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Table of Contents
Skills you'll gain in this topic:
- Explain how plasma membrane structure contributes to selective permeability.
- Describe how molecules move across membranes based on size, charge, and polarity.
- Differentiate between passive and active transport processes.
- Analyze how membrane properties affect ion and water movement.
- Predict changes in cell conditions based on permeability and concentration gradients.
Membrane Permeability
Thanks to the structure of the membrane, with the hydrophobic tails and hydrophilic heads, the cellular membrane has selective permeability. This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.
The membrane acts like a barrier separating the inside of the cell from the external environment of the cell. This separation is super important - it allows the cell to maintain conditions inside that are different from the outside world!
The Fluid Mosaic Model and Selective Permeability
Remember the fluid mosaic model from Topic 2.4? This model explains why membranes are selectively permeable! The phospholipid bilayer creates a hydrophobic barrier in the middle of the membrane, while the embedded proteins create pathways for specific molecules to cross.
Think of the membrane like a bouncer at a club:
- Some molecules (VIPs) can pass right through
- Others need a special escort (transport protein)
- Some aren't getting in no matter what!
What Can Cross the Membrane?
The type of molecule determines how it crosses the membrane:
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Small nonpolar molecules (like O₂, CO₂, and N₂) are the VIPs - they can pass directly through the phospholipid bilayer without any help. These molecules are crucial for respiration and photosynthesis!
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Small polar uncharged molecules (like water/H₂O) can squeeze through the membrane in small amounts, but usually need help. Water molecules are tiny enough that some can slip between phospholipids, but for larger movements of water, cells use special transport proteins called aquaporins.
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Large polar molecules and ions (like glucose, amino acids, Na⁺, K⁺) cannot cross the membrane on their own - they need transport proteins to help them across. These molecules are too hydrophilic to pass through the hydrophobic interior of the membrane.
The hydrophobic fatty acid tails are what controls the movement of substances described above. They repel charged and polar molecules and make it very challenging for them to come across.
Quick Reference Chart: How Molecules Cross the Membrane
| Molecule Type | Examples | Can Cross Freely? | Needs Transport Protein? |
|---|---|---|---|
| Small nonpolar | O₂, CO₂, N₂ | ✅ | ❌ |
| Small polar uncharged | H₂O | ✅ (small amounts) | ✅ (large amounts) |
| Large polar | Glucose, amino acids | ❌ | ✅ |
| Ions | Na⁺, K⁺, Cl⁻, Ca²⁺ | ❌ | ✅ |
Diffusion Across the Plasma Membrane
Cell Walls
Cell walls provide two major functions for cells:
- A structural boundary that gives the cell shape and protection
- A permeability barrier that filters what can reach the plasma membrane
Unlike the plasma membrane, the cell wall is NOT selectively permeable in the same way. Water and most dissolved substances can pass through the cell wall, but it blocks larger particles and some pathogens from reaching the cell membrane.
Cell Wall Composition
Different types of organisms have cell walls made of different materials:
🌱 Plant cell walls: Made primarily of cellulose, a complex carbohydrate. Plant cell walls are arranged in layers and contain other polysaccharides like hemicellulose and pectin.
🦠 Bacterial cell walls: Made of peptidoglycan, a mesh-like layer of sugars and amino acids. The thickness of this layer is what determines if a bacterium is Gram-positive or Gram-negative.
🍄 Fungal cell walls: Mainly composed of chitin, a tough polysaccharide similar to the material in insect exoskeletons.
Each of these materials provides structural support while still allowing for the selective movement of substances to the plasma membrane beneath.
Why Cell Walls Matter for Permeability
The cell wall works together with the plasma membrane to control what enters and exits the cell. While the plasma membrane is the primary permeability barrier, the cell wall:
- Prevents the cell from bursting when water enters (especially important in plant cells)
- Filters out large molecules before they reach the plasma membrane
- Provides an additional layer of protection against the environment
Think of it like this: if the plasma membrane is the bouncer deciding who gets into the club, the cell wall is like the metal detector everyone has to go through first!