Struggling to memorize cell transport mechanisms for your next exam? This AP Biology Unit 2 study guide breaks down everything you need to know about cell membrane structure and function, the fluid mosaic model, and the selective permeability of the lipid bilayer.
What is the Fluid Mosaic Model of the Cell Membrane?
The fluid mosaic model describes the cell membrane as a tapestry of several types of molecules (phospholipids, cholesterols, and proteins) that are constantly moving. This fluidity allows the membrane to maintain its role as a dynamic barrier, regulating what enters and exits the cell.
The Phospholipid Bilayer: Amphipathic Properties
The structural foundation of the plasma membrane is a double layer of phospholipids. Phospholipids are amphipathic molecules, meaning they possess both hydrophilic and hydrophobic regions:
- Hydrophilic Phosphate Heads: These polar, water-loving heads point outward, interacting with the aqueous environments inside and outside the cell.
- Hydrophobic Fatty Acid Tails: These nonpolar, water-fearing tails point inward, facing each other to form a secure, oily barrier that keeps water-soluble substances from crossing freely.
Selective Permeability: What Can Cross the Membrane?
Because of its hydrophobic core, the cell membrane is selectively permeable. It controls the rate and ease with which different molecules pass through:
| Molecule Type | Examples | How It Crosses the Membrane |
|---|---|---|
| Small, Nonpolar | $O_2$, $CO_2$, $N_2$ | Passes freely via Simple Diffusion. |
| Small, Polar / Uncharged | $H_2O$, Urea | Passes slowly in small amounts (Water uses Aquaporins). |
| Large Polar / Charged Ions | Glucose, $Na^+$, $K^+$, $Cl^-$ | Requires transport proteins (Facilitated Diffusion/Active Transport). |
Eukaryotic Cell Wall vs. Cell Membrane
While animal cells only have a plasma membrane, plants, fungi, and some protists have an outer cell wall.
- The cell membrane is flexible, selectively permeable, and composed primarily of lipids and proteins.
- The cell wall is rigid, permeable to most small molecules, and composed of structural carbohydrates (like cellulose in plants or chitin in fungi) to protect against osmotic pressure and provide structural support.
Found This AP Biology Resource Helpful? 📚
I code interactive study calculators and write clean, fluff-free biology guides to keep high-quality education resources open to everyone. Consider supporting my work!
☕ Buy Me a Coffee
