Skip to content
Human Body Study

Cell Membrane

The thin boundary that encloses every cell, separating its interior from the outside world. Also called the plasma membrane.

Schematic of a generalized animal cell A labelled cross-section of a generalized animal cell. Each label links to the article for that part: the cell membrane, cytoplasm, nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes and cytoskeleton. rough ER smooth ER Cell membrane Mitochondria Nucleus Endoplasmicreticulum Lysosomes Cytoskeleton Cytoplasm Ribosomes Golgi apparatus A generalized animal cell — schematic, not to scale. Not every cell contains every structure.

Structure

Section titled “Structure”

A double layer of phospholipids with proteins embedded throughout — often described as a “fluid mosaic” because the components drift within the layer. Cholesterol stiffens it; surface sugars act as identity tags.

What it does

Section titled “What it does”
  • Barrier — keeps the cell’s contents in and unwanted substances out.
  • Selective transport — protein channels and pumps control exactly what crosses, and in which direction.
  • Communication — receptor proteins receive hormones and other signals.
  • Identity — surface markers let the immune system recognize the cell as “self.”

Key idea

Section titled “Key idea”

The membrane is not a passive wall but a selective, active gatekeeper — it is where the cell decides what to admit, expel, and respond to.