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lipids are sequestered away from the water to the

interior of the bilayer in the most thermodynamically

favorable arrangement. Because of such chemical

characteristics, membrane phospholipid molecules

spontanously form bilayers in aqueous environments

and form sealed compartments.

0004 Cellular membranes are organized in mosaic-like

structures which are dynamic and mobile. The terms

‘fluidity’ and ‘motility’ are used to describe properties

of membrane and membrane components, respect-

ively. Fluidy reflects the viscosity within the mem-

brane that depends on both the composition and

temperature within the membrane. The change in vis-

cosity or in phase transition (from liquid to gel or

crystalline state) is determined by the chain length

and saturation of the hydrocarbon chains. A shorter

chain length reduces the tendency of the hydrocarbon

tails to interact with one another and cis double bonds

produce kinks in the hydrocarbon chains that make

them more difficult to pack together, making mem-

branes more fluid at lower temperatures. Phospho-

lipid mobility is determined by intramolecular and

intermolecular motion. Intramolecular motion in-

cludes three main types: (1) rotation or vibration

about each C–C bond within the fatty acyl chain (seg-

mental motion); (2) rotation of the entire molecule

about the long axis perpendicular to the plane of the

bilayer; (3) a pendulum-like motion of the fatty acyl

chains. Intermolecular motion refers to lateral diffu-

sion of complete phospholipid molecules and occurs

by interchange of one lipid molecule for another.

These time motions have a different time scale, i.e.,

segmental motion is very fast, whereas lateral diffu-

sion is slower. Flip-flop or migration of phospholipid

molecules from one monolayer to another is a com-

paratively less common type of intermolecular

motion. These movements are possible because of

the presence of a special class of endoplasmic reticu-

lum (ER) membrane-bound enzyme, called phospho-

lipid exchange proteins or phospholipid translocators,

which catalyze the rapid flip-flop of specific phospho-

lipids from the inner to the outer monolayer of plasma

membrane or from one organelle membrane to an-

other, e.g., from microsomal vesicles to mitochondria.

The phospholipid movements in biological mem-

branes are presented schematically in Figure 2.

0005 Phospholipid fraction of cellular membranes varies

considerably in total amount and in the composition

of the fatty acids. This diversity occurs across species,

among tissues, and even among organelles in the same

cell (Table 1).

0006 The functional role of differences in fatty acid com-

position of cell membranes and the fatty acid asym-

metry between phospholipids from the outer and

the inner membrane have an important functional

significance. In human red blood cell membranes,

most of the PtdSer, which is negatively charged, is

located in the inner monolayer. This position is im-

portant for the activity of the enzyme protein kinase

C which requires the negative charge of phosphati-

dylserine (Figure 2). Similarly, specific inositol phos-

pholipids are concentrated in the cytoplasmatic half

of the plasma membrane where it can be reached by

specific enzymes and the product of the phosphatidy-

linositol molecules serves as intracellular signals.

Biosynthesis of Phospholipids

0007A more direct approach about the biosynthetic path-

ways of phospholipids, known as the Kennedy

pathway, became possible with the utilization of

radioactive and stable isotopes, which permitted ad-

vances in the understanding of phospholipid active

synthesis and breakdown in the cells and their role as

a metabolic pool for cellular signals. These techniques

are now combined with genetic manipulation for

the study of the pathways involved in phospholipid

metabolism.

0008The ER of eukaryotic cells is the predominant site

of phospholipid and sphingolipid assembly, but there

are phospholipid biosynthetic enzymes localized in

the mitochondria and microsomes. The yeast Sac-

charomyces cerevisiae has become the model of

choice for the study of the regulation of phospholipid

biosynthesis, owing to the possibility of a biochem-

ical, molecular, and genetic approach. The model

system of S. cerevisiae has allowed advances in the

study of intracellular transport and assembly of lipids

into membranes, analysis of the genes, and gene prod-

ucts required for lipid metabolism and definition of

the role of lipids in signal transduction. Many of the

enzymes were isolated and purified and metabolic

pathways were targeted by genetic manipulation of

specific genes which were impossible in other eukary-

otic organisms. In addition, the pathways of phos-

pholipid metabolism in S. cerevisiae are, with few

exceptions, similar to those found in higher eukary-

otes. They also display the intracellular compartmen-

talization and subcellular membranes typical of

eukaryotes which are important for the study of lo-

calization of the phospholipid biosynthesis enzymes.

