Biological molecules

• Biological molecules are molecules of life - critically important molecules needed for organisms to survive

   

  Organic molecules - they are three dimensional molecules which contain carbon atoms arranged in rings or chains
• Condensation reaction

  Chemical reaction whereby two or more simple molecules are joined together to form a large biological molecule with the removal of water
• Hydrolysis

  Splitting up of complex biological molecules into its component units with the addition of water molecules
• Carbohydrates

  Made up of carbon, oxygen and hydrogen

  General formula: Cm(H2O)n

  • Hydrogen and oxygen atoms present in ratio 2:1
• Monosaccharides (simple sugar)

  • Simplest form of carbohydrates, cannot be hydrolyzed further into simpler units
  • Most common monosaccharides → sugars with six carbon atoms

  Glucose, fructose, galactose (C6H12O6)

  • Atoms arranged differently within molecules, resulting in different chemical and biological properties
  • Common properties:
  • Generally sweet tasting
  • Soluble in water
  • Able to lower water potential of solutions
• Disaccharides (complex sugar)

  • Made up of two or more monosaccharides through condensation reaction
  • Bond formed between monosaccharides is glycosidic bond → covalent bond
  • Common disaccharides:

  Maltose, lactose and sucrose (C12H22O11)

  • Different structure and properties due to different building units and the way they bond
• Maltose:

  Glucose and glucose

  Lactose:

  Glucose and galactose

  Sucrose:

  Glucose and fructose
•  Polysaccharides (complex carbohydrate)

  • Consist of many monosaccharides joined together through condensation reaction to form one polysaccharide
  • Glycosidic bond between monosaccharides
  • Common type of polysaccharide:

  Starch, glycogen and cellulose

  • Polysaccharides comprised entirely of glucose but properties are different - due to different ways in which the glucose units are linked together
• Storage polysaccharides

  1. Starch - found in plants

  • Formed from condensation reaction of large numbers of glucose molecules
  • Can be in long straight chains (amylose) or branched chains (amylopectin)
• Glycogen - found in mainly animals

  • Formed from condensation reaction of large numbers of glucose molecules
  • Glucose molecules joined up in highly branched chains
  • Mainly stored in liver and muscles of mammals
• Starch and glycogen suitable as storage material:

  • Insoluble in water so they do not affect water potential in cells, do not affect water balance
  • Too large to diffuse through the cell membrane, so they stay in the cell
  • Compact shapes which occupy lesser space than all the individual glucose molecules that make up a glycogen or starch molecule
  • Easily hydrolyzed to glucose when needed
• Cellulose - structural polysaccharides

  • Formed from condensation reaction of large numbers of glucose molecules
  • Glucose molecules that form cellulose bond differently as compared to starch, hence cellulose has a different property
  • Insoluble in water
• Cellulose cell wall

  • Consists of cellulose fibers embedded in a polysaccharide matrix
  • Fully permeable structure
  • Functions:
  • Provide mechanical support for plant cell and to the plant, especially for herbaceous (soft stem) plant
  • Resist expansion when water enters by osmosis, ensuring integrity of plant and to provide turgidity
• General function of carbohydrates

  • Glucose → oxidized during respiration - is a substrate for respiration, to release energy for all cell activities
  • Deoxyribose sugar/ ribose sugar - used for formation of nucleic acids
  • Forms lubricants
  • Forms nectar in some flowers
  • Cellulose - form supporting structure
  • Organisms store the energy contained in monosaccharides by converting them into polysaccharides (insoluble forms) that are deposited into specific storage areas
  • Carbon - hydrogen bonds release energy when oxidized, making carbohydrates well suited for energy storage
• Lipids

  Lipids contain much lesser oxygen as compared to carbon and hydrogen
• Simple lipids - Triglycerides


  Triglyceride contains three molecules of fatty acids and one molecule of glycerol
• Fatty acids

  • A fatty acid is a long hydrocarbon chain that has a carboxyl functional group (COOH)
  • The hydrocarbon chain can be unsaturated (contain one or more carbon-carbon double bonds) or saturated (lack of carbon-carbon double bonds)
  • Do not mix well with water
• Glycerol

  Glycerol is an alcohol

  Formula of glycerol is C3H8O3
• Formation of triglyceride

  Formation of one triglyceride

  • By condensation reaction of 3 molecules of fatty acids and 1 molecule of glycerol with the removal of 3 molecules of water
  • The 3 fatty acids can be the same or different
  • Bond formed between 1 fatty acid and glycerol is an ester bond
• Properties of triglycerides

  Classified as fats and oils, depending on their state: solids (fats) or liquids (oils) at 20 °C

  Oil (unsaturated fatty acids) 

  • Presence of at least 1 carbon-carbon double bond between carbon atoms and has relatively shorter fatty acid chains
  • Lower melting point and molecular weight
  • Cannot be closely packed together to solidify due to kinks in chains

  Fats (saturated fatty acids)

  • Absence of carbon-carbon double bonds and relatively longer fatty acid chains
  • Higher melting point and molecular weight
  • Closely packed together
• Function of triglycerides

  Energy source (main function)

  • Triglyceride contains a greater number of carbon-hydrogen (C-H) bond per gram than starch or glycogen, therefore, one gram of triglyceride yields about twice as much energy than one gram of carbohydrates.
  • Triglyceride has about half the mass of carbohydrates for an equivalent amount of energy stored.

