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Biology LibreTexts

2.E: Chemistry of Life (Exercises)

2.1: The Building Blocks of Molecules

At its most fundamental level, life is made up of matter. Matter occupies space and has mass. All matter is composed of elements, substances that cannot be broken down or transformed chemically into other substances. Each element is made of atoms, each with a constant number of protons and unique properties. Each element is designated by its chemical symbol and possesses unique properties. These unique properties allow elements to combine and to bond with each other in specific ways.

Multiple Choice

Magnesium has an atomic number of 12. Which of the following statements is true of a neutral magnesium atom?

A. It has 12 protons, 12 electrons, and 12 neutrons.
B. It has 12 protons, 12 electrons, and six neutrons.
C. It has six protons, six electrons, and no neutrons.
D. It has six protons, six electrons, and six neutrons.


Which type of bond represents a weak chemical bond?

A. hydrogen bond
B. ionic bond
C. covalent bond
D. polar covalent bond


An isotope of sodium (Na) has a mass number of 22. How many neutrons does it have?

A. 11
B. 12
C. 22
D. 44


Free Response

Why are hydrogen bonds and van der Waals interactions necessary for cells?

Hydrogen bonds and van der Waals interactions form weak associations between different molecules. They provide the structure and shape necessary for proteins and DNA within cells so that they function properly. Hydrogen bonds also give water its unique properties, which are necessary for life.

2.2: Water

Do you ever wonder why scientists spend time looking for water on other planets? It is because water is essential to life; even minute traces of it on another planet can indicate that life could or did exist on that planet. Water is one of the more abundant molecules in living cells and the one most critical to life as we know it. Approximately 60–70 percent of your body is made up of water. Without it, life simply would not exist.

Multiple Choice

Which of the following statements is not true?

A. Water is polar.
B. Water stabilizes temperature.
C. Water is essential for life.
D. Water is the most abundant atom in Earth’s atmosphere.


Using a pH meter, you find the pH of an unknown solution to be 8.0. How would you describe this solution?

A. weakly acidic
B. strongly acidic
C. weakly basic
D. strongly basic


The pH of lemon juice is about 2.0, whereas tomato juice's pH is about 4.0. Approximately how much of an increase in hydrogen ion concentration is there between tomato juice and lemon juice?

A. 2 times
B. 10 times
C. 100 times
D. 1000 times


Free Response

Why can some insects walk on water?

Some insects can walk on water, although they are heavier (denser) than water, because of the surface tension of water. Surface tension results from cohesion, or the attraction between water molecules at the surface of the body of water [the liquid-air (gas) interface].

Explain why water is an excellent solvent.

Water molecules are polar, meaning they have separated partial positive and negative charges. Because of these charges, water molecules are able to surround charged particles created when a substance dissociates. The surrounding layer of water molecules stabilizes the ion and keeps differently charged ions from reassociating, so the substance stays dissolved.

2.3: Biological Molecules

There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), and each is an important component of the cell and performs a wide array of functions. Combined, these molecules make up the majority of a cell’s mass. Biological macromolecules are organic, meaning that they contain carbon (with some exceptions, like carbon dioxide).

Multiple Choice

An example of a monosaccharide is ________.

A. fructose
B. glucose
C. galactose
D. all of the above


Cellulose and starch are examples of ________.

A. monosaccharides
B. disaccharides
C. lipids
D. polysaccharides


Phospholipids are important components of __________.

A. the plasma membrane of cells
B. the ring structure of steroids
C. the waxy covering on leaves
D. the double bond in hydrocarbon chains


The monomers that make up proteins are called _________.

A. nucleotides
B. disaccharides
C. amino acids
D. chaperones


Free Response

Explain at least three functions that lipids serve in plants and/or animals.

Fat serves as a valuable way for animals to store energy. It can also provide insulation. Phospholipids and steroids are important components of cell membranes.

Explain what happens if even one amino acid is substituted for another in a polypeptide chain. Provide a specific example.

A change in gene sequence can lead to a different amino acid being added to a polypeptide chain instead of the normal one. This causes a change in protein structure and function. For example, in sickle cell anemia, the hemoglobin β chain has a single amino acid substitution. Because of this change, the disc-shaped red blood cells assume a crescent shape, which can result in serious health problems.