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Lipids are very diverse in both their respective structures and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are also soluble in other organic solvents such as ether, acetone, and other lipids. Lipids serve a variety of important functions in living organisms. They act as chemical messengers, serve as valuable energy sources, provide insulation, and are the main components of membranes. Major lipid groups include fats, phospholipids, steroids, and waxes.​

Key Takeaways: Lipids

Lipid Soluble Vitamins

Fat-soluble vitamins are stored in adipose tissue and in the liver. They are eliminated from the body more slowly than water-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. Vitamin A is important for vision as well as skin, teeth, and bone health. Vitamin D aids in the absorption of other nutrients including calcium and iron. Vitamin E acts as an antioxidant and also aids in immune function. Vitamin K aids in the blood clotting process and maintaining strong bones.

Organic Polymers

Biological polymers are vital to the existence of all living organisms. In addition to lipids, other organic molecules include:

Carbohydrates: biomolecules that include sugars and sugar derivatives. They not only provide energy but are also important for energy storage.

composed of amino acids, proteins provide structural support for tissues, act as chemical messengers, move muscles, and much more.

Nucleic Acids: biological polymers composed of nucleotides and important for gene inheritance. DNA and RNA are two types of nucleic acids.

Fats are composed of three fatty acids and glycerol. These so-called triglycerides can be solid or liquid at room temperature. Those that are solid are classified as fats, while those that are liquid are known as oils. Fatty acids consist of a long chain of carbons with a carboxyl group at one end. Depending on their structure, fatty acids can be saturated or unsaturated.

Saturated fats raise LDL (low-density lipoprotein) cholesterol levels in the blood. This increases the chances of developing cardiovascular disease. Unsaturated fats lower LDL levels and reduce the risk of disease. While fats have been denigrated to the point that many believe that fat should be eliminated from the diet, fat serves many useful purposes. Fats are stored for energy in adipose tissue, help to insulate the body, and cushion and protect organs.

Phospholipids

A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group, and a polar molecule. The phosphate group and polar head region of the molecule are hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the nonpolar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the water.

Phospholipids are a major component of cell membranes, which enclose and protect the cytoplasm and other contents of a cell. Phospholipids are also a major component of myelin, a fatty substance that is important for insulating nerves and speeding up electrical impulses in the brain. It is the high composition of myelinated nerve fibers that causes white matter in the brain to appear white.

Steroids and Waxes

Steroids have a carbon backbone that consists of four fused ring-like structures. Steroids include cholesterol, sex hormones (progesterone, estrogen, and testosterone) produced by gonads and cortisone.

Waxes are composed of an ester of long-chain alcohol and a fatty acid. Many plants have leaves and fruits with wax coatings to help prevent water loss. Some animals also have wax-coated fur or feathers to repel water. Unlike most waxes, ear wax is composed of phospholipids and esters of cholesterol.

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Lipids are very diverse in both their respective structures and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are also soluble in other organic solvents such as ether, acetone, and other lipids.

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Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose).

Carbohydrates are:

a source of energy for the body e.g. glucose and a store of energy, e.g. starch in plants

building blocks for polysaccharides (giant carbohydrates), e.g. cellulose in plants and glycogen in the human body

components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP

Monosaccharides are the simplest carbohydrates and are often called single sugars. They are the building blocks from which all bigger carbohydrates are made.

Monosaccharides have the general molecular formula (CH ₂O)n, where n can be 3, 5 or 6. They can be classified according to the number of carbon atoms in a molecule:

n = 3

trioses, e.g. glyceraldehyde

n = 5

pentoses, e.g. ribose and deoxyribose ('pent' indicates 5)

n = 6

hexoses, e.g. fructose, glucose and galactose ('hex' indicates 6)

There is more than one molecule with the molecular formula C ₅H ₁₀O ₅ and more than one with the molecular formula C ₆H ₁₂O ₆. Molecules that have the same molecular formula but different structural formulae are called structural isomers.