0009Figure 3 shows the biosynthetic pathways in

S. cerevisiae. The important steps are outlined, to-

gether with the enzymes responsible for specific reac-

tions. The whole biosynthetic process is highly

interconnected to the sorting and delivery of plasma

and subcellular membrane. Synthesis of long-chain

fatty acids is achieved by the coordinated action

of acetyl-coenzyme A (acetyl-CoA) carboxylase, fatty

acid synthase and acyl-CoA synthetases (FAAs),

PHOSPHOLIPIDS/Physiology 4525

Lateral

diffusion

Flip-flop

PtdCho PtdEtn PtdSer PtdIns

Rotation

Flexion

Extracellular space Cytosol

−

fig0002 Figure 2 Schematic representation of a biological membrane section, showing the possible phospholipid movements within each

phospholipid molecule, between molecules and between the inner and outer phospholipid monolayer. The asymmetry of phospholipid

species is also presented, with the predominance of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) facing the

outer half and phosphatidylserine (PtdSer) with a negative charge and phosphatidylinositol (PtdIns) facing the inner half of the

membrane.

tbl0001 Table 1 Phospholipid composition of cell membranes

Membrane PtdCho PtdEth PtdSer PtdIns PtdGro diPtdGro PtdOH

Rat liver

Endoplasmic reticulum 58 17 4 9

Plasma membrane 56 15 10 2

Mitochondrial (inner) <3 3 25 1 6 2 18

Mitochondrial (outer) <5 5 23 2 13 3 3

Nuclear 55 20 3 7

Rat brain

Myelin 11 14 7

Erythrocytes

Rat 31 15 7 2

Human 23 20 8 3

Sheep 1 23 8 1

Escherichia coli plasma membrane 80 15 5

PtdCho, phosphatidylcholine; PtdEth, phosphatidylethanolamine; PtdSer, phosphatidylserine; PtdIns, phosphatidylinositol; PtdGro,

phosphatidylglycerol; diPtdGro, diphosphatidylglycerol (cardiolipin), PtdOH, phosphatidic acid.

Adapted with permission from Jain MK and Wagner RC (1980) Introduction to Biological Membranes, p. 36. New York: John Wiley.

4526 PHOSPHOLIPIDS/Physiology

-elongase(s) and -desaturase(s). The fatty acids are

esterified to sn-1 and sn-2 to glycerol-3-phosphate by

the enzyme glycerol-3-phosphate acyltransferase

(GAT) (Figure 3). Cytidine diphosphate-diacylgly-

cerol (CDP-DAG) synthase (CDS) converts phospha-

tidic acid to CDP-diacylglycerol (CDP-DAG), the

major intermediate in phospholipid synthesis. The

availability of CDP-DAG at specific cellular sites will

direct the synthesis of different phospholipids, such as

cardiolipin (CL), which occurs exclusively in mito-

chondria and phosphatidylinositol (PtdIns) and PtdSer

to distinct subfractions of the ER or the Golgi appar-

atus. There is extensive transfer of intermediates and

cross-compartment integration for the synthesis of

phospholipids. For example, the enzyme phosphati-

dylserine synthase (CHO1) is localized in the ER,

while the steps of conversion of PtdSer to PtdEtn take

place in the inner mitochondrial membrane. The

PtdEtn synthesized in the mitochondria must again

migrate to the ER to insure synthesis of PtdCho, the

most abundant phospholipid of S. cerevisiae.

