Biological Molecules

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Each individual molecule is called a monomer. Monomers can join together to make polymers. Most elements are made up of just carbon, hydrogen, oxygen and nitrogen.

A single monomer is called a monosaccharide. A pair of monosaccharides can be combined to form a disaccharide. Monosaccharides can also combine to form polysaccharides. Maltose is made from glucose and glucose, sucrose is made from fructose and glucose and lactose is made from glucose and galactose. Glucose has the general formula of C6H12O6.

When joining two monosaccharides together, a condensation reaction takes place. The bond is called a glycosidic bond. When breaking two monosaccharides apart a hydrolysis reaction takes place. Polysaccharides are very large moleucle that are insoluble. Examples of polysaccharides are starch, glycogen and cellulose.

Test for reducing sugars - add Benedict's reagant to the sample and heat for 5 minutes. A brick red colour indicates a reducing sugar

Test for non-reducing sugars - add hydrochloric acid to the sample and heat for 5 minutes. Add a reagant to neutralise the solution and perform Benedict's test. A brick red colour indicates a non-reducing sugar

Test for starch - add iodine solution to the sample and the indication of starch is found by a blue-black colouration being present 

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Found in many parts of a plant in the form of small grains. Occurs in seeds and storage organs such as potato tubers. It forms an important component of food and is a major source of energy. 

Made up of alpha glucose and can be branched (amylopectin) or unbranched (amylose). 

The main role of starch is an energy store and it is suited to its function because:

  • It is insoluble so doesn't affect the water potential 
  • Being large and insoluble is doesn't diffuse out of cells 
  • It is compact so lots can be stored in a small space 
  • When hydrolysed it forms alpha glucose which is used in respiration 
  • The branched form has many ends meaning that enzymes can simultaneously act so glucose is released rapidly
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Found in animals and bacteria but never in plant cells. Glycogen has shorter chains and is more highly branched than starch. 

Mainly stored as small granules in the muscles and the liver. Relatively small as fat is the main storage molecule in animals 

Suited for storage because:

  • It is insoluble so doesn't draw in water by osmosis 
  • Being insoluble it doesn't diffuse out of cells 
  • It is compact, so lots can be stored in a small space 
  • It is more highly branched so more of the ends can be acted on by enzymes so it can be broken down more rapidly as animal has a higher metabolic rate
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Made up of beta-glucose.

Cellulose structure is:

  • Forms straight unbranched chains 
  • The chain run parellel allowing hydrogen bonds to form cross-linkages between adjacent chains 
  • Cellulose molecules are grouped together to form microfibrils 
  • Very rigid so is a major component of the cell wall to prevent the cell from bursting 

Cellulose is suited to its function of providing support and rigidity because:

  • Cellulose molecules are made up of beta glucose allowing them to be straight chains 
  • Cellulose molecules run parellel to each other and are crossed linked by hydrogen bons which add collective strength
  • These molecules are grouped to form microfibrils which provide more strenght 
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Roles and Characteristics of Lipids

All lipids have the following characterisitics:

  • Thet contain carbon, hydrogen and oxygen 
  • The proportion of oxygen to carbon and hydrogen is smaller 
  • They are insoluble in water 
  • They are soluble in organic solvents such as alcohols 

Roles of Lipids 

  • Phospholipids contribute to the flexibility in membranes 
  • They provide a source of energy when oxidised as they provide twice the amount of energy 
  • They provide waterproofing as lipids are insoluble so allow plants and insects to conserve water 
  • They are good insulators as fats are slow conductors yet help retain body heat beneath the body surface 
  • They are good protection as fat is often storecd around delicate organs such as the kidney 
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Made up of three fatty acids and combined with a glycerol. Each fatty acid forms an ester bond with the glycerol in a condensation reaction.

Saturated - no double bonds between carbon atoms 

Mono-unsaturated - one double bond between carbon atoms 

Polyunsaturated - more than one double bond between carbon atoms 

Structure Related to their Properties:

  • Have a high ration of energy storing carbon-hydrogen bonds to carbon atoms so is an excellent source of energy 
  • Have a low madd to energy ration so they can be stored in small volumes 
  • Being large, non-polar molecules they are insoluble in water so do not affect the water potential 
  • As they have a high ratio of hydrogen to oxygen atoms they release water when oxidised 
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Phospholipids and the Test for Lipids

Made up of two fatty acids, a glycerol and a phosphate molecule. Made up of two parts:

  • A hydrophilic 'head' - which interacts with water but not with fat 
  • A hydrophobic 'tail' - which orients itself away from water but readily mixes with fat

Structure related to their properties:

  •  Phospholipids are polar molecules meaning they can form bilayers within cell-surface membranes forming a hydrophobic barrier 
  • The hydrophilic heads help to hold the surface of the cell membrane 
  • The sturcture allows them to form glycolipids by combining carbohydrates within the cell-surface membrance. This is important in cell recognition 

Test for Lipids - add ethanol to the sample and shake. Add water and a cloudy white precipitate indicates the presence of a lipid 

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Structure of an Amino Acid

  • Amino group (-NH2)
  • A carboxyl group (-COOH)
  • A hydrogen atoms 
  • An R group

When two proteins are joining together the -OH of a carboxyl group and the -H join together to form a peptide bond 

Primary Structure - the sequence of amino acids in the polypeptide chain 

Secondary Structure - hydrogen bonds form making it coil into an alpha helix or fold into a beta pleated sheet 

Tertiary Structure - hydrogen bonds and ionic bonds form between the positive and negative charges. Disulfide bridges form when a sulphur and a cysteine come close together. 

