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Fructose (a hexose)

Fructose (a hexose)

Sucrose (a disaccharide)

(a-D-glucosido-p-D-fructose)

Fig. 1.7 Some simple sugars and disaccharides. Glucose and fructose are shown in their 'ring' form. Even this representation ignores the true three-dimensional structure, which is 'chair'-shaped: if the middle part of the glucose ring is imagined flat, the left-hand end slopes down and the right-hand end up. Glucose forms a six-membered ring and is described as a pyranose; fructose forms a five-membered ring and is described as a furanose. In solution the a- and P- forms are in equilibrium with each other and with a smaller amount of the straight-chain form. The orientation of the oxygen on carbon atom 1 becomes fixed when glucose forms links via this carbon to another sugar, as in sucrose; a- and P-links then have quite different properties (e.g. cellulose vs starch or glycogen).

H OH OH

Sucrose (a disaccharide)

(a-D-glucosido-p-D-fructose)

Fig. 1.7 Some simple sugars and disaccharides. Glucose and fructose are shown in their 'ring' form. Even this representation ignores the true three-dimensional structure, which is 'chair'-shaped: if the middle part of the glucose ring is imagined flat, the left-hand end slopes down and the right-hand end up. Glucose forms a six-membered ring and is described as a pyranose; fructose forms a five-membered ring and is described as a furanose. In solution the a- and P- forms are in equilibrium with each other and with a smaller amount of the straight-chain form. The orientation of the oxygen on carbon atom 1 becomes fixed when glucose forms links via this carbon to another sugar, as in sucrose; a- and P-links then have quite different properties (e.g. cellulose vs starch or glycogen).

ertheless, some of the chemical properties of sugars can only be understood by remembering that the straight chain form exists. The basic carbohydrate unit is known as a monosaccharide. Monosaccharides may have different numbers of carbon atoms, and the terminology reflects this: thus, a hexose has six carbon atoms in its molecule, a pentose five, etc. Pentoses and hexoses are the most important in terms of mammalian metabolism. These sugars also have 'common names' which often reflect their natural occurrence. The most abundant in our diet and in our bodies are the hexoses glucose (grape sugar, named from the Greek glykys sweet), fructose (fruit sugar, from the Latin fructus for fruit) and galactose (derived from lactose, milk sugar; from the Greek galaktos, milk), and the pentose ribose, a constituent of nucleic acids (the name comes from the related sugar arabinose, named from Gum arabic).

Complex carbohydrates are built up from the monosaccharides by covalent links between sugar molecules. The term disaccharide is used for a molecule composed of two monosaccharides (which may or may not be the same), oli-gosaccharide for a short chain of sugar units, and polysaccharide for longer chains (> 10 units), as found in starch and glycogen. Disaccharides are abundant in the diet, and again their common names often denote their origin: sucrose (table sugar, named from the French, sucre) which contains glucose and fructose (Fig. 1.7); maltose (two glucose molecules) from malt; lactose (galactose and glucose) from milk. The bonds between individual sugar units are relatively strong at normal hydrogen ion concentrations, and sucrose (for instance) does not break down when it is boiled, although it is steadily broken down in acidic solutions such as cola drinks; but there are specific enzymes in the intestine (described in Chapter 3) which hydrolyse these bonds to liberate the individual monosaccharides.

Polysaccharides differ from one another in a number of respects: their chain length, and the nature (a- or 0-) and position (e.g. ring carbons 1-4, 1-6) of the links between individual sugar units. Cellulose consists mostly of P-1,4 linked glucosyl units; these links give the compound a close-packed structure which is not attacked by mammalian enzymes. In humans, therefore, cellulose largely passes intact through the small intestine where other carbohydrates are digested and absorbed. It is broken down by some bacterial enzymes. Ruminants have complex alimentary tracts in which large quantities of bacteria reside, enabling the host to obtain energy from cellulose, the main constituent of its diet of grass. In humans there is some bacterial digestion in the large intestine (see Chapter 3). Starch and the small amount of glycogen in the diet are readily digested (Chapter 3).

