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Biochemistry of Food: Structures, Reactions and Energy


Biological macromolecules are large organic molecules that are built from smaller ones. There are four major classes of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each one performs a diverse set of functions and are crucial for healthy cellular function.  In this lesson we will look into the structure and function of each biological macromolecule and the reactions that build them up and break them down. 


TL; DR

  • Lipids, proteins, carbohydrates, and nucleic acids are the four major biological macromolecules.
  • Monomers can be joined by together to create polymers.
  • Macromolecules are synthesized by dehydration synthesis, a process that removes a water molecule from two individual molecules to create a covalent bond joining them together.
  • Macromolecules are broken down by hydrolysis, a process that adds a water molecule to a molecule separating it into two separate molecules.


Lipids


Lipids include a diverse group of hydrocarbons that are mostly nonpolar. Looking at the image below, you can see what we mean by hydrocarbons: a chain of carbons that are surrounded by mostly hydrogen. Lipids are nonpolar because these the covalent bonds between hydrocarbons are nonpolar. As a result, lipids tend to be hydrophobic or “water fearing”. This means that lipids aren’t soluble in water. To see what this means, you can try an experiment for yourself. Grab a small cup of water and add some oil. Try your best to mix them together by shaking vigourously. For a moment they may have appeared to be mixed, but let the solutions sit for a few minutes and you’ll see that they separate back into their respective solutions. They fail to mix because lipids are nonpolar while water is polar. The reason why nonpolar and polar molecule don’t mix is related to their chemistry.

Importance of Lipids


We tend to think of lipids as a way for cells to store energy, but they’re actually involved many different functions in biology. Lipids are precurosors to the synthesis of many hormones and an they are also a very important constituent of the cell membrane. For some animals and plants, lipids can also provide insulation and protection. Lipids are a diverse class of molecules. Some examples of of molecules that fall into this class are fats, waxes, phospholipids, and steroids.


Triglyceride Structure


One type of lipid molecule that is particularly important for storing energy is the triglyceride. Triglycerides are consist of two main components: glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. Fatty acids have a long chain of hydrocarbons to which a carboxyl (COOH) group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36, but most are between 12–18 carbons. The fatty acids are attached to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom.

    • → Glycerol – structural component of triglycerides; each -OH is available to become dehydrated and bind with a fatty acid.
    • → Fatty Acid – a long hydrocarbon with a terminal -OH group; binds with glycerol to form 
    • → Triglyceride – energy storage molecule; glycerol (1) and fatty acid (3)

 

Lipid Review Questions

Key Terms

  • Lipid
  • Triglyceride
  • Gycerol
  • Fatty Acid
    • Unsaturated
    • Saturated

Carbohydrates


Most people are familiar with carbohydrates, one type of macromolecule, especially when it comes to what we eat. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have enough energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet; grains, fruits, and vegetables are all natural sources of carbohydrates.

Importance of Carbohydrates


A carbohydrate’s main function is to provide energy to the body. Most carbohydrates can be broken down into simple sugars like glucose, that the cell can then use as substrate to generate energy. Carbohydrates can also be stored – in the short term – as a molecule called glycogen. Long term storage of sugars required they be converted to fat first. However, not all carbohydrates are digested or stored.  Non-digestible carbohydrates like fiber help with digestive health, blood sugar regulation and suppressing appetite.


Carbohydrate Structure


Generally, carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule1. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”. Carbo- referencing that one part of the components is carbon (“carbo”) and -hydrate referencing the other component being water (H2O). Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.

      • → Monosaccharides are just a single charbohydrate monomer (glucose)
      • → Disaccharides are two sugar monomers joined together (sucrose)
      • → Polysaccharides are long polymers of repeating sugar molecules connected through glycosidic bonds. These are referred to as either starches or fiber.
      •  
Monosaccharides and disaccharides.

Above: Two monosaccharides – glucose and fructose – come together to form the disaccharide sucrose via the formation of a glycosidic bone through dehydration synthesis.

 

Carbohydrate Review Questions

Key Terms

  • Monosaccharide
  • Disaccharide
  • Polysaccharide
  • Glycosidic Bone
  •  

Proteins


Proteins are one of the most abundant organic molecules in living systems. The reason for this might be because proteins have such a diverse range of functions for all biological life. Proteins provide structure, regulate processes, mediate communication; they’re involved in  transport and storage; proteins are invovled in just about every aspect of cellular function. Each cell in a living system contains thousands of different proteins, each with a unique function. Protein structure – like their functions –  vary greatly. However, all proteins are made from amino acids, with the major difference being the difference in sequence in which the amino acids are arranged.

Amino Acids Make Proteins


Amino acids are the monomers that make up proteins. The name “amino acid” is derived from the fact that they contain both amino group and carboxyl-acid-group in their basic structure. There are 20 common amino acids present in proteins. Each amino acids can be represented by a single upper case letter or a three-letter abbreviation. For example, the amino acid valine is known by the letter V or the three-letter symbol val. In humans, nine of these are considered to be essential amino because the human body cannot produce them and they must be obtained from diet.


