Every cell is wrapped in a thin membrane that decides what gets in and what stays out. This control keeps the inside of the cell very different from the fluid around it.
Structure
The membrane is a double layer of phospholipids with proteins set into it. Small fat-soluble molecules such as oxygen and CO2 pass easily, while ions and glucose need protein channels or carriers.
Diffusion and osmosis
In diffusion, substances move from high to low concentration without using energy. Osmosis is the movement of water across the membrane toward the side with more dissolved particles.
Active transport
Pumps use energy (ATP) to move substances against their gradient. The sodium-potassium pump pushes sodium out and potassium in, which keeps the electrical charge that nerves and muscles need.
Vesicles
Large particles enter by endocytosis, when the membrane wraps around them, and leave by exocytosis. This is how white blood cells swallow bacteria and how cells release hormones.
| Process | Uses energy | Example |
|---|---|---|
| Simple diffusion | No | Oxygen entering cells |
| Facilitated diffusion | No | Glucose entering through carriers |
| Osmosis | No | Water moving into or out of cells |
| Active transport | Yes | Sodium-potassium pump |
At the bedside
Osmosis explains fluid choices: isotonic saline stays outside cells, while a fast drop in blood sodium pulls water into brain cells and makes them swell. Insulin drives potassium into cells, which is why it is used to treat high potassium.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).