Question

Biological molecules are primarily joined by
A. Peptide bond
B. Ionic bond
C. Hydrogen bond
D. Covalent bond

Answer 1

Hint: Biological molecules are those that are formed by the bonds in carbon and hydrogen atoms. Organic or biological molecules refer to carbon, hydrogen, nitrogen, phosphorus, etc. These molecules are joined by bonds in between C and H or between C and C.

Complete answer:
Biological molecules are also known as biomolecules. All the carbon-containing compounds that we get from living tissues are called biomolecules. They are responsible for catalysis of large structures and in the functioning of the cell. They have a wide range of sizes and structures. The major types are: carbohydrates, lipids, nucleic acids, and proteins.
Let us study the above 4 types of bonds:
a. Peptide bond- All the polypeptides or proteins, amino acids are linked through this bond. It is formed when the carboxyl (-COOH) group of one amino acid reacts with the amino (-$NH_2$) group of the next amino acid with the elimination of a water moiety.
b. Ionic bond- It is also called an electrovalent bond that is formed from the electrostatic force of attraction between the charged ions in a chemical compound. Such a bond forms when the valence electrons of one atom that loses the electrons becomes a negatively charged ion.
c. Hydrogen bond- It is an electrostatic force of attraction between a hydrogen atom covalently bonded to a more electronegative atom or to one having a lone pair of electrons. It can exist between atoms in different molecules or in part of the same molecules. These bonds are generally weaker than ionic or covalent bonds but stronger than van der waals forces.
d. Covalent bond-It is formed when an electron is shared between two elements and is the strongest and most common form of chemical bond that is found in living organisms. It is formed between the inorganic molecules such as water, carbon dioxide and oxygen. These bonds do not dissociate in water.
Biological molecules are joined by covalent bonds because these bonds are the strongest to maintain the structural integrity. They will then form a functional cell. Covalent bonds form up the backbone of the structure of biomolecules.

So, the correct answer is option D, Covalent bond.

Note-
All biomolecules share a common fundamental relationship between structure and function which is influenced by many factors like the environment where biomolecules occur. They are joined by covalent bonds to form a long organic molecule.