Revision:Edexcel Biology Unit 1 Molecules and Cells - Nucleic Acids and Enzymes

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Nucleic acids

• A nucleotide is made up of a pentose sugar ,and organic nitrogenous base and a phosphoric acid
• It is formed between the condensation of of a necleoside and a phosphate molecule
• Bonds which link a phosphate group to another phosphate group in a nucleotide is called the phosphodiester bond and the bond which links the phosphate and the nucleoside is the phosphoester bonds.
• There are 2 types of ribonucleotides which are RNA(the one containing ribonucleotides) and DNA(the one containing the deoxyribonucletides)
• There are two types of purines (Adenine, Guanine) and three types of pyrimidines (Uracil ,thymine and cystosine)

DNA replication

• Replication of DNA is semi conservative, which means that the original DNA strand is reused to form new double stranded DNA.
• The process is as follows:
  1. Helicase unwinds the double stranded DNA by breaking the hydrogen bonds.
  2. DNA attracts complementary bases onto the single stranded DNA which was used as a template .(A is attracted to T) and (G is attracted to C) and vice versa in the 5` direction to the 3` direction.
• The phophodiester linkages from other parts are joined by DNA Ligase from the 3` direction to the 5` direction
• In the end, there will be two identical strains of DNA formed as a result.
• This is important in cell divisions of mitosis and meiosis

Protein synthesis

• This process involves the synthesis of protein molecules by using genes found in the DNA
• The process of transcription and translation is assisted by RNA(tRNA ,mRNA and rRNA).They differ in three ways, which are
  1. It is single stranded
  2. It contains a pentose sugar called ribose instead of a deoxyribose
  3. The base thymine is replaced by uracil (pyramidine-single ring structure)
• The process starts off with transcription which involves the unwinding of the double stranded DNA molecule. This is then followed by the enzyme RNA polymerase catalyzing the binding of mRNA bases to the DNA bases which are complementary.
• After this is done, the mRNA is modified by adding a guanine cap at the 5` end and a poly A being added to the 3` end of the mRNA. Thus it moves out of the nuclear envelope and into a ribosome in the cytoplasm or the rough endoplasmic reticulum.
• After the mRNA reaches the ribosomes, a process known as translation occus. mRNA then binds to a subunit of a ribosomes in such a way that the codons (3 units bases) are exposed.
• A unit of ribose consists of rRNA which has an entering point for 2 codons of mRNA whereelse a tRNA is a clover shaped molecules which has 3 anticodons facing outwards.
• As the mRNA reaches the the ribosomes,2 tRNA molecules can be help at one time. The enzyme peptidyl transferase catalyses the formation of a peptide bond between 2 amino acids. After it is formed, tRNA is moves out of the RNA and the product of the amino acids are moved into the golgi apparatus with the help of a transport vesicle for further modification of the amino acids.

Enzymes

• Enzymes are basically tertiary proteins which have an active sites which binds on to a substrate. The reaction causes bonds to be formed and broken much easier. This is because enzymes act as organic catalysts which reduce the activation energy needed for a reaction to go forward.
• The process is summarized as below :
• Enzyme + Substrate ---àenzyme-substrate complex---àenzyme + products
• There are a few factors which affect the rate at which the enzymes react, they are
  1. Temperature
     • The increase in temperature would increase the total kinetic energy of the substrate and enzyme, thus the rate of collision increases and the rate of reaction increases. The opposite is shown if the temperature is reduced
  2. pH
     • The decrease in pH would lead to excess H+ ions which will ionize the side groups in the enzyme`s amino acids residue. This would lead to a change in its molecular structure and cause the enzyme`s active site to be not complementary to the substrate anymore. Thus the lower the pH, the more ionization and a less rate of reaction.
  3. Enzyme and Substrate concentration
     • The increase of any of the substate or concentration would lead to a higher rate of collision and thus increases the rate or reaction
  4. Inhibitors
     • There are two types of inhibitors which are reversible inhibitors and non-reversible inhibitors
     • An irreversible inhibitor are inhibitors which has a shape which is complementary to the active site of an enzyme and therefore competes with the substrate for a reaction to proceed..this decreases the rate of reaction
     • A non competitive inhibitors are inhibitors, this is usually heavy metals which binds on to the enzyme and causes the enzyme to change its shape and loses its catalytic abilities. Therefore the rate of reaction is reduced

Classification of enzymes

• There are 5 types of enzymes which are
  1. Oxidoreductase – catalyses the process of oxidation and reduction
  2. Transferase – catalyses the transfer of one molecule to another
  3. Hydrolases – catalyses the hydrolysis of a molecule
  4. Lyase – catalyses the lysis (breakage) of a molecule
  5. Ligase – catalyses a formation of a bond between molecules

Industrial usages of enzymes

Cellulase

• Catalyses the breakage of cellulose into beta glucose
• Uses include the production of single cell protein and saccharification (sweetening)

Pectinases

• Catalyses the breakage of pectins into galacturonic acid
• Uses includes the clarification of fruit juices and the increase in total fruit juice yield.

Amylases

• Catalyses the hydrolysis of glycosidic bonds in starch into alpha glucose
• Uses includes the clarification of fruit juices and sweetening of fruit juices.

Protease

• Catalyses the breakage of protein into amino acids
• Used for the clarification of beer, biological detergents and the tenderisation of meat.

Lactase

• Catalyses the breakage of lactose into glucose and galactose
• Uses includes sweetening, production of lactose free milk and the production of immobilized enzymes.

Lipase

• Catalyses the breakage of lipids into glycerol and fatty acids.
• Uses includes the enhancement of cheese ripening and biological detergents.

Practical : The production of immobilized enzymes

• Mix sodium alginate solution with desired enzymes in a beaker and place it into a plastic syringe.
• Add this mixture dropwise into a calcium chloride solution for dehydration.
• Strain beads and rinse. Place into a syringe barrel and pour in the substrate.
• Collect the yield which is produced drop by drop

Advantages of immobilized enzymes

• Highly resistant to extreme conditions such as heat and pH
• Immobilized enzymes can be filtered and reused

Disadvantages of immobilized enzymes

• A slower rate of reaction since the beads are now restraint of movement by the alginate solution

Also See

• Edexcel Biology Unit 1 Molecules and Cells - Molecules
• Edexcel Biology Unit 1 Molecules and Cells - Nucleic Acids and Enzymes
• Edexcel Biology Unit 1 Molecules and Cells - Cells
• All A Level Biology Revision Notes

Comments

These notes are aimed at people studying Edexcel Biology A Level, but will be suitable for other exam boards too.

Originally written by MalaysianDude on TSR Forums.