AP Biology Unit 1 Topic 1.2: Elements of Life — Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Sulfur (CHNOPS)

Question 1

Which six elements make up the bulk of living matter?

Accepted Answer

The six main elements in living organisms are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur, often remembered by the acronym CHNOPS. Together, these elements make up roughly 97 percent of the mass of most living things. The remaining 3 percent includes trace elements like calcium, potassium, and iron.

Question 2

Why is carbon central to all biological molecules?

Accepted Answer

Carbon is central because it has four valence electrons and can form four stable covalent bonds with other atoms. This allows carbon to build long chains, branches, and rings, creating an enormous diversity of molecular structures. No other common element matches this versatility, which is why carbon forms the backbone of every macromolecule.

Question 3

What are valence electrons and why do they matter?

Accepted Answer

Valence electrons are the electrons in the outermost shell of an atom and they determine how the atom bonds with others. Atoms tend to gain, lose, or share valence electrons to achieve a full outer shell, usually of eight electrons. The number of valence electrons predicts what kinds of bonds an element will form.

Question 4

What is a covalent bond?

Accepted Answer

A covalent bond is formed when two atoms share one or more pairs of electrons. Covalent bonds are strong and stable, making them the primary type of bond in biological macromolecules. A single bond shares one pair of electrons, a double bond shares two pairs, and a triple bond shares three pairs.

Question 5

What is the difference between a polar and nonpolar covalent bond?

Accepted Answer

In a nonpolar covalent bond, electrons are shared equally between two atoms with similar electronegativity. In a polar covalent bond, one atom pulls the shared electrons more strongly, creating partial charges on both atoms. The water molecule is the classic example of polar covalent bonding.

Question 6

What is an ionic bond?

Accepted Answer

An ionic bond forms when one atom transfers electrons to another, creating two oppositely charged ions that attract each other. Sodium chloride is a classic example, where sodium loses an electron to become positive and chlorine gains it to become negative. Ionic bonds are weaker than covalent bonds in water but important in many biological processes.

Question 7

What is a functional group?

Accepted Answer

A functional group is a specific group of atoms attached to a carbon skeleton that gives a molecule particular chemical properties and reactivity. Examples include hydroxyl, carbonyl, carboxyl, amino, sulfhydryl, and phosphate groups. The functional groups present in a molecule largely determine its behavior in biological reactions.

Question 8

What is the hydroxyl functional group?

Accepted Answer

The hydroxyl group consists of an oxygen bonded to a hydrogen, written as dash O H. It is polar and forms hydrogen bonds with water, making molecules with hydroxyl groups water-soluble. Alcohols and sugars contain hydroxyl groups.

Question 9

What is the carbonyl functional group?

Accepted Answer

The carbonyl group consists of a carbon double-bonded to an oxygen. When the carbonyl is at the end of a carbon chain, the molecule is an aldehyde, and when it is in the middle, the molecule is a ketone. Carbonyls are common in sugars and other biomolecules.

Question 10

What is the carboxyl functional group?

Accepted Answer

The carboxyl group consists of a carbon double-bonded to an oxygen and single-bonded to a hydroxyl group. It acts as an acid because it can donate a hydrogen ion. Amino acids and fatty acids contain carboxyl groups, which is why they can lower pH.

Question 11

What is the amino functional group?

Accepted Answer

The amino group consists of a nitrogen bonded to two hydrogens. It acts as a base because it can accept a hydrogen ion. Every amino acid contains an amino group, which gives proteins their basic character at the amino end of the chain.

Question 12

What is the sulfhydryl functional group?

Accepted Answer

The sulfhydryl group consists of a sulfur bonded to a hydrogen, written as dash S H. Two sulfhydryl groups can form a disulfide bond, which stabilizes protein structure. Cysteine is the amino acid that contains a sulfhydryl group.

Question 13

What is the phosphate functional group?

Accepted Answer

The phosphate group consists of a phosphorus bonded to four oxygens. It carries negative charges, making molecules with phosphate groups acidic and reactive. Phosphate groups are essential in DNA, RNA, ATP, and phospholipids.

Question 14

What is the methyl functional group?

Accepted Answer

The methyl group consists of a carbon bonded to three hydrogens. Unlike most other functional groups, it is nonpolar and does not directly participate in chemical reactions. However, it plays an important role in gene regulation through DNA methylation, which can turn genes on or off.

Question 15

Why are hydrogen bonds important in biological molecules besides water?

Accepted Answer

Hydrogen bonds stabilize the three-dimensional shapes of proteins, hold the two strands of DNA together, and allow enzymes to recognize specific substrates. Although individually weak, large numbers of hydrogen bonds collectively provide significant stability while still being reversible enough to allow biological processes to function.

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