0010In the absence of exogenous choline, PtdCho is

synthesized by the de novo pathway by a three-step

methylation of PtdEtn, which is catalyzed by the

ER methytransferases. In the case of mutants with

defects in phosphatidylserine decarboxylase (PSD)

and methyltransferases, choline and ethanolamine

must be provided in the medium to enter phospho-

lipid biosynthesis via the salvage pathway (Figure 3).

This route also insures recycling of PtdEtn and

FFA

Gro-3-P

PtdOH

INO1

Cho-CDP

Etn-CDP

Salvage pathway

Cho-P

Etn-P

Acyl-CoA

FFA1−4

PAP

DAGTAG

GAT

CDP-DAG

Ins

PtdGro-P

PtdGro

CLS

Ins-1-P

Glc-6-P

Ptd Ins

PtdSer

Ptd Etn

Ptd MMEtn

Ptd DMEtn

Ptd Cho

Bulk membrane

PSD1

PSD2

CDS1

CHO1

CHO2

OPI3

PGP

PIS1

Etn

Cho

fig0003 Figure 3 Phospholipid biosynthetic pathways in Saccharomyces cerevisiae. FFA, free fatty acid; Acyl-CoA, acyl coenzyme A; Gro-3-P,

glycerol-3-phosphate; PtdOH, phosphatidic acid; DAG, diacylglycerol; TAG, triacylglycerol; CDP-DAG, cytidine diphosphate-diacylgly-

cerol; PtdSer, phosphatidylserine; PtdEtn, phosphatidylethanolamine; PtdMMEtn, phosphatidylmonomethylethanolamine; PtdDMEtn,

phosphatidyldimethylethanolamine; PtdCho, phosphatidylcholine Cho, choline; Cho-P, choline phosphate; Cho-CDP, cytidine diphos-

phate-choline. Etn, ethanolamine; Etn-P, ethanolamine phosphate, Etn-CDP, cytidine diphosphate-ethanolamine. Glc-6-P, glucose-6-

phosphate; Ins-1-P, inositol 1-phosphate; Ins, inositol; Ptd Ins, phosphatidylinositol. PtdGro-P, phosphatidylglycerol-phosphate;

PtdGro, phosphatidylglycerol; CL, cardiolipin. Enzymes abbreviations (italic): FFA 1^4, acyl-CoA synthetases 1–4; GAT, glycerol-3-

phosphate acyltransferase; PA P , phosphatidate phosphatase; CDSI, CDP-diacylglycerol synthase; CHO1, phosphatidylserine synthase;

PSD1,2, phosphatidylserine decarboxylase 1,2; CHO2, phosphatidylethanolamine N-methyltransferase; OPI3, phosphatidyl-N-methyl-

transferase; PISI, phosphatidylinositol synthase; INO1, inositol-1-phosphate synthase; PGP, phosphatidylglycerophosphate phosphat-

ase; CLS, cardiolipin synthase.

PHOSPHOLIPIDS/Physiology 4527

PtdCho degradation products and control of local

levels of the potent second messenger diacylglycerol.

Degradation of Phospholipids

0011 Undoubtedly, the most important function of phos-

pholipids in the cell membrane relates to their

breakdown by the action of phospholipases. The

phospholipases are enzymes that hydrolyze phospho-

lipids on water–lipid interfaces and are distinguished

from other esterases by their relatively low activity in

soluble monomeric phospholipids below their critical

micellar concentrations (CMC), but become fully

active in aggregated phospholipid structures above

their CMC, such as in micelle, bilayers, or hexagonal

structures.

0012 As shown in Figure 4, whereas esterases show clas-

sical Michaelis–Menten kinetics, the phospholipases

may reach more than 1000-fold increase in activity as

the substrate phospholipid concentration reaches the

CMC. Important factors responsible for this in-

creased rate of hydrolysis include high local substrate

concentration, amphipatic substrate orientation at

the interface, enhanced diffusion of products from

the enzyme to lipid or aqueous moieties, and con-

formational change of enzyme upon binding to the

interface.

