Partially Permeable Membrane
The cell membrane is described as partially permeable (sometimes called selectively permeable). This means:
- Small molecules (water, oxygen, carbon dioxide) can pass through freely.
- Large molecules (starch, proteins) cannot pass through without help.
- The membrane can control which substances enter and leave.
This property is critical for osmosis: water molecules can pass through the membrane, but solute molecules (like sugar) cannot. This creates a concentration gradient that drives water movement.
The Phospholipid Bilayer (Supplement)
The membrane is made of two layers of phospholipid molecules:
- Each phospholipid has a hydrophilic (water-loving) head and two hydrophobic (water-hating) tails.
- The heads face outward (towards the watery environment on both sides) and the tails face inward (away from water).
- This arrangement forms a stable barrier because the hydrophobic interior repels water-soluble substances.
Membrane proteins are embedded in the bilayer:
- Channel proteins: form pores that allow specific small molecules or ions to pass through (facilitated diffusion).
- Carrier proteins: change shape to transport molecules across the membrane (active transport and facilitated diffusion).
- Receptor proteins: bind to specific molecules (e.g. Hormones) to trigger a response inside the cell.
How Substances Cross the Membrane
There are three main ways substances cross the cell membrane:
| Method | Direction | Energy needed? | Examples |
|---|---|---|---|
| Diffusion | High → low concentration | No (passive) | O₂ into cells, CO₂ out of cells |
| Osmosis | High water potential → low water potential | No (passive) | Water into root hair cells |
| Active transport | Low → high concentration (against gradient) | Yes (from respiration) | Mineral ions into root hair cells |
The membrane proteins play a key role in facilitated diffusion and active transport. Without them, large or charged molecules could not cross the hydrophobic interior of the bilayer.