Glyceraldehyde's molecular formula is C ₃H ₆O ₃. Its structural formula shows it contains an aldehyde group (-CHO) and two hydroxyl groups (-OH). The presence of an aldehyde group means that glyceraldehyde can also be classified as an aldose. It is a reducing sugar and gives a positive test with Benedict's reagent.

CH ₂OHCH(OH)CHO is oxidised by Benedict's reagent to CH ₂OHCH(OH)COOH; the aldehyde group is oxidised to a carboxylic acid and Benedict's reagent is reduced (Cu ²⁺ to Cu⁺).

Pentoses and hexoses can exist in two forms: cyclic and non-cyclic. In the non-cyclic form their structural formulae show they contain either an aldehyde group or a ketone group.

Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. Aldoses are reducing sugars; ketoses are non-reducing sugars. This is important in understanding the reaction of sugars with Benedict's reagent.

However, in water pentoses and hexoses exist mainly in the cyclic form, and it is in this form that they combine to form larger saccharide molecules.

Glucose is the most important carbohydrate fuel in human cells. Its concentration in the blood is about 1 gdm⁻³. The small size and solubility in water of glucose molecules allows them to pass through the cell membrane into the cell. Energy is released when the molecules are metabolised (C ₆H ₁₂O ₆ + 6O ₂ 6CO ₂ + 6H ₂O). This is part of the process of respiration.

There are two forms of the cyclic glucose molecule: α-glucose and β-glucose.

Two glucose molecules react to form the dissacharide maltose. Starch and cellulose are polysaccharides made up of glucose units.

Galactose molecules look very similar to glucose molecules. They can also exist in α and β forms. Galactose reacts with glucose to make the dissacharide lactose.

However, glucose and galactose cannot be easily converted into one another. Galactose cannot play the same part in respiration as glucose.

This comparison of glucose and galactose shows why the precise arrangement of atoms in a molecule (shown by the displayed formula) is so important.

Fructose, glucose and galactose are all hexoses. However, whereas glucose and galactose are aldoses (reducing sugars), fructose is a ketose (a non-reducing sugar). It also has a five-atom ring rather than a six-atom ring. Fructose reacts with glucose to make the dissacharide sucrose.

α-fructose - click on image to open

Ribose and deoxyribose are pentoses. The ribose unit forms part of a nucleotide of RNA. The deoxyribose unit forms part of the nucleotide of DNA.

Monosaccharides are rare in nature. Most sugars found in nature are disaccharides. These form when two monosaccharides react.

Learn more about the disaccharide maltose

A condensation reaction takes place releasing water. This process requires energy. A glycosidic bond forms and holds the two monosaccharide units together.

Learn more about the disaccharide lactose

The three most important disaccharides are sucrose, lactose and maltose. They are formed from the a forms of the appropriate monosaccharides. Sucrose is a non-reducing sugar. Lactose and maltose are reducing sugars.

Disaccharide

Monosaccharides

sucrose

from

α-glucose + α-fructose

maltose

from

α-glucose + α-glucose

α-lactose *

from

α-glucose + β-galactose

* Lactose also exists in a beta form, which is made from β-galactose and β-glucose

Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion. They are broken down in the small intestine during digestion to give the smaller monosaccharides that pass into the blood and through cell membranes into cells.

C ₁₂H ₂₂O ₁₁ + H ₂O C ₆H ₁₂O ₆ + C ₆H ₁₂O ₆

This is a hydrolysis reaction and is the reverse of a condensation reaction. It releases energy.

Monosaccharides are used very quickly by cells. However, a cell may not need all the energy immediately and it may need to store it. Monosaccharides are converted into disaccharides in the cell by condensation reactions. Further condensation reactions result in the formation of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell. These are broken down by hydrolysis into monosaccharides when energy is needed by the cell.

Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. The properties of a polysaccharide molecule depend on:

its length (though they are usually very long)

the extent of any branching (addition of units to the side of the chain rather than one of its ends)

any folding which results in a more compact molecule

whether the chain is 'straight' or 'coiled'

Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose and amylopectin. Both are made from α-glucose. Amylose is an unbranched polymer of α-glucose. The molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a branched polymer of α-glucose. It is completely insoluble in water.

