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Water Molecules

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Water Molecules

Water is a unique biological molecule. The water structure plays a key role in its unique properties. In particular, polarity and hydrogen bonding are two of the most important biological properties of water. Let’s have a closer look at these and how they come about.

Why is water a polar molecule?

Remember that the chemical structure of water is H2O? A single water molecule is made up of one atom of oxygen (O) joined to two atoms of hydrogen (H-O-H) by shared electrons (covalent bonding). The sharing of these electrons between the atoms is uneven because oxygen atoms pull the electrons more strongly than the hydrogen atoms. As a result, the hydrogen atoms have a slightly positive charge (δ+). The oxygen has unshared negative electrons, leaving a slightly negative charge (δ-). Therefore, we can say that water is a polar molecule because it has a partial positively charged region on one side and a partial negatively charged region on the other.

Water Molecule Hydrogen bonding diagram of the hydrogen bonding in a water molecule StudySmarterBall and stick diagram of a water molecule

Why does water undergo hydrogen bonding?

The partially negative oxygen atoms are attracted to the partially positive hydrogen atoms. We call this hydrogen bonding. Hydrogen bonds continuously break and reform; thus, they can be weak in small numbers. However, many hydrogen bonds form a strong structure that contributes to water’s valuable properties.

Water Molecule diagram of a water molecule StudySmarterHydrogen bonding in a water molecule

What are the functions of water molecules in biology?

  • Metabolite: Water acts as a metabolite in many metabolic reactions, including condensation and hydrolysis. For example, the condensation of amino acids results in the formation of various important enzymes (proteins).

  • Solvent: This means that certain substances can dissolve in water. As many metabolic reactions occur in solution to allow reactants to dissolve, this is a useful property. Molecules that dissolve in water can include oxygen, enzymes and sugars.

  • Water transport: Water molecules have a special property called cohesion; they stick together, aiding water transport. Cohesion is essential in plants, because water sticks together to be pulled up in columns, through the xylem vessels, to different regions of the plant. Additionally, water can be moved in and out of cells via osmosis.

  • Temperature control: Water can help with temperature as it has a high specific heat capacity and a high latent heat of vaporisation.

Osmosis is the movement of water molecules from an area of high concentration to an area of low concentration via a partially permeable membrane.

Each function is discussed in the headings below.

How do water molecules function as a metabolite?

Remember that metabolic reactions are just chemical processes occurring within living organisms to sustain life. One of the main biological functions of water molecules is to act as a metabolite. Many of these reactions involve a condensation or hydrolysis reaction. Water plays an important role here because hydrolysis reactions use a water molecule when breaking a bond, such as when energy is released from the biological molecule ATP. On the other hand, condensation reactions release a water molecule as a new bond is formed. A vital example of this is the condensation reaction that joins amino acids to make polypeptides.

How does water act as a good solvent?

As water is a polar molecule, covalently bonded polar substances such as glucose and many ionic compounds can dissolve in it. The negative ion will attract the positively charged region of the water molecule and vice versa. The ions become surrounded, meaning they dissolve.

Let’s take the example of the salt sodium chloride (NaCl) dissolving in water. The Na+ ions are positive, so the partially negative oxygen atom points towards the Na+ ions when water molecules surround the NaCl molecule. On the other hand, the Cl- ions are negative, so the partially positive hydrogen atoms point towards the Cl- ions when water molecules surround the NaCl molecule.

Why is high latent heat of vaporisation of water useful?

When water is exposed to enough heat, it can be vaporised. The process by which this happens is the same as the boiling process for water. First, heat energy is transferred to the molecules of water which causes them to move around at higher speeds. After some time, the water molecules will have too much energy to stay connected as a liquid. Due to this, they will turn into gaseous molecules of water vapour. These then float upwards to the surface of the liquid as bubbles so that they can escape and travel into the air.

Latent heat of vaporisation is the amount of thermal energy required to change one gram of a liquid substance to gas at a constant temperature. To evaporate water, we must put in a lot of energy (heat) to break the numerous hydrogen bonds between the molecules. When a substance requires a lot of energy to vaporise it, it has a high latent heat of vaporisation. We can use water to control temperature. For example, humans sweat to cool down, and only a small amount of water is lost.

How can water molecules act as a buffer to changes in temperature?

Specific heat capacity is the amount of heat needed to raise the temperature of one gram of water by one degree celsius. Water has a high specific heat capacity; a lot of energy is required to raise water temperature because the hydrogen bonds between water molecules can absorb plenty of energy. As a result, water acts as a buffer, i.e. it resists changes in temperature more than other substances.

Because water temperature doesn’t change rapidly, it can be useful in living organisms. For example, the water inside our bodies tends to stay at a stable temperature, allowing us to maintain a constant internal body temperature. Another good example is water habitats; the temperature is likely to be more stable in water than on land, making an ideal environment for some organisms.

