The molecules of life from space
Some meteorites contain amino acids, the building blocks of proteins, made without any living thing. This lesson explains what they are and how scientists know they came from space.
- Describe what amino acids are and how they form proteins.
- Calculate relative formula mass.
- Explain how scientists showed meteorite amino acids are not contamination.
Links to: GCSE chemistry, organic chemistry (separate chemistry): amino acids and the formation of proteins by condensation polymerisation; quantitative chemistry: relative formula mass.
Amino acids and proteins
Amino acids are organic molecules with two functional groups: an amine group (–NH₂) and a carboxylic acid group (–COOH). Amino acids join together by condensation polymerisation, each new bond releasing a molecule of water, to make long chains called polypeptides and proteins. Living things use 20 main amino acids to build their proteins.
The simplest amino acid is glycine, H₂NCH₂COOH, with the molecular formula C₂H₅NO₂.
Murchison
On 28 September 1969 a carbonaceous chondrite fell near Murchison in Australia, and more than 100 kg was collected quickly. Analyses found more than 70 different amino acids, including some used by life and many that life does not use at all. Later work found sugars, including ribose, and all five of the nucleobases used in DNA and RNA in carbonaceous meteorites.
Contamination or space?
Earth is covered with life, so contamination is a constant worry. Several lines of evidence show the meteorite molecules are genuine. Many of the amino acids are types that life on Earth does not use. Their carbon and nitrogen isotope ratios differ from those of Earth life. And samples returned in sealed capsules from the asteroids Ryugu and Bennu, which never touched Earth's surface, also contain amino acids. These molecules form naturally in space, and falling meteorites and dust delivered them to the young Earth.
Relative formula mass of glycine
Calculate Mᵣ for glycine, C₂H₅NO₂. (Aᵣ: C = 12, H = 1, N = 14, O = 16)
- Carbon: 2 × 12 = 24.
- Hydrogen: 5 × 1 = 5.
- Nitrogen: 1 × 14 = 14.
- Oxygen: 2 × 16 = 32.
- Total: 24 + 5 + 14 + 32 = 75.
Answer: Mᵣ = 75. Glycine has been found in meteorites, in dust from a comet returned by the Stardust spacecraft, and in samples from Bennu.
Alanine has the formula C₃H₇NO₂. Calculate its Mᵣ. Then draw two glycine molecules joining to form a dipeptide, circling the atoms that leave as water. Finally, list three pieces of evidence that the amino acids in Murchison came from space.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1Which two functional groups does every amino acid have?
Amino acids contain –NH₂ and –COOH groups.
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Question 2What type of reaction joins amino acids into proteins?
Each bond formed releases a small molecule of water, so it is condensation polymerisation.
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Question 3What is Mᵣ of water, H₂O? (H = 1, O = 16)
2 × 1 + 16 = 18.
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Question 4Which is evidence that meteorite amino acids are not contamination from Earth?
Unusual amino acids and non-terrestrial isotope ratios point to an origin in space.
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Question 5Why are the samples from Ryugu and Bennu important to this question?
Sealed sample-return capsules prove the molecules were present in the asteroids.