Noesis
2.4.6Cell Membrane & Transport

Distinguish passive and active transport

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🇮🇳 Yeh note Hinglish mein padho →

WHAT are we distinguishing?

The deepest dividing line is not "does it use a protein?" (some passive transport uses proteins too). The real line is:

Direction relative to the gradient  ⇒  Energy needed or not\boxed{\text{Direction relative to the gradient} \;\Rightarrow\; \text{Energy needed or not}}

WHY does direction decide the energy cost? (Derive it from scratch)

Everything starts from the free energy of moving one mole of solute from side 11 to side 22:

ΔG=RTln⁡ ⁣C2C1+zF ΔV\Delta G = RT \ln\!\frac{C_2}{C_1} + zF\,\Delta V

Let's build this term by term — why does it look like this?

  1. Concentration term RTln⁡(C2/C1)RT\ln(C_2/C_1).

    • Why a log? Entropy of mixing scales with ln⁡(concentration)\ln(\text{concentration}). Spreading particles out increases disorder, and disorder change is logarithmic in number density.
    • If C2<C1C_2 < C_1 (moving toward the dilute side), then ln⁡(C2/C1)<0\ln(C_2/C_1) < 0, so this term is negative.

  2. Electrical term zF ΔVzF\,\Delta V (for ions, charge zz, Faraday constant FF, voltage difference ΔV\Delta V).

    • Why? Work to move charge across a voltage = charge × voltage.
    • Together with the concentration term it forms the electrochemical gradient.

HOW this answers the question: passive transport always has ΔG<0\Delta G<0, so nature does the work. Active transport faces ΔG>0\Delta G>0, so the cell must "pay" using ATPATP.

HOW: the sub-types you must recognise

Feature Simple diffusion Facilitated diffusion Primary active Secondary active
Class Passive Passive Active Active
Direction down gradient down gradient up gradient up gradient (driven solute)
Protein? No Yes (channel/carrier) Yes (pump) Yes (co-transporter)
Energy source gradient gradient direct ATPATP gradient made by primary pump
Example O2O_2, CO2CO_2 glucose via GLUT, ions via channels Na+/K+Na^+/K^+ ATPase Na+Na^+-glucose symport (SGLT)
Figure — Distinguish passive and active transport

Worked Examples

Common Mistakes (Steel-manned)

Recall Feynman: explain to a 12-year-old

Imagine a hill with marbles. Passive transport is letting marbles roll down the hill — they do it by themselves, you don't push. Active transport is when you want a marble to go up the hill — you have to push it, and pushing makes you tired (that's the cell burning ATPATP "energy snacks"). Some marbles roll down through a slide (that's a protein channel) — still rolling down, still free. So the question is never "is there a slide?" — it's "is the marble going down (free) or up (costs energy)?"

Active-Recall Flashcards

What single factor decides if transport is active or passive?
The direction of movement relative to the electrochemical gradient (down = passive, up = active).
Does facilitated diffusion require ATP?
No — it is passive; the protein just provides a path for downhill movement.
Write the free-energy equation for moving a solute across a membrane.
ΔG=RTln⁡(C2/C1)+zFΔV\Delta G = RT\ln(C_2/C_1) + zF\Delta V.
Sign of ΔG\Delta G for passive vs active transport?
Passive ΔG<0\Delta G<0 (spontaneous); active ΔG>0\Delta G>0 (needs energy input).
What is primary active transport?
Transport directly powered by ATP hydrolysis (e.g. Na⁺/K⁺ ATPase).
What is secondary active transport?
Uphill transport powered by the downhill flow of a second ion whose gradient was built by a primary (ATP-using) pump.
Why is the Na⁺/K⁺ pump active?
It moves both Na⁺ out and K⁺ in toward their already-higher sides (uphill), so ΔG>0, requiring ATP.
Is osmosis active or passive?
Passive — water diffuses down its own gradient via aquaporins, no ATP.
Approx ΔG of ATP hydrolysis used to drive active transport?
About −30.5 kJ/mol.
Why does the concentration term contain a logarithm?
Because the entropy of mixing/spreading particles scales with ln(concentration).

Connections

  • Cell Membrane Structure — fluid mosaic provides the channels and pumps.
  • Facilitated Diffusion — passive but protein-mediated.
  • Sodium-Potassium Pump — the classic primary active example.
  • Osmosis and Water Potential — passive water movement.
  • ATP and Cellular Energy — source of energy for active transport.
  • Electrochemical Gradient — combines concentration + charge.
  • Nernst Equation — quantifies the electrical part zFΔVzF\Delta V.

Concept Map

determines

dG = RT ln C2/C1 + zF dV

dG < 0 spontaneous

dG > 0 non-spontaneous

no energy needed

via proteins

coupled to ATP

drives

builds gradient for

Direction vs gradient

Free energy dG

Electrochemical gradient

Passive transport

Active transport

Simple diffusion

Facilitated diffusion

ATP hydrolysis

Primary active Na/K ATPase

Secondary active symport

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, sabse simple tarika yaad rakhne ka: molecules naturally crowded jagah se khali jagah ki taraf jaate hain — jaise bheed se nikal kar khaali room mein. Isko bolte hain gradient ke "down" jaana, aur ye passive transport hai. Ismein cell ko koi energy kharch nahi karni padti, kyunki nature khud kaam kar deta hai. Oxygen, carbon dioxide, paani (osmosis) — sab passive hai.

Ab agar cell ko molecule ko ulta push karna ho — yaani jahan already zyada concentration hai wahan aur bhejna ho (gradient ke "up" jaana) — to ye apne aap nahi hoga. Iske liye energy lagti hai, aur wo energy aati hai ATP se. Isko bolte hain active transport. Best example hai Na+/K+Na^+/K^+ pump.

Sabse bada confusion: log sochte hain "protein use ho raha hai matlab active transport". Galat! Facilitated diffusion mein bhi protein channel hota hai par wo passive hai, kyunki molecule still neeche (down gradient) ja raha hai. Asli sawaal hai — direction kya hai, up ya down? Down = free = passive, Up = pay karo (ATP) = active.

Formula se confirm: ΔG=RTln⁡(C2/C1)+zFΔV\Delta G = RT\ln(C_2/C_1) + zF\Delta V. Agar ΔG\Delta G negative aaya to spontaneous (passive), positive aaya to ATP chahiye (active). Bas yahi core hai — yaad rakho: "Up jaane ka kiraya dena padta hai."

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Connections

Describe osmosis and water potentialBiology · 2.4.9Describe the fluid mosaic modelBiology · 2.4.1Explain phospholipid bilayer arrangementBiology · 2.4.2Describe selective permeabilityBiology · 2.4.5Explain simple diffusionBiology · 2.4.7Explain facilitated diffusion and channel - carrier proteinsBiology · 2.4.8Describe osmosis and water potentialBiology · 2.4.9