The variety of life, both past and present, is extensive, but the…

The idea

Life on Earth is incredibly diverse, yet the underlying biochemical basis is fundamentally similar across all organisms. Every living thing is constructed from four main groups of carbon-based organic molecules: carbohydrates, lipids, proteins, and nucleic acids. Organic molecules contain carbon bonded to hydrogen. Carbon's ability to form four strong covalent bonds allows it to build complex chains and rings. The same monomers—monosaccharides, amino acids, and nucleotides—are used universally. Furthermore, the same fundamental reactions, condensation and hydrolysis, assemble and dismantle these polymers. This shared biochemistry, including the universal use of DNA, RNA, and ATP, provides strong evidence for evolution and a common ancestor.

What earns the marks

• State that all living organisms share the same fundamental groups of carbon-based organic molecules: carbohydrates, lipids, proteins, and nucleic acids.
• Identify that shared monomers, such as monosaccharides, amino acids, and nucleotides, are common to all life forms.
• Explain that the universal use of DNA, essentially the same genetic code, and ATP provides biochemical evidence for a common ancestor.
• Describe how comparing DNA base sequences, mRNA base sequences, or amino acid sequences can be used to determine evolutionary relationships between diverse species.
• Recognise that organic molecules are carbon-based compounds containing carbon-hydrogen bonds, distinguishing them from inorganic carbon compounds like carbon dioxide.

Common mistakes and how to put them right

• Assuming all carbon-containing compounds are organic.

  Put it right by remembering that simple carbon compounds like carbon dioxide (CO2) and carbonates are inorganic because they lack carbon-hydrogen bonds.

• Misinterpreting a 'similar biochemical basis' to mean organisms have identical molecules.

  Put it right by understanding that while the classes of molecules (e.g., proteins) and monomers (e.g., amino acids) are shared, the specific sequences (e.g., amino acid sequences) differ between species.

• Stating species are related solely because of similar physical appearances when asked for biochemical evidence.

  Put it right: Specifically referencing shared molecules like DNA, RNA, or proteins.

Exam tips

• When evaluating evolutionary relationships, always specify the exact biochemical evidence being compared, such as the 'DNA base sequence' or 'amino acid sequence', rather than just writing 'DNA'.
• The concept of a shared biochemical basis is an excellent linking theme for synoptic essays, connecting biological molecules to classification, evolution, and the genetic code.