How did scientists observe metabolic pathways in organisms?

[Aafia]

how the scientists observed that macromolecules like proteins, carbohydrates and lipids are synthesized from other molecules with intermediate products by living cells. Did they observe this under microscope. I want to know this because I am studying bioenergetics and while studying photosynthesis and cellular respiration I have doubt that whether this all stuff is correct or not. I hope that you will help me to clear this doubt.

 

[Ygggdrasil]

https://en.m.wikipedia.org/wiki/Isotopic_labeling

Most molecules are too small to be seen in typical light microscopes.

 

[jim mcnamara]

One way - a simple answer:
For photosynthesis - Carbon dioxide + water -> glucose + free oxygen.
If you put plant cells in an environment with carbon dioxide and manufacture the carbon dioxide from radioactive carbon you can see that other new molecules in the cell become radioactive. Why? Because the special carbon dioxide got pulled apart and rearranged into brand new molecules by cell metabolism. What you get in terms of newly created molecules is predictable and matches the expected values for given cellular processes. So you see radioactive glucose for example.

Edit:
Darn. Yggdrasil gave a better scientific answer - mine was based on the level of understanding I saw in the post.

 

[J Boogie]

An "observation" is not necessarily something you "see". It can be something you measure.

For example, ethanol is converted to acetaldehyde in the body. How did they determine this was occurring? First, you start with a model organism in a small space, like a bacteria. You add one substance and measure the other. Then what happened? They dissolved the bacteria, removed the cabohydrates and fats and then purified the proteins.

They took each of these purified proteins and added them to a series of vials.

They added ethanol to each one until they found one vial where where acetaldehyde appeared.

Then, they examine the protein, determine its structure. Now they know how it could interact with an ethanol molecule. They run experiments with the purified protein to measure its performance converting one molecule to the other. They block the enzyme with other molecules to affirm their hypothesis. Their observations confirm this biological molecule converts one chemical to another. This is a metabolic pathway.

Using the same isolation technique they purify the proteins from multiple tissue types, for example, the liver. They discover large amounts of alcohol dehydrogenase in the liver. They take isolated liver cells and subject them to ethanol. It converts to acetaldehyde. But then the acetaldehyde becomes acetic acid, through a different process, but they have localized this enzyme activity to a specific body part.

How the concepts in biochemistry were discovered are are best explained as a combination of approaches: cell biology, cell culture, organic and physical chemistry, microbiology, and molecular biology. The explanation above was just a plausible guess as to how you might determine what enzymes might exist in a cell or organism in a lab. The actual story of how any particular metabolic process was discovered is out there.

 

[Andy Resnick]

how the scientists observed that macromolecules like proteins, carbohydrates and lipids are synthesized from other molecules with intermediate products by living cells. Did they observe this under microscope. I want to know this because I am studying bioenergetics and while studying photosynthesis and cellular respiration I have doubt that whether this all stuff is correct or not. I hope that you will help me to clear this doubt.

Optical methods can be used for some things, especially in the mitochondrial respiratory chain- cytochrome redox reactions were studied spectroscopically, with visible light. More typical instrumentation is electrochemical, using species-sensitive electrodes to measure reversal potentials and the like. To be sure, there's a lot more than that- cells need to be fractionated to isolate pure populations of mitochondria, for example.

Since you mentioned bioenergetics, the book "Bioenergetics" by Nicholls and Ferguson is highly readable and provides many experimental details.