Quaternary Structure - some proteins are made up of several different chains due to the way they are assembled. This forms the final 3D structure of the protein 

Test for Proteins -add the buriet solution and a purple colouration indicates the presence of a peptide bond 

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Enzymes are globular proteins that act as catalysts. Catalysts alter the rate of a reaction without undergoing permanent changes to themselves. Enzymes work by lowering the activation energy level allowing reactions to take place at a lower temperature than normal. This enables some metabolic processes to occur rapidly at the hman body temperature.

Enzyme Structure - have a specifice region called the active site that is functional. The molecule the enzyme acts on is the substrate. This fits forming an enzyme substrate complex

Induced Fit Model - can mould itself around a substrate placing a strain on the bonds lowerig the activation energy 

Lock and Key Model - the substrate is the precise shape of the active site 

Effect of Temperature - a rise increase the kinetic energy so they are more likely to collide with each other leading to more successful collisions. To high of a temperature and the enzyme denatures which is a permanent change 

Effect of pH - a change in pH alters the charges on the amino acid on the active site so the substrate can no longer fit in. pH can cause the active site to change shape 

Effect of Low and High Enzyme Concentration - low concentration means there are two many substrate molecules for the enzyme. A high concentration means there are enough enzymes so do not alter the rate of reaction 

Competitive Inhibitors have the same shape as the active site so compete for the site but are not permanent. Non-competitve inhibitors bind to the enzyme and pernanently change the shape of the active site 

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Structure of DNA and RNA

DNA is used to store genetic informaion 

RNA is used to transfer genetic information between DNA and the ribosomes and ribosomes are made of RNA

A nucleotide is made from:

  • A pentose sugar
  • A nitrogen containing base 
  • A phosphate group

Structure of DNA 

  • Pentose sugar of deoxyribose 
  • Phosphate group
  • Adenine, thymine, cytosine and guanine 

Structure of RNA 

  • Pentose sugar of ribose 
  • Phosphate group 
  • Uracil replaces thymine 
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Stability and Function of DNA

DNA is stable because:

  • The phosphodiester backbone protexts the more chemically reactive organic bases 
  • Hydrogen bonds form bridges as there are three bonds between C and G and the higher there proportion the more stable the molecule 

Function of DNA is adapted by DNA being:

  • A very stable molecule which passes from generation to generation without change so only rarely mutates 
  • Two separate strands which allow them to seaperate during DNA replication 
  • Extremely large so carries an immense amount of genetic information 
  • Having base pairs in a helical cylinder, the genetic information is protected from outstide chemical and physical forces 
  • Base pairing leads to DNA being able to replicate and transfer information as mRNA 
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DNA Replication

1) The enzyme DNA helicase breaks the hydrogen bonds linking together the base pairs of DNA 

2) As a result the double helix separates into its two strands and unwinds 

3) Each exposed polynucleotide strand then acts as a template for complementary free nucleotides to bind by specific base pairing 

4) Nucleotides are joined together in a condensation reaction by the enzyme DNA polymerase  

5) Each of the new DNA molecules contain one of the original DNA strands and a new strand so it is semi-conservative replication

Evidence of this comes from Meselson and Stahl 

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Energy and ATP

Structure of ATP - made of an adenine molecule, ribose and phosphates 

How ATP stores energy? - The bonds between the phosphate groups are unstable and have a low activation energy, meaning they are easily broken. When they do break a considerable amount of energy is released. This is a reversible reaction 

ATP + water --> ADP + Pi + energy        This reaction is catalysed by ATP hydrolase.

ATP is a better immediate energy store than glucose because:

  • Release less energy than glucose making it more manageable 
  • The hydrolysis of ATP to ADP releases immediate energy 

ATP is used in energy-requiring processes in cells including:

  • Metabolic processes of making macromolecules from their basic units 
  • Movement as ATP provides energy for muscle contraction 
  • Active transport as ATP changes the shape of the carrier proteins 
  • Secretion as ATP is need to form lysosomes 
  • Activation of molecules as the addition of ATP makes molecules more reactive 
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Water and Its Functions

  • Water is dipole molecule as the oxygen has a slight negative charge whereas the hydrogen has a positive charge so is a good solvent 
  • Water is able to hydrogen bond as the positive pole of one water molecule will be attracted to the negative pole of another water molecule. This allows that molecules to stick together
  • Water has a high specific heat capacity as it takes more energy to separate the bonds between water molecules. Therefore water can act as a buffer against sudden temperature variations in an aquatic environment 
  • Water has a high latent heat of evaporation meaning that sweat is effective in cooling the body down
  • Water is cohesive so can form a surface tension. Cohesion is the tendency for molecules to stick together. This is seen in xylem vessels and the surface tension supporting pond skaters 
  • Water is important is metabolism as is can be used to break down complex molecules. Reactions also take place in an aquaeous environment. 
  • Evaporation of water allows organisms to cool down 
  • Provides support e.g. hydrostatic skeleton of animals 
  • Iron ions are important in haemoglobin 
  • Sodium ions are importan in the transport of glucose and amino acids 
  • Phosphate ions are essentioal in ATP and DNA
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