0013 Phospholipases play a central role in the activation

of various events related to phospholipid degrad-

ation. In general: (1) many phospholipases are digest-

ive enzymes found in high concentrations in venoms,

bacterial secretions, and digestive fluids of higher

animals; (2) many phospholipases have a regulatory

function with their products being cellular medi-

ators such as diacylglycerols, inositol trisphosphate,

platelet-activating factor, and the eicosanoids. The

actions of lipid mediators, known collectively as

second messengers, are highly sensitive and under

rigid control by a variety of anabolic and catabolic

enzymes. Many pathological states, as in inflamma-

tion, allergic reactions, and hypertensive states, are

related to activation of the cascade of events involv-

ing phospholipid degradation and the intracellular

second messenger pathway.

0014Phospholipases are classified as type A

,B,C,

or D, according to where they act on the substrate

phospholipid (Figure 5).

0015The phospholipases A are acyl hydrolases, which

means they remove one acyl group, yielding one fatty

acid and lysophosphatide. Phospholipase A

and

phospholipase A

remove fatty acids at positions

sn-1 and sn-2, respectively. Phospholipase A

is widely

distributed in nature and in mammals the major

sources are found in the pancreas and the brain. Intra-

cellularly, phospholipase A

is dominant in the ER.

The best known examples of phospholipase A

are

snake venom and the pancreatic enzyme that acts

mostly on PtdCho and PtdEdn. It is the major phos-

pholipase of mitochondria. Phospholipase B hydro-

lyzes both acyl groups (sn-1 and sn-2) and also has

high lysophospholipase activity. Phospholipase C acts

on the glycerophosphate bond while phospholipase D

catalyzes the removal of the polar head group. In

mammalian cells, phospholipase C is an important

initiator of the PtdIns cycle. Phospholipase D is the

major phospholipase in many plant tissues and acts

specifically on intact phospholipids to give phospha-

tidic acid and an alcohol. In plants, it seems to be

involved in cell turnover and energy utilization. Phos-

pholipase D is also found in bacterial and mammalian

sources, where it is involved in replacement of the

polar head groups of membrane phospholipids.

Phospholipids as Precursors of Cellular

Signal Transduction

0016In the cell membrane, phospholipids store important

precursors of second messengers. Transduction of sig-

nals from hormones, across a membrane, involves the

coordinated actions of receptor, membrane proteins,

and phospholipids that either stimulate or inhibit the

synthesis of a second messenger. A number of pro-

cesses inside the cell are controlled by the level of

second messengers.

0017Among the best described second messenger

systems is the G-protein and phosphatidylinositide

Substrate concentration

(arbitrar

)

CMC

Activity (arbitrary units)

Esterase

123

fig0004 Figure 4 Esterase exhibits Michaelis–Menden kinetics,

whereas phospholipase needs critical micellar concentration

(CMC) of the substrate for full activity.

4528 PHOSPHOLIPIDS/Physiology

Lysophospholipase

−

OH CH

OH + RCOOH

RCOOH + RCOOH

+ RCOOH

+ XOH

, B

PO CH

Figure 5 The action of phospholipases A

, B, C, and D and lysophospholipase in the hydrolysis of phospholipids. X represents choline, serine, ethanolamine, etc.

fig0005

system. This system is distinctive in that one stimulus

activates membrane reactions that generate two

second messengers. The first experimental observa-

tions noted that administration of neurotransmitter

acetylcholine in pigeon pancreas led to a rapid turn-

over of PtdIns fraction of membrane phospholipids

and release of the digestive enzyme amylase. Similar

observations were made in other systems by hor-

mones, neurotransmitters, or growth factors. Despite

intensive efforts, the understanding of these mechan-

isms had not progressed until the early 1980s. It is

now recognized that the initial events of inositol

phospholipid metabolism occur within 20–30 s of

binding of the agonist to the receptor.

