Anatomy 101: From Muscles and Bones to Organs and Systems, Your Guide to How the Human Body Works

ORGANIC COMPOUNDS: CARBOHYDRATES AND PROTEINS

It Does a Carbon-Based Life Form Good

Most life on the planet is carbon-based. The chemistry associated with carbon-based living organisms is called organic chemistry and focuses on carbohydrates, proteins, lipids, and nucleic acids. These are called “organic” compounds and are used to build everything you need to run that marathon you’re entered in this weekend—from the lungs you use to breathe to the energy you use to power your stride.

Carbohydrates

Also known as “sugars,” carbohydrates (or saccharides) play a major role in energy conservation, transport, transfer, and storage. Plants capture energy from the sunlight and use it to assemble carbon molecules into carbohydrates. When you eat a plant, your body breaks down these complex molecules into individual CO2 molecules and recovers the energy generated from the breaking of the bonds to be used elsewhere in the body. In your body, energy can be stored either as fat or as long chains of carbohydrates (polysaccharides).

A single carbohydrate molecule is often referred to as a monosaccharide with a typical chemical composition of (CH2O)n where n is at least 3. Thus, C3H6O3 is the simplest of monosaccharides (it is called glyceraldehyde). Glucose, one of the most important energy-bearing monosaccharides, is C6H12O6.

Disaccharides are composed of 2 monosaccharides. Sucrose is a disaccharide composed of glucose and fructose, another important monosaccharide for metabolism. The common name for sucrose is table sugar.

Beyond disaccharides

Oligosaccharides are composed of between 3 and 9 monosaccharide units. Polysaccharides (a saccharide of more than 1 monosaccharide) may be much longer.

Saccharides provide energy storage. Glucose is polymerized (the process of linking together molecules) into glycogen, which is stored intracellularly (inside the cell) in muscle and liver cells to be broken down in times of high energy needs and low blood glucose levels, such as when you oversleep and end up running out the door before you even have a chance to eat breakfast.

Proteins

Protein molecules serve as structural elements both inside and outside of the cell, as anchoring molecules to hold cells in place, as adhesive molecules to allow cells to move throughout the body, and as enzymes that facilitate much of the metabolic activity of the cell.

Amino Acids

Amino acids link together to form proteins, using a special linkage called a peptide bond. Because of this, proteins are often called polypeptides. There are twenty types of amino acids, each with a different structure. The final shape and function of a protein is determined by its amino acids. Since the only variable part of an amino acid is the R group, this is the portion of the amino acid that will confer different physical and functional properties to the protein. For instance, several amino acids, such as valine and isoleucine, have hydrocarbon (molecules consisting of only carbon and hydrogen) R groups. These groups will be neutrally charged and will not interact with charged (also called polar) molecules, such as water. Thus, these amino acids are said to be hydrophobic (“afraid of water”) and are often present in regions where a protein will span the plasma membrane (which is also a hydrophobic region of fatty acid hydrocarbon chains). Other amino acids are hydrophilic (attracting water). Some are acid; others are base.

Amino acids and protein folding

The type of amino acid in the protein will have an impact on the shape and folding of proteins into their final structure. A string of amino acids only becomes functional when it folds into a protein. For instance, glycine has the smallest of the R groups, with only hydrogen present. This will allow the protein to fold easily since there isn’t a large R group to physically get in the way.

Protein Structure

Methionine will always be the first amino acid in a protein since it is also the sequence (in the RNA) that signals the start of protein formation. While the protein sequence is usually written in a straight line and is considered the primary structure of the protein, proteins are flexible and typically fold back upon themselves into one of two patterns:

1.     in side-by-side runs of the protein, forming beta pleated sheets (sheetlike regions of the protein)

2.    twisted around neighboring regions of the protein, forming spiraling tubes called alpha helices

Each of these folded patterns, which form the secondary structure of the protein, is held in place by hydrogen bonds between amino acids.

As the protein folds, other amino acids may become closer to each other and form bonds. Cysteine, for example, is an amino acid with a sulfate group. When it is next to another cysteine it may form a disulfide bond. In this way, large loops of protein are held in place. These formations within the protein are called the tertiary structure of the protein.

Lastly, separate protein units may be held together by bonds into large protein aggregates. This quaternary structure (meaning the combination of 2 or more chains that form a final structure) is illustrated in the hemoglobin molecule. Adult hemoglobin (the protein responsible for oxygen transport) is formed from 4 subunit proteins, bonded together in the large single molecule that moves oxygen throughout the blood stream.



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