8.4: Phospholipids
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Phospholipids are similar in structure to triglycerides. They differ from triglycerides only in that a phosphorus-containing substance replaces one of the three fatty acids (see Fig. 8-8). There are several kinds of phospholipids— the phosphorus-containing portion can vary, as can the kinds of fatty acids.
Lipid is the scientific name for fat. Hence the name phospholipid (phosphorus-containing fat).
Lecithin is the most common phospholipid in our body and in food. Its phosphorus-containing portion contains a substance called choline. Lecithin is a fairly familiar name on food ingredient lists, and provides a good example for a discussion of phospholipids.
Lecithin is used as an emulsifier in food preparation. An emulsifier suspends small particles of fat in a watery fluid. In effect, it allows fat and water to “mix.” It does this by acting as a bridge between fat and water—the fat-soluble part of lecithin (the fatty acids) dissolves in fat, and the water-soluble part (the phosphorus-containing part) dissolves in water.
Classic mayonnaise is made of salad oil, egg yolk, and a small amount of lemon juice or vinegar. The lecithin in egg yolk is responsible for the suspension of oil in mayonnaise. In making mayonnaise, the egg yolk and lemon juice are mixed vigorously while the oil is added slowly. The oil is thus finely divided and kept suspended in this emulsion by the egg lecithin. Hollandaise sauce and béarnaise sauce are made in a similar fashion, except butter is used instead of salad oil.
In the body, phospholipids like lecithin make up the membranes of cells because of the same chemical properties that make them useful as emulsifiers. If lecithin is put into water, the choline part dissolves in the water, whereas the fat-soluble part repulses water and attracts the fat-soluble parts of other lecithin molecules. In doing so, the lecithins automatically arrange into spheres, much like globes filled with and surrounded by water (see Fig. 8-9).
Soy lecithin is in soybeans, egg lecithin in eggs, etc. Lecithins vary in their fatty acids. It’s the lecithin in egg yolk that keeps the fat suspended in mayonnaise and hollandaise sauce.
Since the fluid of blood and tissues is water, cell membranes form “naturally” from phospholipids. It’s an ideal cell membrane in that it has a center layer of fat (see Fig. 8-9) which serves as a selective barrier to keep substances out—or in—a cell. Many substances can’t dissolve in fat (i.e., they aren’t fat-soluble) and thus can’t cross this “fat barrier” by themselves. Also, this cell membrane is ideal in that the cell can insert proteins by “simply” pushing them between the phospholipid molecules.