1. Cell Biology Membrane Structure
Alberts, Bruce. Essential Cell Biology. 4th ed. New York, NY: Garland Science Pub., Print.
Copyright © Garland Science 2013

2. Cell membranes act as selective barriers
The plasma membrane separates a cell from the outside and is the only membrane in most bacterial cells. It enables the molecular composition of a cell to differ from that of the cell’s environment.
In eukaryotic cells, additional internal membranes enclose individual organelles. In both cases, the membrane prevent molecules on one side from mixing with those on the other.

3. The plasma membrane is involved in cell communication, import and export of molecules, and cell growth
1 Receptor proteins in plasma membrane act as sensors that enable the cell to receive information about changes in its environment and respond to them.
3 The flexibility of the membrane and its capacity for expansion allow cell growth and cell movement.
2 If a cell is to survive and grow, nutrient must pass inward across the plasma membrane, and waste products must pass out. The highly selective channels and pumps-protein molecules allow specific substances to be imported and others to be exported.

4. Membranes form the many different compartments in a eukaryotic cell
What are the functions of plasma membrane?
Bring nutrients in
Pump waste products out
Protect internal organelles from harmful agents in extracellular environment
Sense activities/changes in the extracellular environment
Move around to change the shape of the cell
Can anything else perform these functions?
Internal membranes around organelles
Both the nucleus and mitochondria are each enclosed by two membranes
Keep everything well organized

5. A typical membrane lipid molecule has a hydrophilic head and hydrophobic tails

6. Different types of membrane lipids are all amphipathic
Hydrophilic Heads
Each of the three types shown here has a hydrophilic head and one or two hydrophobic tails.
The hydrophilic head (blue and yellow) is serine phosphate in phosphatidylserine, an –OH group in cholesterol, and a sugar (galactose) and –OH group in galactocerebroside.

7. A hydrophilic molecule attracts water molecules
Because acetone is polar, it can form favorable interactions with water molecules, which are also polar.
δ– indicates a partial negative charge, and δ+ indicates a partial positive charge. Polar atoms are in red and blue colors.

8. A hydrophobic molecule tends to avoid water
Because the 2-methylpropane molecule is entirely hydrophobic, it cannot form favorable interactions with water, and force adjacent water molecules to reorganize into a cagelike structure around it.

9. Fat molecules are hydrophobic, whereas phospholipids are amphipathic
hydrophilic head
(A) Triacylglycerols, which are the main constituents of animal fats and plant oils, are entirely hydrophobic.
(B) Phospholipids such as phosphatidylethanolamine are amphipathic, containing both hydrophobic and hydrophilic portions.
The hydrophobic parts are shaded red, and the hydrophilic parts are shaded blue and yellow.
The third hydrophobic tail of the triacylglycerol molecule is drawn here facing upward for comparison with the phospholipid, although it is normally depicted facing down.
hydrophobic tail
Hydrophobic
Amphipathic

10. Amphipathic phospholipids form a bilayer in water
The plasma membrane and internal membranes are made of a lipid bilayer.
To separate the aqueous environment inside the cell from the aqueous environment outside the cell.
Most membrane lipids are amphipathic (hydrophobic and hydrophilic).
To create a 2-D layer for membrane components to move around.
If membrane components can’t move, cell can not function properly.

11. Phospholipid bilayers spontaneously close in on themselves to form sealed compartment
The same forces that drive the amphipathic molecules for form a bilayer make the bilayer self-sealing.
The closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable.

12. Phospholipids can move within the plane of the membrane
The fluidity of a cell membrane is important for membrane function and has to be maintained within certain limits
What influences the fluidity of the bilayer?
temperature
heat → increased fluidity
types of hydrocarbon tails (phospholipid composition)
hydrocarbon tails that do not pack closely together increase the fluidity of the membrane
short tails increase the fluidity of the membrane
unsaturated fatty acid tails also tails increase the fluidity

13. Cholesterol stiffens membranes
Cholesterol fits into the gaps between phospholipid molecules in a lipid bilayer, and thereby stiffen the bilayer, making it more rigid and less permeable.

14. Phospholipids and glycolipids are distributed asymmetrically in the plasma membrane lipid bilayer
The two faces of the lipid bilayer:
cytosolic side - faces cytosol
noncytosolic side - faces exterior of cell or interior of internal organelle

15. Flippases play a role in synthesizing the lipid bilayer
Newly synthesized phospholipid molecules are all added to the cytosolic side of the ER membrane.
Flippases then transfer some of these molecules to the opposite monolayer so that the entire bilayer expands.

16. Membranes retain their orientation even after transfer between cell compartments
Membranes are transported by a process of vesicle budding and fusing.
Here, a vesicle is budding from the ER or Golgi and fusing with the plasma membrane.
The original cytosolic surface (red) remains facing the cytosol, and the noncytosolic surface (orange) continues to face away from the cytosol and remains on the lumen side of the ER, golgi, and vesicles and the exterior side of the cell.
Glycolipids are located mainly in the plasma membrane and found only in the noncytosolic half of the bilayer with sugar groups facing the exterior of the cell.
The glycolipids acquire their sugar groups in the Golgi apparatus since the enzymes that add the sugars are confined to the inside of the Golgi apparatus.

