Macromolecules and the Role Carbon in Living Things

Recall that the six main elements found in living things are carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous. These six atoms bond together in a variety of combinations to form the molecules which make up the structures found in the cells of living things. These molecules then join together to form much larger structures known as macromolecules. All macromolecules have carbon as their main element.  The four macromolecules you will be required to know about for this course are carbohydrates, proteins, lipids and nucleic acids.

The Role of Carbon in Living Things

A carbon atom has four electrons in its valence shell. To become stable, carbon will form four covalent bonds to fill its outer valence shell. These bonds may be with other carbon atoms, or with other elements such as hydrogen and oxygen. When two carbon atoms form a covalent bond, each atom can share:

• one electron to form a single bond,
• two electrons to form a double bond or,
• three electrons to form a triple bond.

Carbon atoms bond to each other in different ways to form compounds of various shapes including, straight-chain, branched-chain, or ring compounds. These compounds can have any number of carbon atoms and will contain atoms of other elements, the most frequently occurring of which is hydrogen. This results in a huge number of carbon compounds being synthesized in living organisms.

Monomers and Polymers

Carbon compounds vary greatly in size from a single carbon like that seen in methane (CH₄) to those which contain hundreds or thousands of atoms. The largest carbon-based molecule known to exist is called PG5 and contains around 15 million carbon atoms.

Cells synthesize (make) macromolecules by joining smaller molecules called monomers together. These large macromolecules are called polymers are defined as a long chain of repeating subunits (monomers).  An example of this is seen when amino acids are bonded together to form a polypeptide chain.

Condensation reaction (dehydration synthesis)

Polymers are made through a chemical reaction called condensation. In this reaction, the monomers which are being bonded together have a hydrogen atom (–H) on one end and a hydroxyl group (–OH) group on the other. When the molecules join, the hydrogen atom and hydroxyl group bond and are removed as a water molecule.

The subunits are bonded together by a covalent bond, as the water molecule is released.

The figure below shows the condensation of two glucose molecules to form a maltose molecule.  The covalent bond forms between the glucose units.

When these polymers break apart the reverse reaction called hydrolysis occurs.

Carbohydrates

Carbohydrates as their name suggest consist of carbon, hydrogen and oxygen. The ratio of these atoms is approximately two hydrogen atoms and one oxygen atom to one carbon atom. These macromolecules are the most abundant in the living cell and are used as the primary fuel source for all living cells.

Carbohydrates have various chain lengths and can be divided into three categories: monosaccharides, disaccharides, and polysaccharides.

1. Monosaccharides

Monosaccharides (mono- = “one”; sacchar- = “sugar”) are the simplest type of carbohydrates and typically contain between 3 and 7 carbon atoms. The most common examples in this category are the compounds glucose and fructose.

2. Disaccharides

Disaccharides (di- = “two”) are two-sugar molecules that form when two monosaccharides join via a condensation reaction. For example, when glucose and fructose form a covalent bond, a molecule of sucrose commonly known as table sugar is produced.

3. Polysaccharides

Polysaccharides (poly- = “many”) are the largest carbohydrate molecules. These are long chains of monosaccharides linked together by covalent bonds. Starch, glycogen, and cellulose are examples of polysaccharides that are essential in living organisms. Their structure is shown in the diagram below.

• Starch consists of branched chains of glucose molecules and is utilized as energy storage by plants.
• Glycogen is a highly branched glucose polymer that functions as an energy storage molecule found in the liver and muscle cells of mammals.
• Cellulose is a glucose polymer that is typically found in the plant cell walls and serves as structural

Lipids

Lipids consist of carbon and hydrogen atoms bonded to a small number of oxygen atoms. Lipids are used to store energy, as insulation, form cell membranes, and provide the building blocks for hormones such as testosterone and estrogen. The most common examples of lipids are fats, oils, waxes, and steroids. Lipids are generally insoluble in water because they have a non polar region which is not attracted to water molecules.

Lipids are made of glycerol, a three-carbon molecule that serves as the ‘backbone’ for the structure to which the fatty acids are attached. A fatty acid is a long chain of carbon and hydrogen. If the carbon atoms in the chain are bonded to other carbons with single bonds, the fatty acid is called a saturated fatty acid. When a double bond exists in the chain, the fatty acid is called an unsaturated fatty acid. Fatty acids which contain more than one double bond are called polyunsaturated fatty acids.

Proteins

Proteins are large, complex polymers which are made up of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur.

They are one of the most abundant macromolecules found in living systems. A single living cell contains thousands of proteins that perform unique functions such as the contraction of muscle tissue, transportation of oxygen in the bloodstream, immunity, control of other proteins, and speeding up chemical reactions.

Amino acids are monomers that form the building blocks of a protein molecule. There are 20 naturally-occurring amino acids that, in various combinations produce thousands of proteins, each with their own unique function. An amino acid is composed of a central carbon atom which is bonded to a hydrogen atom, a carboxyl group (–COOH), an amine group (–NH₂), and a variable group (–R) that makes each amino acid different.

Amino acids are bonded together by covalent bonds called a peptide bond. The peptide bond is formed when the –H atom from the amine group of one amino acid and the –OH group from the carboxyl group of another amino acid are removed to release a water molecule as is seen in the reaction below.

As the amino acids join together, they create a long chain known as a polypeptide shown in the diagram below.

The different chemical properties of amino acids cause them to attract and repel each other in different ways. The attraction and repulsion of the amino acids cause the polypeptide chain to fold back and forth on itself into pleated sheets or coiled up helices. The folding is then stabilized by hydrogen bonds. As the side chains interact and continue to fold up, the polypeptide forms a three-dimensional structure. This structure then joins with other coiled up polypeptide chains into the final protein which has a specific role in the cell.  This process is summarized in the diagram below.

Two of the most common types of proteins which act in the human body are enzymes and hormones.

1. Enzymes speed up the rate of a biochemical reaction — for example, salivary amylase acts on starch found in bread, pasta and rice to break it down into monosaccharides such as glucose.
2. Hormones are biochemical messengers that are released by endocrine cells to control specific processes, such as growth, development, metabolism, and reproduction. The hormone insulin controls the amount of glucose present in the blood.

Nucleic acids

Nucleic acids are the genetic material that stores cellular information. These polymers are made up of small building blocks called nucleotides. Nucleotides are composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus atoms arranged as three separate molecules (a nitrogenous base, a sugar, and a phosphate molecule) which have been bonded together.  The structure of the nucleotide is shown below.

The two naturally-occurring varieties of nucleic acid are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA (Deoxyribonucleic acid)

DNA is a double-stranded molecule which is found in the nucleus of the cell. It is the main copy of an organism’s genetic code containing the instructions to create every protein required by the cell. These proteins are then expressed as characteristics or traits in the organism.

RNA (Ribonucleic acid)

RNA is the nucleic acid molecule which forms the copy of DNA that is used to make proteins. Unlike the DNA molecule, RNA is a single-stranded molecule. There are four types of RNA:  messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and regulatory RNAs.

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Unit 1 – Biology Basics

• 1-1  Nature of Science and the Scientific Method

• 1-2  Hypotheses, Theories and Scientific Law

• 1-3  Technological Design Process

• 1-4  Chemistry of Life

  • HS-LS1-6. Construct and revise an explanation based on evidence for how carbon, hydrogen and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.

• 1-5 Properties of Water

  • HS-ESS2-5. Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface process.

• 1-6 Macromolecules

  • HS-LS1-6. Construct and revise an explanation based on evidence for how carbon, hydrogen and oxygen from sugar molecules may combine with other elements to form amino acids and/or other large carbon-based molecules.