Biological molecules

HideShow resource information


  • A fibrous protein
  • Forms supportive tissues in animals
  • Strong and flexible
  • Made of three polypeptide chains-tightly coiled into strong triple helix
  • Chains interlinked by strong covalent bonds
  • Minerals can bind to triple helix to increase rigidity
1 of 28


  • Globular protein
  • Carries oxygen around body
  • Curled up structure-hydrophilic side chains on outside, hydrophobic chains inside
  • Soluble in water-makes it good for transport in blood
2 of 28

Nucleic acids

  • Weak acids found in nuclei of cells
  • Polymers composed of monomers called nucleotides
  • Deoxyribonuleic acid (DNA)
  • Ribonucleic acid (RNA)
  • Contain instructions that make every living organism on the planet
3 of 28


Nucleotides are the monomers that make up nucleic acids (DNA, RNA). They have three parts to them:

  • A phosphric acid
  • A deoxyribose-a 5 carbon pentose sugar
  • A nitrogenous base
    • 5 organic bases
    • All contain elements carbon, hydrogen, oxygen, and nitrogen
    • Two groups-purines (2 rings of carbon and nitrogen atoms) and pyrimidines (1 ring of carbon and nitrogens)
    • The bases are: Adenine, Cytosine, Guanine, Thymine, Uracil
    • Thymine only found in DNA, Uracil only found in RNA
4 of 28



  • One molecule of glycerol with 3 fatty acids attached to it
  • Fatty acids joined by an ester bond
  • Fatty acid molecules have long 'tails' made of hydrocarbons which are hydrophobic.
  • Tails make lipids insoluble in water


  • Mainly used as energy storage for molecules
    • long hydrocarbon chains contain lots of energy-released when broken
  • Lipids contain twice as much energy per gram than carbohydrates 
  • Insoluble (water entering the cell by osmosis would make them swell)
  • Triglycerides bundle together as insoluble droplets
    • hydrophobic tails face inwards and are shielded by their glycerol heads
5 of 28



  • Hydrophilic heads and hydrophobic tails-form a double layer with heads facing out towards water
  • Similar to triglycerides except one fatty acid molecule is replaced by a phosphate group (which is ionised and attracts water)


  • Make up the bilayer of cell membranes
  • Centre of bilayer is hydrophobic
    • water soluble substances can't easily pass through, so the membrane acts as a barrier to those substances
6 of 28


A group of substances used as both energy sources and structural materials in organisms.

  • All carbohydrates contain carbon, hydrogen, and oxygen with the general formula: Cx(H2O)y
  • There are three mains groups:
    • Monosaccharides-simple sugars with the general formula (CH2O)n, where can can be 3-7
    • Disaccharides-'double sugars', formed from 2 monosaccharides
    • Polysaccharides-large molecules formed from many monosaccharides
7 of 28


  • An abundant and very important monosaccharide
  • Contains six carbon atoms-therefore a hexose sugar
  • General formula-C6H12O6
  • Major energy source for most cells
  • Highly soluble
  • Main form in which carbohydrates are transported around the bodies of animals
  • Glucose exists in different forms called structural isomers
    • alpha glucose and beta glucose are two common structural isomers
    • only difference is the position of the -OH group
    • has a major effect of the roles of both
8 of 28

Other monosaccharides

Other hexose monosaccharides:

  • Fructose-sweeter than glucose
  • Galactose-not as souble as glucose

Pentose monosaccharides:

  • Ribose
  • Deoxyribose
    • both important constituents of RNA and DNA
9 of 28


  • Join together with a glycosidic bond
  • This forms between a hydroxyl group on one and a hydroxyl group on another monosaccharide
  • When naming the bond, you must say which carbons are involved
    • in maltose, sucrose, and lactose the bond is a 1-4 glycosidic bond
  • A condensation reaction
10 of 28


  • Polymers containing many monosaccharides linked by glycosidic bonds
  • Formed by condensation reactions
  • Mainly used as energy stores and as structural components of cells
  • The major polysaccharides are starch and cellulose in plants, and glycogen in animals
11 of 28


Made of many alpha glucose molecules arranged into two structural units: amylose and amylopectin.

