A steroid (amphipathic, with a polar –OH head, rigid ring body, nonpolar tail).
Why is cholesterol called a "fluidity buffer"?
At high T it decreases fluidity (rings restrain tails); at low T it increases fluidity (rings block tight packing) — buffering both ways.
At HIGH temperature, what does cholesterol do to fluidity and why?
Decreases it; its rigid rings restrain the rapidly moving phospholipid tails.
At LOW temperature, what does cholesterol do to fluidity and why?
Increases it; its rings sit between tails and stop them packing/freezing solid.
How is cholesterol oriented in the bilayer?
–OH head at the membrane surface (near phospholipid heads), rigid rings against upper tails, hydrocarbon tail in the hydrophobic core.
What happens to membrane permeability when cholesterol is added?
It decreases — cholesterol fills gaps between tails, making a tighter barrier to small polar/ionic molecules.
What are lipid rafts?
Cholesterol-rich micro-domains of membrane that organise certain proteins (e.g. for signalling).
What do plants use instead of cholesterol?
Other sterols like sitosterol/stigmasterol (plus a rigid cell wall).
On a fluidity-vs-temperature graph, what does cholesterol do to the curve?
Flattens it — removes the sharp melting transition, keeping fluidity intermediate over a wide temperature range.
Recall Feynman: explain it to a 12-year-old
Imagine a crowd of people holding hands in lines (the phospholipids). When it's hot, everyone jumps around wildly and the lines fall apart — too messy. When it's freezing, everyone stands stiff and stuck — can't move at all. Now add some short, stubborn referees (cholesterol) standing between people. When it's hot they grab arms and say "calm down" → less chaos. When it's cold they wedge between people so nobody can fully freeze in place → keeps things moving. Same referees fix both problems. That's cholesterol: a calm-it-down-and-keep-it-loose buffer.
Dekho, cell membrane ek "sea of phospholipids" hai jisme tails wiggle karti rehti hain. Problem yeh hai ki garmi mein yeh membrane bahut zyada fluid (runny) ho jaati hai, aur thand mein bahut zyada stiff (jam jaati hai). Cholesterol ka kaam hai is fluidity ko buffer karna — yaani dono taraf se control karna. Isliye ise membrane ka "thermostat" bolte hain.
Cholesterol amphipathic molecule hai: ek chhota –OH polar head, ek rigid four-ring body, aur ek chhoti nonpolar tail. Yeh fixed orientation mein baithta hai — head surface ki taraf, rigid rings phospholipid tails ke upar wale (stiff) part ke against, aur tail core mein. Yahi orientation iske kaam ka raaz hai.
High temperature pe iske stiff rings tails ko pakad ke rakhte hain → zyada wiggle nahi hone dete → fluidity kam. Low temperature pe yahi rings tails ke beech ghus jaate hain → tails ko tight pack hone se rokte hain → membrane freeze nahi hoti → fluidity bani rehti. Ek hi molecule, do opposite kaam — isiliye "buffer".
Ek important point exam ke liye: cholesterol membrane ki permeability kam karta hai chhote polar/ionic molecules ke liye, kyunki yeh gaps bhar deta hai. Common galti yeh hai ki students bolte hain "cholesterol always increases fluidity" — galat! Yeh bidirectional hai. Yaad rakho: Cold? Hold open the lines. Hot? Hold on, less wiggle.
