BIO 181 MODULE 3, HOW MEMBRANES FORM CELLS
COMPREHENSIVE STUDY GUIDE & REVISION NOTES 2026
Module 3.1 (How Molecules Move)
Objective
Qualitatively predict the net diffusion of solutes and water through a cell
membrane.
Cell Membrane
a fluid barrier that prevents certain solutes from passing into or out of a cell
Phospholipid
extracellular fluid
intracellular fluid
moving through cells
Phospholipid Structure
hydrophilic head
interacts with ions and polar molecules
hydrophobic tails
interacts with nonpolar molecules
Amphiphilic
a molecule having a nonpolar, hydrophobic region and a polar, hydrophilic
region
Phospholipid Bilayer
H₂O present inside and outside the membrane
Membrane Dimensions
extracellular fluid
Module 3: How Membranes Form Cells 1
, intracellular fluid
1 nm / 3 nm / 1 nm structure
Permeability
a measure of how easily a solute passes through a membrane
Permeability Factors
Size: small, medium, large
Polarity: nonpolar, polar, charged
very likely ↔ very unlikely to cross
Examples of Solutes
sodium ion (Na⁺)
oxygen molecule
water molecule
glucose molecule
testosterone molecule
Osmosis
The net diffusion of water through a membrane
Osmolarity
the total number of solutes per liter of solvent
osmolarity = concentration of water
Osmotic Environments
hyperosmotic environment (outside > inside)
isosmotic environment (outside = inside)
hypoosmotic environment (outside < inside)
Key Point
A membrane of phospholipids creates a barrier that prevents ions and large
polar molecules from entering or leaving the cell.
Module 3: How Membranes Form Cells 2
COMPREHENSIVE STUDY GUIDE & REVISION NOTES 2026
Module 3.1 (How Molecules Move)
Objective
Qualitatively predict the net diffusion of solutes and water through a cell
membrane.
Cell Membrane
a fluid barrier that prevents certain solutes from passing into or out of a cell
Phospholipid
extracellular fluid
intracellular fluid
moving through cells
Phospholipid Structure
hydrophilic head
interacts with ions and polar molecules
hydrophobic tails
interacts with nonpolar molecules
Amphiphilic
a molecule having a nonpolar, hydrophobic region and a polar, hydrophilic
region
Phospholipid Bilayer
H₂O present inside and outside the membrane
Membrane Dimensions
extracellular fluid
Module 3: How Membranes Form Cells 1
, intracellular fluid
1 nm / 3 nm / 1 nm structure
Permeability
a measure of how easily a solute passes through a membrane
Permeability Factors
Size: small, medium, large
Polarity: nonpolar, polar, charged
very likely ↔ very unlikely to cross
Examples of Solutes
sodium ion (Na⁺)
oxygen molecule
water molecule
glucose molecule
testosterone molecule
Osmosis
The net diffusion of water through a membrane
Osmolarity
the total number of solutes per liter of solvent
osmolarity = concentration of water
Osmotic Environments
hyperosmotic environment (outside > inside)
isosmotic environment (outside = inside)
hypoosmotic environment (outside < inside)
Key Point
A membrane of phospholipids creates a barrier that prevents ions and large
polar molecules from entering or leaving the cell.
Module 3: How Membranes Form Cells 2
