATP provides the energy for protein pumps to move
  molecules from low to high concentrations across
  the membrane