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Written by tutor Jill L.

Metabolism is a term that pops up a lot these days. It’s in every biology textbook, as well as most nutrition and fitness articles – where people are urged to ‘speed up their metabolism’ - but what does it really mean? Really simply, metabolism is a whole group of chemical reactions that keep us alive, energized, and growing. Every organism, from an amoeba to a human being, needs energy for activity, and also has to be able to produce new material such as proteins and nucleic acids to grow over time. The main idea behind the metabolic process is the ingestion of food (nutrients) which is then broken down and digested by the body to supply both energy and raw material for building!

Metabolism can be split into two main divisions. Catabolism is the breakdown of various types of molecules – such as fats, proteins, or carbohydrates - to provide chemical energy in the form of ATP, FADH2, or NADH molecules inside the body; anabolism then uses this energy to build components of a cell, such as proteins or nucleic acids (DNA). Different types of organisms use different methods, or metabolic pathways (including some bacteria that like to eat hydrogen sulfide which smells like rotten eggs!), to achieve this, but there are some factors common to all of them.

Catabolic processes seek to break down organic molecules and form as many of these energy molecules possible for each molecule being broken down. In general, carbohydrates, proteins, and fats are divided into their simplest and smallest units, which are then recombined to make structures the body needs in the anabolic part of metabolism. Of the three previously mentioned types of molecules, carbohydrates are the main energy source of most organisms, as they’re far easier to break down than others. This is why athletes ‘carboload’ before a big race or game. Carbohydrates can be easily broken down into monosaccharides (or ‘simple sugars’) such as glucose to produce many ATP molecules, through the processes of glycolysis, the Kreb’s cycle, and cellular respiration.

However, let’s not forget about proteins and fats!

Proteins are broken down through proteolysis into their base unit, amino acids. An interesting point here is that there are twenty of these amino acids that naturally occur on Earth. The problem is that mammals only make 11 of them, so we need to make sure we ingest the rest!

Fat molecules go through a process called hydrolysis, where they’re sliced into fatty acids and glycerol. The really cool part about metabolism here is that the glycerol molecules actually enter the glycolysis pathway, and the fatty acids can enter into the citric acid cycle (or Kreb’s) after being broken down a little bit more. This means the body can still metabolize fats for energy. It takes a little bit more work, or energy, to do this, but it’s usually worth it because fat has twice the stored chemical energy per gram than either carbohydrates or proteins.

Does this mean all we need to worry about for our daily intake is carbohydrates, fats, and proteins? Well, no... we also need our vitamins and minerals. Essential minerals are elements such as calcium, zinc, iron, and many more, whereas vitamins are organic molecules that our bodies need, but can’t synthesize – like Vitamin A, B1-12, and niacin.

So now we have our energy, and all of our base building blocks... but we need to grow too!

Remember, anabolism is where the body takes raw materials and uses them to construct whatever it needs. We mentioned glucose as a basic unit for carbohydrates, and this can be joined together with other simple sugars to form a polysaccharide, which is a long string of sugars. Common examples of this include starch (like a potato), glycogen (what athletes need for a long day), or even cellulose, which is the fiber in plants.

Fat gets a really bad rap when it comes to nutrition, but they are very useful molecules in anabolic processes. Fats are used to construct things like cell membranes which protect the interiors of cells from damaging molecules and viruses, but they also protect and insulate our internal organs as well as help with the absorption of fat-soluble vitamins.

Proteins are possibly the most multi-faceted aspect of catabolism, as they fulfill so many roles in the body. Blood cells, hormones, hair, DNA... all proteins. Amino acids are joined together in a long chain by peptide bonds, and then the growing protein gets folded into a very complex structure at the end, which helps it perform whatever function it is destined for.

The body is a very complex system, but very efficient at taking in and modifying all the material it needs! Understanding the nutritional aspects of metabolism helps make sense of some of the internal processes going on inside.

Metabolism Practice Quiz

Why are carbohydrates such an important energy source?

A. Most foods have more carbohydrates than proteins or fats
B. They’re broken down more straightforwardly by the body than other types of molecules
C. They tend to be cheaper to buy.
D. The body doesn’t need oxygen to fully metabolize them.
The correct answer here would be B.

The main difference between catabolism and anabolism can be summed up as:

A. Catabolism is building and growth in the body, anabolism is the breaking down of molecules.
B. Both involve separating molecules into their base units.
C. Anabolism is building and growth in the body, catabolism is the breaking down of molecules.
D. Catabolism involves carbohydrates, whereas anabolism deals with both proteins and fats.
The correct answer here would be C.

True or False: Mammals can produce all 20 amino acids required by the body.

A. True
B. False
The correct answer here would be B.

Which of the following is not a role fats play in the body?

A. Contribute essential amino acids
B. Insulate organs
C. Compose part of the cellular membrane
D. Join up the Kreb’s cycle in catabolism
E. Help with the absorption of certain vitamins
The correct answer here would be A.
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