The structure of glycogen is illustrated in Fig. 1.8. It is a branched polysaccharide. Most of the links between sugar units are of the a-1,4 variety but after every 9-10 residues there is an a-1,6 link, creating a branch. Branching makes the molecules more soluble, and also creates more 'ends' where the enzymes of glycogen synthesis and breakdown operate. Glycogen is stored within cells, not simply free in solution but in organised structures which may be seen as gran-

Proglycogen Macroglycogen

Fig. 1.8 Structure of glycogen. Left-hand side: each circle in the upper diagram represents a glucosyl residue. Most of the links are of the a-1,4 variety. One of the branch points, an a-1,6 link, is enlarged below. Amylopectin, a component of starch, has a similar structure. Amylose, the other component of starch, has a linear a-1,4 structure. Right-hand side: glycogen is built upon a protein backbone, glycogenin. The first layer of glycogen chains forms proglycogen, which is enlarged by addition of further glucosyl residues (by glycogen synthase and a specific branching enzyme, that creates the a-1,6 branch-points), to form macroglycogen. When glycogen is referred to in this book, it is the macroglycogen form that is involved. Pictures of proglycogen and macroglycogen taken from Alonso, M.D., Lomako, J., Lomako, W.M. & Whelan, W.J. (1995) A new look at the biogenesis of glycogen FASEB J 9; 1126-1137. With permission of the publisher.

Fig. 1.8 Structure of glycogen. Left-hand side: each circle in the upper diagram represents a glucosyl residue. Most of the links are of the a-1,4 variety. One of the branch points, an a-1,6 link, is enlarged below. Amylopectin, a component of starch, has a similar structure. Amylose, the other component of starch, has a linear a-1,4 structure. Right-hand side: glycogen is built upon a protein backbone, glycogenin. The first layer of glycogen chains forms proglycogen, which is enlarged by addition of further glucosyl residues (by glycogen synthase and a specific branching enzyme, that creates the a-1,6 branch-points), to form macroglycogen. When glycogen is referred to in this book, it is the macroglycogen form that is involved. Pictures of proglycogen and macroglycogen taken from Alonso, M.D., Lomako, J., Lomako, W.M. & Whelan, W.J. (1995) A new look at the biogenesis of glycogen FASEB J 9; 1126-1137. With permission of the publisher.

ules on electron microscopy. Each glycogen molecule is synthesised on a protein backbone, or primer, glycogenin. Carbohydrate chains branch out from glycogenin to give a relatively compact molecule called proglycogen. The glycogen molecules that participate in normal cellular metabolism are considerably bigger (see Fig. 1.8), typically with molecular weights of several million. The enzymes of glycogen metabolism are intimately linked with the glycogen granules.

The carbohydrates share the property of relatively high polarity. Cellulose is not strictly water-soluble because of the tight packing between its chains, but even cellulose can be made to mix with water (as in paper pulp or wallpaper paste). The polysaccharides tend to make 'pasty' mixtures with water whereas the small oligo-, di- and monosaccharides are completely soluble. These characteristics have important consequences for the metabolism of carbohydrates, some of which are as follows.

(1) Glucose and other monosaccharides circulate freely in the blood and interstitial fluid, but their entry into cells is facilitated by specific carrier proteins.

(2) Perhaps because of the need for a specific transporter for glucose to cross cell membranes (thus making its entry into cells susceptible to regulation), glucose is an important fuel for many tissues, and an obligatory fuel for some. Carbohydrate cannot be synthesised from the more abundant store of fat within the body. The body must therefore maintain a store of carbohydrate.

(3) Because of the water-soluble nature of sugars, this store will be liable to osmotic influences: it cannot therefore be in the form of simple sugars or even oligosaccharides, because of the osmotic problem this would cause to the cells. This is overcome by the synthesis of the macromolecule glycogen, so that the osmotic effect is reduced by a factor of many thousand compared with monosaccharides. The synthesis of such a polymer from glucose, and its breakdown, are brought about by enzyme systems which are themselves regulated, thus giving the opportunity for precise control of the availability of glucose.

