Lipid is a general term for substances that are non-polar and insoluble in water. Lipids include a range of compounds differing in chemical structure. Because of their low affinity for water, lipid molecules aggregate to remove themselves from an aqueous environment. However, many lipids present in organisms have intramolecular polar groups. In humans, a high amount of lipids is present in adipose tissues and the brain. The high amount of lipids in the brain is a result of abundance of cell membranes because all membranes are composed of lipids and proteins.
Prokaryotic and eukaryotic cells are separated from the external environment by a cell membrane that is composed of lipid molecules and situated between the intracellular and extracellular environments (membrane structure is covered in see Selection of 1 Chapter 11). In simple words, a cell is a vesicle covered by a lipid membrane. In eukaryotic cells, intracytoplasmic organelles are separated by membranes.
Although the main lipids contained in organisms are broadly classified according to their chemical structure as glycerolipids, sphingolipids, and steroids, we begin by describing a component of lipids called fatty acid.
Fatty acids are hydrocarbon chains with a carboxyl group at one end. Because the carboxyl group becomes a polar group by releasing a hydrogen ion in water and assumes a negative charge, fatty acids with short hydrocarbon chains (e.g., formic acid and acetic acid) are well soluble in water. Miscibility and solubility decrease and liposolubility increases with an increase in the length of the hydrocarbon chain. Fatty acids that comprise organisms generally have linear chains, with branched fatty acids being extremely rare. A carbon number of 16–18 is common. Carbon numbers are generally restricted to even numbers because fatty acids are synthesized from two-carbon compounds as starting materials and elongated by simultaneous addition of two carbons.
Saturated fatty acids do not have double bonds in the hydrocarbon chain (saturated with hydrogen), while unsaturated fatty acids possess double bonds in the hydrocarbon chain (Table 6-2). Interestingly, this double bond is generally found in the cis form rather than the energetically stable trans form. Thus, hydrocarbon chains with unsaturated bonds bend at the double bond. Saturated hydrocarbon chains easily assume a crystal structure when aggregated, while unsaturated hydrocarbon chains make orderly structure formation difficult.
Trans Unsaturated Fatty Acids
Margarine (fat) is produced when hydrogen is added to liquid oil. For example, corn can be used as a starting ingredient to produce corn margarine. It is cheaper than butter and easy to spread on bread because its firmness can be adjusted. It is also healthier because it has more unsaturated fatty acids than butter. However, in this process, a very small proportion of cis unsaturated fatty acids is converted to trans unsaturated fatty acids, thus contributing to arteriosclerosis and ischemic heart diseases. Harmful substances being incorporated into the cell membrane can have ill effects. However, even if we can qualitatively speak of some danger, the degree of risk to a normal life needs to be more quantitatively and comprehensively evaluated.
Glycerol contains three hydroxyl groups and produces esterified glycerolipids on binding to fatty acids by ester bonds. These are broadly categorized into fats, phospholipids, and glycolipids.
Fats (neutral fats) comprise reserve lipids, including adipocytes in animal adipose tissue and the endosperm and cotyledon in plant seeds. They play a key role in energy storage. Reserve fats do not form membranes. One, two, or three fatty acids can be joined to the three hydroxyl groups of glycerol, with majority of glycerol within the body being bound to three. This ester is termed triacylglycerol (Fig. 6-4A) and is the main constituent of fats. While animal fats are solid at room temperature, plant fats are liquid. This is because the former contains numerous saturated fatty acids, while the latter contains numerous unsaturated fatty acids.