0018 Today, the role of specific inositol phospholipids

in the phosphoinositide family, namely, phosphati-

dylinositol 4,5-bisphosphate (PI(P

)), as a mem-

brane-associated storage form of two second

messengers, is quite clearly understood.

0019 Figure 6 shows a simplified scheme of the events

involved. When an agonist binds to a receptor, mem-

brane G-protein is stimulated to bind guanosine

triphosphate. The activated G-protein then acts on

the membrane-bound enzyme, phospholipase C,

which in turn cleaves PI(P

) to yield two products,

sn-1,2-diacylglycerol (DAG) and inositol 1,4,5-

trisphosphate (IP

). Both of these products are second

messengers. The function of IP

is to stimulate the

release of calcium ion from intracellular stores,

largely from the ER. The increased calcium concen-

tration originates a cascade of events in the intracel-

lular metabolism, including the activation of the

membrane-bound protein kinase C (C from calcium).

This enzyme requires calcium and PtdSer for its activ-

ity. The specific role of the second messenger DAG is

to increase the affinity of the protein kinase C for

calcium ions. Activated protein kinases will then

phosphorylate target proteins inside the cell, which

will then be activated and proceed within the cascade.

Since many metabolic events are controlled by cal-

cium fluxes and by phosphorylation of specific pro-

teins, the phosphoinositol system shows great

ingenuity as a control mechanism. Some of the

known target proteins are the insulin receptor, b-

adrenergic receptor, and glucose transporter. It is

now quite clear that PtdIns is not the only source of

DAG which can activate protein kinase C. PtdCho

and PtdEtn also seem to serve this role. PtdCho is also

a major source of arachidonic acid for the biosyn-

thesis of eicosanoids.

0020Another well-described example of a regulatory

function involving phospholipids in membrane is the

generation of another second messenger, known as

arachidonate cascade. As described earlier, when an

Cytosol

Extracellular space

Receptor

Stimulus

AcylCoA

G-protein

PLA

Cellular

response

Active

protein

Eicosanoids

LPI (P

)

PLA

PLC

PKC

PLC

PI (P

)

Ptd Ser

PtdCho

DAG

fig0006 Figure 6 The role of phospholipids in signal transduction. PLA

, PLA

, PLC, phospholipases A

, C; PI(P

), phosphatidylinositol 4,5

bisphosphate; IP

, inositol 1,4,5-trisphosphate; AcylCoA, acyl coenzyme A; DAG, diacylglycerol; FA, fatty acids; AA, archidonic acid;

CP, choline phosphate; PKC, protein kinase C; Ptd Ser, phosphatidylserine; PtdCho, phosphatidylcholine; LPI, lysophosphatidylino-

sitol.

4530 PHOSPHOLIPIDS/Physiology

agonist stimulates a membrane receptor, a series of

events occurs which may lead to the release of 20

carbon polyunsaturated fatty acids from membrane

phospholipids, most commonly arachidonic acid

(Figure 6). These are tissue-specific stimulants by

hormones such as bradykinin or epinephrine, or

proteases such as thrombins, to name but a few.

Pathological release can occur if membranes are per-

turbed; for example, the bee sting may stimulate the

release of arachidonate from local cell membrane and

cause inflammation. This release involves the action

of a specific phospholipase A

on PtdCho or PtdEdn,

yielding arachidonate, and the action of a phospho-

lipase C on PtdIns, yielding a diacylgycerol, which in

turn undergoes cleavage to give free arachidonate,

which is then converted into a specific eicosanoid in

the cell. Eicosanoids, such as prostaglandins, leuko-

trienes, and thromboxanes are compounds with

potent physiological properties which are formed

from 20-carbon unsaturated fatty acids into one of a

series of eicosanoids according to the enzyme present

in the cell and the unsaturated fatty acid released

from the membrane. Long-chain polyunsaturated

fatty acids of the n-3 series, especially from marine

origin, when present in the diet will be incorporated

into the cell membrane and may replace arachidonic

acid in the signal pathway. All eicosanoids are metab-

olized very rapidly. We still know relatively little

about the subsequent effects of eicosanoids at a mo-

lecular level, though recent evidence points to their

function in the communication pathway with nuclear

receptors.

