Macromolecules

Characteristics of macromolecules

The characteristics of macromolecules are made up of smaller molecules that are covalently bonded. The small molecules inside the macromolecules are known as monomers, and the macromolecules are known as polymers.

Monomers and polymers are primarily made up of carbon (C), but they can also have hydrogen (H), nitrogen (N), oxygen (O), and potentially traces of additional elements.

Macromolecules and micromolecules

Micromolecules are another name for the monomers of macromolecules.

• Carbohydrate micromoelcules are monosaccharides, also known as simple sugars.

• Protein micromolecules are amino acids.

• Lipid micromolecules are glycerol and fatty acids.

• Nucleic acid monomers are nucleotides.

Types of macromolecules

There are many different types of macromolecules. The four we will focus on are carbohydrates, proteins, lipids (fats) and nucleic acids.

Carbohydrates

Carbohydrates are made of hydrogen, carbon, and oxygen.

Carbohydrates can be broken down into two categories: simple carbohydrates and complex carbohydrates.

Simple carbohydrates are monosaccharides and disaccharides. Simple carbohydrates are small molecules composed of only one or two molecules of sugars.

• Monosaccharides are composed of one molecule of sugar.

  • They are soluble in water.

  • Monosaccharides are building blocks (monomers) of larger molecules of carbohydrates called polysaccharides (polymers).

  • Examples of monosaccharides: glucose, galactose, fructose, deoxyribose, and ribose.

• Disaccharides are composed of two molecules of sugar (di- stands for 'two').
  • Disaccharides are soluble in water.
  • Examples of the most common disaccharides are sucrose, lactose, and maltose.
  • Sucrose is composed of one molecule of glucose and one of fructose. In nature, it is found in plants, where it is refined and used as table sugar.
  • Lactose is composed of one molecule of glucose and one of galactose. It is a sugar found in milk.
  • Maltose is composed of two molecules of glucose. It is a sugar found in beer.

Complex carbohydrates are polysaccharides. Complex carbohydrates are molecules composed of a chain of sugar molecules that are longer than simple carbohydrates.

• Polysaccharides (poly- means 'many') are large molecules composed of many molecules of glucose, i.e., individual monosaccharides.
  • Polysaccharides are not sugars, even though they are composed of glucose units.
  • They are insoluble in water.
  • Three very important polysaccharides are starch, glycogen, and cellulose.

Proteins

Proteins are one of the most fundamental molecules in all living organisms. Proteins are made of amino acids, and are present in every single cell in living systems, sometimes in numbers larger than a million, where they allow for various essential chemical processes, such as DNA replication. There are four different types of proteins depending on the structure of the protein itself.

Lipids

There are two main types of lipids: triglycerides and phospholipids.

Triglycerides

Triglycerides are lipids that include fats and oils. Fats and oils are the most common types of lipids found in living organisms. The term triglyceride comes from the fact that they have three (tri-) fatty acids attached to glycerol (glyceride). Triglycerides are entirely insoluble in water (hydrophobic).

The building blocks of triglycerides are fatty acids and glycerol. Fatty acids that build triglycerides can be saturated or unsaturated. Triglycerides composed of saturated fatty acids are fats, while those consisting of unsaturated fatty acids are oils. They help with energy storage.

Phospholipids

Like triglycerides, phospholipids are lipids built of fatty acids and glycerol. However, phospholipids are composed of two, not three, fatty acids. Like in triglycerides, these fatty acids can be saturated and unsaturated. One of the three fatty acids that attach to glycerol is replaced with a phosphate-containing group.

The phosphate in the group is hydrophilic, meaning it interacts with water. This gives phospholipids one property that triglycerides don't have: one part of a phospholipid molecule is soluble in water. Phospholipids aid in cell recognition.

Nucleic acids

Nucleic acids store and maintain genetic information within an organism. There are two forms of nucleic acids, DNA and RNA. DNA and RNA are made up of nucleotides, the monomers for nucleic acids.

Examples of macromolecules

While macromolecules are found in all foods, different foods will have higher amounts of macromolecules than other foods. For example, meat would have more protein than an apple.

Macromolecule functions

Different macromolecules have different functions, but they all have the same goal of keeping an organism alive!

Carbohydrates functions

Carbohydrates are essential in all plants and animals as they provide much-needed energy, mostly in the form of glucose.

Proteins functions

Proteins have a vast array of functions in living organisms. According to their general purposes, we can group them into fibrous, globular, and membrane proteins.

