Chapter 15

Given that bacteria change the composition of their cellular membranes, which of the following would you predict to happen to the membrane composition of a bacterial culture that was shifted from growth at 37 °C to 10 °C?

-Fatty acids with more cis double bonds would be found in the membrane. -The percentage of glycerophospholipids with shorter chain fatty acids would increase. -More unsaturated glycerophospholipid fatty acids would be found in the membrane. The bacterial membrane would need to stay fluid for survival. Thus, the intermolecular interactions between the fatty acid chains of lipids in the membrane would need to be reduced to lower the melting point. More glycerophospholipids with shorter chain fatty acids, more unsaturated fatty acids, and fatty acids with more cis double bonds would all accomplish this.

If the new hot springs bacterial species were found to be deficient in certain enzymes that allowed membrane composition to be changed, would you predict that this species would be culturable in a laboratory under conditions typical for E. coli (37 °C, for example)?

-No, because the bacterial membrane is likely to be poorly suited to survival at 37 °C, potentially transitioning to a solid. If the bacteria could not change membrane composition, the significant change in environmental temperature would have a detrimental effect. It is likely that the thermophilic bacteria would not survive since their membrane is likely to be poorly suited to survival at 37 °C, potentially transitioning the bacterial membrane to a solid.

A new bacterial species is found on a thermal vent in a boiling hot springs. Which of the following do you predict would be true?

-The membrane composition would contain more saturated fatty acids than a terrestrial counterpart. To offset the high temperature and maintain membrane integrity, the melting point of bacterial lipids in such an environment would be elevated compared to terrestrial counterparts. Thus, the membrane composition would contain more saturated fatty acids than at lower temperatures.

Fatty Acid Nomenclature

1) Start from carboxyl C, then number C's 2) # after ":" is # of double bonds 3) Δ# is where double bonds are Omega (ω) - start from opposite end

Fat storage in animals serves three distinct functions:

1. Energy production: most fat in animals is oxidized for generation of ATP --> metabolic processes 2. Heat production: specialized cells (i.e. in brown fat of warm-blooded animals) oxidize triacylglycerols for heat production, rather than to make ATP 3. Insulation: in animals that live in a cold environment, layers of fat cells under the skin serve as thermal insulation. The blubber of whales is one obvious example.

Why are triacylglycerides well suited as energy storage compounds?

1. Fatty acids at higher reduction state relative to glycogen- more energy 2. Hydrophobic- don't need to be solvated when stored

3 main types of membrane lipids

1. Glycerophospholypids 2. Sphingolyipids (including some glycolipid forms) 3. Cholesterol

Lipids

Biomolecules that are soluble in organic solvents but are poorly soluble in water. Include fats, oils, vitamins, hormones, and non-protein components of membranes.

Sort the following into cis or trans fatty acids.

Carbon-carbon double bonds in fatty acids can be in either the cis or trans configuration. Cis means that the hydrogens are on the same side of the double bond, which introduces a kink in the otherwise linear structure. The trans configuration, with the hydrogens on the opposite side, is not kinked. Only the illlustration in answer C is linear, so it is the only trans fatty acid.

For humans, ω-3 is an essential fatty acid, which means it is obtained from which of the following?

Dietary sources Being essential, ω-3 fatty acids cannot be synthesized by humans. Thus, they must be obtained from dietary sources.

Which of the two figures above depicts an oil and why?

Figure B depicts an oil since there are numerous unsaturated lipids with cis double bonds.

Omega-3 fatty acids

First unsaturated bond 3 from the carbon head

How do fatty acids work in soaps?

Form micelles, agitated to break up micelles, hydrophobic reactions occur, micelles surround greasy dirt particles and reform micelle shape. Rinsed out with water.

Free fatty acids inside cells would also act like soap and disrupt biological membranes, except that fatty acids inside cells are chemically neutralized by covalent linkage to what small molecule for energy storage?

Glycerol. Triacylglycerols are the lipids used for fat storage. They are composed of three fatty acids attached to glycerol.

Why is tallow harder than butter, if it is more unsaturated?

Greatest content of long chain saturated fatty acids compared to short chain.

Kinks in chains and MP

Kinks lead to less surface area for packing and therefore less energy required for melting

What do sites of unsaturation create?

Kinky lipid chains

Longer tail length and MP

Longer tail length = higher melting temp

The effect of cholesterol on membrane fluidity at low temperatures is to:

Maintain membrane fluidity through its disruption of fatty acid packing. At low temperatures, cholesterol maintains membrane fluidity through its disruption of fatty acid packing.

