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=Rl5EmUQdkuI

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 mid-

sections 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. Transport

2. Enzymatic

3. Signal transduction

4. Cell-cell recognition

5. Intercellular joining

6. 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)