(4) Glycogen in an aqueous environment (as in cells) is highly hydrated; in fact, it is always associated with about three times its own weight of water. Thus, storage of energy in the form of glycogen carries a large weight penalty (discussed further in Chapter 8).

Just as there are many different sugars and carbohydrates built from them, so there are a variety of types of fat. The term fat comes from Anglo-Saxon and is related to the filling of a container or vat. The term lipid, from Greek, is more useful in chemical discussions since 'fat' can have so many shades of meaning. Lipid materials are those substances which can be extracted from tissues in organic solvents such as petroleum or chloroform. This immediately distinguishes them from the largely water-soluble carbohydrates.

Amongst lipids there are a number of groups (Fig. 1.4). The most prevalent, in terms of amount, are the triacylglycerols or triglycerides, referred to in older literature as 'neutral fat' since they have no acidic or basic properties. These compounds consist of three individual fatty acids, each linked by an ester bond to a molecule of glycerol. As discussed above, the triacylglycerols are very non-polar, hydrophobic compounds. The phospholipids are another important group of lipids - constituents of membranes and also of the lipoprotein particles which will be discussed in Chapter 9. Steroids - compounds with the same nucleus as cholesterol (see Fig. 1.6) - form yet another important group and will be considered in later chapters, steroid hormones in Chapter 5 and cholesterol metabolism in Chapter 9.

Fatty acids are the building blocks of lipids, analogous to the monosaccha-rides. The fatty acids important in metabolism are mostly unbranched, long-

chain (12 carbon atoms or more) carboxylic acids with an even number of carbon atoms. They may contain no double bonds, in which case they are referred to as saturated fatty acids, one double bond (mono-unsaturated fatty acids), or several double bonds - the polyunsaturated fatty acids. Many individual fatty acids are named, like monosaccharides, according to the source from which they were first isolated. Thus, lauric acid (C12, saturated) comes from the laurel tree, myristic acid (C14, saturated) from the Myristica or nutmeg genus, palmitic acid (C16, saturated) from palm oil, and stearic acid (C18, saturated) from suet (Greek steatos). Oleic acid (C18, mono-unsaturated) comes from the olive (from Latin: olea, olive, or oleum, oil). Linoleic acid, C18 with two double bonds, is a polyunsaturated acid common in certain vegetable oils; it is obtained from linseed (from the Latin linum for flax and oleum for oil).

The fatty acids mostly found in the diet have some common characteristics. They are composed of even numbers of carbon atoms, and the most abundant have 16 or 18 carbon atoms. There are three major series or families of fatty acids, grouped according to the distribution of their double bonds (Box 1.2).

Differences in the metabolism of the different fatty acids are not very important from the point of view of their roles as fuels for energy metabolism. When considering the release, transport and uptake of fatty acids the term non-esteri-fied fatty acids will therefore be used without reference to particular molecular species. In a later section (Box 9.5) some differences in their effects on the serum cholesterol concentration and propensity to heart disease will be discussed.

It will be seen from Figs 1.4 and 1.9 that saturated fatty acids, such as palmitic (16:0), have a natural tendency to fit together in nice orderly arrays. The unsaturated fatty acids, on the other hand, have less regular shapes (Fig. 1.9). This is reflected in the melting points of the corresponding triacylglycerols - saturated fats, such as beef suet with a high content of stearic acid, 18:0, are relatively solid at room temperature, whereas unsaturated fats such as olive oil are liquid. This feature may have an important role in metabolic regulation, although its exact significance is not yet clear. We know that cell membranes with a high content of unsaturated fatty acids in their phospholipids are more 'fluid' than those with more saturated fatty acids. This may make them better able to regulate metabolic processes - for instance, muscle cells with a higher content of unsaturated fatty acids in their membranes respond better to the hormone, insulin, probably because the response involves the movement of proteins (insulin receptors, glucose transporters) within the plane of the membrane (discussed in Box 2.4), and this occurs faster if the membrane is more fluid.

