There are a few elements that are essential for all known life. In this chapter you will learn what the three most important elements are, and why they are so important.

Essential elements for life

As you can see from the above table, Carbon, Hydrgen, and Oxygen are the three most important elements, making up more than 63 kg of a typical 70 kg human! Let's go through the importance of each one of these.

# The importance of Carbon

A carbon atom has 4 electrons in its outermost shell, which can hold a maximum of 8 electrons. Thus Carbon's outermost shell is like a glass half-full or half-empty, depending on your perspective, and that has important consequences. Because of this, Carbons can bind to a whole range of other elements in all sorts of unique combinations, leading to a host of compounds, most importantly the 4 basic molecules of life.

Carbon atom
Periodic Table

The 4 basic molecules of life are: carbohydrates, fats, amino acids (the building blocks of proteins) and nucleic acids (DNA, RNA). A carbohydrate molecule is made of carbon, hydrogen and oxygen. Sugars or many sugars linked together form carbohydrates. Our body is amazing in processing and releasing energy from sugar to do work. In the figure below, on the left you see how energy is released from sugars, in multiple steps of small activation energies where sugar is released and stored in carrier molecules. On the right you see what happens when all the sugar is oxidized at once. First of all, you need a large oxidation energy. This is similar to burning wood. If you've ever been camping and started a fire, you would know that the fire takes a long time to start. And unless you stop the fire externally, it continues until all the wood is burnt. However, that is not the case in the body. The body manages to store and release energy in small bursts, making it energy efficient. If you want to understand how the energy released can be used for work, take a look at this video below, which shows how heat energy released from burning gasoline powers cars.

Energy released from sugar

# What do Hydrogen and Oxygen make?

Hydrogen and oxygen make water! Without water, life as we known it would not have been possible. See the 5 major properties of water below:

Properties of Water

## The anomalous expansion of water

Most materials become more dense as temperature decreases. This makes sense as when temperature decreases, molecules become closer together. Too close and the state of matter changes i.e. from gas to liquid or liquid to solid. But water is special. There is a special temperature at which water is most dense: 40Celsius, whereas it freezes at 00C, and this makes all the difference in the world (where there is life)! As you can see below, water at 40C rises above frozen water. Meaning there is a thin layer where life can survive and not be frozen, even if temperatures above water get below 00C! This is the anolmalous nature of water, wherein the density vs temperature is an inverted U shape graph. The reason why water is so special has to do with hydrogen bonding: bonds fromed between the hydrogen atom in one water molecule and oxygen of an adjacent water molecule. These bonds are weaker than regular bonds within water molecules, and cause water at lower temperatures (below 40C), to become more 'puffy', or less dense.

Anomalous expansion of water
Anomalous expansion of water

## Water as the universal solvent

Due to the polarity of water, it can pull apart many polar molecules, making it a great solvent - for example, dissolving common table salt (NaCl). But don't be fooled, water can't dissolve everything. Take oil for example. Oils are non polar, meaning it is largely composed of Carbons and doesn't have much charge separation among molecules, because of which it does not dissolve in water.

Water dissolving Nacl

## High specific heat of water

Ever noticed that the pavement concrete gets so hot that it burns your feet, but the pool is never too hot? You can observe the same phenomena when you go to the beach as well. The reason for this is the large specific heat of water. Specific heat is defined as the amount of heat required to rise a specific amount of substance by a degree. Water has a specific heat of 1 calorie/gram °C = 4.186 joule/gram °C, which is higher than any other common substance. This also plays an important role in maintaining your body temperature. Think of what might happen to your organs if your body were to get as hot as the outside temperature on a hot summer day!

Specific heat

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