Fibrous proteins are structural proteins that are, as the name suggests, responsible for the firm structures of various parts of cells, tissues, and organs. They do not participate in chemical reactions but strictly operate as structural and connective units.

Globular proteins are functional proteins. They perform a much wider range of roles than fibrous proteins. They act as enzymes, carriers, hormones, receptors, etc. Essentially, globular proteins carry out metabolic functions.

Lipids functions

Lipids have numerous functions that are significant for all living organisms:

• Energy storage (Fatty acids are used to store energy in organisms, they are saturated in animals and unsaturated in plants)

• Structural components of cells (Lipids make up the cell membranes in organisms)

• Cell recognition (Glycolipids aid in this process by binding to receptors on neighboring cells)

• Insulation (Lipids found under the skin are able to insulate the body and maintain a constant internal temperature)

• Protection (Lipids are also able to provide an extra layer of protection, for example, vital organs will have fat surrounding them to protect them from harm)

• Hormone regulation (Lipids are able to help regulate and produce necessary hormones in the body such as leptin, a hormone that prevents hunger)

Nucleic acids functions

Depending on whether it is RNA or DNA, nucleic acids will have differing functions.

DNA functions

The chief function of DNA is to store genetic information in structures called chromosomes. In eukaryotic cells, DNA can be found in the nucleus, the mitochondria, and the chloroplast (in plants only). Meanwhile, prokaryotes carry DNA in the nucleoid, which is a region in the cytoplasm, and plasmids.

RNA functions

RNA transfers genetic information from the DNA found in the nucleus to the ribosomes, specialized organelles comprised of RNA and proteins. The ribosomes are especially important as translation (the final stage of protein synthesis) occurs here. There are different types of RNA, such as messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), each with its specific function.

Macromolecules structures

Macromolecule structures play a vital role in their function. Here we explore the various macromolecule structures of each type of macromolecule.

Carbohydrate structure

Carbohydrates are composed of molecules of simple sugars - saccharides. Therefore, a single monomer of carbohydrates is called a monosaccharide. Mono- means 'one,' and -sacchar means 'sugar.' Monosaccharides can be represented by their linear or ring structures. Disaccharides will have two rings and polysaccharides will have multiple.

Protein structure

The basic unit in the protein structure is an amino acid. Amino acids are joined together by covalent peptide bonds, which form polymers called polypeptides. Polypeptides are then combined to form proteins. Therefore, you can conclude that proteins are polymers composed of amino acids and monomers.

Amino acids are organic compounds composed of five parts:

• the central carbon atom, or the α-carbon (alpha-carbon)
• amino group -NH₂
• carboxyl group -COOH
• hydrogen atom -H
• R side group, which is unique to each amino acid

Also, based on the sequence of amino acids and the complexity of the structures, we can differentiate four structures of proteins: primary, secondary, tertiary, and quaternary.

The primary structure is the sequence of amino acids in a polypeptide chain. The secondary structure refers to the polypeptide chain from the primary structure folding in a certain way in specific and small sections of the protein. When the secondary structure of proteins starts to fold further to create more complex structures in 3D, the tertiary structure is formed. The quaternary structure is the most complex of them all. It forms when multiple polypeptide chains, folded in their specific way, are bonded with the same chemical bonds.

Fig. 2. The four protein structures.

Lipids structure

Lipids are composed of glycerol and fatty acids. The two are bonded with covalent bonds during condensation. The covalent bond that forms between glycerol and fatty acids is called the ester bond. Triglycerides are lipids with one glycerol and three fatty acids, while phospholipids have one glycerol, a phosphate group, and two fatty acids instead of three.

Nucleic acids structure

Depending on whether it is DNA or RNA, nucleic acids can have different structures.

DNA structure

The DNA molecule is an anti-parallel double helix formed of two polynucleotide strands. It is anti-parallel, as the DNA strands run in opposite directions to each other. The two polynucleotide strands are joined together by hydrogen bonds between complementary base pairs, which we will explore later. The DNA molecule is also described as having a deoxyribose-phosphate backbone - some textbooks may also call this a sugar-phosphate backbone.

RNA structure

The RNA molecule is a little different to DNA in that it is made of only one polynucleotide which is shorter than DNA. This helps it carry out one of its primary functions, which is to transfer genetic information from the nucleus to the ribosomes - the nucleus contains pores that mRNA can pass through due to its small size, unlike DNA, a larger molecule. Below in Figure 4, you can visually see how DNA and RNA differ from each other, both in size and the number of polynucleotide strands.

Fig. 4. DNA vs RNA structure.