IB Biology HLProteins & Their StructurePaper 1 & 2~12 min read
Globular & Fibrous Proteins
Fold a polypeptide into a ball and you get a molecule that dissolves and does chemistry. Stretch it into a long strand and you get a molecule that holds your body together. Same building blocks, two completely different families.
📚 What you need to know
Globular proteins are compact, roughly spherical and generally soluble in water.
They fold that way because non-polar hydrophobic R groups point into the centre and polar hydrophilic R groups face outwards.
Their solubility means they can be transported around organisms and take part in metabolic reactions, so they have physiological roles.
Their specific shapes let enzymes catalyse specific reactions and immunoglobulins respond to specific antigens.
Insulin: a globular hormone from the pancreas, 2 chains (21 and 30 amino acid residues) held by three disulfide bridges. It was the first protein to have its sequence determined.
Fibrous proteins are long strands with cross-linkages, a repetitive amino acid sequence, little or no tertiary structure, and are insoluble.
Collagen: three polypeptide chains held by hydrogen bonds in a triple helix, cross-linked into fibrils with staggered ends, and bundled into fibres.
Compare the two using SAFES: Shape, Amino acid sequence, Function, Examples, Solubility.
Two families, one difference
The difference is not the amino acids — it is what the R groups make the chain do.
Globular proteins
Globular proteins are compact and roughly spherical, and they are generally soluble in water. That shape is not a coincidence; it is the direct result of the tertiary folding you met in the last topic.
The non-polar hydrophobic R groups are turned towards the centre of the protein, away from the aqueous surroundings.
The polar hydrophilic R groups end up on the outside.
Water molecules can surround those polar groups on the surface, which is what makes the protein soluble.
Solubility is what allows globular proteins to play physiological roles. They can be transported around the organism in blood or cytoplasm, and they can take part in metabolic reactions in solution.
The folding also gives each one a very specific shape:
Enzymes have an active site shaped to catalyse one specific reaction.
Immunoglobulins have binding sites shaped to respond to specific antigens.
Some globular proteins are conjugated, containing a prosthetic group — haemoglobin and its haem group being the standard example.
Insulin
Insulin was the first protein to have its sequence determined by scientists.
It is a globular protein produced in the pancreas, with an important role in the control of blood glucose concentration.
It consists of two polypeptide chains: chain A has 21 amino acid residues, chain B has 30.
The chains are held together by three disulfide bridges — two joining the two chains and one within chain A.
Why insulin can be made in a lab. Because the sequence is short and known, insulin was one of the first proteins to be produced by genetically modified bacteria. That is only possible because the primary structure alone determines the final shape — the bacterium does not need to know how to fold it.
Fibrous proteins
Fibrous proteins are long strands of polypeptide chains with cross-linkages between them, usually due to hydrogen bonds.
They have little or no tertiary structure — they do not fold into a ball, they stay stretched out.
Because of their large number of hydrophobic R groups, they are insoluble in water.
They contain a limited number of different amino acids, and the sequence is usually highly repetitive.
A repetitive sequence creates very organised, regular structures that are strong. Combined with being insoluble, this makes them ideal for structural roles.
Examples include keratin, which makes up hair, nails, horns and feathers, and collagen, the connective tissue found in skin, tendons and ligaments.
Think about why insolubility is a feature and not a fault. You would not want the protein in your tendons dissolving into your blood. Fibrous proteins are built to stay exactly where they are put.
Collagen in detail
Collagen is the most common structural protein in vertebrates. It has a flexible structure and forms connective tissue in tendons, cartilage, ligaments, bones, teeth, skin, blood vessel walls and the cornea of the eye.
Strength is built up in stages here, the same way a rope is stronger than the threads it is twisted from.
Collagen is an insoluble fibrous protein formed from three polypeptide chains held closely together by hydrogen bonds to form a triple helix. Those hydrogen bonds give it great tensile strength.
Each chain is itself a helix and contains about 1000 amino acids, with glycine, proline and hydroxyproline the most common.
Alongside the hydrogen bonds between the three chains, there are also covalent bonds.
Covalent cross-links form between R groups of amino acids in neighbouring triple helices when they lie parallel to each other. These cross-links hold collagen molecules together to form fibrils.
Inside a fibril, the molecules are positioned so that there are staggered ends, which adds strength.
Many fibrils arranged together form collagen fibres, and those fibres are lined up with the direction of the forces they have to withstand.
