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1.2 Elements of Life

Keywords

English Term 中文翻译 Definition & Explanation
Macromolecule 大分子 A very large molecule, such as a protein or nucleic acid, commonly created by the polymerization of smaller subunits.
Carbohydrate 碳水化合物 A biological molecule consisting of carbon, hydrogen, and oxygen atoms, primarily used for short-term energy and structural support.
Lipid 脂质 A diverse group of hydrophobic molecules (including fats and phospholipids) consisting primarily of carbon, hydrogen, and oxygen.
Protein 蛋白质 A functional biological molecule consisting of one or more polypeptides folded into a specific 3D structure.
Nucleic Acid 核酸 Polymers (DNA and RNA) made of nucleotide monomers that store and transmit genetic information.
Phospholipid 磷脂 A lipid made of glycerol, two fatty acids, and a phosphate group; it forms the cell membrane bilayer.
Monomer / Polymer 单体/聚合物 A monomer is a building block molecule; a polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds.

1. Exchanging Matter with the Environment

Living organisms cannot create matter out of nowhere. Atoms and molecules from the environment are necessary to build new molecules. Organisms must constantly exchange matter with their surroundings to grow, reproduce, and maintain organization.

Core Concept: The CHNOPS Elements

While there are over 100 elements in the periodic table, life primarily relies on just a few: Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S).

Among these, Carbon, Hydrogen, and Oxygen are the most prevalent elements. They form the foundational "skeleton" for all major biological macromolecules:

  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic Acids
Overview of four major macromolecules

2. Allocation of Elements in Biological Molecules

Different biological macromolecules require specific elements from the environment to be synthesized. Understanding which elements go into which macromolecules is crucial.

A. Carbon, Hydrogen, and Oxygen (C, H, O)

As mentioned, these three elements are ubiquitous. They are found in all classes of macromolecules. Carbon's unique ability to form four covalent bonds allows it to build large, complex, and diverse molecules.

B. Nitrogen (N)

Nitrogen is acquired from the environment (often through the nitrogen cycle by plants) and is primarily used in the building of:

  • Nucleic Acids: Nitrogenous bases (Adenine, Thymine, Cytosine, Guanine, Uracil) are key components of DNA and RNA.
  • Proteins: Every amino acid contains an amino group (\(\ce{-NH2}\)).

C. Phosphorus (P)

Phosphorus is highly reactive and is a critical component of:

  • Nucleic Acids: The sugar-phosphate backbone of DNA and RNA requires a phosphate group.
  • Phospholipids: A special type of lipid containing a phosphate group, essential for forming all cell membranes.

D. Sulfur (S)

Sulfur is found in a much smaller subset of molecules, but plays a crucial structural role:

  • Proteins: Certain amino acids, specifically cysteine and methionine, contain sulfur. The sulfur in cysteine can form disulfide bridges, which are strong covalent bonds that help maintain the 3D shape of proteins.

Biological Application: Isotope Tracing

Because of the specific elemental makeup of macromolecules, scientists often use radioactive isotopes to track molecules in biological systems. For example, in the famous Hershey-Chase experiment:

  1. Radioactive Phosphorus-32 (\(^{32}P\)) was used to label DNA (because proteins lack P).
  2. Radioactive Sulfur-35 (\(^{35}S\)) was used to label Proteins (because DNA lacks S).

Quiz

Source: Campbell Biology Practice Test - Chapter 2 (The Chemical Context of Life)