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Like carbohydrates, lipids, too, contain carbon, hydrogen, and oxygen, but the number of oxygen atoms in lipids is much lower than the number of hydrogen atoms in carbohydrates. This is why lipids (fats and oils) typically do not dissolve in water, or dissolve in water only very sparsely. Oxygen atoms make a substance polar because the oxygen atoms force electrons in covalent bonds to spend more time at the oxygen atom than at the other atom in the covalent bond. And because water itself is a highly polar substance, it dissolves substances that also are polar (likes dissolve likes). While lipids are not soluble in a polar liquid as water, they are soluble in nonpolar liquids such as chloroform and ether.

About 18 to 25 percent of the body of a lean adult human consists of lipids, and in an obese person, the lipid percentage is higher, depending on the degree of obesity.

Lipids come in a number of forms: as fatty acids, as triglycerides, as steroids, as phospholipids, as eicosanoids, as lipoiproteins, and as some vitamins.

Fatty acids are long chains of hydrocarbons with a carboxyl group (-COOH) at one end. The carboxyl group usually defines an organic acid. Hydrocarbons are methane, ethane, butane, propane, and so on. A fatty acid is saturated if it does not contain any carbon-carbon double bonds. If it contains one carbon-carbon double bond it is monounsaturated, if it contains more than one carbon-carbon double bond, it is polyunsaturated.

Olive oil and peanuts oil are monounsaturated fatty acids, and for that reason are considered conducive to the health of the cardiovascular system. A diet high in saturated fatty acids (animal fats), as well as high in cholesterol, is believed to contribute to the formation of fatty plaques in arteries (a condition referred to as atherosclerosis).

Triglycerides are lipids that combine a glycerol molecule with three fatty acids through dehydration synthesis. Glycerol is related to propanol (a three-carbon alcohol). However, in glycerol, three hydroxyl groups (-OH) are bonded to the three carbons of the carbon skeleton of glycerol. With propanol, it's only one hydroxyl group.

Triglycerides are stored in adipose tissue as energy reserves.

Phospholipids are similar to triglycerides. They are also build from a glycerol molecule and fatty acids. However, while triglycerides contain three fatty acids, phospholipids only have two. The third spot is occupied directly by a phosphate functional group (-PO42-), to which then another molecular group is connected that contains a nitrogen atom. A nitrogen atom often can accept a hydrogen proton more than its valence shell suggests, and thus create a positively charged group. Thus, with the phosphate group being singly negative charged, and the nitrogen-containing group being positively charged, a phospholipid has a strongly polar head (which is hydrophilic), while the tail of the two long fatty acids is nonpolar (and hydrophobic). This condition (a molecule having both a nonpolar and a polar region) is referred to as amphipathic. Because they are amphipathic, phospholipids are used by cells to form membranes.

Steroids are lipids with a structure that is completely different from the structures of triglycerides and phospholipids. Steroids contain four rings of carbon atoms, to which certain other molecules are attaches. In the human body, many hormones are steroids. All steroid hormones are synthesized from cholesterol, which is formed by the body in the liver. Sex hormones (hormones that occur in different concentrations in men and women, e.g. testosterone and estrogens) are steroids. Actually, testosterone and estradiol (the primary estrogen) are very similar inn chemical structure. Testosterone only contains an additional methyl group, while estradiol has a hydroxyl group where testosterone features an oxygen atom with a double bond to a carbon atom in the fourth ring. Cortisol is also a steroid hormone.

Eicosanoids are lipids which the body uses in a fashion similar to that of steroid hormones. However, while hormones relay chemical signals to target cells at a distance, the eicosanoids synthesized by cells relay signals only to the immediate surrounding of the cells that secret them. There are to groups of eicosanoids: prostaglandins and leukotrienes. Chemically, eicosanoids are 20-carbon fatty acids.

