BOTANY, VOLUME 1: CELL BIOLOGY. ANATOMY. MORPHOLOGY — 2007

1. MOLECULAR BASIS — THE BUILDING BLOCKS OF CELLS

1.5. Lipids

Although Lipids cannot be classified as macromolecules, we discuss them here because of their crucial role in building Cell membranes and, consequently, in cellular Structure. Alongside structural lipids, Cells also contain storage lipids, which serve as the primary form of organic carbon accumulation (for Lipid Biosynthesis, see 6.11; for Lipid METABOLISM, see 6.12) in fat-storing seeds (such as those of sunflower and flax).

1.5.1. Storage Lipids

Serving as temporary depots for energy and metabolic carbon, storage lipids primarily consist of non-polar and therefore Water-insoluble triacylglycerols (= triglycerides: Fig. 1.21). When solid at room Temperature, they are referred to as fats; when liquid, as oils. A triacylglycerol molecule consists of one glycerol molecule whose three hydroxyl groups have formed ester bonds with Fatty acids. While these fatty acids can be identical, they usually exhibit certain variations. Storage lipids contain both saturated fatty acids (palmitic and stearic acids) and Unsaturated fatty acids (oleic, linoleic, and linolenic acids). Saturated Fatty acids are alkanecarboxylic acids, whereas unsaturated ones are alkenecarboxylic acids containing one or more C=C double bonds (Fig. 1.21; for fatty acid synthesis, see section 6.11.1). Storage lipids accumulate either as oil bodies 0.5 — 2 µm in diameter (oleosomes; see Fig. 6.101) within the Cytoplasm of fat-storing cells, or as lipid droplets (plastoglobules) in Plastids. Oleosomes consist of an oil droplet enclosed by a simple lipid membrane derived from the smooth Endoplasmic reticulum (ER), the site of triacylglycerol synthesis in The Cell (see 6.12, Fig. 6.101). Specialized Proteins called oleosins are embedded in this membrane, and their function is to facilitate the mobilization of storage lipids (see 6.11.3; 6.12).

Class="center">Fig. 1.21. Structures of storage and Membrane Lipids

Storage lipids are non-polar (hydrophobic) triglycerides. Membrane lipids are amphiphilic molecules whose polar (hydrophilic) "heads" are shown in color (for the proportion of lipids in the construction of various cellular membranes, see Table 1.4).

Table 1.4. Proportion (%) of various lipid classes in the composition of cellular membranes

* Abbreviations as in Fig. 1.21 (after J. Joyard, H.W. Heidt).

Storage lipids are hydrophobic. Their molecules are excluded from the polar aqueous phase, and they disrupt water structure due to their inability to form Hydrogen Bonds (see 1.1). This accounts for the immiscibility of non-polar organic Solvents (such as benzene, gasoline, and petroleum ether) with water. The lowest-energy and most stable state of a mixture of hydrophilic and hydrophobic liquids is achieved when phase Separation minimizes the contact area between the hydrophilic (polar) and hydrophobic (non-polar) phases. This principle underlies, for example, the deposition of storage lipids in cells as spherical oleosomes. The aggregation of hydrophobic substances in a hydrophilic environment is termed the hydrophobic effect: hydrophobic molecules gather in the smallest possible space within an aqueous medium, acting as if they were strongly attracted to one another. In reality, intermolecular attraction among non-polar compounds is very weak. Nevertheless, it is sufficient for non-polar compounds to aggregate not only in the liquid state but even in the solid state. First proposed by J. Van der Waals and explained in 1930 by F. London, the existence of Intermolecular Forces (commonly known as London dispersion forces or van der Waals forces) arises from weak electrical dipoles generated by the uneven distribution of shared electrons. The protein-containing single-layer membranes of oleosomes prevent triglyceride droplets within the cell from coalescing into a single large drop via the hydrophobic effect, as a larger surface area is advantageous for the enzymatic Mobilization of reserve fat (see 6.12). The layer of

oleosin proteins also prevents oil droplets from fusing together.

The strength of chemical bonds can be quantified as the energy required to break them (bond energy): covalent bonds require more than 100 kJ mol^-1 (for instance, the exceptionally strong triple bond in the nitrogen molecule (N₂) requires 946 kJ mol^-1), hydrogen bonds require 12 to 25 kJ mol^-1, and London dispersion forces require 4 to 8 kJ mol^-1, which is just slightly above the thermal motion energy of molecules at physiological temperatures (2.5 kJ mol^-1).

1.5.2. Structural Lipids — Formation of the Membrane Lipid Bilayer

Unlike non-polar storage lipids, Introduction/36.html">Biological Membranes are constructed from amphipoliteral (synonyms: amphiphilic, amphipathic) molecules of structural lipids (see Fig. 1.21). They possess both hydrophobic and hydrophilic groups. This structural feature endows membrane lipids with The ability to form flat structures in a liquid medium. At the water surface, molecules orient themselves such that their hydrophilic groups are immersed in water, forming Hydration shells, while the non-polar radicals avoid contact with the aqueous environment. When lipid molecules are packed tightly enough, this occurs even in a monomolecular lipid layer (monolayer). However, within the aqueous phase, a bimolecular lipid layer is formed by the arrangement of two monomolecular layers such that the hydrophilic "heads" of the amphiphilic membrane lipids are hydrated and face the aqueous phase, whereas the hydrophobic "tails" point toward one another into the interior of the double layer, which is devoid of water (Fig. 1.22) and stabilized by non-polar interactions (London dispersion forces).

Fig. 1.22. Lipid monolayers and bilayers

The contact surface with the aqueous medium is formed by hydrophilic heads, while the non-polar fatty acid residues are directed mostly perpendicular to this surface. Bottom right, in the aqueous phase: a liposome; on the left: an oil droplet (oleosome) whose surface is formed by a monolayer of membrane lipids, whereas the interior is dominated by disordered neutral lipids. Liposomes can be prepared experimentally by exposing suitable mixtures of membrane lipids to ultrasound. Membrane Proteins can be incorporated into the Artificial Membranes of liposomes, a technique utilized in experiments to determine The properties of transport proteins.

Because structural lipids (unlike storage lipids) exhibit strong adhesion to water due to their hydrophilic "heads," their contact surface is maximized rather than minimized, resulting in The formation of extremely thin, flat lipid films. Although lipid molecules in bilayers are oriented uniformly, they are not as rigidly ordered as molecules in a crystal lattice. Instead, lipid layers are generally fluid, meaning that the lateral mobility of lipid molecules is very high. Conversely, the translocation of a membrane lipid molecule to the opposite side of the bilayer occurs rarely (flip-flop, with a half-exchange time between layers of several hours). Consequently, the Lipid Composition of the two leaflets of a lipid bilayer is typically asymmetric.

Membrane lipids, much like storage lipids, are represented by glycerolipids. Here, two adjacent hydroxyl groups of glycerol form ester bonds with fatty acids, while the third hydroxyl group bears a polar "HEAD." When the latter is an ester of phosphoric acid, the molecule is termed a phospholipid; when a glycoside is formed with a sugar, it is a glycolipid. In plant cells, Glycolipids are found in plastid membranes. The sugar component is typically galactose (galactolipids) or sulfoquinovose (sulfolipids) (see Fig. 1.21). The lipid composition of different cellular membranes can vary considerably, as clearly demonstrated by the data presented in Table 1.4.