LEQ: How does the cell membrane function as a selectively-permeable barrier?

1

• LEQ How does the cell membrane function as a
  selectively-permeable barrier?
• Reading 3.4-3.5
• Activator Word splash
• Write three sentences about cells biomolecules
  using the following terms
• Hydrophilic, hydrophobic, phospholipid, fatty
  acid, water, polar, nonpolar,
• Key terms fluid mosaic model, selective
  permeability,

2

• The lipid membrane hypothesis describes how cells
  became unique, sustained chemical systems

• Cells evolved when a membrane could enclose a
  unique chemical system
• Phospholipids

3

• Cell membranes are barriers that separate the
  inside of a cell from the external environment.
• phospholipid bilayer
• Two sided w/ heads facing toward water ?
  hydrophilic
• Fatty acid tails in middle ? hydrophobic!

4

• Two major functions
• Barrier enclosing unique chemistry
• regulates passage of chemicals

outside cell
inside cell
5

• The fluid mosaic model describes the membranes
  chemical behavior.
• assemblage of small pieces
• Membranes behave like oil at room temp (fluid).
• Biomolecules float in the membrane

6

• The cell membrane is selectively permeable.

Small nonpolar molecules can cross the membrane
while hydrophilic molecules cannot.
7
(No Transcript)
8

• Summary
• Why do phospholipids form a bilayer when they are
  dissolved in water?
• If membranes solidified why would homeostasis be
  disrupted in a cell?
• How does this relate to the characteristics of
  organisms?

9

• Phospholipids form a double layer in response to
  the presence of polar water molecules surrounding
  them. The polar heads of phospholipids interact
  with the water inside and outside the cell,
  forming transient hydrogen bonds. The nonpolar
  tails are repelled by water and interact with
  each other inside the membrane
• Membrane receptors bind to a signal molecule on
  the outside of a cell. Upon binding, the
  membrane receptor changes shape, which sends a
  message inside the cell.
• Both enzymes and receptors are proteins that bind
  to a specific ligand or substrate, change shape
  in response to binding, and cause some sort or
  action or response.
• If proteins were rigid, they would be unable to
  change shape. Therefore, they could not
  effectively transmit a message to the cells
  interior.

10

• The body would no longer receive the message from
  insulin. Cells would not take up sugar, blood
  sugar levels would rise, and death would result.

11

• Chemical signals are transmitted across the cell
  membrane.
• Signal (ligand) binds to a protein receptor
• Membrane receptors hydrophilic signals (left)
• Intracellular receptors hydrophobic signals
  (right)