Describe selective permeability — Biology notes

WHAT is selective permeability?

WHY does selectivity matter? A cell must keep its internal chemistry different from the outside (e.g. high K⁺ inside, high Na⁺ outside) to power nerves, make ATP, and stay alive. If everything leaked freely, those gradients would collapse → no life. Selectivity is how the cell chooses what enters and leaves.

HOW does the membrane achieve selectivity?

The structure causes the function. Recall the membrane is a phospholipid bilayer — two sheets of phospholipids, each with a hydrophilic (water-loving) phosphate head facing outward and hydrophobic (water-fearing) fatty-acid tails facing inward.

Three deciding factors

Factor	Crosses easily?	WHY
Small + nonpolar (O₂, CO₂, N₂)	✅ Yes, freely	Tiny and lipid-soluble — dissolve straight through the oily core
Small + polar, uncharged (H₂O, urea, glycerol)	⚠️ Slowly	Polar but small; squeeze through or use aquaporins
Large + polar (glucose, amino acids)	❌ Not alone	Too big and polar — need a carrier protein
Ions (Na⁺, K⁺, Cl⁻, H⁺)	❌ Not alone	Charged — strongly repelled; need channel proteins

Figure — Describe selective permeability

Quantifying "how easily" — the permeability coefficient

Deriving $P$ from first principles (so it isn't a magic number):

A molecule crossing must first partition into the membrane oil. Define the partition coefficient $K = \dfrac{\text{conc. in membrane}}{\text{conc. in water}}$ . Lipid-loving → big $K$ .
Inside the membrane it diffuses with diffusion coefficient $D$ (small molecules → big $D$ ).
It must travel across the membrane thickness $\Delta x$ . Thicker → slower.

Fick's law inside the membrane gives flux $J = \dfrac{D}{\Delta x}(C^{mem}_{out}-C^{mem}_{in})$ . Convert membrane concentrations to water concentrations using $C^{mem}=K\,C^{water}$ : $J = \frac{D K}{\Delta x}(C_{out}-C_{in}) \quad\Rightarrow\quad \boxed{P=\frac{D\,K}{\Delta x}}$

Worked examples

Common mistakes (Steel-manned)

Recall Feynman: explain it to a 12-year-old (click to open)

Imagine your skin is made of oil. If you throw an oily drop of paint at it, the paint sinks in — they like each other. If you throw water, it beads up and rolls off — they don't mix. The cell's wall is a thin layer of oil. So oily, small things slip through, but watery, electric (charged) things can't — unless a special door-shaped protein lets them in. That choosiness is "selective permeability": the wall picks who gets in.

Flashcards

What does "selectively permeable" mean?

The membrane lets some substances cross while restricting/blocking others, based on size, polarity, and lipid solubility.

Which membrane structure is responsible for selective permeability?

The phospholipid bilayer — its hydrophobic (oily) inner core blocks polar/charged molecules.

Name three molecules that cross the membrane freely.

Small nonpolar molecules: O₂, CO₂, N₂ (also small uncharged like H₂O slowly).

Why can't Na⁺ ions diffuse freely through the bilayer?

They are charged, carry a hydration shell, and are repelled by the nonpolar core (partition coefficient K ≈ 0); they need channel proteins.

Write the permeability formula and define each term.

P = DK/Δx; D = diffusion coeff (size), K = partition coeff (lipid solubility), Δx = membrane thickness.

What drives net flux across the membrane (uncharged solute)?

The concentration difference: J = P(C_out − C_in); zero difference → zero net flux.

Why is glucose unable to cross alone but CO₂ can?

Glucose is large and polar (needs GLUT carrier); CO₂ is small and nonpolar (dissolves through).

Does selective permeability itself require energy?

No — it is a passive property of the barrier; only active transport against a gradient needs ATP.

Steel-man: why ions are NOT fast despite being small.

Charge dominates over size; K≈0 makes P≈0, so size only matters once a molecule can enter the oily core.

Connections

Phospholipid Bilayer Structure — the oily core that causes selectivity
Simple Diffusion — how nonpolar molecules cross
Facilitated Diffusion — carriers/channels for polar & charged solutes
Osmosis — selective permeability applied to water
Active Transport — moving against gradients (needs ATP), uses selectivity + energy
Membrane Transport Proteins — the "bouncers" that extend selectivity
Fick's Law of Diffusion — the physics behind $P=DK/\Delta x$

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, cell membrane ek selectively permeable wall hoti hai — matlab woh sabko andar nahi aane deti, sirf kuch chosen molecules ko. Iska reason structure mein chhupa hai: membrane ek phospholipid bilayer hai jiska beech ka hissa oily/nonpolar hota hai. Ab jo molecule oil ko pasand karta hai (small aur nonpolar, jaise O₂ aur CO₂), woh seedha ghul ke nikal jaata hai. Lekin jo polar ya charged hai (jaise glucose ya Na⁺ ion), woh oily core se repel ho jaata hai aur akela cross nahi kar paata — usko ek protein "bouncer" chahiye.

Teen cheezein decide karti hain: size (chhota = easy), polarity/charge (charged = hard), aur lipid solubility (oily = easy). Ek line mein yaad rakho: "Small & oily slide in; big & charged are denied." Yahi pura concept hai.

Maths bhi same baat bolti hai: $P = \frac{DK}{\Delta x}$ . Yahan $D$ chhote molecule ke liye bada, $K$ oily molecule ke liye bada, aur $\Delta x$ membrane ki motai. Aur net flow $J = P(C_{out}-C_{in})$ — yaani jab dono taraf concentration barabar ho jaayega tab net movement zero ho jaata hai (equilibrium). Ion ke liye $K \approx 0$ , isliye $P \approx 0$ — formula khud bata raha hai ke ion akela nahi nikal sakta.

Important: yeh choosiness free hai, ATP nahi lagti. Energy sirf tab chahiye jab cell ko molecule ko gradient ke against push karna ho (active transport). Exam mein common galti — "semi-permeable matlab sirf paani" — ye galat hai; selective permeability har molecule ke liye apply hoti hai, paani toh sirf ek example hai kyunki woh chhota aur uncharged hota hai.

Test yourself — Cell Membrane & Transport

Connections