Download 3.3 Cell Membrane

CHAPTER 3 CELLS
3.3 Cell Membrane
KEY CONCEPT The cell membrane is a barrier that
separates a cell from the external environment.
The cell membrane has two major
functions
• Aka Plasma
Membrane
• Forms a boundary
between inside and
outside of the cell
• Controls import and
export of materials
cell membrane
inside cell
outside cell
Cell membranes are made of
Phospholipids
• Phospholipids
• Molecule with a
phosphate head region
and two fatty acid tails
• Head is polar = charge
• (Hydrophilic)
• Tails are non-polar = no
charge
• (Hydrophobic)
Phospholipid Bilayer
• Properties of the polar
heads and nonpolar tails
cause the phospholipids
to arrange themselves in
layers
• Like a sandwich
• Heads like bread
• Interact with watery
environment inside and
outside the cell
• Tails
• Like filling protected from water
Other Membrane Molecules
• Cholesterol
• Strengthens the cell
membrane
• Protein Channels
• Allow specific molecules into
the cell
• Cytoskeletal Protein
• Help cell maintain its shape
• Carbohydrates
(Glycoproteins)
• Carb chain serves as an ID
badge for the cell
Fluid Mosaic Model
• Describes the arrangement of molecules that make up a
cell membrane
Fluid Mosaic Model
• Takes it’s name from 2
characteristics
• Fluid = Membrane is
flexible not ridged
• Allows phospholipids and
proteins to move side to
side and slide past each
other
• Proteins cannot flip end to
end though
• Mosaic = because of
the variety of scattered
molecules
Selectively Permeable
• Some molecules can
cross over, while others
cannot
• Helps to maintain
homeostasis –
equilibrium – by
monitoring what enters
and exits cells. For
Example:
• Small molecules and larger
hydrophobic molecules
move through easily.
Examples: O2, CO2, H2O
Selectively Permeable
• Small non-polar
molecule
• Pass right through the
membrane
• Small polar molecules
• Transported by channel
proteins
• Large molecules (polar
and non-polar)
• Moved in vesicles by
Endocytosis/Exocytosis
Chemical signals are transmitted
across the cell membrane
• Receptors bind with
ligands and change
shape.
• Two types
of receptors
Intracellular Receptor
• Found totally within
cell
• Ligand moves across
the membrane, binds
with receptor inside
cell, and receptorligand complex cause
something to happen
• Ex. Many Hormones
work this way
Membrane receptor
• Ligand cannot enter cell
• Ligand bind to external
receptor
• Receptor changes
shape
• Shape change causes
molecules in cell to
respond