An important feature of the fatty acids is that, as their name implies, they have within one molecule both a hydrophobic tail and a polar carboxylic acid group. Long-chain fatty acids (12 carbons and more) are almost insoluble in water. They are carried in the plasma loosely bound to the plasma protein albumin. Nevertheless, they are more water-miscible than triacylglycerols, which are carried in plasma in the complex structures known as lipoproteins. The simpler transport of non-esterified fatty acids is perhaps why they serve within

Box 1.2 The structures and interrelationships of fatty acids

In the orthodox nomenclature, the position of double bonds is counted from the carboxyl end. Thus, a-linolenic acid (18 carbons, three double bonds) may be represented as ds-9,12,15-18:3, and its structure is:

CH3 - CH2 - CH = C15H - CH2 - CH = C12H - CH2 - CH = C9H - (CH2)7 - C1OOH

(where the superscripts denote the numbering of carbon atoms from the carboxyl end). However, this is also known as an n-3 (or sometimes as an ra-3) fatty acid, since its first double bond counting from the non-carboxyl (ra) end is after the third carbon atom. On the latter basis, unsaturated fatty acids can be split into three main families, n-3, n-6 and n-9.

The saturated fatty acids can be synthesised within the body. In addition, many tissues possess the desaturase enzymes to form cis-6 or cis-9 double bonds, and to elongate the fatty acid chain (elongases) by addition of 2-carbon units at the carboxyl end. But these processes do not alter the position of the double bonds relative to the ra end, so fatty acids cannot be converted from one family to another: an n-3 fatty acid (for instance) remains an n-3 fatty acid. Oleic acid (cis-9-18:1, n-9 family) can be synthesised in the human body, but we cannot form n-6 or n-3 fatty acids. Since the body has a need for fatty acids of these families, they must be supplied in the diet (in small quantities). The parent members of these families, that need to be supplied in the diet, are linoleic acid for the n-6 family and a-linolenic acid for the n-3 family. These are known as essential fatty acids. They can be converted into other members of the same family although there seem to be health benefits of consumption of other members of the n-3 family, particularly 20:5 n-3 (eicosapentaenoic acid) and 22:6 n-3 (docosahexaenoic acid), found in high concentrations in fish oils. This is discussed further in Box 9.5. Some patients receiving all their nutrition intravenously have become deficient in essential fatty acids. The problem may be cured by rubbing sunflower oil into the skin!

Table 1.2.1

Family

Source

Typical member Simplified structure

Saturated n-9 n-6 n-3

Diet or synthesis

Diet or synthesis

Diet

Diet

Myristic

Palmitic

Stearic

Oleic

Linoleic a-linolenic

14:0

16:0

18:0

Fig. 1.9 Pictures of the molecular shapes of different fatty acids: (a) a saturated fatty acid, stearic acid (18:0), (b) a monounsaturated fatty acid, oleic acid (18:1n-9). From Gurr et al. (2002).

Fig. 1.9 Pictures of the molecular shapes of different fatty acids: (a) a saturated fatty acid, stearic acid (18:0), (b) a monounsaturated fatty acid, oleic acid (18:1n-9). From Gurr et al. (2002).

the body as the immediate carriers of lipid energy from the stores to the sites of utilisation and oxidation; they can be released very rapidly from stores when required and their delivery to tissues is regulated on a minute-to-minute basis.

But non-esterified fatty acids would not be a good form in which to store lipid fuels in any quantity. Their amphipathic nature means that they aggregate in micelles (small groups of molecules, formed with their tails together and their heads facing the aqueous environment); they would not easily aggregate in a very condensed form for storage. Triacylglycerols, on the other hand, do so readily; these hydrophobic molecules form uniform lipid droplets from which water is completely excluded, and which are an extremely efficient form in which to store energy (in terms of kJ stored per g weight). This is illustrated in Fig. 1.10. Thus, in brief, triacylglycerols are the form in which fat is mostly stored in the human body, and in the bodies of other organisms; hence they are the major form of fat in food. Non-esterified fatty acids, on the other hand, are the form in which lipid energy is transported in a highly regulated manner from storage depots to sites of utilisation and oxidation.