Section of the amylose molecule

Section of the amylopectin molecule

Glycogen is amylopectin with very short distances between the branching side-chains. Starch from plants is hydrolysed in the body to produce glucose. Glucose passes into the cell and is used in metabolism. Inside the cell, glucose can be polymerised to make glycogen which acts as a carbohydrate energy store.

Cellulose is a third polymer made from glucose. But this time it's made from β-glucose molecules and the polymer molecules are 'straight'.

Section of a cellulose molecule

Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of glucose units in the polymer chain and the hydrogen-bonding between neighbouring chains.

Cellulose is not hydrolysed easily and, therefore, cannot be digested so it is not a source of energy for humans. The stomachs of Herbivores contain a specific enzyme called cellulase which enables them to digest cellulose.
Take quiz on Carbohydrates

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Carbohydrates (also called saccharides) are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose). Carbohydrates are: a source of energy for the body e.g.

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Amino acids are a crucial, yet basic unit of protein, and they contain an amino group and a carboxylic group. They play an extensive role in gene expression process, which includes an adjustment of protein functions that facilitate messenger RNA (mRNA) translation (Scot et al., 2006).

There are over 700 types of amino acids that have been discovered in nature. Almost all of them are α-amino acids. They have been found in:
* bacteria
* fungi
* algae
* plants.

The amino acids are essential components of peptides and proteins. Twenty important amino acids are crucial for life as they contain peptides and proteins and are known to be the building blocks for all living things on earth. They are used for a protein synthesis. The amino acids are controlled by genetics. Some unusual amino acids are found in plant seeds.

The amino acids are imperative for sustaining the health of a human body. They largely promote the:
* hormones production
* muscles structure
* nervous system
* vital organs working
* cellular functioning.

If amino acids are deficient the protein synthesis would stop.

Without alpha-amino acids human may experience fatigue, irritability, hormonal imbalances and sometimes even depression.

Abbreviations

To understand the amino acids' abbreviation, it is important to know why their names have been shorten in the first place. A reason is to make them easy to identify and to use more manageable three-letter system. For instance, the simplest amino acid, glycine is depicted as H-Gly-OH, with the "H" and the "OH" being "H2O", which represents the H2O at the time of amino acid condensing in order to form a peptide.

Another way to look at the three-letter abbreviation system is that it captures the amino acid residual state, which comprises proteins and peptides. When the system was introduced, it was thought primarily to save space, rather than simplify amino acid names. It is important to know that, when one-letter system is used, such as "G" for glycine, which is more commonly used nowadays, it is referring to synthesized peptides from the coded amino acids groups.

How Amino Acids were Discovered

The amino acids are a result of protein hydrolysis. Throughout the centuries, amino acids have been discovered in a variety of ways, though primarily by way of chemists and biochemists of high intelligence who possessed the greatest skills and patience and who were innovative and creative in their work.

Protein chemistry is age-old, with some dating back thousands of years ago. Processes and technical applications such as glue preparation, cheese manufacturing and even the discovery of ammonia via the filtering of dung, occurred centuries ago. Moving forward in time to 1820, Braconnot prepared glycine directly from gelatin. He was attempting to uncover whether proteins acted like starch or whether they are made of acids and sugar.

While progress was slow at that time, it has since gained plenty of speed, although the complicated processes of protein composition have not entirely been uncovered even to this day. But many years have gone by since Braconnot first initiated such observations.

Much more should be discovered in the analysis of amino acids as well as finding new amino acids. The future of protein and amino acids chemistry is lying in biochemistry. Once that is accomplished-but only until then will our knowledge of amino acids and proteins be satiated. Yet it is likely that day will not come anytime soon. This all adds to the mystery, complexities and strong scientific value of amino acids.