0021 Figure 6 presents events as a coordinated response

involving membrane phospholipids and their degrad-

ation products. This figure exemplifies the events

associated with arachidonic acid release (top) and

the mobilization of calcium ions and protein kinase

C (bottom). The action of lipid mediators is highly

sensitive and under rigid control and when this regu-

lation is not maintained, a number of pathological

states, such as those mentioned above, may result.

0022Current knowledge on the mechanisms involved in

intracellular signaling is rapidly advancing and it

underscores the importance of the physiological,

pathological, as well as the pharmacological prop-

erties of this diverse and ubiquitous group of com-

pounds, known as phospholipids. The nutritional

significance of phospholipid physiology may be sum-

marized by the fact that foods are the main sources of

essential elements of this system.

See also: Choline: Properties and Determination;

Essential Oils: Properties and Uses; Fatty Acids:

Properties; Metabolism; Gamma-linolenic Acid; Fish:

Dietary Importance of Fish and Shellfish; Fish Oils:

Dietary Importance; Fats: Classification; Occurrence;

Prostaglandins and Leukotrienes; Vegetable Oils:

Dietary Importance

Further Reading

Carman GM and Henry AS (1989) Phospholipid bio-

synthesis in yeast. Annual Review of Biochemistry 58:

635–669.

Eyster KM (1998) Introduction to signal transduction – a

primer for untangling the web of intracellular messen-

gers. Biochemistry and Pharmacology 55: 1927–1938.

Kohlwein SD, Daum G, Schneiter R and Paltauf F (1996)

Phospholipids: synthesis, sorting, subcellular traffic – the

yeast approach. Trends in Cell Biology 6: 260–266.

Mead JF, Alfin-Slater RB, Howton DR and Popja

k G (1986)

Lipids: Chemistry, Biochemistry and Nutrition. New

York: Plenum.

Sim E (1982) Membrane Biochemistry. Outline Studies in

Biology. London: Chapman and Hall.

Vance DE (1991) Phospholipid metabolism and cell signal-

ling in eucaryotes. In: Vance DE and Vance J (eds)

Biochemistry of Lipids, Lipoproteins and Membranes –

New Comprehensive Biochemistry, vol. 20, pp. 205–

239. Netherlands: Elsevier.

Waite M (1991) Phospholipases. In: Vance DE and Vance J

(eds) Biochemistry of Lipids, Lipoproteins and Mem-

branes – New Comprehensive Biochemistry, vol. 20,

pp. 269–295. Netherlands: Elsevier.

PHOSPHOLIPIDS/Physiology 4531

PHOSPHORUS

Contents

Properties and Determination

Physiology

Properties and Determination

A N Garg, Indian Institute of Technology, Roorkee,

U.A., India

Introduction

0001 The extensive and varied chemistry of phosphorus

transcends the traditional boundaries of inorganic

chemistry because of its vital role in the biochemistry

of all living organisms as a constituent of adenosine

triphosphate (ATP) and phosphoproteins. It was first

isolated from urine by Hennig Brandt in 1669 as a

white waxy substance. The spontaneous chemilumin-

escent reaction of white phosphorus with moist air

was the first observed property and was also the

origin of its name (Greek: ‘phos,’‘light’; ‘pherein,’

‘bearing’). It was also the ancient name for the planet

Venus, when it appeared before sunrise.

