Chemical elements and water

Carbon, hydrogen, oxygen and nitrogen are the most frequently occurring chemical elements in living things.

A variety of other elements are needed by living organisms, including sulfur, calcium, phosphorus, iron and sodium.

Sulfur: Needed for the synthesis of two amino acids.

Calcium: Acts as a messenger by binding to calmodulin and a few other proteins which regulate transcription and other processes in the cell.

Phosphorus: Is part of DNA molecules and is also part of the phosphate groups in ATP.

Iron: Is needed for the synthesis of cytochromes which are proteins used during electron transport for aerobic cell respiration.

Sodium: When it enters the cytoplasm, it raises the solute concentration which causes water to enter by osmosis.

Thermal properties of water include heat capacity, boiling and freezing points and the cooling effect of evaporation.

Water has a large heat capacity which means that a considerable amount of energy is needed to increase it’s temperature. This is due to the strength of the hydrogen bonds which are not easily broken. This is why the temperature of water tends to remain relatively stable. It is beneficial for aquatic animals as they use water as a habitat.

Water has a high boiling and freezing point. It boils at 100 C because the strong hydrogen bonds. All these hydrogen bonds between the water molecules need to break for the liquid to change to gas. Water becomes less dense as it gets closer to the freezing point and so ice always forms on the surface first. The high boiling point of water is vital for life on earth as if water boiled at a lower temperature the water in living organisms would start to boil and therefore these organisms would not survive.
The fact that water becomes less dense as it freezes is beneficial to organisms as ice will always form at the surface of lakes or seas and by doing so it insulates the water underneath, maintaining a possible habitat for organisms to live in.

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur.

Cohesion is the effect of hydrogen bonds holding the water molecules together. Water moves up plants because of cohesion. Long columns of water can be sucked up from roots to leaves without the columns breaking. The hydrogen bonds keep the water molecules sticking to each other.

The solvent properties of water mean that many different substances can dissolve in it because of its polarity.

Water can evaporate at temperatures below the boiling point. Hydrogen bonds need to break for this to occur. The evaporation of water cools body surfaces (sweat) and plant leaves (transpiration) by using the energy from liquid water to break the hydrogen bonds. The solvent properties of water mean that many different substances can dissolve in it because of its polarity. This allows substances to be carried in the blood and sap of plants as they dissolve in water. It also makes water a good medium for metabolic reactions.

Organic compounds are compounds that are found in living organisms and contain carbon. Inorganic compounds are the ones that don’t contain carbon. Although, there are a few compounds found in living organisms which also contain carbon but are considered as inorganic compounds. These include carbon dioxide, carbonates and hydrogen carbonates. 

• Glucose, galactose and fructose are all monosaccharides.
• Maltose, lactose and sucrose are all disaccharides.
• Starch, glycogen and cellulose are all polysaccharides. 

In animals, glucose is used as an energy source for the body and lactose is the sugar found in milk which provides energy to new borns until they are weaned. Finally, glycogen is used as an energy source (short term only) and is stored in muscles and the liver. 

In plants, fructose is what makes fruits taste sweet which attracts animals and these then eat the fruits and disperse the seeds found in the fruits. Sucrose is used as an energy source for the plant whereas cellulose fibers is what makes the plant cell wall strong. 

• Lipids can be used for energy storage in the form of fat in humans and oil in plants.
• Lipids can be used as heat insulation as fat under the skin reduces heat loss. 
• Lipids allow buoyancy as they are less dense than water and so animals can float in water. 

Carbohydrates and lipids can both be used as energy storage however carbohydrates are usually used for short term storage whereas lipids are used for long term storage. Carbohydrates are soluble in water unlike lipids. This makes carbohydrates easy to transport around the body (from and to the store). Also, carbohydrates are a lot easier and more rapidly digested so their energy is useful if the body requires energy fast. As for lipids, they are insoluble which makes them more difficult to transport however because they are insoluble, lipids do not have an effect on osmosis which prevents problems within the cells in the body. They also contain more energy per gram than carbohydrates which makes lipids a lighter store compared to a store of carbohydrates equivalent in energy. 

A nucleotide is made of the sugar deoxyribose, a base (which can be either adenine, guanine, cytosine or thymine) and a phosphate group. Below is a representation of a nucleotide. 

• Adenine
• Cytosine 
• Guanine
• Thymine

Below is a diagram showing how nucleotides are linked to one another to form a strand. A covalent bond forms between the sugar of one nucleotide and the phosphate group of another nucleotide.

DNA is made up of two nucleotide strands. The nucleotides are connected together by covalent bonds within each strand. The sugar of one nucleotide forms a covalent bond with the phosphate group of another. The two strands themselves are connected by hydrogen bonds. The hydrogen bonds are found between the bases of the two strands of nucleotides. Adenine forms hydrogen bonds with thymine whereas guanine forms hydrogen bonds with cytosine. This is called complementary base pairing. Below is a digram showing the molecular structure and bonds within DNA. 

DNA replication is semi-conservative as both of the DNA molecules produced are formed from an old strand and a new one. The first stage of DNA replication involves the unwinding of the double strand of DNA (DNA double helix) and separating them by breaking the hydrogen bonds between the bases. This is done by the enzyme helicase. Each separated strand now is a template for the new strands. There are many free nucleotides around the replication fork which then bond to the template strands. The free nucleotides form hydrogen bonds with their complimentary base pairs on the template strand. Adenine will pair up with thymine and guanine will pair up with cytosine. DNA polymerase is the enzyme responsible for this. The new DNA strands then rewind to form a double helix. The replication process has produced a new DNA molecule which is identical to the initial one.

Semi Conservative replication 

• large number of enzymes are involved in the process of replication. For these enzymes to gain access to the DNA the helix has to be unwound and separated by the enzyme DNA Helicas
   
• Nucleotides hydrogen bond to their complementary bases within the template
• DNA polymerase links the phosphate of the newest nucleotide to the sugar of the nucleotide before it by a covalent bond 

Complementary base pairing is very important in the conservation of the base sequence of DNA. This is because adenine always pairs up with thymine and guanine always pairs up with cytosine. As DNA replication is semi-conservative (one old strand an d one new strand make up the new DNA molecules), this complementary base pairing allows the two DNA molecules to be identical to each other as they have the same base sequence. The new strands formed are complementary to their template strands but also identical to the other template. Therefore, complementary base pairing has a big role in the conservation of the base sequence of DNA.

DNA replication is semi-conservative. 

Which means that it uses the old strand from the mother DNA to bind with the new Daughter DNA to make two new strands. Using both the original and the copy is called semi-conservative. 

DNA and RNA both consist of nucleotides which contain a sugar, a base and a phosphate group. However there are a few differences. Firstly, DNA is composed of a double strand forming a helix whereas RNA is only composed of one strand. Also the sugar in DNA is deoxyribose whereas in RNA it is ribose. Finally, both DNA and RNA have the bases adenine, guanine and cytosine. However DNA also contains thymine which is replaced by uracil in RNA.