---
title: "Phospholipid Bilayer — AP Bio Definition & Exam Guide"
description: "The phospholipid bilayer is the double-layered sheet that forms cell membranes, with hydrophilic heads out and hydrophobic tails in, creating selective permeability for AP Bio Unit 2."
canonical: "https://fiveable.me/ap-bio/key-terms/phospholipid-bilayer"
type: "key-term"
subject: "AP Biology"
---

# Phospholipid Bilayer — AP Bio Definition & Exam Guide

## Definition

The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that form a cell membrane, the hydrophobic lipid ends facing inward and the hydrophilic phosphate ends face outward.

## Related Study Guides

- [2.4 Membrane Permeability](/ap-bio/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi)
- [4.2 Introduction to Signal Transduction](/ap-bio/unit-4/intro-signal-transduction/study-guide/VAotQCiNsYQzCcmUBt3D)
- [2.7 Tonicity and Osmoregulation](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B)
- [2.1 Cell Structure and Function](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9)
- [2.9 Cell Compartmentalization](/ap-bio/unit-2/mechanisms-transport/study-guide/sMlGCA9J4sGVE7S24xBM)

## Review

### Related Terms

- Hydrophilic Head: These are water-loving parts of molecules; they tend to be attracted to water due to their polar properties.
- Hydrophobic Tail: These are water-fearing parts of molecules; they tend to repel away from water due to their non-polar properties.
- Amphipathic Molecule: An amphipathic molecule has both hydrophilic (water-loving) and hydrophobic (water-hating) parts. Phospholipids, which make up the bilayer of cell membranes, are amphipathic.