BackFacilitated Diffusion and Membrane Transport
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Facilitated Diffusion and Membrane Transport
Facilitated Diffusion
Facilitated diffusion is a passive, non-energetic process that enables the movement of molecules across biological membranes with the help of specific proteins. This process is essential for transporting substances that cannot freely diffuse through the lipid bilayer due to their polarity, size, or charge.
Definition: Facilitated diffusion is the passive movement of molecules across a membrane via the aid of membrane proteins, without the expenditure of cellular energy (ATP).
Key Proteins Involved:
Carrier (Transporter) Proteins: Undergo conformational changes to move a solute across the membrane.
Channel Proteins: Form aqueous pores that allow membrane-spanning transport of specific molecules or ions.
Example: Carrier transporters and pores such as aquaporins and ion channels facilitate the movement of water and ions across membranes.
Types of Membrane Transport
Simple Diffusion: Movement of molecules from high to low concentration without the assistance of membrane proteins.
Facilitated Diffusion: Passive transport requiring membrane proteins for molecules that cannot diffuse freely.
Active Transport: Movement of molecules against their concentration gradient, requiring energy input (usually ATP).
Key Structures in Facilitated Diffusion
Porins: Channel proteins that allow the passage of certain molecules through membranes, especially in bacteria and mitochondria.
Aquaporins: Specialized channel proteins that facilitate rapid water transport across cell membranes.
Ion Channels: Proteins that allow the selective passage of ions such as Na+, K+, and Cl- across membranes.
Practice Questions
What type of transport does not require the assistance of a membrane protein?
Answer: Simple diffusion
Movement of water across a membrane is facilitated by proteins called ______.
Answer: Aquaporins
Summary Table: Types of Membrane Transport
Transport Type | Energy Required? | Protein Required? | Example |
|---|---|---|---|
Simple Diffusion | No | No | O2 diffusion across membrane |
Facilitated Diffusion | No | Yes | Glucose transport via GLUT1 |
Active Transport | Yes | Yes | Na+/K+ ATPase pump |
Key Equations
Fick's Law of Diffusion:
Where J is the flux, D is the diffusion coefficient, and is the concentration gradient.
Additional info: Facilitated diffusion is crucial for maintaining cellular homeostasis and allows cells to regulate the internal concentration of ions, nutrients, and water.