Other lipids in the body are beta-carotenes (which also cause the orange, yellow, or red color of egg yolk, carrots, and tomatoes), vitamins D, E, and K, as well as lipoproteins.

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Proteins are among the largest and most complicated organic molecules, and for the formation of living organisms, including humans, proteins are the most important structural elements.

Proteins fulfill a wide range of functions in the human body. Not only is much tissue protein-based (e.g. collagen and keratin). Proteins also serve as transport vehicle (e.g. hemoglobin in blood), as regulatory chemical (e.g. the hormone insulin), they are essential for movement (e.g. the muscle proteins myosin and actin), as well as for the immune response (e.g. antibodies and interleukins); and last not least, most enzymes are proteins, and thus proteins are essential for metabolism.

About 12 to 18 percent of the adult human body are proteins.

Like complex carbohydrates and nucleic acids, proteins, too, are polymers built up from monomers. The monomers of proteins are amino acids. All in all, 20 amino acids are used to synthesize all the different proteins of the human body. Amino acids consist of a central carbon atom to which three groups are covalently bonded: 1. an amino group, -NH2; 2. a carboxyl group, -COOH; 3. a rest group, -R. It is the rest group that differentiates the amino acids among each other. For the simplest amino acid, glycine, the rest group is just a single hydrogen atom.

Just like with carbohydrates, amino acids (the monomers of proteins) are joined by dehydration synthesis during which the -OH group of the carboxyl group and a hydrogen of the amino group are removed to form a water molecule. While the dehydration synthesis bonds among amino acids are called "peptide bonds", they are practically the same as the bonds that joined carbohydrates. And just as is the case with carbohydrates, breaking peptide bonds needs a molecule of water in a process referred to as "hydrolysis".

While in the case of sugars, we speak of monosaccharides, disaccharides, and polysaccharides, we differentiate between peptides, dipeptides, tripeptides, and polypeptides. When a polypeptide is sufficiently complex or a lasting or functional structure, it may be called a protein rather than a polypeptide, but there is no definite differentiation between the terms protein and polypeptide. However, when several chains of amino acids link to each other, only the term protein is used for identification.

While both, complex carbohydrates and proteins, are assembled from monomers, the bonds that can exist between peptides are more varied than those among carbohydrate monomers. Amino acids (the monomers of proteins) can bond to each other in four distinct ways, and therefore, we speak of the primary, secondary, tertiary, and quaternary structure of proteins.

The primary structure of a protein is just the sequence of amino acids it is made of. The bonds in the primary structure are all just peptide bonds.

The secondary structure refers to the way, the chain of amino acids is folded. Hydrogen bonds between subsequent amino acids cause the chain of aminoacids to fold in a particular manner, for example as alpha helix or as pleated sheet.

The tertiary structure is the overall form of an amino acid. A number of environmental factors play a role in the overall shape of amino acids. For amino acids in a watery environment, it will often be the case that hydrophobic sections of an amino acid chain will be found in the center of a polypeptide structure. Apart from environmental factors, so-called sulfide bridges also influence the tertiary structure of a protein. Sulfide bridges form between the side chains of the amino acid cysteine, which contains sulfur atoms. Disulfide bridges between the side chains of distant cysteine monomers within one polypeptide chain force a protein into a distinct three-dimensional shape.

The quaternary structure of a protein is only present in proteins that consist of two or more polypeptide chains, which are intertwined. Hydrogen bonds and disulfide bridges can form between two or several separate but intertwined polypeptide chains which together form a specific protein. However, not all proteins have a quaternary structure; only those that consist of more than one polypeptide chain do.

The bonds and environmental factors that account for the secondary, tertiary, and quaternary structure of proteins are much weaker than the peptide bonds of the primary structure. Nevertheless, a change in the secondary, tertiary, and quaternary structure makes a protein just as dysfunctional as an error in the primary structure. This is the case because many of the functions of proteins depend on a precise shape. When this shape is damaged, for example by heat, then the protein is denatured.


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