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First teaching 2023

First exams 2025

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The Variety of Proteins (SL IB Biology)

Revision Note

Cara Head

Author

Cara Head

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Peptide Chain Diversity

  • There is a large variety of proteins available to living organisms
  • This is because:
    • There are 20 naturally occurring amino acids that form the basic structure of a polypeptide chain
    • Polypeptides can vary in length from a few to thousands
    • The structure and amino acid sequence can also vary
    • The genetic code, meaning DNA base sequence, codes for the number and order of amino acids in a polypeptide, and there is a huge variety of options for DNA base sequence
  • 20 amino acids can give an almost infinite number of polypeptides
  • Polypeptides are assembled at a ribosome by condensing individual amino acids onto a growing chain, one by one
  • This allows a choice of 20 amino acids each time one is added
  • The mRNA codon determines which amino acid is added
  • For a polypeptide chain of 50 amino acids in length (considered a very short protein), there would be 2050 possible combinations of amino acids
    • This gives 1.13 x 1065 combinations
  • Given that the average length of a protein is 300 amino acids, the number of possible combinations is so large, we can consider it to be infinite

Role of proteins

  • The range of proteins available means that they are very versatile so that they have many different roles in cells, tissues and organs, such as:
    • Speeding up cellular reactions, or catalysis, is performed by enzymes
    • Blood clotting, where blood proteins interact with oxygen to form a gel-like scab across a wound
    • Strengthening fibres in skin, hair, tendons, blood vessels e.g. collagen, keratin
    • Transport of vital metabolites e.g. oxygen which is carried by haemoglobin
    • Formation of the cytoskeleton, a network of tubules within a cell that cause chromosomes to move during the cell cycle
    • Cell adhesion, where cells in the same tissue stick together
    • Hormones, chemical messengers that are secreted in one part of the body to have an effect elsewhere
    • Compaction of DNA in chromosomes for storage, caused by histone proteins
    • The immune response produces antibodies, the most diverse group of proteins
    • Membrane transport channel and carrier proteins that determine which substances can pass across a membrane
    • Cell receptors, which are binding sites for hormones, chemical stimuli such as tastes, and for other stimuli such as light and sound

Examples of polypeptides

Rubisco

  • Ribulose Bisphosphate Carboxylase
  • An enzyme that catalyses the fixing of CO2 from the atmosphere during photosynthesis
  • Composed of 16 polypeptide chains as a globular protein
  • This is the source of all organic carbon, so Rubisco is arguably the most important enzyme in nature!
  • The most abundant enzyme on Earth as it's present in all leaves
  • Rubisco is a very slow catalyst, but it's the most effective to have evolved so far to fulfil this vital function

Insulin

  • A hormone produced and secreted by β-cells in the pancreas
  • Binds to insulin receptors (on liver, fat and muscle cells) reversibly, causing absorption of glucose from the blood
  • Composed of 2 polypeptide chains as a short, globular protein

Immunoglobulins

  • Also known as antibodies
  • They have a generic 'Y' shape, with specific binding sites at the two tips of the 'Y'
  • They bind to specific antigens
  • The binding areas of immunoglobulins are highly variable, meaning that antibodies can be produced against millions of different antigens
  • Immunoglobulins (as the name suggests) are globular and are the most diverse range of proteins

Rhodopsin

  • A pigment in the retina of the eye
  • A membrane protein that is expressed in rod cells
  • Contains a light-sensitive part, retinal, which is derived from Vitamin A
  • A photon of light causes a conformational change in rhodopsin, which sends a nerve impulse along the optic nerve to the central nervous system

Collagen

  • A fibrous protein made of three separate polypeptide chains
  • The most abundant protein in the human body - approximately 25%
  • Fibres form a network in skin, blood vessel walls and connective tissue that can resist tearing forces
  • Plays a role in teeth and bones, helping to reduce their brittleness

Spider Silk

  • The silk used by spiders to suspend themselves and create the spokes of their webs is as strong as steel wire though considerably lighter
  • Contains rope-like, fibrous parts but also coiled parts that stretch when under tension, helping to cause extension and resist breaking
  • Does not denature easily at extremes of temperature
  • Has many attractive aspects for engineering and textile product design thanks to its strength and low weight
  • Can be genetically engineered to be expressed in goats' milk as spiders can't be farmed on a large enough scale
  • Other kinds of spider silk protein are tougher though lack the tensile strength, e.g. the silk they use to encase their prey after capture

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding