Chapter 11: lipids and membranes

general characteristics of lipids:

- Lipids can be defined as substances which are soluble in nonpolar organic solvents such as ether, chloroform, acetone and sparingly soluble in water.
- They differ widely both in their structure and functions.
- They are structural components of cell membranes, serve as rich energy source, and also serve as important component of cells such as chemical signals, vitamins or pigments.
- They also serve as protective and water proof outermost
layer in many cells.

classes of lipids: fatty acids and their derivatives, triacyl glycerols, wax esters, phospholipids, sphingolipids, and, isoprenoids. 

phospholipids:  Phosphoglycerides and Sphingomyelin

sphingolipids: Molecules other than sphingomyelin
that contain the amino alcohol sphingosine

isoprenoids: Molecules that consist of multiple copies of a branched five-carbon hydrocarbon unit called isoprene.

fatty acids and their derivatives structure and reactivity: They are structurally monocarboxylic acids with hydrocarbon chains of variable lengths. They react with alcohol to form esters.

fatty acids and their derivatives carbons: Most naturally occurring fatty acids have an even number of carbon atoms that form unbranched chain, although some species do contain unusually branched and ring-containing chains. 

saturated fatty acids: contain single bond between their carbons.

unsaturated fatty acids: contain one or more double bonds between their carbons.

double bond containing fatty acids: are rigid and they can be found in
cis (identical groups on one side of the double bond) or trans isomeric forms.
Naturally occurring fatty acids are usually cis.

unsaturated fatty acid characteristics: o not pack very well due to the presence of double bonds as compare to saturated fatty acids. Therefore the melting point of unsaturated fatty acids is much lower than saturated fatty acids.

plants and bacteria synthesizing fatty acids: synthesize their fatty acids from acetyl-CoA

mammals obtaining fatty acids: obtain most of their fatty acids from dietary sources. Mammals do synthesize some saturated and monounsaturated fatty
acids (non essential fatty acids). They can also modify dietary fatty acids by
adding two carbon units or introducing some double bonds.

triacyglycerols: the esters of glycerol with three fatty acid molecules

fatty acids in triacyglycerols: can be saturated, unsaturated, or a mixture of both

carboxyl group of fatty acids (in triacyglycerols): used in covalent bond formation, there is no charge on these molecules and are also referred to as neutral fats. 

Fat triacyglycerols: solid at room temperature (high saturated) 

oil triacyglycerols: liquid at room temperature (high unsat.) 

Fatty acids: serve as rich energy reserve. their hydrophobicity they coalesce into compact droplet within cells. in adipose tissues and occupy small volume (1/8 volume of glycogen).

energy release of fatty acids: since they are less oxidized compare to glycogen, they release much more energy on degradation (9.3 kcal/mole vs. 4.1 kcal/mole). being a poor conductor of heat provides insulation in low temperatures. 

wax esters: complex mixtures of nonpolar lipids. due to their hydrophobicity thety form protective layer on leaves, stems, and fruits of plants and the skin and fur of animals. 

example of wax esters: carnauba wax, produced by the leaves of brazilian wax palm, and beeswax. the important constituent of carnauba wax is the was ester melissyl cerotate. 

phospholipids: important components of living organisms. most importantly they are the structural components of membranes. amphipathic molecules consisting of hydrophobic and hydrophilic domains. 

hydrophobic component of phospholipids: consists largely of hydrocarbon chain of fatty acids. when suspended in water phospholipids spontaneously rearrange into ordered structures. these components are buried inside the polar group exposed to water.

hydrophilic component of phospholipids:  consists largely of polar head group which contains phosphate and other charged polar groups. 