Classifications of Amino Acids

Experts classify amino acids based on a variety of features, including whether people can acquire them through diet. Accordingly, scientists recognize three amino acid types:
1. Nonessential
2. Essential
3. Conditionally essential

However, the classification as essential or nonessential does not actually reflect their importance as all 20 amino acids are necessary for human health.

Eight of these amino acids are essential (or indispensable) and cannot be produced by the body. They are:
* Leucine
* Isoleucine
* Lysine
* Threonine
* Methionine
* Phenylalanine
* Valine
* Tryptophan

Histidine is an amino acid that is categorized as semi-essential since the human body doesn't always need it to properly function therefore dietary sources of it are not always essential. Meanwhile, conditionally essential amino acids aren't usually required in the human diet, but do become essential under certain circumstances.

Finally, nonessential amino acids are produced by the human body either from essential amino acids or from normal protein breakdowns. Nonessential amino acids include:
* Asparagine
* Alanine
* Arginine
* Aspartic acid
* Cysteine
* Glutamic acid
* Glutamine
* Proline
* Glycine
* Tyrosine
* Serine

An additional amino acids' classification depends upon the side chain structure, and experts recognize these five as:
* Cysteine and Methionine (amino acids containing sulfur)
* Asparagine, Serine, Threonine, and Glutamine (neutral amino acids)
* Glutamic acid and Aspartic acid (acidic); and Arginine and Lysine (basic)
* Leucine, Isoleucine, Glycine, Valine, and Alanine (aliphatic amino acids)
* Phenylalanine, Tryptophan, Tyrosine and Histidine (aromatic amino acids)

One final amino acid classification is categorized by the side chain structure that divides the list of 20 amino acids into four groups - two of which are the main groups and two that are subgroups. They are:
1. Non-polar
2. Polar
3. Acidic and polar
4. Basic and polar

For example, side chains having pure hydrocarbon alkyl or aromatic groups are considered non-polar, and these amino acids are comprised of Phenylalanine, Glycine, Valine, Leucine, Alanine, Isoleucine, Proline, Methionine and Tryptophan. Meanwhile, if the side chain contains different polar groups like amides, acids and alcohols, they are classified as polar. It includes Tyrosine, Serine, Asparagine, Threonine, Glutamine, and Cysteine. If the side chain contains carboxylic acid, the amino acids in the acidic-polar classification are Aspartic Acid and Glutamic Acid. Furthermore, if the side chain consists of a carboxylic acid and basic-polar, these amino acids are Lysine, Arginine, and Histidine.

Properties of Amino Acids

The properties of α-amino acids are complex, yet simplistic in that every molecule of an amino acid involves two functional groups: carboxyl (-COOH) and amino (-NH2).

Each molecule can contain a side chain or R group, e.g. Alanine is an example of standard amino acid containing methyl side chain group. The R groups have a variety of shapes, sizes, charges, and reactivities. This allows amino acids to be grouped according to the chemical properties of their side chains.

Table of common amino acid abbreviations and properties

Amino acids are crystalline solids which usually are water soluble and only sparingly dissoluble in organic solvents. Their solubility depends on the size and nature of the side chain. Amino acids have very high melting points, up to 200-300°C. Their other properties varying for each particular amino acid.

20 Amino Acids and their Functions

Only 20 amino acids are found in the human peptides and proteins. These naturally occurring amino acids are used by cells to synthesize peptides and proteins. They are typically identified by generic formula: H ₂ NCHRCOOH.

The primary difference between the 20 amino acids is a different structure of R group. Below the essential amino acids and their respective functions are shown.

Non-polar, aliphatic residues

Aromatic residues

Polar, non-charged residues

Positively charged residues

Negatively charged residues

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Amino acids are a crucial, yet basic unit of protein, and they contain an amino group and a carboxylic group. They play an extensive role in gene expression process, which includes an adjustment of protein functions that facilitate messenger RNA (mRNA) translation (Scot et al., 2006).

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Chemistry of amino acids and protein structure

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TLC Procedure