The Plasma Membrane

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The Plasma Membrane
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Plasma Membrane

• The outer layer of a cell is semi-permeable.
• Semi-permeable means that only certain molecules
  can pass through it.

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Plasma Membrane

• Has two layers of lipid molecules -Phospholipid
  bilayer
• Polar heads point to the outside (Phosphate head)
• Non-polar (straight) tails form the inside of the
  membrane

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Fluid Mosaic Membrane

• Its called a fluid mosaic membrane because it is
  a mosaic of different kinds of molecules. These
  include
• lipids
• protein
• carbohydrate chains

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The 3 Ways a Cell regulates what comes into a
cell

• Simple Diffusion some materials move into and
  out of the cell without using energy.
• Carrier Facilitated Diffusion movement into the
  cell through a protein in the plasma membrane
• Active Transport use energy in the form of ATP
  to move other molecules into and out of the cell

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Comparison of Active Transport and Passive
Transport (continued)

• Most solutes can cross the membrane only if there
  is a carrier protein or a channel protein to
  transfer them.
• Passive transport, in the same direction as a
  concentration gradient, occurs spontaneously,
  whereas transport against a concentration
  gradient (active transport) requires an input of
  energy. Only carrier proteins can carry out
  active transport, but both carrier proteins and
  channel proteins can carry out passive transport

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Diffusion

• In a solution, molecules move constantly.
• They collide with each other and tend to spread
  out randomly.
• The molecules tend to move from an area where
  they are more concentrated to an area where they
  are less concentrated, a process known as
  diffusion.

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Diffusion

• Diffusion "goes from the higher concentration
  side to the lower concentration side".
• There is no energy input to cause diffusion
• It simply occurs due to the random motion of the
  substance moving across the plasma membrane.

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Diffusion

• Even after the solution has reached equilibrium,
  individual molecules continue to move rapidly
  across the membrane in both directions.

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Osmosis

• Diffusion of water through a semi-permeable
  membrane.
• Not all substances can cross cell membranes.
• If something can cross the cell membrane then the
  membrane is permeable.
• The cell membrane is described as being
  selectively permeable.
• Cell membranes are selectively permeable, which
  means that some substances can pass across them
  and others cannot.

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Osmosis

• Osmosis the diffusion of water through a
  selectively permeable membrane.

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Carrier Facilitated Diffusion

• Facilitated Diffusion movement of specific
  molecules across cell membranes through protein
  channels

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Active Transport

• Active Transport - when material moves from an
  area of lower concentration to an area of greater
  concentration
• Active transport requires the cell to spend
  energy, usually in the form of ATP.

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Review Questions

1. The ____ of a phospholipids is polar, and the
   long _______ are non-polar.
2. What does the term selectively permeable mean
   in regards to a cell membrane.
3. What are the three major ways of taking in or of
   regulating the passage of materials into and out
   of the cell?
4. True or False, Osmosis of water is a form of
   diffusion.

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Warm-Up Label the diagram with as much detail
as possible.

• Phospholipid
• Phosphate Head
• Hydrophilic
• Fatty Acid Tails
• Hydrophobic