Why is the cohesion and adhesion of water a useful property?

When we say water is cohesive, all we mean is that water molecules tend to stick together because they are polar. Strong cohesion implies that water has a high surface tension when it comes into contact with the air, which is why pond skaters can ‘walk’ along the water’s surface. This strong cohesion property also allows water to flow, so it is ideal for transporting substances, e.g. movement through blood vessels of animals or columns of water in the xylem of plants.

Adhesion refers to water's ability to stick to other substances. Let's use the same example of xylem. Water will adhere to the walls of xylem vessels which allows it to move up the plant towards the pleaves where it will evaporate. You can find out more about this in our Xylem article.

Water - Key takeaways

  • A water molecule consists of one oxygen atom and two hydrogen atoms with an uneven sharing of electrons, which causes water molecules to be polar. They are also able to form hydrogen bonds.

  • Water is an important metabolite as water molecules are released and used during hydrolysis and condensation reactions.

  • Water is a good solvent due to its polar properties, which is useful in the transport of substances and for allowing chemical reactions to take place

  • Another important property of water is its high specific heat capacity due to its many hydrogen bonds. It is a suitable habitat for some organisms and ensures we maintain a constant internal temperature.

  • Water has a high latent heat of vaporisation (again due to its many hydrogen bonds); helpful for controlling temperature via water loss.

Frequently Asked Questions about Water Molecules

Water is a polar molecule because of the uneven sharing of electrons between atoms. This occurs because the oxygen atoms pull the electrons more strongly than the hydrogen atoms. As a result,  the other side of the hydrogen atoms has a slightly positive charge (δ+). The oxygen has unshared negative electrons, leaving a slightly negative charge (δ-).

There are 3 molecules in water. 2 hydrogen molecules and 1 oxygen molecule.

One

First, heat energy is transferred to the molecules of water which causes them to move around at higher speeds. After some time, the water molecules will have too much energy to stay connected as a liquid. Due to this, they will turn into gaseous molecules of water vapour. These then float upwards to the surface of the liquid as bubbles so that they can escape and travel into the air.

Water is important in acting as a solvent and metabolite. It is also key for temperature control and water transport.

Water is a biological molecule consisting of 2 hydrogen atoms bonded to 1 oxygen atom. It is a polar molecule and can form hydrogen bonds. It has several functions, such as acting as a solvent and metabolite. It is also helpful in controlling temperature and allowing substances to be transported.

Final Water Molecules Quiz

Question

What are the four key functions of water that we need to know for A-Level Biology?

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Answer

Water can act as a metabolite and as a solvent. It is key during water transport and also temperature control.

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Question

Why is the sharing of electrons between atoms in a water molecule uneven?

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Answer

The oxygen atoms pull the electrons more strongly than the hydrogen atoms.

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Why is water considered a polar molecule?


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Answer

The hydrogen atoms have a slightly positive charge (δ+). The oxygen has unshared negative electrons, leaving  a slightly negative charge (δ-).

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Why does water undergo hydrogen bonding?

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Answer

The partially negative oxygen atoms are attracted to the partially positive hydrogen atoms.

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How is water involved in metabolic processes?


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Answer

Many chemical processes that occur within living organisms require condensation or hydrolysis reactions, both of which involve the use of a water molecule

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What is the condensation reaction?


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A reaction that releases a water molecule as a new bond is formed, e.g. during the joining of amino acids to make polypeptides.

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What is a hydrolysis reaction?


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Answer

A reaction that uses a water molecule when breaking a bond, e.g. during the release of energy from the biological molecule ATP

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What does it mean when we say that water has a high latent heat of vaporisation?


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Answer

A lot of energy is required to vaporise water, i.e. turn it from a liquid to a gas

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Why does the property of having a high latent heat of vaporisation make water useful?


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Answer

It means that water can be used to control temperature. For example, humans sweat to cool down.

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What do we mean when we say that water has a high specific heat capacity?


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Answer

A lot of energy is required to raise the temperature of water.

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What property of water causes it to have a high specific heat capacity?


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Answer

Water has hydrogen bonds

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How does water act as a good solvent?


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Answer

Water is a polar molecule; it can dissolve ionic compounds and covalently bonded polar substances such as glucose.

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How does water dissolve ionic compounds?


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Answer

The positively charged region of the water molecule will be attracted to the negative ion and vice versa. Therefore, the ions become surrounded by water molecules, so they are dissolved.

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What does it mean when we say water is cohesive?


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Answer

All water molecules tend to stick together

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What two properties of water arise from the fact that it is cohesive?


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Answer

Strong cohesion means that water has a high surface tension and allows water to flow, making it ideal for transporting substances.

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Question

Give some examples of where the cohesion of water molecules helps with transport.


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Answer

E.g. movement of substances through the blood vessels of animals or through the columns of water in the xylem of plants.

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