Download Structure and Function of Membranes

Structure and Function of
Membranes
Chapter 7
What you need to know!
• Why membranes are selectively
permeable.
• The role of phospholipids, proteins, and
carbohydrates in membranes.
Plasma Membrane (PM) of
animal/plant cells and organelles
• Barrier between interior and exterior
• Maintains homeostasis
• Allow for compartmentalization
(eukaryotes)
• PM is selectively permeable for gas,
H2O, entering nutrients, exiting waste
• Blocks harmful substances,
microorganisms
• Controls ion exchange
• Made from phospholipids
Phospholipids
• Phospholipids form a bilayer in
aqueous solutions
• the heads (phosphate groups) are
polar (hydrophilic) and will form
the two outer faces
• the fatty acid tails are hydrophobic
and point toward the inside
Fluid Mosaic
Model
• Structure can be
observed with EM
• Mosaic of floating
phospholipids with
cholesterol, proteins,
glycoproteins, and
glycolipids embedded
• Held together by hydrophobic/hydrophilic
interaction and cytoskeleton attached to
desmosomes.
• Flexible and in constant motion
– Think of a soap bubble skin rather than saran wrap
• http://www.youtube.com/watch?v=Rl5EmUQd
kuI
Fluidity:
• Phospholipid molecules move around
constantly
• Fluidity regulated by different kinds of fatty
acid (FA) tails:
• More unsaturated FA, membrane stays
fluid at lower temp (winter)
• More saturated FA, membrane is more
stable at high temperatures (summer)
• Cholesterol embedded in animal
membranes, keeps FA tails from twisting
together
Membrane Molecules
•
Integral – proteins that are embedded in the
bilayer
–
–
•
contain hydrophilic ends and hydrophobic midsections to mimic the phospholipids
Transmembrane proteins span the entirety of the
bilayer (stick out on both ends)
Peripheral proteins: not embedded but
loosely attached (usually to integral proteins)
Transmembrane Proteins
1.
2.
3.
4.
5.
6.
Transport
Enzymatic
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular
matrix (ECM)
Membrane Molecules
•
•
Peripheral Proteins loosely connected on only one
side of the membrane
Membrane bound pigments in plants
•
•
Chlorophylls
Carotenoids
Both sides of the membrane are not identical:
• Cytoplasmic side
• Extracellular side
Animals: Extracellular Matrix (ECM)
• Glycoproteins = carbohydrates attached to proteins
for self recognition/immune system
• Collagen fibers for connectivity
Plants: Cell wall (1st, middle lamella, 2nd)
Intrercellular Connections:
Animals:
• Tight Junctions: allowing
movement of material
across cell layer/preventing
movement of material
between cells: digestive
system, epithelial cells
• Desmosomes: tight
connection of adjacent
cells under high physical
stress (muscle, cartilage),
attached to cytoskeleton
• Gap-Junctions:
intercellular ion and small
molecules transfer
Intrercellular Connections:
Plants:
• Plasmodesma: channel between plant
cells where ER goes through
(desmotubule)