A glycerophospholipid is a lipid formed when two fatty acids bind to glycerol and a range of compounds bind to the remaining hydroxyl group through phosphate. Glycerophospholipids are major phospholipids in nature and the main component of biomembranes (Fig. 6-4B). They are often simply called phospholipids. The fatty acid bound to the hydroxyl group at position 2 of the phospholipid glycerol is usually an unsaturated fatty acid. Unsaturated fatty acids containing phospholipids are important in maintaining the balance between appropriate solidity and fluidity within the membrane structure. They are often depicted schematically as shown in Fig. 6-4C because of the contrast between the high polarity of phosphate and the bound molecule preceding it and the non-polarity of the two fatty acid chains (hydrocarbon chains). Phospholipids form a lipid bilayer (Chapter 11) with the polar groups on the outside and the non-polar groups on the inside. This bilayer is the main component of the cell membrane structure throughout Monera, Animalia, and Plantae kingdoms (Table 6-3). While the surface on both sides is hydrophilic and suited to the surrounding aqueous environment, ions and hydrophilic molecules cannot easily pass through the membrane because of the hydrophobic interior of the membrane. Many of the molecules making up organisms are hydrophilic, including amino acids, carbohydrates, and nucleic acids, and require transporter proteins in the cell membrane for their passage.
Although they are rare in animals, glycoglycerolipids such as monogalactosyldiacylglycerol are widespread in plants and bacteria (including archaea), comprising half of all lipids in the chloroplast membrane.
Fig. 6-4 Ester Glycerolipids
(A) Triacylglycerol (B) Glycerophospholipid (C) Membrane-forming phospholipid molecule. Rn indicates fatty acids
A fatty acid bound to a compound called sphingosine produces a ceramide. A range of molecules can bind to the hydroxyl group of ceramide to produce a sphingolipid. A sphingophospholipid (Fig. 6-5A) is produced when compounds are bound to the hydroxyl group through a phosphate, with a carbohydrate attached to the hydroxyl group termed a sphingoglycolipid (Fig. 6-5B). Because lipids containing carbohydrates in kingdom Animalia are predominantly sphingoglycolipids, they are often simply called glycolipids. The carbohydrate moieties of glycolipids are hydrophilic because of the abundance of hydroxyl groups. Similar to phospholipids, glycolipids are a component of membrane lipids (Table 6-3) because of their carbohydrate chains having a high affinity for water and their two long hydrocarbon chains. The carbohydrate chains are almost completely present in the exterior of the membrane facing the exterior of the cell, with almost none facing the interior of the cell. Glycolipid distribution is asymmetrical within the lipid bilayer of the cell membrane. It is not necessary to memorize the structure in Fig. 6-5, but you should understand its complexity.
Fig. 6-5 Sphingolipids
(A) Sphingomyelin is a representative sphingophospholipid. (B) GM3, which is present in all vertebrate cells and has the simplest ganglioside structure, and GQ1b, which is a ganglioside abundantly present in nerve cells and contains four sialic acids, are shown as representative sphingoglycolipids. The substructure is schematically shown according to the method generally used for monosaccharides (▲: glucose, ●: galactose, ♦: sialic acid, ■: N-acetylglucosamine).
Archaea are considered to be in the same form as that during the emergence of life. They include several organisms that can survive in various harsh environments, including high temperature, acidity, alkalinity, and salt concentration. Archaeal cell membranes comprise branching isoprenoids instead of linear fatty acid chains and ether bonding instead of ester bonding between glycerols. Characteristic of this cell is the inclusion of ether glycerolipids (Column Fig. 6-1). With four ether bonds, tetraether lipids form lipid monolayer cell membranes (as opposed to lipid bilayer) with a high degree of heat resistance. Archaea living in slightly cooler locales contain diether lipids.
Steroids are compounds with a distinctive structure consisting of three cyclohexane rings attached to one cyclopentane ring (Fig. 6-6A). Cholesterol (Fig. 6-6B) is a major component accounting for as much as 25% of membrane lipids in eukaryotic cells and is important for maintaining membrane fluidity. However, it is seldom part of cell membranes in prokaryotic cells. Steroid derivatives are essential components of bile acid and steroid hormones (Fig. 6-6C), including adrenocortical hormones, estrogen, and androgen.