  • Amylose contains glucose molecules joined mainly by alpha 1-4 glycosidic bonds
  • Amylose is an unbranched molecule and forms a helical structure (compact)
  • Amylopectin mainly contains 1-4 glycosidic bonds, but also contains many more alpha 1-6 glycosidic bonds
  • Most starch is amylose but does contain some amylopectin
  • Amylopectin has a highly branched structure
  • Starch usually stored as intracellular starch grains in organelles called plastids
    • plastids include green chloroplasts and colourless amyloplasts
  • Starch is produced from glucose and is broken down during respiration
12 of 28


Another polysaccharide and is the main part of plant cell walls. It is the most abundant organic polymer.

  • Very strong (unlike starch) and prevents cells from bursting when they take in excess water
  • Consists of long chains of beta glucose molecules joined by beta 1-4 glycosisdic bonds
  • Chains of glucose form rope-like microfibrils which are layered to form a network
  • Every other glucose molecule rotates through 180 degrees so that hydroxyl groups on each molecule are adjacent to each other in the structure of cellulose
  • Results in long unbranched chains
  • Hydrogen bonding between chains gives cellulose molecules great tensile strength-there are thousands of chains and thousands of bonds
13 of 28


  • Has a similar structure to amylopectin, containing more aplha 1-6 glycosidic bonds that produce an even more branched structure
  • Stored as small granules
  • Less dense and more soluble than starch and is broken down more rapidly-this indicates the higher metabolic requirements of animals compared with plants
  • A very energy dense molecule
  • Branches in the molecule are for enzymes to attach more glucose to the glycogen molecule or for glucose to be broken off from it
  • Because glycogen is not soluble it does not affect the water potential of the cell-if it was stored as glucose the cell would burst as it draws in more water
14 of 28

Amino acids and proteins

  • Proteins are unbranched molecules made of amino acids joined together by peptide bonds
  • There are 20 different amino acids
  • Singles chains of amino acids are polypeptides
    • they can be folded in 3d shapes to form globular proteins
    • or formed into simpler shapes such as helicer to form fibrous proteins

Uses of proteins

  • 50% of the living matter of a cell
  • Large molecules made of C, H, O, and N. Some also contain S.
  • Functions:
    • structural
    • membrane carriers and pores
    • enzymes
    • many hormones
    • antibodies
15 of 28

Amino acid structure


16 of 28

Amino acids and proteins (continued)

  • R-groups are what create the differences in amino acids
  • Some R-groups are larger than the C-N-N part of the molecule
  • Some R-groups are +, some -, or they can be hydrophobic or hydrophilic

Plants and animals

  • Plants can manufacture AAs, using nitrogen from the soil converted into amino groups
  • Animals must ingest proteins in their diet
  • Some (8-10 of the 20) AAs are called essential amino acids
    • most essential AAs are found in meat or soya.
  • AAs that are surplus to the body's requirements cannot be stored
  • Urea is toxic due to the amine group in AAs.
  • They are processed in the liver, where urea is formed.
17 of 28

Joining amino acids

  • Condensation reaction
    • between the acid group of one amino acid and the amine group of another
  • Forms a covalent bond
  • A water molecule is produced
  • New bond is called a peptide bond
  • New molecule is called a dipeptide
  • A peptide bond can be broken in a hydrolysis reaction
18 of 28

Making proteins

  • Proteins may consist of hundreds of amino acids, or even more than one polypeptide chain bonded to form a larger molecule
  • Synthesised in the cell on ribosomes
  • Uses mRNA to put the amino acids in the right order
  • The mRNA passes through the ribosome joining the AAs together one at a time
  • The formation of different polypeptides uses different mRNA molecules.