Amino Acid Structure


Each amino acid has the same fundamental structure. A central carbon atom known as the alpha (α) carbon shares four bonds. One bond is shared with an amino group $-NH2$. A second bond is shared with a carboxyl group $-COOH$. A third bond is shared with a lonely hydrogen. And a fourth bond is shared with a variable group known as a side chain (denoted by R in the image). For each amino acid, the side chain is going to be different. The chemical composition of the side chain helps determine the physical properties of the amino acid. This means that whether an amino acid is polar, nonpolar, acid, basic, et cetera, is determined by what atoms are in this sdie chain.

    • → α-carbon – central part of the amino acid; bonded to the side chain.
    • → Amino Group – ‘nitrogen side’ of the amino acid; forms peptide bond to the carboxyl side of another amino acid.
    • → Carboxyl Group – COOH ‘side’ of the amino acid; forms a peptide bond with the amino side of another amino acid.
    • → Side Chain – differs between amino acid; provides chemical properties like polar, charged, and non-polar.

Amino acid molecule

 

Peptide Bonding


The sequence and the number of amino acids ultimately determine the protein’s shape, size, and function. Each amino acid is attached to another amino acid by a covalent bond known as a peptide bond. These bonds are formed by a dehydration synthesis reaction which we’ll review here shortly.This reaction combines two amino acids by joining the carboxyl group of one amino acid with the amino group of another amino acid. The product formed by joining amino acids are called peptides. As more amino acids are added, the growing chain is known as a polypeptide.


Each polypeptide has a free amino group at one end called the N-terminus and a free carboxyl group at the other end,  known as the C-terminus. Taking a look at the dipeptide in the image on the right, can you locate the N-terminus? 


While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that have combined together. After protein synthesis (translation), most proteins are modified. These are known as post-translational modifications. They may undergo cleavage or phosphorylation, or may require the addition of other chemical groups. Only after these modifications is the protein completely functional.

Two amino acids forming a dipeptide

 

Protein Review Questions

Key Terms

  • Amine Group
  • Carboxyl Group
  • Side Chain
  • α-Carbon
  • Peptide
  • Protein
  • Amino Acid

Dehydration Synthesis


Most macromolecules are made from single sub units – or building blocks – called monomers. However, we typical find macromolecules to exist as polymers. In fact, most macromolecules are fairly large molecules of repeating monomers. To achieve such a structure, we need a way to join together two separate molecules. A process known as dehydration synthesis does exactly this. It creates a bond between two molecules by removing atoms from each of the molecules. More specifically, two hydrogen atoms and a single oxygen atom are removed, which form a molecule of $H_2O$. Hence, this process removes water (dehydration) to synthesize a new molecule (synthesis). 


To see an example, let’s take a look at the image below. To the left of the arrow are two molecules, each of these molecules are individual monosaccharides.  The right of the arrow indicates what a reaction has taken place, what do you notice that is different between the two sides of the reaction?

Dehydration synthesis of sugars

Left Side of the Equation

Two monosaccharides
Note: the atoms colored in red indicate which atoms will be removed to form the product.

Right Side of the Equation

One disaccharides
Water molecule is removed.

 

The two individual molecules have joined to become a single molecule, called a disaccharide. However, a disaccharide is not quite a polymer. Polymers general consist of many repeating units; molecules with a couple of repeat units are called oligomers. You may have noticed is that $-OH$  from one suagr and $-H$ from the other have been removed. Taken together, a net of a single $H_2O$ moecule is removed. 

To create larger molecules, this process can repeat itself and continue to add sugars to a growing chain.  Glucose monomers are the constituents of large polysaccharides like starch, glycogen, and cellulose. Dehydration synthesis is not limited to sugars. Its actually a very common process in biology and it’s the same reacting use in the synthesis of proteins, lipids and DNA.

 

Hydrolysis


Polymers can also be broken down into monomers by the process called hydrolysis, which literately mean “to split with water.” Hydrolysis is a reaction in which a water molecule is used during the breakdown of another compound. During these reactions, the polymer is broken into two components: one part gains a hydrogen atom (H+) and the other gains a hydroxyl molecule (OH–) from a split water molecule. The hydrolysis reaction below show a disaccharide (maltose) broken down into two glucose monomers with the addition of a water molecule.

Hydrolysis of sugars

One example is our digestive system. Here, food is broken down (hydrolyzed) into smaller molecules by enzymes. The enzymes speed up the reactions, making it easier for cells in the intestines to absorb nutrients.

  1. Zedalis J, Eggebrecht J. 3.3 Lipids. “Biology for AP® Courses.” OpenStax. 8 March 2018. https://openstax.org/books/biology-ap-courses/pages/3-3-lipids. License: CC BY 4.0 | License Terms: Edited & Adapted | Access for free at https://openstax.org/books/biology-ap-courses/pages/1-introduction
  2. Zedalis J, Eggebrecht J. 3.2 Carbohydrates. “Biology for AP® Courses.” OpenStax. 8 March 2018. https://openstax.org/books/biology-ap-courses/pages/3-2-carbohydrates. License: CC BY 4.0 | License Terms: Edited & Adapted | Access for free at https://openstax.org/books/biology-ap-courses/pages/1-introduction
  3. Zedalis J, Eggebrecht J. 3.4 Proteins. “Biology for AP® Courses.” OpenStax. 8 March 2018. https://openstax.org/books/biology-ap-courses/pages/3-4-proteins. License: CC BY 4.0 | License Terms: Edited & Adapted | Access for free at https://openstax.org/books/biology-ap-courses/pages/1-introduction
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