Atoms and Molecules in Biochemistry

Atoms and Molecules in Biochemistry

Biochemistry is fundamentally the chemistry of life. All biological structures from small metabolites to macromolecules like proteins and nucleic acids are composed of atoms arranged into molecules. Understanding atomic structure and molecular formation is essential for grasping biochemical reactions, enzyme action, and metabolic pathways.

1. What is Matter and it’s Forms?

Definition of Matter

Matter is anything that has mass and occupies space. All living and non-living things are made of matter.

Forms of Matter

Matter exists mainly in three physical states, depending on temperature and pressure.

State	Characteristics	Example (Biochemical relevance)
Solid	Fixed shape & volume	DNA crystals, proteins
Liquid	Fixed volume, variable shape	Cytoplasm, blood
Gas	No fixed shape or volume	O₂, CO₂

Types of Matter (Chemical Classification)

Type	Description	Example
Element	Pure substance, one kind of atom	C, H, O, N
Compound	Two or more elements chemically combined	H₂O, NaCl
Mixture	Physical combination	Cytosol

 

CSIR-NET Memory Points

• Living systems are matter + energy
• Elements combine to form compounds via chemical bonds
• Water is the most abundant compound in cells

2. Structure of Atom

An atom is the smallest unit of an element that retains chemical properties.

Basic Structure

Subatomic Particle	Charge	Location	Relative Mass
Proton	+1	Nucleus	1 amu
Neutron	0	Nucleus	1 amu
Electron	−1	Orbitals	~0

Atomic Model (Modern Concept)

• Nucleus contains protons + neutrons
• Electrons exist in probability regions (orbital)

CSIR-NET Memory Points

• Atomic mass ≈ protons + neutrons
• Chemical behavior depends on electrons
• Nucleus contributes most of the mass

3. Atomic Number and Mass Number

Atomic Number (Z)

• Number of protons
• Determines element identity
• In neutral atoms: protons = electrons

Mass Number (A)

• Total number of protons + neutrons

Relation Formula

A=Z+N,

Example Table

Element	Atomic Number (Z)	Neutrons	Mass Number (A)
Carbon	6	6	12
Oxygen	8	8	16
Nitrogen	7	7	14

 

CSIR-NET Memory Points

• Atomic number never changes for an element
• Mass number varies → isotopes
• Biologically important elements: CHNOPS

4. Isotopes and Radioisotopes

Isotopes

Atoms of the same element having same Z but different A.

Examples

• ¹²C, ¹³C, ¹⁴C
• Same chemical properties, different physical properties

Radioisotopes

Isotopes with unstable nuclei that emit radiation.

Radioisotope	Radiation	Biochemical Use
¹⁴C	β	Tracer in metabolism
³H (Tritium)	β	DNA labeling
³²P	β	Nucleic acids, ATP
¹²⁵I	γ	Protein labeling

 

CSIR-NET Memory Points

• Radioisotopes used as biochemical tracers
• Half-life is critical for experiments
• ¹⁴C used in carbon dating & metabolic studies

5. Arrangement of Electrons (Electronic Configuration)

Electrons are arranged in energy levels (shells) and subshells (orbitals).

Shell Capacity Rule

Shell	Symbol	Max Electrons
1st	K	2
2nd	L	8
3rd	M	18

 

Orbital and Shapes

Orbital	Shape	Max Electrons
s	Spherical	2
p	Dumbbell	6
d	Cloverleaf	10

 

CSIR-NET Memory Points

• Valence electrons determine bonding
• Most biological atoms obey octet rule
• Carbon forms 4 covalent bonds → backbone of life

6. What is a Molecule?

A molecule is a stable group of two or more atoms held together by chemical bonds.

Types of Molecules

Type	Description	Example
Homoatomic	Same atoms	O₂
Heteroatomic	Different atoms	H₂O
Macromolecules	Large biomolecules	Proteins, DNA

 

Chemical Bonds in Molecules

Bond	Strength	Example
Covalent	Strong	C–C, C–H
Ionic	Electrostatic	NaCl
Hydrogen	Weak	DNA base pairing
Van der Waals	Very weak	Protein folding

 

CSIR-NET Memory Points

• Covalent bonds dominate bio molecules
• Weak bonds stabilize 3D structures
• Hydrogen bonds crucial for DNA & proteins

CSIR-NET Rapid Revision Table

Concept	Key Focus
Matter	States & types
Atom	Subatomic particles
Atomic number	Identity
Mass number	Isotopes
Isotopes	Tracers
Electrons	Chemical behavior
Molecules	Basis of life

 

Conclusion

Atoms form molecules, molecules form bio molecules, and bio molecules drive life. A solid understanding of atomic structure, isotopes, and molecular bonding is essential for mastering biochemistry and excelling in competitive exams like CSIR-NET Life Sciences.

 

🧬 Biochemistry Interactive Quiz

Click on the options to check your answer!

1. Which subatomic particle determines the chemical identity of an atom?

A) Neutron

B) Proton

C) Electron

D) Positron

2. Which radioisotope is used to label DNA in replication studies?

A) Carbon-14

B) Sulfur-35

C) Tritium (3H)

D) Iodine-125

3. What is the maximum number of electrons the 'L' shell can hold?

A) 2

B) 8

C) 18

D) 32

4. Disulfide bonds in proteins are formed by which element?

A) Phosphorus

B) Nitrogen

C) Sulfur

D) Oxygen

5. Which bond is responsible for the secondary structure (alpha-helix) of proteins?

A) Ionic Bond

B) Covalent Bond

C) Hydrogen Bond

D) Van der Waals

6. Isotopes differ in the number of:

A) Protons

B) Neutrons

C) Electrons

D) Valence electrons

7. The most abundant compound in a living cell is:

A) Protein

B) Water

C) DNA

D) Glucose

8. Which of the following is a Homoatomic molecule?

A) H2O

B) CO2

C) O2

D) NH3

9. Carbon is the backbone of life because it can form:

A) 2 bonds

B) 4 covalent bonds

C) Ionic complexes

D) Hydrogen bonds only

10. 32P is used to label which macromolecule?

A) Proteins

B) Lipids

C) Nucleic Acids

D) Carbohydrates