2.2.1—
Lipids

Of the lipids present in plant cells the various phospholipids, glycolipids and sterols are of the greatest significance in membrane construction. The relative abundance of the components can be quite variable (Table 2.1) depending on the part of the plant analysed, the species and prevailing environmental conditions (see p. 29).

2.2.1.1—
Phospholipids

The phospholipid molecule can be separated into a charged or polar 'head' region and an uncharged or non-polar 'tail'. Such a molecule is described as amphipathic, and as we shall see later on, this property is of crucial importance in determining membrane structure (p. 36). Phospholipids are generally thought to be restricted to membranes but the extremely rapid rate at which membranes can be taken apart and re-assembled, as in cell plasmolysis and de-plasmolysis, makes it probable that there are stores of phospholipid within the cell.

Phospholipids are readily extracted from macerated plant tissues by a mixture of chloroform and methanol (2:1) and can be separated by thin layer chromatography using a variety of solvent systems (see Hitchcock & Nichols, 1971, for a review of techniques).

The commonest phospholipids in plant membranes are derivatives of phosphatidic acid (Fig. 2.2); thus, lecithin is the choline ester of phosphatidic acid. Other common derivatives are also shown in Fig. 2.2. Phosphatidic acid (PA) itself is generally said to occur only in minute quantities in membranes or not at all, indeed, its presence in an extract is often taken as an indication of the activity of phospholipase D (Mazliak, 1973). There is a report, however, in which phosphatidic acid is said to be one of the major constituents of the plasmalemma of oat (Avena sativa ) root, (Keenan et al., 1974). Unfortunately,

Figure 2.2
Structural formulae of phospholipids commonly found in plants.

detailed analyses of the plasmalemma from other plants are not available for comparison. In passing it might be noted that a great deal remains to be done, firstly in preparing pure sub-cellular fractions of the plasmalemma and of other membranes from plants and subsequently in determining their lipid composition. Table 2.2 presents some of the available information on the distribution of the different types of phospholipid. The information on the composition of the

membranes of mitochondria and chloroplasts is the most detailed and reliable since these organelles can be separated with relative ease and high purity during cell fractionation. The predominant phospholipid in a given membrane may be characteristic, e.g. phosphatidylglycerol (PG) is a major component of chloroplast membranes while it is only a minor component of the inner mitochondrial membrane where diphosphatidylglycerol (DPG) is predominant. In general extracts of shoots and roots neither of these phospholipids is as abundant as lecithin (PC) or phosphatidyl ethanolamine (PE). A small quantity of phosphatidyl inositol (PI), usually less than 10% of the total phospholipid, is found in all membranes.

In Fig. 2.2 the exact chain length of the acyl groups R₁ and R₂ which make up the hydrophobic tail, is not defined precisely. In nature it can vary considerably even in one type of phospholipid from a given tissue. The chain may be made from 12 to 22 carbon atoms and may contain up to three or, rarely, six double bonds. The chain is straight in all eukaryotic organisms and has been found to be branched only in certain bacteria (Asselineau, 1966). Variation in both the length and unsaturation (i.e. the number of double bonds) of the hydrocarbon chain influences its melting point; shorter and unsaturated chains melt at much

lower temperatures than longer and saturated ones. As an example of this consider the effect of double bonds on the melting of free fatty acids containing 18 carbon atoms; the saturated stearic acid (C_18:0 ) melts at 69ºC, the monounsaturated oleic acid (C_18:1 ) at 5°C and the double unsaturated linoleic acid (C_18:2 ) at –12ºC. Organisms which live in warm conditions and warm blooded animals are generally found to have phospholipids with an abundance of fatty acids which tend to be fully saturated (e.g. the thermophilic alga Cyanidium caldarium, see Kleinschmidt & McMahon, 1970). By contrast, organisms which are exposed to lower temperatures have either more unsaturated acids or ones with shorter average chain lengths or a combination of both of these (e.g. in Acholeplasma laidlawii, see Huang et al., 1974) to give phospholipids whose tails remain fluid. The significance of the maintenance of membrane fluidity will become apparent later (p. 41). The process of hardening plants against injury from frost or chilling is accompanied by changes in the degree of unsaturation of the membrane lipids (Wilson & Crawford, 1974).