2 types of phospholipids: phosphogylcerides and sphingolipids that contain sphingosine instead of glycerol. 

simplest phosphoglyceride: phosphatidic acid, precursor for all other phosphoglyceride molecules. 

phosphatidic acid structure: composed of glycerol-3-phosphate that is esterified with two fatty acids. classified on the basis of the alcohol which is esterified to the phosphate group. C1 of glycerol usually have saturated fatty acid while C2 has unsaturated fatty acids. 

sphingolipids: structural components of both plant and animal cells. all of them are long chain amino-alcohol. the core is ceramide, which is a fatty acid amide derivative of sphingosine. 

shingomyelin: 1-hydroxy group of ceramide is esterfied with the phosphate group of phosphorylcholine or phosphorylethanolamine. 

sphingomyelin: found in animal cell membrane but also in abundance in nerve cells facilitating rapid transmission of impulses. 

ceramides: also precursors of glycolipids. The hydroxyl group is linked with O-glycolipids are, cerebrosides, the sulfatides and gangliosides. many glycolipids serve as receptor for bacterial  cells as well as their toxins such as botulism and tetanus. 

isoprenoids: wide variety of biomolecules that contain repeating five-carbon structural units known as “isoprene” units. 

precursor of isoprenoids: isopentenyl pyrophosphate which is formed from Acetly-CoA 

isoprenoids consist of two groups of compounds: terpenes and steroids 

terpenes: huge group of molecules that are found largely in essential oils of plants used in perfumed and medicines. classified according to the numbers of isoprene residues they contain. 

steroids: derivatives of complex hydrocarbons ring system. 

monoterpenes: contain two units of isoprene. 

only example of of tetraterpenes: carotenoid, the orange color pigments found in plants. 

Natural rubber composition: polyterpene composed of 3,000 to 6,000 isoprene units. 

Mixed terpenoids: many important biomolecules consist of nonterpene groups attached to isoprenoid groups. these molecules are referred to as “________” terpenoids and important examples of such molecules are vitamin E, ubiquinone, vitamin K, and some cytokinins (plant hormones). 

steroids: complex derivatives of triterpenes. they are found in all eukaryotes and few bacteria. distinguished on the basis of placement of carbon-carbon double bonds and various substituents like hydroxyl, carbonyl, and, alkyl groups. 

cholesterol: example of a steroid, important component of cell membrane, also serves as precursor in the biosynthesis of all steroid hormones, vitamin D, and bile salts. 

Lipoproteins: protein is covalently linked to lipid groups. the lipid group may be fatty acids or prenyl groups. 

function of lipoproteins in blood plasma: they transport lipid molecules like triacylglycerols, phospholipids, and cholesterol from one organ to another.

antioxidant: lipoproteins also contain lipid soluble components, several carotenoids. 

apolipoprotein or apoproteins: protein part of lipoproteins. 

amount of protein in lipoproteins: varies from 2%-55%. 

quality used to classify lipoproteins: density 

VLDL: very low density lipoprotein 

LDL: low density lipoprotein

HDL: high density lipoproteins 

LDL function: formed from VLDL, carry cholesterol to the tissues. important in plaque formation.

HDL function: important in scavenging excessive cholesterol from cell membrane. 

plasma lipoproteins: contains a neutral lipid core composed of cholesteryl esters and/or triacylglyercols. This core is surrounded by a layer of phospholipid, cholesterol and protein. charged and polar residues on the surface of a lipoprotein enable it to dissolve in blood. 

membranes: most of the properties of living cells like movement, growth, metabolism, and reproduction depend directly or indirectly on membranes. all of these have the same general structure. according to the currently accepted fluid mosaic model. membrane is a lipid bilayer within which proteins float. proteins determine largely the biological functions of membranes. 

structure of membrane lipids: phospholipid, and amphipathic molecules, when suspended in water, they spontaneously rearrange into ordered structure. 

lipid membrane responsibilities: membrane fluidity, selective permeability, self sealing capability, and, asymmetry (each half of bilayer is different). 

membrane protein classification: classified on the basis of function, structural components, enzymes, hormone receptor, or transport proteins. also classified on the basis of their location as integral or peripheral membrane proteins. 

most important functions of biological membrane: transport of molecules and ions in and out of the cells / organelles and the binding of hormones and other molecules. 

membrane transport: vital to organisms since it provides nutrients and also takes care of waste disposal. biological transports are classified on the basis of their energy need. 

passive transport: does not require energy, can be simple or facilitated diffusion. 

active transport: this type of transport requires energy which is in most cases provided by ATP hydrolyses. 
  

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