IB Biology SLTopic 2 — Proteins & Their StructurePaper 1 & 2Structure and function~10 min read
The Variety of Proteins
Twenty amino acids does not sound like much to work with. But you are not picking twenty proteins — you are picking one of twenty at every position in a chain that might be three hundred long. The number of possible proteins that falls out of that is so large it stops being a number you can picture. This page shows where the variety comes from and what a few real proteins do with it.
📘 What you need to know
There are 20 amino acids to choose from at every position in a polypeptide chain.
Chains vary in length (a few to thousands) and in sequence, so the number of possible proteins is enormous.
The DNA base sequence codes for the number and order of amino acids, and DNA itself can vary hugely.
Polypeptides are assembled at a ribosome, one amino acid at a time, with the mRNA codon deciding which one is added.
A 50-amino-acid chain has 2050 possible sequences — about 1.1 × 1065.
Proteins are globular (rounded, often soluble — enzymes, hormones, antibodies) or fibrous (long strands, insoluble — collagen, keratin, silk).
Their roles include catalysis, transport, structure, movement, signalling, immunity and packaging DNA.
Where the variety comes from
Four things stack up, and they multiply rather than add:
There are 20 amino acids available for every position.
Chains vary in length, from a handful of amino acids to thousands.
The sequence can be anything — swap two amino acids and you have a different protein.
The genetic code sets that sequence, and DNA base sequences vary almost without limit.
At the ribosome, amino acids are added to a growing chain one at a time. Each time one is added, the mRNA codon decides which of the 20 it will be. That is a fresh choice of twenty at every single step.
Five positions already gives more possible chains than there are people in a large country. Stretch that to 300 positions and the answer is bigger than the number of atoms in the observable universe.
This is a maths point disguised as a biology point, and it is worth being comfortable with. If you have n positions and 20 options at each, the total is 20n. Not 20 × n. The difference between those two is the whole answer.
What proteins actually do
Because there are so many possible shapes, proteins have ended up doing almost every job in a cell.
Catalysis — enzymes speed up reactions.
Transport — haemoglobin carries oxygen around the body.
Structure — collagen and keratin strengthen skin, hair, tendons and blood vessel walls.
Movement — the cytoskeleton moves chromosomes during the cell cycle.
Blood clotting — blood proteins form a gel-like scab across a wound.
Signalling — hormones are made in one place and act somewhere else.
Immunity — antibodies bind to specific antigens.
Membrane transport — channel and carrier proteins control what crosses.
Receptors — binding sites for hormones, tastes, light and sound.
Packing DNA — histone proteins wind DNA up for storage.
Cell adhesion — cells in a tissue stick to each other.
Two broad shapes
A globular protein needs to dissolve and move around to reach its target. A fibrous one needs to sit still and take a load. The shape follows the job in both cases.
Some proteins worth knowing by name
Protein
Type
Where it is
What it does
Rubisco
Globular enzyme, 16 chains
Every green leaf
Fixes CO2 from the air in photosynthesis. Slow, but it is the source of nearly all organic carbon, and the most abundant enzyme on Earth
Insulin
Short globular hormone, 2 chains
Made by beta cells in the pancreas
Binds reversibly to receptors on liver, muscle and fat cells, causing glucose to be taken up from the blood
Immunoglobulins
Globular, Y-shaped
Blood and body fluids
Antibodies. The binding sites at the tips of the Y are highly variable, so they can target millions of different antigens
Rhodopsin
Globular membrane protein
Rod cells in the retina
Holds retinal, a light-sensitive group made from vitamin A. Light changes its shape, which triggers a nerve impulse to the brain
Collagen
Fibrous, 3 chains
Skin, tendons, blood vessels, teeth and bone
Forms a network that resists tearing. About a quarter of all the protein in your body
Spider silk
Fibrous
Spider webs
As strong as steel wire but far lighter. Rope-like parts give strength and coiled parts stretch under tension
🤔 Why spider silk interests engineers
It combines two things that usually trade off against each other: it resists breaking, and it stretches instead of snapping when pulled. It also holds its shape at temperatures that would denature most proteins. The problem is supply — spiders cannot be farmed in numbers, so the gene has been engineered into goats so that the protein is produced in their milk and can be collected.
🧠 A way to remember the two shapes
Globular = globe = ball = dissolves and does chemistry.Fibrous = fibre = rope = holds things together. If a question describes something long, insoluble and strong, it is fibrous every time.
Worked examples
WORKED EXAMPLE
Calculate the number of different tripeptides that could be made from the 20 amino acids. [2]
Step 1: choices at each position
A tripeptide has 3 positions and 20 options at each.
Step 2: multiply, do not add20 × 20 × 20 = 20³= 80008000 different tripeptides20 × 3 = 60 is the classic wrong answer. It is a power, not a product.
WORKED EXAMPLE
Explain how such a large variety of proteins can be produced from only 20 amino acids. [3]
Point 1
Any of the 20 amino acids can be used at each position in the chain.
Point 2
Chains also vary in length, from a few amino acids to thousands.
Point 3
The number of possible sequences is 20 to the power of the chain length, which is effectively unlimited; the DNA base sequence decides which one is made.
Choice at every position × variable length = near-infinite varietyMention length as well as sequence. It is usually a separate mark.
WORKED EXAMPLE
A protein is described as long, insoluble, and made of three polypeptide chains wound around each other, found in tendons. State its type and suggest its function. [2]
Step 1: read the clues
Long, insoluble, several chains together → not a globular protein.
Step 2: match to a role
Tendons take pulling forces, so the protein must resist tearing.
A fibrous protein (collagen), giving structural strength“Structural” alone is thin. Say what force it resists.
💡 Exam tip
For combination questions use 20n, where n is the number of amino acids in the chain.
Give at least two sources of variety: sequence and length. Adding “the DNA codes for it” earns a third mark.
When naming a role, name a real example with it — haemoglobin for transport, collagen for structure.
Globular usually means soluble; fibrous usually means insoluble. Say which in comparison questions.
Insulin is a hormone, not an enzyme. It binds to receptors, it does not catalyse anything.
Rubisco is the enzyme of photosynthesis that fixes carbon dioxide. That one line is usually enough.
⚠ Common mix-up
Multiplying instead of using a power. A chain of 4 gives 204 = 160 000, not 80.
Saying there are 20 proteins. There are 20 amino acids, and an almost unlimited number of proteins.
Thinking all proteins are enzymes. Enzymes are one job among many.
Calling antibodies antigens. Antibodies are the proteins; antigens are what they bind to.
Saying rhodopsin is in the brain. It is a membrane protein in the rod cells of the retina.
Mixing up globular and fibrous. Globular is round and soluble; fibrous is long and strong.
Up next: Protein Structure: pH & Temperature — what actually holds a protein in its shape, and what happens when heat or acid takes that shape away.
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