IB Biology SL

Revision Notes

2.3.3 The Variety of Proteins

Functions of Proteins

  • Living organisms synthesize many different proteins with a wide range of functions
  • Proteins are so versatile that they have many different roles in cells, tissues and organs
  • All of the following functions are performed by proteins:
    • 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 eg. collagen, keratin
    • Transport of vital metabolites eg. 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

Exam Tip

Many exam questions focus on enzymes but don’t forget all the other types of protein when discussing protein functions.

Examples of Proteins


  • 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


  • 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


  • 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


  • 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


  • A fibrous protein made of three separate polypeptide chains
  • The most abundant protein in the human body – approx 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, eg. the silk they use to encase their prey after capture


  • The proteome is the full range of proteins that a cell or organism is able to produce
  • By contrast, a genome is the complete set of genes present in a cell/organism
    • The full genome is present within every cell of an organism, but not every gene is expressed in every cell. Which genes are expressed, depends on the cell type
  • The proteome is usually larger than the genome of an organism
  • Every individual has a different proteome
    • Because of small differences in the amino acid sequence of proteins
  • The proteome varies during an organism’s lifetime as certain proteins are not needed throughout the organism’s life
      • An example is fetal haemoglobin. The gene for that protein is not expressed after the baby is around 3 months old, as the baby expresses adult forms of haemoglobin, which are encoded by separate genes
    • This is also due to a large amount of modification of proteins that can take place after synthesis (often in the Golgi apparatus)
      • For example, adding a carbohydrate part to form glycoproteins, which are important in cell signalling
  • Splicing of RNA during transcription can allow one gene to code for many proteins

Exam Tip

You don’t need to know the details of splicing for Standard Level but it accounts for several proteins being produced from just one gene. Even though a lot of genes do not code for proteins, the proteome is larger than the genome because of the sheer range of proteins that can be produced from the DNA code.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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