How does facilitated diffusion differ from simple diffusion regarding the types of molecules transported?

Simple diffusion involves small, nonpolar molecules (like $O_2$) passing directly through the bilayer. Facilitated diffusion is required for large, polar, or charged molecules (like glucose or ions) that are repelled by the hydrophobic interior.

When comparing facilitated diffusion and active transport, what is the most critical difference regarding energy?

Facilitated diffusion is passive and relies on the existing concentration gradient (no ATP). Active transport requires metabolic energy (ATP) to move substances against their concentration gradient.

What is the difference between the mechanism of a channel protein and a carrier protein?

Channel proteins provide a fixed, water-filled pore for molecules to flow through. Carrier proteins must bind to the solute and undergo a physical change in shape to move the substance across the membrane.

Why is it a mistake to say that all protein-mediated transport is 'active'?

Because facilitated diffusion uses proteins (channels and carriers) but is entirely passive. The protein simply provides a path for the molecule to follow its natural concentration gradient.

What happens to the rate of facilitated diffusion if all transport proteins are currently occupied?

The rate reaches a plateau or 'saturation point.' Unlike simple diffusion, the speed cannot increase further because the transport machinery is working at its maximum capacity.

Does facilitated diffusion require the cell to be alive and respiring?

Not directly for the process itself, as it is passive and requires no ATP. However, the cell must be alive to maintain the membrane structure and produce the necessary transport proteins.

Define 'Aquaporin' and its role in the cell.

An aquaporin is a specialized channel protein that facilitates the rapid transport of water molecules across the cell membrane, significantly increasing the rate of osmosis.

What is a 'Gated Channel'?

A gated channel is a transport protein that can open or close in response to specific stimuli, such as a change in voltage or the binding of a chemical signal (ligand).

What is meant by 'Conformational Change' in the context of carrier proteins?

It refers to a temporary change in the three-dimensional shape of the protein triggered by solute binding, which allows the solute to be released on the opposite side of the membrane.

Why can't ions like $Na^+$ pass through the membrane via simple diffusion?

Ions are charged and therefore highly hydrophilic. They cannot interact with or pass through the nonpolar, hydrophobic fatty acid tails that make up the core of the phospholipid bilayer.

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Biology

Unit 2: Cell Structure & Function

Facilitated Diffusion

Summary

Facilitated diffusion is a specialized form of passive transport that enables the movement of large, polar, or charged substances across the hydrophobic cell membrane via specific transmembrane proteins. Unlike active transport, it relies entirely on the kinetic energy of particles moving down their concentration gradient, requiring no metabolic energy (ATP) from the cell.

1. Definition & Core Concepts

Facilitated diffusion is the process by which molecules or ions that cannot cross the phospholipid bilayer directly are moved across the membrane with the help of specialized transport proteins.

This process is strictly passive, meaning it occurs down a concentration gradient from an area of high concentration to an area of low concentration.

It is essential for maintaining cellular homeostasis by allowing the entry of nutrients like glucose and the exit of waste products or ions that are otherwise repelled by the membrane's nonpolar interior.

Diagram showing a channel protein (blue) and a carrier protein (orange) facilitating the movement of red molecules across a membrane bilayer.

2. Underlying Principles

The driving force behind facilitated diffusion is the concentration gradient, representing a state of high potential energy that naturally dissipates as molecules spread out.

Transport proteins provide a hydrophilic pathway through the hydrophobic core of the lipid bilayer, lowering the activation energy required for polar molecules to cross.

The rate of facilitated diffusion is limited by the number of available transport proteins; once all proteins are occupied, the rate reaches a maximum, known as saturation.

3. Methods & Protein Types

Channel Proteins

These act as fixed hydrophilic pores that allow specific ions or water molecules to flow through the membrane rapidly.
Aquaporins are a specialized type of channel protein dedicated exclusively to the rapid transport of water molecules.

Carrier Proteins

These proteins possess specific binding sites for a target molecule; once bound, the protein undergoes a conformational change (shape shift).
This change in shape physically moves the molecule from one side of the membrane to the other before releasing it and returning to its original state.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions