The Question is from neet-botany cell-biology , and it was created by the author Mr. NARESH KUMAR K

Question

A researcher is studying the transport of a novel molecule across a synthetic lipid bilayer.  They observe that the molecule moves down its concentration gradient without the need for any membrane proteins or energy input.  However, the rate of transport is significantly slower than predicted for simple diffusion.  Which of the following BEST explains this observation?

Mr. NARESH KUMAR K · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Membrane transport and permeability
 **Related Topics**: Lipid bilayers, simple diffusion, hydrophobic interactions, polar molecules
 **Prerequisites**: Understanding of cell membrane structure and the principles of diffusion.
## SOLUTION PATHWAY
**Given Information**:
  * A novel molecule is transported across a synthetic lipid bilayer.
  * Transport occurs down the concentration gradient (passive transport).
  * No membrane proteins or energy are involved (simple diffusion).
  * Transport rate is slower than expected for simple diffusion.
**Method & Steps**:
 1. Analyze the given information: The molecule moves passively across the lipid bilayer without the aid of proteins, indicating simple diffusion. However, the slower-than-expected rate suggests a hindrance to this process.
  Reasoning: Simple diffusion is faster for small, nonpolar molecules that can easily traverse the hydrophobic core of the bilayer.  Anything that impedes this passage will slow down the rate.
 2. Evaluate the options:
  * A: Large size restricts diffusion. While size does affect diffusion, the question specifies that the transport is *slower* than predicted, implying a specific interaction with the bilayer, not just size.
  * B: Charge repels the hydrophobic core. Charged molecules are indeed repelled by the hydrophobic core and would likely not cross the membrane by simple diffusion at all. This option suggests that the molecule would not cross at all.
  * C: Polarity hinders passage through the hydrophobic core. Polar molecules experience interactions with water molecules, making them less likely to interact favorably with the hydrophobic lipid tails. This creates a barrier and reduces the rate of diffusion.
  * D: Binding to a receptor slows diffusion. This scenario would involve a specific interaction and wouldn't be considered simple diffusion.
 3. Select the best explanation: Option C best explains the observation. The molecule's polarity creates a barrier to its passage through the hydrophobic core of the bilayer, resulting in a slower rate of diffusion than expected for a small, nonpolar molecule.
  Result: C
  Verification: The other options either do not explain the slow diffusion or suggest mechanisms inconsistent with the given information (i.e., the absence of proteins or energy input).
 ## OPTION ANALYSIS
A :x:  While size does affect diffusion rate, it doesn't fully explain why the rate is *slower* than predicted for simple diffusion.  It's a contributing factor but not the primary reason.
B :x: A charged molecule would likely be completely blocked from crossing the membrane by simple diffusion, not just slowed down.
C ✓: Polar molecules interact unfavorably with the hydrophobic core of the lipid bilayer, slowing their diffusion rate. This aligns perfectly with the observation.
D :x: Receptor binding implies facilitated diffusion or active transport, which is explicitly ruled out by the problem statement.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing simple diffusion with facilitated diffusion or active transport:  Remember that simple diffusion requires no energy input or membrane proteins.
  2.Overlooking the role of polarity and hydrophobicity in membrane permeability: Polar molecules have difficulty crossing the hydrophobic core of the lipid bilayer.
 **Quick Tips**:
  1.Consider the properties of the lipid bilayer (hydrophobic core) when analyzing membrane transport.
  2.Remember that the rate of simple diffusion depends on the size and polarity of the molecule.

Mr. NARESH KUMAR K · Answer

The molecule is very large and its size restricts diffusion through the membrane.

Mr. NARESH KUMAR K · Answer

The molecule is charged and is repelled by the hydrophobic core of the bilayer.

Mr. NARESH KUMAR K · Answer

The correct Option is "The molecule is polar and its passage is hindered by the hydrophobic core of the bilayer."

Mr. NARESH KUMAR K · Answer

The molecule is binding to a specific receptor on the membrane surface, slowing its diffusion.

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