Cell Membrane and Cytoplasm
Simple Explanation
The cell membrane is a thin, flexible barrier that surrounds every cell, controlling what enters and leaves. Inside it, the cytoplasm is the jelly-like substance that fills the cell and holds all the organelles in place.
Why Do We Need It?
Without a membrane to control what crosses in and out, a cell couldn't keep its internal chemistry stable enough to survive — and without cytoplasm, its organelles would have nowhere to sit or interact.
See It
A labelled diagram of an animal cell showing the cell membrane, nucleus, mitochondria, ribosomes, endoplasmic reticulum and Golgi apparatus
Worked Example
Identify the structure controlling water loss
A cell placed in salty water loses water and shrinks. Which structure is responsible for controlling this water loss?
Why Does This Work?
The cell membrane is selectively permeable — it lets some substances cross freely while restricting others — which is exactly the property needed to keep the cell's internal environment different from (and more stable than) its surroundings.
Real-Life Example
Why cucumbers go soft in salty water
A cucumber slice left in a strong salt solution becomes limp instead of staying crisp.
Water moves out of the cucumber's cells, across their cell membranes, into the saltier solution outside — a direct, visible example of the membrane controlling water movement.
Practice
What is the main function of the cell membrane?
EasyCommon mistake
Confusing the cell membrane with the cell wall — every cell (plant or animal) has a cell membrane, but only plant (and some other) cells have a cell wall as well.
Quick Review
- The cell membrane surrounds every cell and controls what crosses in and out.
- Cytoplasm is the jelly-like substance filling the cell.
- Organelles sit within the cytoplasm.