  ** It is therefore a light-weight energy source for animals that move by speed or flight and seeds dispersed by wind or insects.
• Why is carbohydrates still the most utilised energy source than lipids

  Carbohydrate is still the most direct source of energy in living things because they are metabolised quickly, both in aerobic (when oxygen is readily available) and anaerobic (when oxygen is not available) conditions
• Where triglycerides are found in for energy storage

  • Oil in seeds
  • Fats in mammals - Mammals have specialised fat cells for storing fat under their skin; cells are grouped together to form adipose tissue
  • Hibernating animals store fats as food reserve in their body because in very cold seasons, it is difficult for these animals to hunt for food
• Function of triglycerides

  Heat insulator

  • Fats conduct heat slowly. Excellent heat insulator against heat loss from deeper regions of the body to the outside

  Where is it found

  • Huge and thick blubber found in polar bears and whales
• Buoyancy in water

  • Less dense than water (insoluble in water)
  • Found in:

  Large animals that live in cold seas, such as whales and seals, often have very thick layers of adipose tissue called blubber.
• Protective layer

  • Ability to absorb shock

  Found in:

  Surrounding delicate and vital organs, e.g. heart, kidneys
• Important component of myelin sheath

  Act as electrical insulator, allowing rapid transmission of electrical impulses along myelinated neurones.

  Found in:

  Nerve cells
• Provides metabolic water

  Oxidation of triglycerides produces metabolic water

  • Triglycerides release twice as much water as carbohydrates when oxidized during respiration
  • Water produced from respiration is known as metabolic water.
  • The hydrogen atoms of triglycerides and carbohydrates are used in the formation of metabolic water during oxidation.
  • Triglycerides contain more hydrogen atoms than carbohydrates, so it is a better source of metabolic water.

  Found in:

  This is important for animals, e.g. camels and kangaroo rats, those that live in dry, hot desert where water is scarce
• As a solvent

  A solvent for fat-soluble vitamins (A, D, E and K) and other vital substances (e.g. hormones)
• Compound lipids - Phospholipids

  Structure of phospholipids

  • Phospholipids are a group of compound lipids that contain two molecules of fatty acids, one molecule of glycerol and a phosphate group.
  • Phospholipids have a hydrophilic ("water-loving") and a hydrophobic ("water-fearing") component - the phosphate group is hydrophilic and the fatty acid tails are hydrophobic.
• Structure of phospholipids:

  The hydrophilic phosphate group is attracted to the water molecules in their environment and the hydrophobic fatty acid tails are repelled by the water molecules. The hydrophilic heads face the aqueous environment (exterior) while the hydrophobic tails face the inside of the bilayer, away from water in their environment, forming the interior of the membrane.
• Function of phospholipids

  Phospholipids are a major component of biological membranes. Due to the nature of phospholipids, the cell membrane is made up of two layers of phospholipids - phospholipid bilayer.
• Proteins

  • Proteins are made up of the elements carbon, hydrogen, oxygen and nitrogen. Sulfur is sometimes present.
  • Each type of protein has a unique three-dimensional shape thus proteins have diverse functions.
  • When heated, the weak bonds in proteins are broken, the protein is denatured (loses its three-dimensional shape)
  • Proteins are polymers made up of monomers - amino acids
• Amino Acids

  • Monomers (building units) of the protein.
  • There are 20 naturally occurring amino acids.
  • Every amino acid molecule has a central carbon atom bonded covalently to four groups:

  1. at least one amino group (-NH2)
  2. a carboxyl group (-COOH)
  3. a hydrogen atom
  4. a variable group, the R group (also known as the side chain)

  • Different amino acids have different R groups - gives different properties and functions
• Formation of polypeptide

  • Amino acids join together to form polypeptide through condensation reaction with the removal of water molecules.
  • Amino group (-NH2) of one amino acid reacts with the carboxyl group (-COOH) of another amino acid, with the removal of water.
  • The covalent bond that is formed is a peptide bond.
  • Continued condensation reactions result in the formation of a long chain of amino acids - polypeptide.
  • Conversely, hydrolysis can break down peptide bonds of polypeptides into amino acids with the addition of water molecules.
• Bonds in a protein structure

  • Peptide bonds join the amino acids to form a polypeptide chain.
  • The polypeptide chain fold into a particular three-dimensional shape as a result of four other types of bonds, namely disulfide bonds / bridges, ionic bonds, hydrogen bonds, and hydrophobic interactions 
• Functions of proteins 

  • Used in the synthesis of new cells, for growth and repair of worn-up cells.
  • Biological catalysts - enzymes, to speed up chemical reactions.
  • Chemical messenger - Hormones such as insulin to stimulate target organs to perform a certain function.
  • Transport proteins such as haemoglobin transport oxygen in red blood cells.
• Functions of proteins

  • Structural protein e.g. collagen is a component of skin and bones while keratin is a component of hair, nails, and feathers.
  • Defence of body - Antibodies are used to help fight infections in the body.
  • Protein is source of energy during starvation and is oxidised after all the carbohydrates and lipids are used up
• Kwashiorkor is a form of protein deficiency disease. Children with this disease, caused by malnutrition, will have swollen stomachs. Their skins crack and become scaly.
• Polysaccharides, polypeptides, proteins and polynucleotides are also known as polymers.

  Polymer is a macro-molecule (complex), composed of many repeated subunits