Chemical Properties

0002 Phosphorus is a typical nonmetal placed in group

VA of the periodic table. The element phosphorus

(P) has an atomic number of 15 and atomic

weight of 30.97 with electrons distributed as

and atomic energy levels

as shown in Figure 1. Thus, three unpaired electrons,

together with the availability of low-lying vacant 3d

orbitals, account for the predominant oxidation

states III and V in phosphorus chemistry. The most

important biological form is the pentavalent oxygen

compound phosphate PO

3

. Phosphorus exists in

three main allotropic forms; white, red, and black,

each of these being polymorphic. There are at least 11

known modifications, some amorphous, others of

some indefinite identity, and all but three of unknown

structure. Of these, white phosphorus is soft, waxy,

most reactive, and thermodynamically least stable. It

has a melting point of 44.1



C, a boiling point of



C and a specific gravity of 1.82 g cm

3

. Red and

black forms are heavier with specific gravities of

2.2 and 2.69 g cm

3

, respectively. White phosphorus

reacts with moist air and gives out light. It ignites

spontaneously in air at about 35



C and is therefore

stored in water to prevent combustion. It is soluble in

organic solvents such as CS

and benzene. If white

phosphorus is heated to about 250



C, or a lower

temperature in the presence of sunlight, then red

phosphorus is formed. It is a polymeric solid with a

melting point of 280.5



C and sublimes at 430



C. It

is much less reactive than white phosphorus and does

not phosphoresce in air. Unlike white phosphorus,

red phosphorus need not be stored under water. Its

structure is extremely complex, involving a crisscross

packing of infinite tubular chains of P atoms. When

white phosphorus is heated under high pressure, a

highly polymerized form called black phosphorus is

obtained. It is also obtained by heating at 220–370



for 8 days in the presence of mercury as a catalyst and

with a seed of black P. This is thermodynamically the

most stable allotrope. It is inert and has a double layer

structure with P atoms being bound to three neigh-

bors, as shown in Figure 2. The entire structure con-

sists of a stacking of these double layers with the

closest P–P distances within each layer being 2.17–

2.20 A

and the shortest P–P distance between layers

at 3.87 A

. Therefore, the crystals are flaky like graph-

ite. All forms of phosphorus melt to give the same

liquid, which consists of symmetric P

molecules with

phosphorus atoms occupying corners of a regular

tetrahedron. Each atom is directly bonded to the

other three atoms P–P ¼ 2.21 + 0.02 A

. The same

molecular form exists in the gas phase at >800



C and

low pressure. The bonding orbitals in P

have only

2% of 3s and 3d character. It is most likely that pure

7.5 eV

9eV

~2 eV

fig0001Figure 1 Atomic energy levels in phosphorus.

4532 PHOSPHORUS/Properties and Determination

3p orbitals are involved, even though the bonds in P

are bent with ﬀP–P–P ¼ 60



and a strain energy of

96 kJ mol

1

, which accounts for its high reactivity.

Electronic spectral studies suggest a resonance struc-

ture with strong p bonds. The regions of high electron

density in the P

molecule are shown in Figure 3. The

moment of inertia about any of its three major axes

is 2.5 10

40

gcm

. Thus, the P

molecule might be

expected to have donor ability.

0003 The only naturally occurring isotope,

P, has a

nuclear spin I ¼ ½h/2p and a large magnetic moment

(1.13 NM) but no quadrupole moment. It is suitable

for nuclear magnetic resonance (NMR) spectroscopy.

In a 10-kG field

P resonates at 17.24 MHz. Phos-

phorus forms compounds with all elements except

tin, bismuth, and the inert gases. It also reacts readily

with heated aqueous solutions to give a variety of

products. The bonding and stereochemistry of the

phosphorus atom are varied, essentially due to

empty d orbitals. It is known in at least 14 coordin-

ation geometries with coordination numbers up to 9,

though the most frequently met coordination numbers

are 3, 4, 5, and 6. Some typical geometries along with

orbitals used in the formation of bonds are given in

Table 1. Phosphorus has a significant tendency to

catenation, forming a series of cyclic compounds

(RP)

where n ¼ 3–6, as well as some R

PPR

type

of compounds. (See Spectroscopy: Nuclear Magnetic

Resonance.)