17. Plasma membrane proteins have many functions

18. Membrane proteins associate with lipid bilayer in different ways
Transmembrane proteins can extend across the bilayer as a single α helix, as multiple α helices, or as a rolled-up β sheet (called a β barrel).
Membrane proteins anchored to cytosolic surface by an amphipathic α helix.
Others are attached to either side of the bilayer solely by a covalent attachment to a lipid molecule (red zigzag lines).
Finally, many proteins are attached to the membrane only by relatively weak, noncovalent interactions with other membrane proteins.
α helix
Amphipathic
α helix
β barrel
(A rolled-up β sheet)

19. The peptide bonds are polar and thereby hydrophilic
The peptide bonds (shaded in gray) that join adjacent amino acids together in a polypeptide chain are polar and therefore hydrophilic.
The partial charges (δ– indicates a partial negative charge and δ+ indicates a partial positive charge) allow these atoms to hydrogen bond with one another when the polypeptide folds into an α helix that spans the membrane.

20. A transmembrane hydrophilic pore can be formed by multiple α helices
Five transmembrane α helices form a water-filled channel across the lipid bilayer.
The hydrophobic amino acid side chains (green) on one side of each helix contact the hydrophobic hydrocarbon tails, while the hydrophilic side chains (red) on the opposite side of the helices form a water-filled pore.

21. Porin proteins form water-filled channels in the membrane of a bacterium
The protein consists of a 16-stranded β sheet curved around on itself to form a transmembrane water- filled channel, as shown in this 3-D structure, determined by X-ray crystallography.

22. Bacteriorhodopsin acts as proton pump during the light-activated pumping cycle
When retinal absorbs a photon of light, it changes shape, and cause the protein a series of small conformational changes.
The changes result in the transfer of one H+ moves across the bilayer along the pump channel.
In the presence of sunlight, thousands of bacteriorhodopsin pump H+ out of the cell, generating a concentration gradient of H+ across
the plasma membrane, which can be converted into ATP.
polar amino acid side chains

23. Human red blood cells have a distinctive flattened shape
The best understood cell cortex is that of human red blood cells, which has a relatively simple and regular structure.
Because a cell membrane is extremely thin and fragile, most of cell membrane are thereby strengthened and supported by a framework of proteins, attached to the membrane via transmembrane proteins.
The cell cortex is a meshwork of fibrous proteins that is attached to the cytosolic surface of the membrane.
The cell cortex determines the shape of a cell and the mechanical properties of plasma membrane.
Scanning electron micrograph of red blood cells

24. A spectrin meshwork forms the cell cortex in human red blood cells
The main component of the cell cortex is the protein spectrin, a long, thin, flexible rod and attached to the cytosolic surface of the membrane.
Spectrin dimers, together with a smaller number of actin molecules, are linked together into a netlike meshwork that is attached into the plasma membrane by the binding of at least two types of attachment proteins to two kinds of transmembrane proteins.

25. The lateral mobility of plasma membrane proteins can be restricted in several way
Proteins (green) can be tethered:
cell cortex extracellular matrix
to the cell cortex
inside the cell,
to extracellular matrix molecules outside the cell,
to proteins on the surface of another cell.
Diffusion barriers (black bars) can restrict proteins to a particular membrane domain.
surface proteins
diffusion barriers

26. A membrane protein is restricted to a particular domain of plasma membrane of a gut epithelial cell
Protein A (in the apical membrane) and Protein B (in the basal membrane) can diffuse laterally in their own domains but are prevented from entering the other domain by a specialized cell junction called a tight junction.
Apical surface - the lumen of the intestine
Basal surface - the supporting connective tissue and muscle

27. The eukaryotic cell surface is coated with sugars
Oligosaccharides (short chains of sugars)
Long polysaccharide chains
Sugars are attached to both lipids and proteins in extracellular face only
Adsorb water, give the cell a slimy surface, prevent blood cells from sticking to one another or to the walls of blood vessels
Play an important role in cell-cell recognition and adhesion.

28. The cell-surface carbohydrate on neutrophils is recognized by the cells lining the blood vessels at sites of infection
Oligosaccharides (short sugar chains) carried by glycolipids and glycoproteins on the extracellular faces of neutrophils (white blood cells).
Recognized by lectins on the endothelial cells lining the blood vessels at the site of infection.
Provide “ID” tags for cells, which are recognized by immune system, and unfortunately, also by pathogens.

29. Movie: Membrane behaves as a two dimensional fluid
SDS and Triton X-100 are two commonly used detergents in the laboratory
Sodium dodecyl sulfate (SDS) is a strong ionic detergent (that is, it has an ionized group at its hydrophilic end), and Triton X-100 is a mild nonionic detergent (that is, it has a nonionized but polar structure at its hydrophilic end).
The hydrophobic portion of each detergent is shown in blue, and the hydrophilic portion in red.
The bracketed portion of triton X is repeated about eight times.
Strong ionic detergents like SDS not only displace lipid molecules from proteins but also unfold the proteins.

30. Membrane proteins can be solubilized by a mild detergent such as Triton X-100
If you want to break up a lipid bilayer, you need a detergent.
Small, amphipathic lipid-like molecules
One hydrophobic tail
Spontaneously aggregate into clusters, or micelles, in water
Able to squeeze in between lipids and proteins in the plasma membrane and separate them and keep them in water- soluble complexes

31. Mild detergents can be used to solubilize and reconstitute functional membrane protein
Researchers often wish to study the behavior of a particular protein in isolation, in the absence of molecules that might restrain its movement or activity.
For such studies, membrane proteins can be removed from cells and reconstituted in artificial phospholipid vesicles.