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Biology 2: Chemical Bases of the Life 1

A Biomedical science student, certificated in english, spanish, french and japanese language, who also writes diverse topic articles.



• Introduction to chemical bases of life

• Dehydration synthesis

• Hydrolysis

• Carbohydrates

• Lipids

Introduction to chemical bases of life

Macromolecules, being the large biological molecules, perform a wide variety of functions within each organism. These macromolecules can be divided into carbohydrates, lipids, nucleic acids, and proteins. Some carbohydrates contain energy that can be used or stored within the organism, some lipids are essential structural components of cell membranes, nucleic acids store and transfer hereditary information, among other functions. Proteins probably have the widest range of functions: some provide structural support, but many carry out specific jobs in a cell, such as catalyzing metabolic reactions or receiving and transmitting signals.

Most large biological molecules are polymers, that is, long chains made up of repeating structural units, called monomers. Carbohydrates, nucleic acids and proteins are found in nature in the form of long polymers, therefore they are classified as macromolecules. The lipids, which are not normally polymers, but simple molecules and smaller than the other three that are not formally considered as macromolecules because they do not reach the size of one. However, many other sources use the term "macromolecule" more generally, as a common name for the four types of biological molecules.

Dehydration synthesis

Macromolecules are formed from monomers, and are usually formed by dehydration synthesis reactions, in which a monomer is covalently linked to another monomer to form a growing chain of monomers, during this process a water molecule is released into the process, which is why it is known as dehydration synthesis.

One of the molecules loses an H, the other loses an OH, and a water molecule is released while a new covalent bond is formed between the two molecules. As more monomers are added by the same process, the chain becomes longer and longer, and a polymer is formed.

Although polymers are formed by repeating monomeric units, there is plenty of varied shapes and compositions. Carbohydrates, nucleic acids, and proteins can contain several different types of monomers, the composition and sequence of them are important to their function. For example, human DNA has four types of nucleotide monomers, as well as 20 types of amino acid monomers proteins. Even a single type of monomer can form different polymers with different properties. For example, starch, glycogen, and cellulose are carbohydrates made up of glucose monomers, but they have different binding and branching patterns.


Polymers break down into monomers through hydrolysis reactions, in which a bond is broken when a water molecule is incorporated into the reaction.
During a hydrolysis reaction, a molecule composed of several subunits splits into two: one of the new molecules gains a hydrogen atom, while the other gains a hydroxyl group (-OH), both of which are donated by water.

Hydrolysis reactions degrade molecules and often release energy. Carbohydrates, proteins and nucleic acids are degraded by these types of reactions, although the monomers involved are different in each case.

In the organisms, enzymes have different functions, one of them is to catalyze, or accelerate, dehydration synthesis reactions and hydrolysis reactions; the enzymes involved in bond breaking are often given names that end with -ase. For example, the enzyme maltase breaks down maltose, lipase break down lipids, and peptidases break down peptide bonds (proteins).



Carbohydrates are biological molecules made up of carbon, hydrogen and oxygen in a ratio of approximately one carbon atom (C) to each molecule of water (H2O). This composition is what gives carbohydrates their name: carbon (carbo-) plus water (-hydrate). Carbohydrate chains have different lengths, and biologically important carbohydrates fall into three categories: monosaccharides, disaccharides, and polysaccharides.

  • Monosaccharides [mono (one), sacchar (sugar)] are simple sugars, the most common of which is glucose. They have a formula of (CH2O)n and usually contain three to seven carbon atoms.
  • Most of the oxygen atoms in monosaccharides are found in hydroxyl groups (OH), but one of them is part of a carbonyl group (C = O).
  • The position of the carbonyl group (C = O) can serve to classify sugars, for example, if the sugar has an aldehyde group (if the C carbonyl is the last in the chain), it is called aldose; if the carbonyl is inside the chain (it has other carbons on both sides), it forms a ketone group and the sugar is called ketosis.
  • Sugars are also named according to the number of carbons they are made of: some of the most common types are trioses (three carbons), pentoses (five carbons), and hexoses (six carbons).

An important monosaccharide is glucose, a six-carbon sugar with the formula C6H12O6.

Other common monosaccharides are galactose (which is part of lactose or milk sugar) and fructose (which is found in fruits).

Glucose, galactose and fructose have the same formula (C6H12O6), but they differ in the organization of their atoms, so they are isomers, that is, molecules with identical molecular formulas (the same number of atoms of each element) but different arrangements of atoms in space. Fructose is a structural isomer of glucose, while galactose has its atoms linked in a different order.

Glucose and galactose are stereoisomers, which means, their atoms are linked in the same order, but they have a different organization around one carbon. This difference is enough for the enzymes to be able to distinguish between them.

Glucose and galactose are stereoisomers, which means, their atoms are linked in the same order, but they have a different organization around one carbon. This difference is enough for the enzymes to be able to distinguish between them.

Sugars are actually represented in the form of rings despite being carbon chains, although they can be presented in both ways. These forms exist in equilibrium with each other, although annular forms are more common, especially in aqueous media. For example, in an aqueous solution, the main configuration of glucose is a six-carbon ring, which is an hexose; and all glucose is in this form.

During ring formation, the carbonyl oxygen will be trapped "above" on the same side as the CH2OH group or "below" the opposite side of this ring group, becoming a hydroxyl group. When the hydroxyl is down, glucose is said to be in its alpha (α) form and when it is up, it is said to be in its beta (β) form.