How does melting point change from saturated to unsaturated fatty acids?

Melting point is highest for saturated for fatty acids, and decreases more and more each time an unsaturated bond is added.

Many people do not realize that animal fat is used to make most soap, which is interesting because we use soap to remove animal fat from our hands and clothing. Soap is made by subjecting animal lard to saponification, in which heat and a strong alkali (NaOH or KOH) release fatty acids in the form of sodium or potassium salts from triacylglycerols. When mixed with water, the fatty acids in soap function as amphipathic molecules forming what?

Micelles. Fatty acids have a single hydrocarbon chain with polar carboxylic acid at one end. The carboxylic acid is hydrophilic, whereas the fatty acyl tail is hydrophobic. Thus, the hydrophobic regions will be sequestered away from water in the interior of a micelle when fatty acids are mixed with water.

What is an unsaturated fatty acid with one carbon-carbon double bond called?

Monounsaturated Being an unsaturated fatty acid means that one or more carbon-carbon double bonds is present. Given that only one double bond is present, the fatty acid is called monounsaturated.

Van Der walls packing of fatty acids

More surface area, stronger intermolecular interactions (i.e. longer saturated tails)

Unsaturated bonds and MP

More unsaturation = lower MP

What composes a membrane?

Mostly lipids and proteins, with a bit of sugar.

What is the charge of storage lipids?

Neutral. Membrane lipids are polar.

Membrane lipids components

Polar head group and a hydrophobic chain Head has formal neg charge

The effect of cholesterol on membrane fluidity at physiological temperatures is to:

Prevent lateral movement of phospholipids and thereby decrease membrane fluidity in that region. Cholesterol has different impacts on membrane fluidity at different temperatures. At physiological temperatures, cholesterol prevents lateral movement of phospholipids and thereby decreases membrane fluidity.

What is a fatty acid that contains fully reduced methylene groups called?

Saturated If fully reduced, a fatty acid does not have any double bonds. If no double bonds are present, the fatty acid is termed saturated because it is saturated with hydrogens.

Sort the following fatty acids into the appropriate categories.

Saturation of fatty acids is determined by the presence or absence of double bonds. If no carbon-carbon double bonds are present, then the fatty acid is saturated with hydrogens. The scientific notation for fatty acids indicates double bonds after the colon. If that integer is one or greater, then the fatty acid is unsaturated.

Glycogen is a polysaccharide and has many water molecules associated with it due to its polar nature. Triacylglycerols are nonpolar. What is the relative energy storage density (kJ/g) of triacylglycerols to glycogen?

Significantly greater, 6:1 A considerable amount of water is associated with the polar carbohydrates of glycogen, reducing its energy density. Also, the carbons of triacylglycerols are more highly reduced and hence release more energy upon oxidation; triacylglycerols therefore have considerably more energy per unit volume than glycogen. The ratio of energy density is approximately 6:1.

Triaglycerols

Storage form of fatty acids. Formed by tri-esterification of glycerol with fatty acids. Typically found in fats and oils from both plants and animals.

Determine whether the melting point will increase, decrease, or remain unchanged for the given circumstances.

Stronger intermolecular forces require greater kinetic energy to cause a phase transition from solid to liquid. Therefore, anything that strengthens the intermolecular forces between fatty acids will raise the melting point, keeping the fat solid at higher temperatures. Longer saturated fatty acid chains, which are linear, increase the van der Waals forces, thereby increasing the melting point. Conversely, shorter linear fatty acid chains and those with more cis double bonds reduce the van der Waals interactions and thereby lower the temperature necessary to keep the fat in its liquid state.

Sort the following into either ω-3 or ω-6 fatty acids. Remember that numbering starts from the "omega" end for these fatty acids.

The difference between ω-3 and ω-6 fatty acids is the position of the first carbon-carbon double bond. For ω-3 fatty acids, it occurs at C-3. For ω-6 fatty acids, it occurs at C-6. For the examples shown, only B and D are ω-6 fatty acids. The others are ω-3 fatty acids.

Sort the following lipids by their major role in the cell.

Though all are related, different lipids have different properties leading to different roles in the cell. Being more energy dense than carbohydrates and chemically stable over long periods, fatty acids are utilized in energy storage (as well as other roles). Triacylglycerols, composed of three fatty acids attached to glycerol, are also utilized for energy storage. Cell membranes are composed of amphipathic lipids. In a general sense, membrane lipids have two fatty acids and a polar head group. Certain lipids also play roles in biosignaling, particularly eicosanoids and steroids.