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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.
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.
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.
Systems include the respiratory system, which allows you to breathe and turn the food you've eaten into energy; the reproductive system that allows humans to create new life; the circulatory system, which moves blood around the body, supplying the whole body with water and energy, and taking the waste where it needs to go; and the excretion system, which manages the waste and moves it out of the body in urine and feces. There are many other systems: the nervous system, the skeletal system, the muscular system, the immune system... and they all function by millions of individual cells working together.
A living organism is a living system, such as a vertebrate, insect, plant, or bacterium. A living organism is usually considered to be capable of reproducing, growing, responding to stimuli, and self-regulating.
The most complex molecules that make us up contain carbon bonded to other elements: carbon bonded to oxygen or hydrogen or nitrogen. For this reason, life on Earth is known as carbon-based life, or, in other words, 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.
Those atoms combine together to form complex molecules of various types: proteins, carbohydrates, lipids, and nucleic acids. And those are in turn 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.
Systems include the respiratory system, which allows you to breathe and turn the food you've eaten into energy; the reproductive system that allows humans to create new life; the circulatory system, which moves blood around the body, supplying the whole body with water and energy, and taking the waste where it needs to go; and the excretion system, which manages the waste and moves it out of the body in urine and feces. There are many other systems: the nervous system, the skeletal system, the muscular system, the immune system... and they all function by millions of individual cells working together.
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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.
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.
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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.
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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.
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Physical Science: Middle School9 chapters | 61 lessons
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