  • Disaccharides are formed when two monosaccharides join together through a dehydration reaction, also known as a condensation reaction or dehydration synthesis. In this process, the hydroxyl group of one monosaccharide combines with the hydrogen of another, liberates a water molecule, and forms a covalent bond known as a glycosidic bond.
  • Each carbon atom in a monosaccharide is assigned a number, starting with the terminal carbon closest to the carbonyl (CO) group (when the sugar is in its linear form). In this way it can be determined in which way the monosaccharides are attached.
  • Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide made up of glucose and galactose and is found naturally in milk. Maltose, or malt sugar, is a disaccharide made up of two glucose molecules. The most common disaccharide is sucrose (table sugar), which is made up of glucose and fructose.
  • A long chain of monosaccharides linked by glycosidic bonds is called a polysaccharide. The chain can be branched or unbranched and can contain different types of monosaccharides. The molecular weight of a polysaccharide can be very high, 100,000 daltons or more if enough monomers are attached. Starch, glycogen, cellulose, and chitin are some of the main examples of important polysaccharides in living organisms.


Lipids are hydrophobic molecules, nonpolar and made up mainly of chains of fatty acids and carbohydrates. Different types of lipids can have different structures and therefore different functions in organisms. For example, storing energy, providing thermal insulation, shaping cell membranes, forming impermeable layers in leaves, and building the building blocks for hormones such as testosterone.

Fats and oils: A fat molecule consists of two parts, a glycerol backbone and three fatty acid tails. Glycerol is a small organic molecule with three hydroxyl groups (OH), while a fatty acid consists of a long chain of carbohydrates attached to a carboxyl group. A typical fatty acid has between 12 and 18 carbons, although some may have as little as 4 or as many as 36.

To form a fat molecule, each of the hydroxyl groups on the glycerol backbone must react with the carboxyl group on the fatty acid through a dehydration synthesis reaction. This process produces a fat molecule with three fatty acid tails attached to the glycerol backbone via ester bonds (containing an oxygen atom along with a carbonyl or C = O group). Triglycerides can have three identical or different fatty acid tails (differing in length or double bond pattern).

Fat molecules are also known as triacylglycerols or triglycerides.

Fat molecules are also known as triacylglycerols or triglycerides.

Saturated and unsaturated fatty acids: As we mentioned, the three fatty acid tails of a triglyceride do not necessarily have to be identical, since in addition to the difference in length, the fatty acid chains also differ in their degree of unsaturation. Which means, if there are only single bonds between carbons in the carbohydrate chain, a fatty acid is saturated (fatty acids are saturated with hydrogen; in a saturated fat, there are as many hydrogen atoms attached to the carbon skeleton as may be possible). On the other hand, when the carbohydrate chain contains a double bond, the fatty acid is said to be unsaturated as it now has less hydrogens. If there is only one double bond in a fatty acid, it is monounsaturated, while if there are several double bonds, it is polyunsaturated.

Double bonds in unsaturated fatty acids, like other types of double bonds, can exist in a trans or cis configuration. In the cis configuration, the two hydrogens associated with the bond are on the same side, while in the trans configuration they are on opposite sides. A cis double bond generates a taper or bend in the fatty acid, a characteristic that has important consequences for the behavior of fats.

The saturated fatty acid tails are straight, so, saturated fat molecules can be packed tightly, producing solid fats. For example, most of the fat in butter is saturated.
In contrast, in cisunsaturated fatty acids, the tails are doubled due to the presence of a cis double bond. This makes it difficult to compact fat molecules with one or more cis-unsaturated fatty acid tails, so they tend to be in a liquid state. These molecules are commonly known as oils. For example, olive oil is composed mainly of unsaturated fats.


Omega fatty acids: There are different types of omega-3 and omega-6 fatty acids, but they all come from two basic precursor forms, alpha-linolenic acid (ALA) for omega-3 and linoleic acid (LA) for omega- 6. Some fish, like salmon, and some seeds, like chia and flaxseed, are good sources of omega-3 fatty acids. Omega-3 and omega-6 fatty acids have at least two cis-unsaturated bonds, giving them a curved shape.

Omega-3 and omega-6 fatty acids perform various functions in organisms. They are the precursors (raw material) for the synthesis of a number of important molecules, such as those that regulate inflammation and mood (hormones). Omega-3 fatty acids in particular can reduce the risk of sudden death from heart attacks, lower blood triglycerides, lower blood pressure, and prevent blood clots.


Phospholipids: Cells are surrounded by a structure called the plasma membrane, which serves as a barrier between the interior of the cell and its environment. The main components of the plasma membrane are specialized lipids called phospholipids. Like fats, they are typically made up of fatty acid chains attached to a glycerol backbone. However, instead of having three fatty acid tails, they have only two and the third carbon of the glycerol backbone is occupied by a modified phosphate group.

Different phospholipids have different modifiers on the phosphate group; the most common examples are choline (a nitrogen compound) and serine (an amino acid). Different modifiers provide phospholipids with different characteristics and functions in cells.

The fatty acid chains that form the phospholipid tails are hydrophobic as they do not interact with water, while the phosphate-containing group is hydrophilic and easily interacts with water. In a membrane, phospholipids are arranged in a structure called a bilayer, with their phosphate heads on the water side and their tails pointing inward. This organization prevents hydrophobic glues from coming into contact with water, making it a stable arrangement.


Steroids: Steroids are another class of lipid molecules, identified by their four-molecule structure in the form of fused rings. Although structurally they do not resemble other lipids, steroids are included in this category because they are also hydrophobic and insoluble in water. All steroids have four linked carbon rings and several of them, like cholesterol, also have a short tail. Many steroids have an -OH functional group attached to a specific site, as shown in the cholesterol in the illustration below; These steroids are also classified as alcohols and are called sterols.


© 2021 Daniela Alejandra Rodríguez Cerda


E Randall from United States on April 16, 2021:

Thank you for this, reading this made me feel like I am taking chemistry 1 all over again. Thanks again for this.

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