1.2.2.3 Proteins

Proteins are chains of amino acids linked through peptide bonds. Individual proteins are distinguished by the number and order of amino acids in the chain - the sequence, or primary structure. Within its normal environment, the chain of amino acids will assume a folded, three-dimensional shape, representing the secondary structure (local folding into a-helix and P-sheet) and tertiary structure (folding of the complete chain on itself). Two or more such folded peptide chains may then aggregate (quaternary structure) to form a complete enzyme or other functional protein.

In terms of energy metabolism, the first aspect we shall consider is not how this beautiful and complex arrangement is brought about; we shall consider how it is destroyed. Protein in food is usually denatured (its higher-order structures disrupted) by cooking or other treatment, and then within the intestinal tract the disrupted chains are broken down to short lengths of amino acids before absorption into the bloodstream. Within the bloodstream and within

Fig. 1.10 The comparison of fat and carbohydrate as fuel sources. Raw potatoes (right) are hydrated to almost exactly the same extent as glycogen in mammalian cells. Olive oil (left) is similar to the fat stored in droplets in mature human adipocytes. The potatoes (1.05 kg) and olive oil (90 g) here each provide 3.3 MJ on oxidation. This emphasises the advantage of storing most of our energy in the body as triacylglycerol rather than as glycogen.

Fig. 1.10 The comparison of fat and carbohydrate as fuel sources. Raw potatoes (right) are hydrated to almost exactly the same extent as glycogen in mammalian cells. Olive oil (left) is similar to the fat stored in droplets in mature human adipocytes. The potatoes (1.05 kg) and olive oil (90 g) here each provide 3.3 MJ on oxidation. This emphasises the advantage of storing most of our energy in the body as triacylglycerol rather than as glycogen.

tissues we shall be concerned with the transport and distribution of individual amino acids. These are mostly sufficiently water-soluble to circulate freely in the aqueous environment of the plasma. Only tryptophan is sufficiently hydrophobic to require a transporter; it is bound loosely (like the non-esterified fatty acids) to albumin. Amino acids, not surprisingly, do not cross cell membranes by simple diffusion; there are specific transporters, carrying particular groups of amino acids (see Chapter 2, Table 2.3).

Protein is often considered as the structural material of the body, although it should not be thought of as the only structural material; it can only assume this function because of the complex arrangements of other cellular constituents, especially phospholipids forming cell membranes. Nevertheless, apart from water, protein is the largest single component in terms of mass of most tissues.1 Within the body, the majority of protein is present in the skeletal muscles, mainly because of their sheer weight (around 40% of the body weight) but also because each muscle cell is well packed with the proteins (actin and myosin) which constitute the contractile apparatus. But it is important to remember that most proteins act in an aqueous environment and are therefore associated with water. This is relevant if we consider the body's protein reserves as a form of stored chemical energy. Since protein is associated with water, it suffers the same drawback as a form of energy storage as does glycogen; with every gram of protein are associated about 3 grams of water. It is not an energy-dense storage medium. Further, although protein undoubtedly represents a large source of energy that is drawn upon during starvation, it should be remembered that there is, in animals, no specific storage form of protein; all proteins have some function other than storage of energy. Thus, utilisation of protein as an energy source involves loss of the substance of the body. In evolutionary terms we might expect that this will be minimised (i.e. the use of the specific storage compounds glycogen and triacylglycerol will be favoured) and, as we shall see in later chapters, this is exactly the case.

1.3 Some physiological concepts

The emphasis of this book on the integration of metabolism in different tissues and organs is more closely related to physiology than to molecular biology. This short section is intended to fill in some physiological concepts for those from more biochemical backgrounds.

1.3.1 Circulation, capillaries, interstitial fluid

Blood is pumped around the body by the heart (Fig. 1.11). Strictly, it is pumped by the left ventricle, out into the aorta - the main artery - and its various

Fig. 1.11 The circulatory system. Oxygenated blood from the lungs returns in the pulmonary veins to the left heart, from where it is pumped through the aorta and its various branches (arteries) to the tissues and organs. It returns from the tissues and is pumped to the lungs for reoxygenation and expiration of CO2. The key feature from the point of view of integration of metabolism is that blood returning from all tissues (and from endocrine glands) is mixed within the heart and lungs, and then redistributed to tissues. Thus, the bloodstream ('the circulation') acts as an efficient means for interchange of nutrients, metabolites and hormones between tissues.

