Molecules in Living Organisms: Number & Size

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  • 0:03 Molecules and Living Organisms
  • 1:07 Types of Molecules
  • 2:09 Number and Size of Molecules
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain how molecules form part of living organisms, list the most common types of molecules in organisms like humans, and describe how abundant they are in the human body. A short quiz will follow.

Molecules and Living Organisms

Molecules consist of two or more atoms chemically bonded (or connected) together. For example, two oxygen atoms bond together to form O2, and a carbon and two oxygen atoms bond together to form carbon dioxide (CO2). Molecules are among the most basic units found inside living organisms.

A living organism is a living system, such as a vertebrate, insect, plant or bacterium. A living organism is usually described as being capable of reproducing, growing, responding to stimuli, and self-regulating. Molecules are one of the basic building blocks that make them up.

The most complex molecules that make up organisms (including humans) contain carbon bonded to other elements: carbon bonded to oxygen, carbon bonded to hydrogen, carbon bonded to nitrogen. For this reason, life on Earth is known as carbon-based life, or life that contains building blocks that are made of combinations of carbon and other elements. But as well as carbon, living organisms also contain a lot of hydrogen, oxygen, nitrogen, sulfur, and phosphorous.

Types of Molecules

But what actual molecules make up the human body? What are the most common kinds?

There are many types of molecules, but the most common kinds in biological organisms, like humans, are proteins, carbohydrates, lipids, and nucleic acids. These form the building blocks of cells, and biological organisms are entirely composed of cells that work together to make the organism work.

A protein is a compound containing large chains of amino acids. A carbohydrate is an organic compound containing hydrogen and oxygen in the same ratio as water (two hydrogen atoms for every oxygen). A lipid (or fat) is a compound containing a hydrocarbon chain and a carboxyl group, which sounds complex, but just means that it is made of exactly the right combination of hydrogen, carbon, and oxygen atoms. Last of all, nucleic acids make up DNA, which store the instructions for creating new life.

All four of these extremely common types of molecules are vital for life in biological organisms.

Number and Size

The size of the molecules that make up living organisms varies hugely, from tiny molecules like water (which is H2O -- it contains two hydrogen atoms bonded to one oxygen atom) to huge molecules like the proteins and nucleic acids we've already talked about. It takes this mix of large and small to make up a full organism.

The number of each kind of molecule also varies a lot. Water is by far the most common type of molecule in the human body -- it makes up 65% of your total mass. There aren't very many protein molecules by comparison, but proteins are next by mass because they're huge, making up 20% of your body weight. This is followed by lipids (fats) at 12%, with everything else -- RNA, DNA, and other kinds of molecules -- being the remaining 3%.

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