Biological Molecules & Covalent Bonds Quiz

Covalent bonds involve the sharing of electrons between atoms. When the electronegativity difference between the atoms is small, the bond is nonpolar and electrons are shared equally. When the difference is larger, the bond becomes polar, and electrons are shared unequally. Because both polar and nonpolar bonds involve electron sharing, they are both classified as covalent bonds.

Carbon–oxygen bonds are polar because oxygen has a much higher electronegativity than carbon. As a result, electrons are pulled closer to oxygen, creating a partial negative charge on oxygen and a partial positive charge on carbon. This polarity allows molecules containing C–O bonds to interact with water and participate in hydrogen bonding.

Polar bonds form when atoms have different electronegativities, causing electrons to be shared unequally. The atom with higher electronegativity pulls electrons closer, creating partial charges. This uneven distribution of charge produces a dipole moment. If the electronegativity difference is very large, the bond may become ionic instead of covalent.

Nitrogen is more electronegative than hydrogen, so N–H bonds involve unequal sharing of electrons. This makes N–H bonds polar rather than nonpolar. The polarity of amino groups affects protein structure, hydrogen bonding, and interactions with water, which are critical for protein folding and biological function.

Sulfur is incorporated mainly into proteins because some amino acids, such as cysteine and methionine, contain sulfur atoms. DNA, RNA, and carbohydrates do not contain sulfur. Therefore, when radioactive sulfur is added to cells, it becomes incorporated primarily into newly synthesized proteins.

Proteins are polymers made of amino acid monomers. Amino acids are linked together by peptide bonds, which form between the carboxyl group of one amino acid and the amino group of another. The sequence of amino acids determines protein structure and function.

Each strand of DNA is made of nucleotide monomers called deoxynucleotides. Each deoxynucleotide contains a deoxyribose sugar, phosphate group, and nitrogenous base. These nucleotides are linked together by phosphodiester bonds to form the DNA backbone.

Cells synthesize macromolecules through enzyme-catalyzed chemical reactions. Enzymes speed up reactions by lowering activation energy and ensuring proper molecular interactions. Diffusion and digestion do not build macromolecules inside cells.

Proteins perform most cellular work, including catalyzing reactions, providing structure, transporting substances, and regulating processes. While other macromolecules have important roles, proteins are the most functionally diverse.

Peptide bonds are characteristic of proteins. They form between amino acids during dehydration synthesis, creating long polypeptide chains that fold into functional proteins.

Glycosidic bonds link monosaccharides together to form carbohydrates. These bonds determine carbohydrate structure and function, including energy storage and structural support.

Peptide bonds covalently link amino acids. The bond forms between the amino group of one amino acid and the carboxyl group of another, releasing water in the process.

Ester bonds covalently link glycerol and fatty acids in lipids. These bonds create triglycerides and phospholipids, which are essential for energy storage and membrane structure.

Carbon can form both polar and nonpolar bonds depending on the atom it bonds with. When carbon bonds with atoms of similar electronegativity, such as hydrogen, the bond is nonpolar. When carbon bonds with more electronegative atoms like oxygen or nitrogen, electrons are shared unequally, creating polar bonds. This versatility allows carbon to form many biologically important molecules.

Carbon–hydrogen bonds are considered nonpolar because carbon and hydrogen have very similar electronegativity values. This similarity means electrons are shared almost equally between the atoms, resulting in no significant partial charges. Because of this, hydrocarbons tend to be nonpolar and hydrophobic, which strongly influences the structure and behavior of biological molecules such as lipids.

Oxygen is more electronegative than carbon, so when a carbon–oxygen bond forms, the shared electrons spend more time near the oxygen atom. This unequal sharing creates partial charges, making the bond polar. Therefore, it is incorrect to say electrons spend more time near carbon. This polarity is important in determining molecule reactivity and solubility.

Phosphate groups contain phosphorus–oxygen bonds. Oxygen is significantly more electronegative than phosphorus, making P–O bonds polar. This polarity contributes to the negative charge of phosphate groups at physiological pH and plays a major role in energy transfer, signaling, and the structure of nucleic acids and phospholipids.

At cellular pH, phosphate groups typically lose protons and carry a negative charge. Hydroxyl groups can also lose hydrogen under certain conditions, contributing to polarity. Amino groups often gain a proton and become positively charged. These charges affect molecular interactions, protein folding, enzyme activity, and membrane structure inside cells.

Nucleic acids are composed of carbon, hydrogen, oxygen, nitrogen, and phosphorus. These elements form nucleotides, which include a sugar, phosphate group, and nitrogenous base. Sulfur is not present in DNA or RNA, although it is found in certain amino acids within proteins.

Polymers are formed when monomers are linked together by covalent bonds through dehydration synthesis reactions. Hydrogen bonds are weak intermolecular attractions and do not permanently link monomers into polymers. Therefore, hydrogen bonds cannot be used to build polymer chains.

Hydrolysis is a reaction that breaks polymers into monomers by adding water. The water molecule splits into hydrogen and hydroxyl groups, which attach to the separated monomers. This process occurs during digestion and cellular recycling of macromolecules.

RNA is a polymer composed of ribonucleotide monomers. Each nucleotide contains a ribose sugar, phosphate group, and nitrogenous base. RNA differs from DNA in sugar type and base composition, but both are nucleic acid polymers.

Starch, glycogen, and chitin are polysaccharides, meaning they are polymers made of sugar monomers. These sugars are linked by glycosidic bonds. Although their functions differ, they all serve structural or energy-storage roles in living organisms.

Lipids are not considered true polymers because they are not built from repeating monomer units linked in long chains. Although they may contain glycerol and fatty acids, their structure lacks the repeating pattern seen in proteins, carbohydrates, and nucleic acids.

Not all macromolecules require a template. DNA, RNA, and proteins rely on templates to determine monomer order, but carbohydrates and lipids are assembled without templates. Their structure depends on enzyme activity rather than sequence information.

DNA’s primary role is to serve as a template for RNA synthesis during transcription. RNA then carries genetic information used for protein production. Without DNA templates, accurate gene expression would not be possible.

Cell membranes and cell walls are structurally different. Membranes are flexible phospholipid bilayers, while cell walls are rigid structures made of carbohydrates like cellulose or peptidoglycan. Their composition and function are not similar.

Ester bonds are characteristic of lipids. They form between the hydroxyl group of glycerol and the carboxyl group of fatty acids. These bonds create fats and oils used for energy storage and membrane structure.

Phosphodiester bonds link nucleotides together in DNA and RNA. They connect the phosphate group of one nucleotide to the sugar of the next, forming the backbone of nucleic acid strands.

Phosphodiester bonds link nucleotides in nucleic acids. These strong covalent bonds form the sugar-phosphate backbone essential for DNA and RNA stability.