CO2 02

vena cava)

CO2 02

vena cava)

Capillary bed (in tissues)

Capillary bed (in tissues)

branches, which supply blood to all tissues. Within tissues, the arterial vessels supplying blood divide into smaller and smaller vessels, and eventually into the capillaries - small vessels whose interior lumen is approximately 0.01 mm diameter, just large enough for red blood cells to pass through in single file.

The density of capillaries (numbers of capillaries per unit area when the tissue is examined in cross-section under the microscope) varies between different tissues, but in most tissues at least one capillary is in close proximity to each cell. The inner walls of the capillaries are lined with flat endothelial cells, but in most tissues there are gaps between the endothelial cells, and/or 'fenestrations' (passages) through the endothelial cells - not large enough to let red blood cells through, but large enough for proteins and other molecules such as metabolites and hormones to pass. Outside the capillaries, surrounding the cells of the tissue, is an aqueous medium known as the interstitial fluid. For the most part, it is believed that substances diffuse from cells through the interstitial fluid into the capillaries, and from the capillaries through the interstitial fluid to cells, following concentration gradients (Fig. 1.12). Thus oxygen, at its highest concentration in the blood supply at the arterial end of the capillary, will diffuse towards cells which are using it and thus depleting its local concentration in interstitial

Cells of tissue

Cells of tissue

Fig. 1.12 Diffusion of chemical substances through the interstitial fluid. A typical tissue is shown (schematically) in cross-section. The diffusion of oxygen from erythrocytes to cells in the tissue is shown as an example. Oxygen diffuses down a concentration gradient, from the erythrocytes, via the plasma and the interstitial fluid, into the cells where its concentration is depleted as it is used in mitochondrial oxidation. CO2 diffuses back to the plasma in the same way. The interstitial fluid occupies the space between cells known as the extracellular space; this is not a true empty space, but in reality is occupied by glycoproteins and other molecules joining the cells. Nevertheless, it offers a path for diffusion of substances.

Fig. 1.12 Diffusion of chemical substances through the interstitial fluid. A typical tissue is shown (schematically) in cross-section. The diffusion of oxygen from erythrocytes to cells in the tissue is shown as an example. Oxygen diffuses down a concentration gradient, from the erythrocytes, via the plasma and the interstitial fluid, into the cells where its concentration is depleted as it is used in mitochondrial oxidation. CO2 diffuses back to the plasma in the same way. The interstitial fluid occupies the space between cells known as the extracellular space; this is not a true empty space, but in reality is occupied by glycoproteins and other molecules joining the cells. Nevertheless, it offers a path for diffusion of substances.

fluid; carbon dioxide will diffuse from cells which are generating it, and thus creating a high local concentration, into the capillaries where the concentration is lower because it is continuously being removed by the flow of blood. There are some substances for which this cannot be entirely true, especially the non-esterified fatty acids; this will be discussed in more detail later.

There are different types of capillaries: those with abundant fenestrations in the endothelial cells occur in tissues where there are high rates of exchange with the cells, for instance the mucosa (absorptive lining) of the small intestine, where substances are absorbed, and in endocrine tissues where there is rapid secretion of hormones. In the brain the endothelial cells are tightly joined to one another, and this is believed to be the structural basis of the 'blood-brain barrier'; a number of substances, including non-esterified fatty acids and many drugs, are thus denied access to the cells of the brain.

The capillaries in turn lead to larger and larger vessels, merging to form the major veins, through which blood returns to the heart. The returning blood enters the right ventricle, from where it is pumped through the lungs, collecting O2 and losing CO2; it then returns to the left heart and starts its journey anew.

