Note
Studied by 12 peopleCell Membrane Intro
The Cell Membrane
When you swim or shower, your cells don't fill up with water, nor do their contents leak out, thanks to the cell membrane. This outer layer of the cell shields it from the environment and controls what enters and exits, a characteristic known as semi-permeability.
Phospholipids: The Building Blocks
The primary component of the cell membrane is phospholipids. These have three key parts:
A phosphate head group
A glycerol backbone
Two fatty acid tails
Think of a phospholipid as a balloon with two strings: the "balloon" is the phosphate head, and the "strings" are the fatty acid tails. The glycerol backbone isn't usually drawn, but it connects the fatty acid tails to the phosphate head.
Hydrophilic vs. Hydrophobic
Phospholipids exhibit a unique property:
The phosphate head is hydrophilic (polar), meaning it loves water.
The fatty acid tails are hydrophobic, meaning they fear water.
A molecule with both hydrophobic and hydrophilic sections is called amphipathic.
In water, phospholipids cluster together, with the hydrophilic heads near water and the hydrophobic tails shielded away. This arrangement forms a phospholipid bilayer, the basic structure of the cell membrane.
The inside section of the bilayer is hydrophobic. In real cells, the phospholipid bilayer forms a sphere, with water both inside and outside the cell. No matter where the cell membrane touches water, it is always the hydrophilic phosphate head groups that interact with water.
🧪 Semi-Permeability: What Can Pass Through?
The cell membrane's semi-permeability dictates which molecules can pass through. Consider a cell membrane separating the extracellular (outside) and intracellular (inside) environments. The cell membrane consists of phospholipids packed closely together.
Factors Affecting Permeability
Factor | Description |
|---|---|
Size | Small molecules generally pass through more easily. |
Polarity | The inside of the cell membrane is hydrophobic, so nonpolar molecules are preferred. |
Examples of Molecular Movement
Small, Nonpolar Molecules: Gases like $O_2$ and $CO_2$ pass through easily via passive diffusion because they are small and nonpolar.
Small, Polar Molecules: Water and ethanol can pass through, but slowly, as the hydrophobic region of the cell membrane resists them.
Large, Nonpolar Molecules: Benzene can pass through, albeit slowly, because it is nonpolar and interacts well with the hydrophobic region.
Large, Polar Molecules: Sugar (glucose) cannot pass through because it is large and polar, the opposite of what the cell membrane prefers.
Charged Molecules: Ions (like chloride or sodium) and amino acids cannot pass through due to their charge.
In summary, the cell membrane protects cells and controls what enters and exits due to its semi-permeability. Made of phospholipids, it prefers small, nonpolar molecules because of its large hydrophobic region and the close packing of phospholipids.