Viking Metabolism Experiments

Unit 1: Theoretical Pillars of Biology

Lecture 1.5: Introduction to Metabolism

John D. Nagy

BIO 181: General Biology for Majors, Scottsdale Community College

2019 Revision

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Viking Metabolism Experiments

Outline

1 NASA’s Viking missions

2 Viking’s search for metabolism on Mars

3 Viking’s experimentsLR methodsLR results

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Viking Metabolism Experiments

Viking missions to Mars

Carl Sagan and a mockup of the Viking Lander.

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First color image from VL1

Is there life in the soil?

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NASA’s working description of life on Earth

“Life is a self-sustaining chemical sys-

tem capable of Darwinian evolution.”

From The limits of Organic Life in Planetary Systems [?]:

Life on Earth is fundamentally cellular.

Life is chemical: Living things use covalent bonding propertiesof C, H, N, O, P, and S and the ability of O and N to modulatehydrocarbon reactivity.

Biomolecules have evolved to function when dissoved in water.

Metabolism is controlled by enzymes that are inheritedthrough reproduction.

Living systems adapt to changing environments via evolutionby natural selection (Darwinian evolution).

Life exploits thermodynamic disequilibrium (homeostasis).

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First clues—Lavoisier and Laplace

Where does the carbon come from?

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Basis of metabolism

The source of carbon is organic compounds.

Example:

Sugar (glucose) + oxygen → carbon dioxide.

An equation of metabolism?

C6H12O6 + O2 → CO2 + H2O (?).

Is this equation balanced?

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Summary of terrestrial energy metabolism

Basic energy metabolism of all organisms on Earth:

These pathways transfer energy from glucose to energycarrying molecules.

All organisms use glycolysis to produce pyruvate.If no O2: cells use fermentation;If O2: cells use the Krebs cycle and other pathways.

Basic equation of oxidative metabolism

C6H12O6 + 6O2 → 6CO2 + 6H2O

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Viking Landers

Two misstions,Viking I and VikingII.

Viking I launchedAug 20, 1975; landedon Mars July 20,1976 in ChrysePlanitia.

Viking II launchedSept 9, 1975; landedon Mars Sept. 3,1976 in UtopiaPlanitia.

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Schematic of the Viking biology package

Contains 3 experiments: Labeled Release (LR), PyrolyticRelease (PR), and Gas Exchange Experiment (GEX)

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Details of the LR experiment

Chamber is opened; scoop of Martialsoil is deposited at the bottom.

Vessel sealed; headspace cleared withHe.

Radiolabeled (with 14C) nutrient brothadded to the soil sample.

Dependent variable (measured):amount of radioactivity (from 14C) inthe headspace.

If microbes in the soil metabolizeorganic compounds in the nutrientbroth, they are expected to releaseradioactive CO2 gas (hence, “LabeledRelease”).

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How do we interpret the LR experiment?

What would be a positive result inthis experiment?

What would be a negative result inthis experiment?

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How do we interpret the LR experiment?

What would be a positive result inthis experiment?

Radioactivity (14C) in the headspace⇒ microbes in the soil.

What would be a negative result inthis experiment?

No radioactivity (14C) in the headspace⇒ no microbes in the soil.

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Controls

Before we fly this experiment to Mars, we have to test it tomake sure it works.

The third principle of scientific thought

Experiments must always be thoroughly tested to make surethey are working properly.

Definition: Experimental control

An experimental control is a test of the validity of anexperiment. One performs the experiment under controlledconditions to force the experiment to give an expected result. Ifthe experiment fails to produce the expected result, it fails thecontrol and the experiment is deemed invalid.

How will we test this experiment using experimental controls?

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Positive and negative controls

Definition: Positive control

A positive control in an experiment run under controlledconditions to force a positive result. If the positive control failsto give a positive result, the experiment is invalid.

Definition: Negative control

A negative control is an experiment run under controlledconditions to force a negative result. If the negative control failsto give a negative result, the experiment is invalid.

Example: How should we test a patient for HIV disease?

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LR positive and negative controls on Earth

Should we risk flying this experiment to Mars based on thesedata?

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Actual data from LR experiment on Viking 1

Are these results consistent or inconsistent with the hypothesisof organic life on Mars?

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Actual data from LR experiment on Viking 2

Are these results consistent or inconsistent with the hypothesisof organic life on Mars?

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