67. Which combination of profiles in the following figure accurately represents the transport rate
of glycerol and oxygen into 𝐸. coli cells as a function of their extracellular concentration?
(A) glycerol-(ii) and oxygen-(iii)
(B) glycerol-(ii) and oxygen-(i)
(C) glycerol-(iii) and oxygen-(i)
(D) glycerol-(i) and oxygen-(ii)

Introduction

Transport of molecules across the bacterial cell membrane is a fundamental concept in cell biology and frequently tested in competitive exams. Different molecules enter E. coli cells through different transport mechanisms, and this directly affects how their rate of transport changes with extracellular concentration.

The given figure shows three transport profiles—(i), (ii), and (iii)—each representing a distinct relationship between extracellular concentration and rate of transport. The question asks which combination correctly represents glycerol and oxygen transport into E. coli cells.

Understanding the Transport Curves

From the graph:

• Profile (i):
  A rapid increase in transport rate followed by a plateau
  → Indicates saturable transport (carrier-mediated)

• Profile (ii):
  A straight-line relationship
  → Indicates simple diffusion (non-saturable)

• Profile (iii):
  A sigmoidal curve
  → Indicates cooperative or regulated transport, uncommon for small molecules like oxygen or glycerol in bacteria

Transport Mechanism of Each Molecule

1. Glycerol in E. coli

• Glycerol enters E. coli through facilitated diffusion

• Transported via the GlpF protein (glycerol facilitator)

• Carrier-mediated → saturable

• Therefore, glycerol follows Profile (i)

2. Oxygen in E. coli

• Oxygen is small, nonpolar, and lipid-soluble

• Crosses the membrane by simple diffusion

• No carrier involved → non-saturable

• Transport rate increases linearly with concentration

• Therefore, oxygen follows Profile (ii)

Correct Answer

✅ Option (D): glycerol–(i) and oxygen–(ii)

This option correctly matches:

• Glycerol → Saturable facilitated diffusion (i)

• Oxygen → Simple diffusion (ii)

Explanation of All Options

Option (A): glycerol–(ii) and oxygen–(iii) ❌

• Glycerol does not show linear diffusion

• Oxygen does not use cooperative transport

• Incorrect

Option (B): glycerol–(ii) and oxygen–(i) ❌

• Glycerol transport is not linear

• Oxygen transport is not saturable

• Incorrect

Option (C): glycerol–(iii) and oxygen–(i) ❌

• Glycerol transport is not sigmoidal

• Oxygen does not show saturation

• Incorrect

Option (D): glycerol–(i) and oxygen–(ii) ✅

• Correctly reflects the biological transport mechanisms

• Correct answer

Key Takeaway

• Facilitated diffusion → Saturation curve

• Simple diffusion → Linear curve

• In E. coli:

  • Glycerol uses a carrier → Profile (i)

  • Oxygen diffuses freely → Profile (ii)