What Are Living Organisms Made Of? - Molecules & Functions

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  • 0:01 What Are Living Organisms?
  • 0:35 What Are They Made Of?
  • 1:52 Molecular Functions
  • 2:43 Lesson Summary
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Lesson Transcript
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.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

After watching this video, you will be able to explain what living organisms are made of, the elements, the important molecules, and how they build to perform biological functions. A short quiz will follow.

What Are Living Organisms?

A living organism is a living system, such as a vertebrate, insect, plant, or bacterium, containing lots of parts that are interdependent. But that seems a bit vague, so let's go deeper. What is it to be alive? What makes an organism be declared as living?

Unfortunately, we don't have a great answer to that question and it is still hotly debated to this day. But many scientists would contend that a living organism is capable of reproducing, growing, responding to stimuli, and self-regulating.

What are They Made of?

Although we don't know if it's the same elsewhere, living things on Earth are said to be carbon based. The most complex molecules that make us up contain carbon bonded to other elements: carbon bonded to oxygen, carbon bonded to hydrogen, carbon bonded to nitrogen. Almost everything in our bodies and the bodies of other animals and plants is made of carbon. 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.

There are certain key molecules that are a big part of our bodies and the bodies of other living organisms. Proteins for example, form almost our entire bodies, and proteins on Earth are based on carbon. Nucleic acids are vitally important to animal life, and indeed also contain carbon. Carbohydrates and lipids (or fats) are also major parts of the bodies of animals like us.

But it's not just carbon. Living organisms on Earth are made of a mixture of carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous. This includes water, which has the chemical name H2O and contains two hydrogen and one oxygen atoms. In fact, our bodies are made of similar materials to the atmosphere of the Earth, which shouldn't be too surprising considering we breathe in the air every day.

Molecular Functions

Those elements that make up the body are the building blocks for proteins, carbohydrates, lipids, and nucleic acids. And those in turn are the building blocks for cells. Each cell inside the body has a particular function, and all the cells of your body work together in systems to complete their job.

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Additional Activities

Review of Molecules of Living Organisms

In the video lesson, you learned that living organisms are carbon-based. A majority of the complex molecules that make up humans and other organisms contain carbon (C) atoms bonded to other elements, most often oxygen (O), hydrogen (H), or nitrogen (N). Living organisms also contain a lot of sulfur (S) and phosphorous (P) atoms.

The six atoms just mentioned make up more than 98% of the tissue (excluding the bones of the skeletal system) of most living organisms. There are four types of complex carbon-based molecules in all living organisms: proteins, carbohydrates, lipids, and nucleic acids. We will examine each of these in a bit more molecular detail in this lesson extension.

Complex Molecules of Life

Proteins, nucleic acids, carbohydrates, and lipids often are very large molecules built from the chemical associations of many smaller building blocks or subunits. The large molecules and their subunits are listed in the table below.

The images below are examples of the subunits of proteins (A), nucleic acids (B), complex carbohydrates (C), and large lipids (D). Examine them for similarities and differences in terms of the atoms that compose them.

An Amino Acid

A Nucleotide

A Sugar

A Fatty Acid

  1. Which three atoms (elements) are found in all of the subunits that make up complex molecules of organisms? _____ _____ _____
  2. Which two subunits and the complex molecules they build have Nitrogen (N)? _____ _____
  3. Which one of the subunits has a large amount of oxygen (O) and phosphorus (P)? _____
  4. Which one of the subunits has mostly carbon (C) and (H) atoms? _____
  5. Which one of the subunits has a 1:2:1 ratio of C:H:O? _____

None of the subunit molecules shown included sulfur (S) atoms. Sulfur is common in proteins because several amino acids include this element. Fill in the table below to summarize which atoms (elements) are present in each type of complex molecule. Use the one-letter abbreviations for each of the elements.

Answer Key for Instructors

  1. carbon (C), hydrogen (H), oxygen (O)
  2. amino acids and proteins, nucleotides and nucleic acids
  3. nucleotides
  4. fatty acids
  5. sugars

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