Why glycine, and why so much of it? Glycine has the smallest R group of all — just a hydrogen atom. It appears at every third position in a collagen chain, and that is the only way three helices can wind tightly enough to touch. A bulkier R group would get in the way.
Comparing the two
Feature
Globular
Fibrous
Shape
Roughly circular, compact
Long strands
Amino acid sequence
Irregular, with a wide range of R groups
Repetitive, with a limited range of R groups
Function
Physiological and functional
Structural
Examples
Haemoglobin, enzymes, insulin, immunoglobulins
Collagen, keratin, myosin, actin, fibrin
Solubility
Generally soluble in water
Generally insoluble in water
Tertiary structure
Highly developed — it is what makes the ball
Little or none
🧠
SAFES
Shape, Amino acid sequence, Function, Examples, Solubility. Five headings, five comparison marks. If a question says “compare globular and fibrous proteins”, work down the list and you cannot miss a point.
“Compare” means you must say something about both in the same sentence. “Globular proteins are soluble” is half an answer. “Globular proteins are generally soluble in water whereas fibrous proteins are generally insoluble” is a whole one.
Worked examples
WE 1
Why globular proteins dissolve
Explain why globular proteins are generally soluble in water. (3 marks)
Point 1: how they fold
As the chain folds, the non-polar hydrophobic R groups turn inwards, away from the water.
Point 2: what ends up outside
The polar hydrophilic R groups are left on the outer surface of the molecule.
Point 3: the result
Water molecules can surround those polar groups, so the protein dissolves — which is why globular proteins can be transported and take part in metabolic reactions.
Hydrophilic outside, hydrophobic inside, therefore solublefinish with the consequence — solubility is only interesting because of what it allows
WE 2
Comparing globular and fibrous proteins
Compare and contrast globular and fibrous proteins. (4 marks)
Point 1: shape
Globular proteins are compact and roughly spherical, whereas fibrous proteins are long strands.
Point 2: sequence
Globular proteins have an irregular sequence with a wide range of R groups, whereas fibrous proteins have a repetitive sequence with a limited range.
Point 3: solubility and function
Globular proteins are generally soluble and have physiological roles; fibrous proteins are insoluble and have structural roles.
Point 4: what they share
Both are polypeptides built from the same 20 amino acids and joined by peptide bonds.
SAFES: Shape, Amino acid sequence, Function, Examples, Solubility“compare and contrast” wants similarities too, so always add one shared feature
WE 3
Collagen and tensile strength
Explain how the structure of collagen makes it suitable for its role in tendons. (4 marks)
Point 1: the triple helix
Three polypeptide chains are held together by hydrogen bonds in a triple helix, giving great tensile strength.
Point 2: the cross-links
Covalent cross-links form between R groups of neighbouring triple helices, holding the molecules together as fibrils.
Point 3: the staggered ends
Molecules within a fibril are arranged with staggered ends, so there is no line of weakness across the fibril.
Point 4: the fibre
Many fibrils bundle into fibres, and the fibres line up with the direction of the force they have to resist. Being insoluble, they stay in place in the tissue.
Helix → cross-linked fibril → aligned fibre = resists tearingwork up through the levels in order — it is the clearest way to score all four
💡 Exam tips
Use SAFES for any comparison question, and write both sides of each point.
Explain solubility through R groups, not just by saying “globular proteins are soluble”.
Say “generally soluble” and “generally insoluble” — there are exceptions, and the word costs nothing.
Learn the insulin numbers: 2 chains, 21 and 30 residues, 3 disulfide bridges.
Learn the collagen chain: triple helix → cross-links → fibrils with staggered ends → fibres.
Remember which examples go where: haemoglobin, enzymes, insulin and immunoglobulins are globular; collagen, keratin, myosin, actin and fibrin are fibrous.
⚠ Common mistakes
Saying fibrous proteins have no bonds holding them together. They have plenty — hydrogen bonds and covalent cross-links. What they lack is a folded tertiary shape.
Calling collagen globular because it is a helix. A triple helix is still a long strand.
Saying insolubility is a disadvantage. For a structural protein it is exactly what is needed.
Mixing up fibrils and fibres. Cross-linked molecules make fibrils; many fibrils make a fibre.
Forgetting the staggered ends. They are a favourite exam detail, and the reason the fibril has no weak point.
Writing only about one type. A comparison question needs both, point by point.
That completes Proteins & Their Structure. The whole sub-topic is one argument: DNA sets the sequence, the sequence sets the R groups, the R groups set the folding, and the folding sets the job. If you can tell that story in order, you can answer almost any protein question the exam gives you.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
