SME
Grade 9 Science · Problem type
The sodium–potassium pump
Use this for “How many Na⁺ and K⁺ ions does the pump move for 2 ATP?”
Diagram
The pump carries Na⁺ from inside (8) to outside (20): low to high, against the gradient, so it uses ATP.
At the same time it carries K⁺ the other way, into the cell.
6 crossing now.
Na⁺ outside after: 26
Na⁺ inside after: 2
This is an example. Type your own values to start a new problem.
s
=
3
×
a
k
=
2
×
a
o
Na⁺ outside
example
i
Na⁺ inside
example
o₂
Na⁺ outside after
worked out (not typed)
i₂
Na⁺ inside after
worked out (not typed)
Formulas
s
=
3
×
a
6 = 3 × 2
k
=
2
×
a
4 = 2 × 2
o₂
=
o
+
s
26 = 20 + 6
i₂
=
i
−
s
2 = 8 − 6
Assumptions
•
Each cycle, the pump splits one ATP to move 3 Na⁺ out of the cell and 2 K⁺ in.
•
It moves Na⁺ from the side with fewer to the side with more, against the gradient: that needs energy.
•
Only the Na⁺ counts are drawn; the K⁺ move the opposite way.
Step-by-step
WE KNOW
o = 20, i = 8, a = 2
FIND
Na⁺ pumped out (s), K⁺ pumped in (k), Na⁺ outside after (o₂), Na⁺ inside after (i₂)
Step 1 · Find Na⁺ pumped out (s)
Use s = 3 × a
Each ATP split moves 3 Na⁺ out.
s
=
3
×
2
s
=
6
Step 2 · Find K⁺ pumped in (k)
Use k = 2 × a
Each ATP split brings 2 K⁺ in.
k
=
2
×
2
k
=
4
Step 3 · Find Na⁺ outside after (o₂)
Use o₂ = o + s
The Na⁺ pumped out join the outside.
o₂
=
20
+
6
o₂
=
26
Step 4 · Find Na⁺ inside after (i₂)
Use i₂ = i − s
The Na⁺ pumped out leave the inside.
i₂
=
8
−
6
i₂
=
2
Check
Put the numbers back in. Both sides should match.
6 = 3 × 2 ✓
4 = 2 × 2 ✓
26 = 20 + 6 ✓
2 = 8 − 6 ✓
← Previous lesson
Cell membranes and transport: diffusion, osmosis and active transport
Next lesson →
Which kind of transport is it?
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
Related lessons
Cell membranes and transport: diffusion, osmosis and active transport
Main lesson
Which kind of transport is it?
Use this for “Is it passive or active transport, and does it need a protein?”
Cells in hypotonic, isotonic and hypertonic water
Use this for “Onion cells in salt water shrink from their walls. Why?”