Identify membrane proteins (integral, peripheral)
WHAT are we classifying?
HOW do you tell them apart? (3 lenses)
Lens 1 — Hydrophobicity (the chemistry WHY)
The bilayer core is oily (hydrophobic). A protein region can only stay buried there if its surface is also hydrophobic (made of nonpolar amino acids like Leu, Ile, Val, Phe).
- Integral proteins have hydrophobic stretches (often an α-helix of ~20 nonpolar residues) that match the core.
- Peripheral proteins are hydrophilic overall → repelled by the oil → stay on the surface.
Lens 2 — How they're extracted (the lab test HOW)
| Treatment | Removes peripheral? | Removes integral? | Why |
|---|---|---|---|
| High salt / pH change | ✅ Yes | ❌ No | breaks weak ionic/H-bonds only |
| Urea (denaturant) | ✅ Yes | ❌ No | unfolds surface attachments |
| Detergent (e.g. SDS, Triton) | ✅ Yes | ✅ Yes | dissolves the lipid core itself |
Lens 3 — Location & spanning
- Transmembrane (integral) → both faces; can form channels/transporters/receptors.
- Peripheral → one face only; often enzymes, signal relays, cytoskeleton anchors.
WHY hydrophobicity predicts spanning (mini "derivation")
Worked examples
Common mistakes (Steel-man + fix)
Recall Feynman: explain to a 12-year-old
Imagine the membrane is a chocolate bar floating in water — the oily middle is the chocolate. Integral proteins are like nuts stuck inside the chocolate; you can only get them out by melting the chocolate (that's the detergent). Some nuts go all the way through (those are transmembrane). Peripheral proteins are like sprinkles just resting on top — you can knock them off by gently shaking or rinsing (salt water), because they were never inside the chocolate. So the whole trick is: is it stuck in the oily part, or just sitting on the surface?
Flashcards
Where do integral proteins sit relative to the bilayer?
What is the ONLY effective way to remove an integral protein?
What removes a peripheral protein?
What is a transmembrane protein?
Are all integral proteins transmembrane?
Why can integral proteins stay in the core?
Why are peripheral proteins kept out of the core?
How many residues does an α-helix need to span the ~30 Å core?
On a hydropathy plot, what marks a transmembrane segment?
Which test DISTINGUISHES integral from peripheral, and why?
Why do detergents free integral proteins without exposing them to water?
Typical roles of transmembrane proteins?
Connections
- Fluid Mosaic Model — proteins are the "mosaic" tiles in the fluid lipid sea.
- Phospholipid Bilayer — its hydrophobic core is exactly what defines integral vs peripheral.
- Membrane Transport Proteins — channels & carriers are integral/transmembrane.
- Hydrophobic Effect — the thermodynamic reason nonpolar surfaces hide in the core.
- Amino Acid Properties — nonpolar residues (Leu, Ile, Val, Phe) build membrane helices.
- Cell Signaling & Receptors — peripheral relay proteins on the cytoplasmic face.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, cell membrane ek "fluid mosaic" hai — matlab oily phospholipid bilayer jisme proteins tairte rehte hain. Sabse important sawaal yahi hai: ye protein membrane ke oily beech wale hissa (hydrophobic core) ko touch karta hai ya sirf surface pe baitha hai? Agar core ke andar ghusa hua hai to wo integral (intrinsic) protein hai. Agar sirf surface pe, polar heads ya kisi doosre protein ko pakad ke baitha hai, to wo peripheral (extrinsic) hai.
Pehchaan ka asaan tarika lab test hai. Integral protein ko nikalne ke liye detergent chahiye, kyunki detergent hi oily bilayer ko dissolve kar pata hai (micelle banake protein ko free karta hai). Peripheral protein halke se nikal jaata hai — sirf namak (high salt) ya pH change se, kyunki wo to bas weak ionic/H-bonds se chipka hota hai. Yaad rakho: detergent dono ko nikaal deta hai, isliye difference batane ke liye hamesha mild test dekho.
Chemistry ka logic: oily core me sirf wahi reh sakta hai jiska surface bhi hydrophobic (nonpolar amino acids — Leu, Ile, Val, Phe) ho. Isliye integral proteins me ek hydrophobic α-helix hoti hai. Core ~30 Å motha hai aur helix har residue pe 1.5 Å aage badhti hai, to ~20 residues chahiye ek baar membrane cross karne ke liye — yahi transmembrane segment ka signature hai (hydropathy plot me 20-residue wide hydrophobic peak).
Ek common galti: "integral matlab poori membrane cross karta hai" — galat! Cross karna (transmembrane) to ek type hai; integral ke liye sirf core me ghusna kaafi hai. Aur "peripheral membrane ke andar thoda kam depth pe hota hai" — bhi galat, peripheral core me bilkul nahi ghusta. Bas itna yaad rakho: integral = IN the oil (detergent), peripheral = perimeter/surface (gentle wash).