Table 2.3 shows fatty acid analyses of individual phospholipids extracted from various sources. Bearing in mind that there is a great deal of room for manoeuvre in selecting the fatty acids to suit the prevailing environmental temperature the values for the relative abundance of fatty acids should be considered only as very general guides to the types of acid found in nature. Thus, the predominant fatty acids have even numbers of carbon atoms, the saturated acids found most frequently are palmitic (16:0) and stearic (18:0), and the principal unsaturated acids are linoleic (18:2) and the triply unsaturated linolenic (18:3). The fatty acid composition of lecithin can depend very strongly on its origin. For example, the lecithin in the outer mitochondrial membrane is much richer in palmitic acid (16:0), and perhaps is a less fluid component than in the inner mitochondrial membrane where triply unsaturated linolenic (18:3) is the most abundant fatty acid.

2.2.1.2—
Glycolipids

In several respects the glycolipids resemble phospholipids. The molecule is amphipathic, the polar group being a galactosyl derivative of a diglyceride, the non-polar part of the molecule being a pair of long, straight-chain fatty acids. Glycolipids are unusually rich in the triply unsaturated linolenic acid (C_18:3 ) which may make up more than 90% of the fatty acid (Table 2.3).

The two most abundant glycolipids are mono- and di-galactosyl diglyceride, the structural formulae of which are illustrated in Fig. 2.3. They are characteristic of photosynthetic tissues since they are the major lipid component of chloroplast lamellae, largely replacing the phospholipids. Ongun et al., (1968) showed that more than 80% of all of the glycolipid in leaf cells was present in the chloroplasts. The probable orientation of these molecules in the chloroplast lamellae is much like that described for phospholipids (see 2.2.1.1.) with the fatty acid tails inserted into the central region of the membrane with the polar

galactosyl groups at the membrane surface protruding into the stroma (Weier & Benson, 1967). Because the fatty acid is so highly unsaturated the membranes of lamellae probably remain fluid even at sub-zero temperatures—thus any photosynthetic reaction, or molecular reorientation, which depends on membrane fluidity may have a wide temperature range in which it can occur.

A sulphur-containing glycolipid is found as a minor component of most membranes. It is known as sulpholipid (Fig. 2.3) and its structure and occurrence in chloroplasts was reported by Benson (1963); the acyl groups are mainly palmitic with a preponderance of linolenic acid, thus resembling the other

Figure 2.3
Structural formulae of glycolipids commonly found in plants.

galactosyl lipids. Sulpholipid represents only 1% of the total lipid in most tissues and organelles but in chloroplasts it may be as much as 10–15% of the lipid (Ongun & Mudd, 1968).

2.2.1.3—
Sterols

A number of sterols can be extracted from plant tissues and fungi as well as from isolated membrane fractions. The conventional example of a sterol of common biological origin is cholesterol (Fig. 2.4); in practice plant cells contain relatively little of this sterol in comparison with animal cells. Such meagre quantitative data as is available show that sterols having 29 carbon atoms, e.g. b -sitosterol (Fig. 2.4), are the most abundant in higher plants, while in fungi the C-28 sterol, ergosterol (Fig. 2.4) is often dominant. All of these molecules have an extended concertina-like configuration (known as the 'chair' or 'boat'),

Figure 2.4
Structural formulae of sterols commonly found in biological membranes.

seem metabolically inert and are synthesized and turned over very slowly, especially in comparison with the other lipid components of membranes (Nes, 1974). Their function in membranes is not well understood but it is likely that they have an architectural role concerned with the maintenance of structure or order in the lipid domain. In this respect all of them probably function in the same was as cholesterol (see p. 38) because Butler et al., (1970) found that the structural order of bilayer membranes synthesized from lipids of ox brain tissue was stabilized equally well by cholesterol, b -sitosterol of plant origin and ergosterol. In the plasmalemma of the animal and plant cell there is a much higher proportion of sterols and sterol esters relative to phospholipid than in other

membranes (Table 2.4). It should be noted, however, that the membranes of intracellular organelles contain much more protein than do plasmamembranes (see p. 35). To some extent this protein, much of ₂ which is bonded hydrophobically to the lipid, may function in a way similar to sterol in maintaining the structural order of the membrane interior.