'm' in mRNA stands for messanger

19 of 28

Primary structure of proteins

  • The primary structure is the structure first determined by the specific amino acid sequence
  • All proteins have an amine group at one end, and an acid group at the other, no matter the length
  • Function is determined by structure 
  • There are around 10,000 different proteins that we know of, each with its own function
20 of 28

Biochemical tests


  • Add Biuret Reagent to sample
  • If positive, change from pale blue to lilac

Reducing sugar

  • Heat with Benedict's solution
  • If positive, change from blue to orange-red precipitate

Non-reducing sugar

  • Boil with hydrochloric acid, cool solution and add sodium hydrogencarbonate or sodium carbonate solution and repeat Benedict's test
  • If positive (once Benedict's test repeated) change from blue to orange-red precipitate
21 of 28

Biochemical tests (continued)


  • Mix sample with ethanol and then pour the liquid into water contained in a clean test tube
  • If positive, cloudy white emulsion will form near top of water


  • Add iodine to sample
  • If positive, change from yellow-brown to blue-black
22 of 28


  • All globular proteins
  • 3D shape usually has hydrophobic AA R-groups in the centre of the ball, and hydrophilic AA R-groups outside the ball.
  • Soluble in water
  • Catalyst
  • Specific

Active site

  • Only a tiny proportion of the AAs make up the active site (often fewer than 10 AAs)
  • Very specific shape
  • The job of the rest of the amino acids is to keep the enzymes this specific shape
23 of 28

Enzymes (continued)

Biological catalysts

  • Speed up chemical reactions
  • Usually work much faster than inorganic catalysts
  • Specific to one catalytic reaction
  • Do not produce a range of unwanted by-products
  • Area of industrial research

Enzymes in cells

  • Metabolism is enzyme driven
  • Bond formation and breakage is usually enzyme driven
  • Protein synthesis, digestion, respiration, and photosynthesis all require a number of enzymes
  • Estimated that each cell contains over 100 different enzymes
  • Substrate and products (substrate=molecule that binds with enzyme)
  • Named after substrate with suffix -ase 
24 of 28

Enzymes-nutrition and digestion

  • Bond breaking happens in digestion
  • Internal digestion
    • enzymes are extracellular or intracellular
    • extracellular enzymes are released outside cells to perform catalysis
    • intracellular enzymes are found in the cytoplasm or attached to cell membranes
  • Enzymes are also used as protection - white blood cells and bacteria
25 of 28

Enzymes-lock and key

  • Substrate fits into active site
  • Substrate=key
  • Active site=lock
  • Substrate held in place so reaction can go ahead
26 of 28

Enzymes-induced fit hypothesis

  • Substrate collides with enzyme's active site, the enzyme molecule changes shape slightly
  • Active site fits more closely around substrate
  • Substrate held more closely by oppositely charged groups on substrate and active site
  • Forms enzyme-substrate complex (ESC)
  • Change in the enzyme places strain on substrate molecule
  • This destabilises the substrate molecule so the reaction occurs more easily
  • This produces a product, and is now the enzyme-product complex
  • Products formed are a different shape from the substrate
  • Products no longer fit active site-move away
  • Active site reverts to original shape
  • Enzyme is now able to catalyse the same reaction with another substrate molecule
27 of 28

Factors affecting enzyme activity


  • Too high-lots of OH- ions, hydrogen bonds break, ionic bonds break, tertiary structure unfolds. Denatures.
  • Too low-lots of H+ ions (same happens again). Denatures.


  • Increases-rate of reaction increases.
  • Too high-bonds in enzyme molecule break, active site changes shape. Denatures.
  • Too low-not enough kinetic energy, fewer collisions-rate of reaction decreases.


  • Increased substrate concentration-increased rate of reaction, to a point (working at optimum level).
  • Increased enzyme concentration has the same effect.
28 of 28


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules, organic chemistry and biochemistry resources »