Cell Biology- Chapter 11

The Lipid Bilayer

Lipid bilayers separate intracellular spaces from the outside environment

. A cell consists of spaces that can regulate exchange

. an ideal barrier would be naturally impermeable to the majority of molecules present in both the environment and the cell

• most biological molecules are hydrophilic, but a purely hydrophobic barrier would not be stable in an aqueous environment

A Cell membrane is a multi-functional structure

. not a rigid barrier, but a living structure

. a cell uses it’s membrane to:

• Evaluate and communicate with its environment

• Acquire molecules it needs and discard molecules it doesn’t

• Traverse its environment

Internal membranes compartmentalize cells

• organelles isolated by own membranes

• • Compartmentalization allows sets of

  different reactions to be contained to

  discrete regions of the cell where they

  won’t interfere with each other.

  • ER – lipid sorting and membrane-

  bound/secreted protein synthesis

  • Peroxisome – oxidation of long fatty

  acids

  • Lysosome – hydrolysis of unneeded (or

  consumed) macromolecules

  • Golgi apparatus – protein modification

  and trafficking

  • Mitochondrion – RedOx reactions

  related to energy extraction and

  metabolic intermediates

A cell membrane consists of lipid bilayers and proteins

• mostly phospholipids+ proteins

• some can function as “rafts” when they isolate particular membrane regions

Phospholipids are the main component of lipid bilayers

Different membranes are amphipathic

. form bilayers in water

. when in water, amphipathic lipids will arrange into bilayers that maximize polar group interaction with water while minimizing contact between water and hydrophobic tails

. charged and polar molecules cannot easily cross a lipid bilayer

Sphere formation limits contact between hydrophobic layer with water,

Phospholipid position within a bilayer is dynamic

. lateral diffusion

. flexion

. rotation

. flip-flop: movement of a phospholipid from one leaflet to another

• facilitated by flippase

Membrane composition influences fluidity

. fluidity is the degree of freedom of movement among membrane lipids

. the freer, the softer the membrane.

. membrane fluidity is a function of temperature and composition

• a membrane with a higher concentration of unsaturated lipids will be more fluid than one with more saturated lipids

• cholesterol is a fluidity buffer; it prevents increases in fluidity with temperature by obstructing lipid movement. It prevents decreases in fluidity at lower temperatures by interfering with stacking.

Membrane assembly begins in the ER

. membranes around the cell are the descendants of endoplasmic reticulum membrane

. phospholipid synthesis begins in the cytosol and precursor phospholipids incorporate into the cytosolic faces of the ER membrane

. after modification by ER enzymes, scramblase catalyzes random distribution of different lipid types on the luminal and cytosolic faces of the ER membrane

. after randomization, there is no distinction in lipid composition between the two faces of the ER membrane

Asymmetric distribution of membrane lipids is established at the Golgi

Membrane orientations are maintained as they are transferred between compartments

. in addition to phospholipid distribution, orientation of transmembrane proteins is also maintained.

.transmembrane proteins are synthesized on the ER membrane, and lumen-facing domains of transmembrane proteins become extracellular space-facing if they end up on the plasma membrane

.secreted proteins are contained in the lumen of the ER

Membrane lipids redistributed asymmetrically in the plasma membrane

. functionally significant

. glycolipids are restructured to the extracellular leaflet. some cells are recognized on the basis of sugars on their membrane lipids and proteins.

Plasma membrane outer leaflet composition reflects on cell health

. phosphatidylserine flips to extracellular leaflet during apoptosis

. Annexin V can bind phosphatidylserine and identifies apoptotic cells.

• propidium iodide is a DNA-binding due that stains dead cells

Membrane Proteins

membrane proteins perform multiple functions

. transporters, anchors, receptors, enzymes

Different proteins associate with membranes in different ways

. integral:

• transmembrane

• monolayer associated lipid linked

.peripheral

• protein attached

. transmembrane proteins span the entire lipid layer and have faces on either side of the membrane

. hydrophobic residues allow insertion into only one leaflet

Membrane spanning protein domains are rich in hydrophobic side chains

. during folding, hydrophobic residues are typically pushed to core of protein

. membrane spanning proteins invert this, regions of the protein that reside in the hydrophobic later of a membrane project hydrophobic residues outward

. outward positioning of R groups by alpha and beta helices limits interactions between lipid tails and the polar peptide backbone

Aqueous pore proteins create hydrophilic spans across membranes

. not all residues in membrane spanning regions hydrophobic

. circular formations of beta sheets and groups of alpha helices can project hydrophilic residues inwards to create a polar pathway through which hydrophilic molecules can pass

Mild detergents can release membrane proteins from membranes

. Amphipathic detergents can release proteins by covering hydrophobic regions

. proteins released from the membrane can be studied individually

The 3-D structures of only a few membrane proteins have been determined

. classic method is x-ray crystallography

• requires formation of multiple protein molecules into an ordered crystal for x-ray scattering experiments

• the hydrophobic regions of membrane proteins resist crystallization in their native formation

• modern proteomics methods such as cryo-electron microscopy and nuclear magnetic resonance have recently revealed the structure of more membrane proteins

. bacteriorhodopsin is a light driven proton transporter (pump) hound in halobacterium

The cell cortex is a network of proteins that support cell shape

. a cell’s shape is maintained by cytoskeleton, attaches to proteins in the plasma membrane

. erythrocytes, like many other cells, have a cell cortex where the cytoskeleton forms a network with membrane anchor proteins

. actin can form a focal complex with transmembrane and attachment proteins. each complex linked by spectrin tetramers

. some of the transmembrane proteins can adhere the cell to extracellular matrix or other cells.

Unrestricted proteins can move freely about the plasma membrane

. if they aren’t contained, membrane proteins will more everywhere

FRAP measures protein movement

Proteins show diverse patterns of movement

. unrestricted- move freely

. confined- move freely in small space

. tethered- do not move much

Individual proteins can be inserted into reconstituted bilayers

. the function of an individual membrane is subjected to modulation by other proteins within the membrane

The movement of many plasma membrane proteins is restricted

. among polarized cells, protein restriction is common

. restriction of proteins to specific regions leads to membrane domains

. restriction mechanisms

• A- tethering to intracellular proteins

• B- tethering to extracellular proteins

• C- Interactions with proteins on an adjacent cell

• D- Diffusion barriers at tight junctions that allow free movement in a limited membrane domain (example, claudin and occlaudin proteins)

Epithelial cells exhibit polarity with proteins restricted to specific regions

. form sheets attached to basal lamina

. tight junctions form diffusion barriers that separate polarized cells into apical and basolateal domains

. diffusion barriers on intestinal epithelial cells allow selective permeability

Eukaryotic cells are coated with sugars

. multiple molecules can be substrates for glycosylation

. on the plasma membrane, lipids, transmembrane proteins, and peripheral proteins are most commonly glycosylated

. proteoglycans are a special type of glycoproteins that have many long linear chains of repeating disaccharides called glycosaminoglycans (GAGs) attached to them

Cells can recognize each other through binding of specifc sugars

. The glycocalyx can protect cells from physical harm, but also plays a role in recognition

. affected tissues have endothelial cells of the serving capillaries that respond to local inflammatory signals.

• increase their expression of surface lectins

.Lectins in capillary beds of inflamed tissues attach to sugars on passing cells

additional concepts

. purified proteins can be inserted into artificial bilayers to investigate protein function

. transport proteins are more easily studied in liposomes