SME
Grade 9 Science · Biology
Cell membranes and transport: diffusion, osmosis and active transport
Use this for “Oxygen is at 20 units outside a cell and 8 inside. Which way does it diffuse, and where does it stop?”
Diagram
20 O₂ outside, 8 inside: they diffuse into the cell, from high to low concentration.
6 crossing now.
This is an example. Type your own values to start a new problem.
o₂
=
o
−
m
i₂
=
i
+
m
d
Gradient (outside − inside)
worked out (not typed)
Formulas
d
=
o
−
i
12 = 20 − 8
o₂
=
o
−
m
14 = 20 − 6
i₂
=
i
+
m
14 = 8 + 6
Assumptions
•
Particles move both ways at random, so the net flow runs from the side with more to the side with fewer.
•
Small nonpolar molecules such as O₂ and CO₂ cross the bilayer with no protein and no ATP.
•
At equal counts particles still cross, but the net movement is zero: dynamic equilibrium.
Step-by-step
WE KNOW
o = 20, i = 8, m = 6
FIND
gradient (outside − inside) (d), O₂ outside after (o₂), O₂ inside after (i₂)
Step 1 · Find gradient (outside − inside): d
Use d = o − i
The gradient is the difference across the membrane: outside minus inside.
d
=
20
−
8
d
=
12
Step 2 · Find O₂ outside after (o₂)
Use o₂ = o − m
The particles that cross in leave the outside.
o₂
=
20
−
6
o₂
=
14
Step 3 · Find O₂ inside after (i₂)
Use i₂ = i + m
The particles that cross in join the inside.
i₂
=
8
+
6
i₂
=
14
Check
Put the numbers back in. Both sides should match.
12 = 20 − 8 ✓
14 = 20 − 6 ✓
14 = 8 + 6 ✓
Topic check
See what you know: 5 questions on this topic
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