Individual carbon atoms have an incomplete outermost electron shell. With an atomic number of 6 six electrons and six protons , the first two electrons fill the inner shell, leaving four in the second shell. Therefore, carbon atoms can form up to four covalent bonds with other atoms to satisfy the octet rule. The methane molecule provides an example: it has the chemical formula CH 4. Each of its four hydrogen atoms forms a single covalent bond with the carbon atom by sharing a pair of electrons.
This results in a filled outermost shell. Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane CH 4 described above. We often use hydrocarbons in our daily lives as fuels—like the propane in a gas grill or the butane in a lighter. The many covalent bonds between the atoms in hydrocarbons store a great amount of energy, which is released when these molecules are burned oxidized. The geometry of the methane molecule, where the atoms reside in three dimensions, is determined by the shape of its electron orbitals.
The carbons and the four hydrogen atoms form a shape known as a tetrahedron, with four triangular faces; for this reason, methane is described as having tetrahedral geometry. As the backbone of the large molecules of living things, hydrocarbons may exist as linear carbon chains, carbon rings, or combinations of both. Furthermore, individual carbon-to-carbon bonds may be single, double, or triple covalent bonds, and each type of bond affects the geometry of the molecule in a specific way.
This three-dimensional shape or conformation of the large molecules of life macromolecules is critical to how they function.
Hydrocarbon chains are formed by successive bonds between carbon atoms and may be branched or unbranched. The hydrocarbons ethane, ethene, and ethyne serve as examples of how different carbon-to-carbon bonds affect the geometry of the molecule.
Thus, propane, propene, and propyne follow the same pattern with three carbon molecules, butane, butane, and butyne for four carbon molecules, and so on. Double and triple bonds change the geometry of the molecule: single bonds allow rotation along the axis of the bond, whereas double bonds lead to a planar configuration and triple bonds to a linear one.
These geometries have a significant impact on the shape a particular molecule can assume. So far, the hydrocarbons we have discussed have been aliphatic hydrocarbons , which consist of linear chains of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons , consists of closed rings of carbon atoms. Examples of biological molecules that incorporate the benzene ring include some amino acids and cholesterol and its derivatives, including the hormones estrogen and testosterone.
The benzene ring is also found in the herbicide 2,4-D. Benzene is a natural component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and aromatic portions; beta-carotene is an example of such a hydrocarbon. Biochemistry is the discipline that studies the chemistry of life, and its objective is to explain form and function based on chemical principles.
Organic chemistry is the discipline devoted to the study of carbon-based chemistry, which is the foundation for the study of biomolecules and the discipline of biochemistry. The most abundant element in cells is hydrogen H , followed by carbon C , oxygen O , nitrogen N , phosphorous P , and sulfur S. Some elements, such as sodium Na , potassium K , magnesium Mg , zinc Zn , iron Fe , calcium Ca , molybdenum Mo , copper Cu , cobalt Co , manganese Mn , or vanadium Va , are required by some cells in very small amounts and are called micronutrients or trace elements.
All of these elements are essential to the function of many biochemical reactions, and, therefore, are essential to life.
Unlike carbon, nitrogen forms up to three bonds, oxygen forms up to two, and hydrogen forms one. These traits in combination permit the formation of a vast number of diverse molecular species necessary to form the structures and enable the functions of living organisms. Living organisms contain inorganic compounds mainly water and salts and organic molecules. In doing so, monomers release water molecules as byproducts.
In a dehydration synthesis reaction Figure , the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a water molecule. At the same time, the monomers share electrons and form covalent bonds. As additional monomers join, this chain of repeating monomers forms a polymer. Different monomer types can combine in many configurations, giving rise to a diverse group of macromolecules.
Even one kind of monomer can combine in a variety of ways to form several different polymers. For example, glucose monomers are the constituents of starch, glycogen, and cellulose. Polymers break down into monomers during hydrolysis. A chemical reaction occurs when inserting a water molecule across the bond. Breaking a covalent bond with this water molecule in the compound achieves this Figure. These reactions are similar for most macromolecules, but each monomer and polymer reaction is specific for its class.
For example, catalytic enzymes in the digestive system hydrolyze or break down the food we ingest into smaller molecules. This allows cells in our body to easily absorb nutrients in the intestine. A specific enzyme breaks down each macromolecule. For instance, amylase, sucrase, lactase, or maltase break down carbohydrates.
Humans do not have digestive enzymes to break down cellulose in food, which is also called dietary fiber. However, dietary fiber consumption helps to maintain a healthy gut flora, which in turn contributes to the health of digestive and immune systems 1. Similar to plants, some animals and fungi use another polysaccharide, chitin, as a structural molecule. Arthropods use chitin to build and maintain their exoskeletons, whereas fungi incorporate it into their cell walls to maintain rigidity.
The second class of biological macromolecules are lipids, which include fats, oils, and waxes. Lipids are hydrophobic molecules that are almost entirely made up of carbon and hydrogen atoms. Often, lipids are grouped in three major categories; triglycerides, phospholipids, and steroids. The most common type of lipid is triglycerides, which include fats from animals and oils from plants.
Triglycerides generally serve as long-term energy storage molecules, except indigestible waxes, which are instead used as a waterproofing substance in both plants and animals.
Triglycerides contain three fatty acid chains, which can be either saturated or unsaturated, connected to a glycerol molecule. Saturated fatty acid chains are linear molecules with a maximum number of hydrogen atoms, where every carbon in the chain is connected via a single bond.
On the other hand, unsaturated fatty acid chains have kinks due to the presence of at least one double bond. While trans fats occur naturally, they are generated during industrial production of saturated vegetable oils with hydrogen.
Similar to saturated fatty acids, trans fats stack very well due to their relative linearity. However, trans fats cause problems for human heart health, such as the damaging the lining of arteries and causing inflammation when digested 2.
Phospholipids are similar to triglycerides, however, one of the fatty acid chains is replaced with a phosphate-containing polar group. Therefore, phospholipids have a hydrophilic head and two hydrophobic fatty acid tails. These properties of phospholipids are crucial to the cell membrane structure and function. Steroids are lipids that are composed of fused carbon rings with varying functional groups.
Cholesterol is a steroid that is also a cell membrane component. Moreover, cholesterol is used to synthesize other steroids, including sex hormones such as estrogen and testosterone.
Although cholesterol is essential for cell membrane structure and hormone synthesis, high levels of plasma cholesterol are implicated in plaque accumulation inside blood vessels and causing coronary disease 3. The third class of biological macromolecules are proteins, which are made up of chains of amino acids. These groups link together, N-terminal to C-terminal, in a chain connected by peptide bonds.
Proteins are important for maintaining body functions as enzymes, hormones, structural components and transport molecules, and play vital roles in muscle contractibility, immunity and blood clotting. However, issues can arise in protein structure and function, and these issues are often genetic. For instance, normal red blood cells are round, but in people affected by sickle cell anemia, cells have a curved shape with an exposed hydrophobic region, caused by a mutation in a protein called hemoglobin S.
In this chapter, these questions will be explored. There are four major classes of biological macromolecules carbohydrates, lipids, proteins, and nucleic acids ; each is an important cell component and performs a wide array of functions.
Carbohydrates provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. Carbohydrates also have other important functions in humans, animals, and plants.
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