Forms in Foods

0004Plants need phosphate for healthy growth, especially

for the development of roots, flowers, fruits, and

seeds, though the requirement is less compared with

that for nitrogen and potassium. All foods contain

phosphorus in the form of the phosphate anion

ðPO

3

Þ, and is consumed by living organisms as

such. Very few natural compounds contain phos-

phorus in any other form. A summary of the types

of biologically important phosphorus compounds is

presented in Table 2. It is picked up from the soil,

where it is present as organic and inorganic phos-

phates (soluble as well as insoluble) in nucleopro-

teins, nucleic acids, and the coenzymes nicotinamide

adenine dinucleotide (NAD) 2-phosphate, ATP, and

other high-energy phosphates. Organic phosphates

include sugar phosphates such as glucose 6-phosphate

(Figure 4), phospholipids, and pigments. Phosphate

in the form of nucleotides serves as a source of a high-

energy bond and performs an important function in

conserving and providing bursts of metabolic energy.

fig0002 Figure 2 Arrangement of P atoms in the double layers in crystalline black phosphorus.

fig0003 Figure 3 Three-dimensional distribution of valence shell elec-

tron density in the P

molecule. From Hart RR, Robin MB and

Kuebler NA (1965) Journal of Chemical Physics 42: 3631–3638, with

permission.

PHOSPHORUS/Properties and Determination 4533

0005 Most of the phosphorus in foods is in the form of

organic phosphates that are digested in the intestines

to form inorganic phosphates of sodium, calcium,

and potassium. A special regulatory mechanism, the

parathyroid hormone (PTH)–vitamin D–thyro calci-

tonin axis is involved in the control of both calcium

and phosphorus balance. This hormonal axis controls

the absorption rate in the gut, the excretion rate in the

kidney, and the storage capacity of bones. Therefore,

phosphorus is involved in a multitude of process in

the entire life cycle, serving both a structural and

catalytic function.

0006In egg yolk and fish, it occurs in the form of phos-

pholipids, phosphatidylcholine, and phosphatidyl-

ethanolamine. In certain foods such as cereal grains

and proteins from vegetable sources, 50–80% of the

phosphorus occurs in the form of phytin, which is

usually the calcium/magnesium salt of phytic acid

(the hexaphosphate ester of inositol). Natural starch,

particularly of potato, contains phosphoric acid as

an ester (Figure 5). (See Phospholipids: Properties

and Occurrence; Phytic Acid: Properties and Deter-

mination; Starch: Structure, Properties, and Deter-

mination.)

0007The phosphate cycle in water is controlled by the

biocycle. The inorganic part of the cycle consists of

HPO

2

in solution equilibrium with H



, ðPO

3

Þ,

and H

. Planktonic algae can absorb inorganic

phosphates. Some algae also have a method of

tbl0001 Table 1 Types of phosphorus compounds and their molecular structure

Numbers of bonds Hybridization Geometry Shape Examples

3sp

(with one lone pair) Pyramidal PH

4sp

Tetrahedral PH

, POCl

, PO(OH)

–p

bonding in

compounds with PO)

5sp

d Trigonal bipyramid PCl

,PF

,Ph

6sp

Octahedral PF



tbl0002 Table 2 Type of biologically important phosphorus compounds

and their functions

Type Example Function

Nucleic acids DNA Storage of genetic

information

RNA Transcription of DNA and

protein synthesis

Phospholipid Phosphatidyl

choline

Structural components of

membranes

Bone salt Hydroxyapatite Bone structure and function

Phosphoproteins Casein Nutrient storage

Glutaminase NH

production; acid–base

regulation

Stathmin Cell proliferation and

differentiation

Sugar phosphate Glucose-6-P Glycolysis

Nucleotides ATP Energy transformation,

molecular activation

cAMP Second messenger

Uridine di-P

glucose

Glycogen synthesis

−

OH H

fig0004Figure 4

4534 PHOSPHORUS/Properties and Determination