The bloodstream is the major means of carrying substances from one tissue to another - for instance, it carries non-esterified fatty acids liberated from adipose tissue to other tissues where they will be oxidised, and it carries hormones from endocrine organs to their target tissues. The term the circulation is often used to mean 'the bloodstream'; we speak of a substance being carried in the circulation, or even of circulating glucose (for instance), meaning glucose in the bloodstream. In the metabolic diagrams used extensively later in this book, the clear area in which different organs and tissues sit is meant to represent the bloodstream, and it may be assumed that substances will be efficiently carried across these blank spaces from one tissue to another.

1.3.2 Blood, blood plasma and serum

The blood itself is an aqueous environment, consisting of the liquid plasma - a solution of salts, small organic molecules such as glucose and amino acids, and a variety of peptides and proteins - and the blood cells, mostly red blood cells (erythrocytes). The erythrocyte membrane is permeable to, or has carriers for, some molecules but not others. Glucose, for instance, partially equilibrates across the erythrocyte membrane. Its concentration is somewhat lower inside the cell than outside, since the erythrocyte uses some for glycolysis and transport across the cell membrane must be somewhat limiting for this process. But nevertheless, glucose and some amino acids are carried around both in blood cells and in the plasma. On the other hand, lipid molecules are excluded from red blood cells and carried in the plasma. On the whole, the term 'in the plasma' will be used for those substances confined to that compartment, and 'in the blood' or 'in the bloodstream' for those which are carried in both compartments.

If blood is allowed to clot and then centrifuged, a yellow fluid can be removed: this is serum. It is like plasma but lacks the protein fibrinogen, which is used in the clotting process. Serum is often collected from patients for measurement of the concentration of cholesterol or triacylglycerol, mainly because it is convenient to let the blood clot. The term 'serum cholesterol', for instance, then simply refers to the concentration of cholesterol in the serum; it would be almost exactly the same as the plasma cholesterol concentration.

1.3.3 Lymph and lymphatics

The interstitial fluid is formed by filtration of the blood plasma through the endothelium (vessel lining), as described earlier. Some of the fluid which leaves the bloodstream in this way will naturally find its way back to the blood vessels, but some is drained away from tissues in another series of vessels, the lymphatics. These are for the most part smaller than blood vessels. The fluid within them, the lymph, resembles an ultrafiltrate of plasma - i.e. it is like plasma but without red blood cells and without some of the larger proteins of plasma. The lymphatic vessels merge and form larger vessels and eventually discharge their contents into the bloodstream. We shall be concerned with one particular branch of the lymphatic system - that which drains the walls of the small intestine. The products of fat digestion enter these lymphatic vessels, which collect together and form a duct running up the back of the chest, known as the thoracic duct. The thoracic duct discharges its contents into the bloodstream in the upper chest. The lymphatic system also plays an important role in defence against infection, but this immunological role is beyond the scope of this book.

1.4 Further reading

General metabolic biochemistry and nutrition: other useful textbooks

Fell, D. (1997) Understanding the Control of Metabolism. London: Portland Press.

Salway, J.G. (1999) Metabolism at a Glance, 2nd edn. Oxford: Blackwell

Scientific Publications. Murray, R.K., Granner D.K., Mayes, P.A. & Rodwell, V.W. (2000) Harper's

Biochemistry, 25th edn. Stamford, CT: Appleton & Lange. Bender, D.A. (2002) Introduction to Nutrition and Metabolism, 3rd edn. London: Taylor & Francis.

You may also need one of the 'classical' biochemistry textbooks to give more detail of pathways and structures, for example:

Nelson, D.L. & Cox, M.M. (2000) Lehninger Principles of Biochemistry, 3rd edn. New York: Worth Publishers. Stryer, L. (1995) Biochemistry, 4th edn. New York: W.H. Freeman.

Harper's Biochemistry (Murray et al. 2000, above) may be a good compromise as it takes a 'physiological' approach to metabolism.

Note

1 Two important exceptions are mature white adipose tissue, in which triacylglycerol is the major constituent by weight, and the brain, of which 50-60% of dry weight is lipid (mostly phospholipid).

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