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### Course: AP®︎/College Biology>Unit 1

Lesson 2: Elements of life

# Elemental building blocks of biological molecules

Common elemental building blocks of biological molecules: Carbon, Oxygen, Hydrogen, Nitrogen and Phosphorus.

## Want to join the conversation?

• When we say something like the body is 65% oxygen, what does that mean? Does that mean 65% of atoms in the body are oxygen atoms? 65% of our weight? Thank you!
• I think it would be safe to assume that our body is 65% oxygen by mass.

In general, our body is 65-70% water by mass. Water has a molar mass of 18 g/mol, and the majority of that mass comes from oxygen, not hydrogen (oxygen molar mass = 16 g/mol). 16/18=89%, and if 70% of body weight is water, than 70%*0.89=62.3% of body weight is oxygen. However, water is not the only source of oxygen; considering oxygen from other body organs as well, our body would be approximately 65% of oxygen by mass.

Also, as you can see from hydrocarbon chains of macromolecules, there are a LOT of hydrogen atoms attached. To calculate percentage of number of oxygen atoms, the formula would be:
(oxygen atom number) / (total atom number)
However, hydrogens would make total atom numbers increasingly large, so it would be difficult for oxygen to take up 65% of the number of atoms.

Of course, all these calculations came up from my head...since there isn't any reference for this, it would be better to ask your science teachers if anyone sees this.
• Hello, I have what might be a stupid question.
I understand how to tell which functional groups molecules have. How do I use those to determine what type of molecule it is? (protein, nucleic acid, carbohydrate, phospholipid, etc.)
• Carbohydrates are made up of C, H and O. The ratio of H:O is 2:1. An example for this is glucose (C₆H₁₂O₆).
Lipids are made up of C, H and O. However, the ratio of H:O is >2:1.
Proteins are made up of C, H, O, N, P and S. The units which make up proteins are known as amino acids. These amino acids are made up of an amino group (-NH2), a carboxyl group (COOH) and a unique "R" (side chain) group. The side chain (R) can differ based on the type of amino acid.
Nucleic acids are made up of C, H, O, N and P. The units which make up nucleic acids are known as nucleotides. Nucleotides consist of a sugar (such as deoxyribose for DNA and ribose for RNA), a phosphate group, and nitrogenous bases (adenine, guanine, cytosine, thymine) [Uracil is present instead of thymine in RNA].

Hope this helps :)
• What is a biological system and molecules in simple terms? Im pretty confused in understanding what Sal explained. Thanks in advance!
• Biological system: think everything from your nervous and digestive systems, to a cell in your body. Basically anything that has a way to maintain itself and its functions.

Molecule: is a group of two or more atoms. Think water molecules H2O.

The molecules in this video are specifically *macromolecules*, DNA, amino acids, ATP, etc. are the macromolecules that build up the human body and every complex living being (So all life today other than bacteria). They are made of a lot of atoms, and they can repeat in chains, which is why they are called "macro"molecules = large molecules.
• I see some of these trace elements are the same used in energy drinks. Is too much of these trace elements dangerous for your health?
• Since our bodies cannot naturally synthesize these elements we must consume them in our diets. But, to answer your question, too much of it just like many other good things, is bad for you. In this case, excess consumption could result in toxic side effects.
• how come O makes up 65% of our body and hydrogen makes up much less when H2O is made up of 2 H and only 1 O?
• It's 65 percent by mass not number of atoms. An oxygen atom has 8 times the mass of a hydrogen atom
• At 3.19 Sal says Carbon makes up about 0.04% of our atmosphere. doesn't he mean CO2? if so carbon would make up an even smaller percentage around 0.01%
• No, carbon forms many more molecules in our atmosphere, such as carbon monoxide, and other molecules.
• How many types of elements can be found in bio molecules ?
• About six elements consisting of carbon, nitrogen, hydrogen, oxygen, sulfur, and phosphorus.
• When Sal mentioned Triglyceride, he called it a 'fat', but wouldn't one of the components of Triglyceride be glycerin or glucose? Just intrigued :)
(1 vote)
• Triglyceride is made up of 1 molecule of glycerol bound to 3 fatty acids.

Hope this helps :)