Biology · Introductory biology · Worked example

Name the transport mechanism

Name the way each substance crosses the membrane, and say whether the cell spends energy. (a) $O_{2}$ entering a muscle cell from the blood. (b) Glucose entering a red blood cell from plasma, where glucose is more concentrated, through a carrier protein. (c) ${Na}^{+}$ leaving a nerve cell, where ${Na}^{+}$ is already more concentrated outside. (d) Glucose entering a cell lining the gut, where glucose is already more concentrated inside, together with ${Na}^{+}$ . (e) A white blood cell taking in a whole bacterium. Then find how many ATP and $K^{+}$ go with $300 {Na}^{+}$ pumped out.

$O_{2}$ entering a muscle cell

$O_{2}$ is small and nonpolar, so it crosses the bilayer itself, down its gradient from the blood into a cell that uses it up. That is simple diffusion, and it costs no energy.

Glucose entering a red blood cell

Glucose is polar and cannot cross the bilayer, so it needs a protein. It moves down its gradient, from plasma into the cell, through a carrier. That is facilitated diffusion, and it costs no energy.

${Na}^{+}$ leaving a nerve cell

${Na}^{+}$ is already more concentrated outside, so pushing more out goes up its gradient. Only a pump can do that, here the sodium-potassium pump, which spends ATP. That is primary active transport.

Glucose entering a gut cell

Glucose moves up its gradient, into a cell that already holds more. The cotransporter lets ${Na}^{+}$ flow in down its own gradient and carries glucose with it. That is secondary active transport, paid for by the ATP the pump spent building the ${Na}^{+}$ gradient.

A whole bacterium

A bacterium is far too large for any channel or carrier. The white blood cell wraps membrane around it and pinches off a vesicle. That is phagocytosis, a kind of endocytosis, and it uses energy.

Count the pump’s cycles

Each cycle moves $3 {Na}^{+}$ out and $2 K^{+}$ in for one ATP. $300 {Na}^{+}$ out takes $100$ cycles, so $100$ ATP, and brings $200 K^{+}$ in.

\begin{array}{l} \frac{300}{3} = 100 \\ 2 (100) = 200 \end{array}

Result

(a) Simple diffusion, no energy. (b) Facilitated diffusion through a carrier, no energy. (c) Primary active transport by the sodium-potassium pump, using ATP. (d) Secondary active transport, powered by the ${Na}^{+}$ gradient. (e) Phagocytosis, a form of endocytosis, using energy. Pumping $300 {Na}^{+}$ out uses $100$ ATP and brings $200 K^{+}$ in.

Your turn

1. One like the example

Name the way each crosses, and whether it costs energy. (a) ${CO}_{2}$ leaving a cell. (b) $K^{+}$ flowing out of a cell through open $K^{+}$ channels, down its gradient. (c) ${Ca}^{2 +}$ pumped out of a cell, up its gradient, using ATP.

Show the answer and explanation

(a) Simple diffusion, no energy. (b) Facilitated diffusion through a channel, no energy. (c) Primary active transport, using ATP.

${CO}_{2}$ is small and nonpolar, so it crosses the bilayer itself. $K^{+}$ is an ion, so it needs a channel, but it moves down its gradient. ${Ca}^{2 +}$ moves up its gradient, which only a pump spending energy can do.

2. A variation

A sodium-potassium pump completes $150$ cycles. How many ${Na}^{+}$ leave, how many $K^{+}$ enter, how many ATP are used, and how many positive charges does the cell lose overall?

Show the answer and explanation

$450 {Na}^{+}$ out, $300 K^{+}$ in, $150$ ATP, and a net $150$ positive charges out.

Each cycle moves $3 {Na}^{+}$ out and $2 K^{+}$ in for one ATP, so $150$ cycles move $450$ out and $300$ in. Each cycle carries one more positive charge out than in, so the cell loses $150$ .

\begin{array}{l} 3 (150) = 450 \\ 2 (150) = 300 \\ 450 - 300 = 150 \end{array}

3. Somewhere new

A poison stops the sodium-potassium pump. The sodium–glucose cotransporters in the gut are not harmed. Why does glucose uptake by gut cells still stop after a while?

Show the answer and explanation

The cotransporters run on the ${Na}^{+}$ gradient, and without the pump that gradient runs down.

${Na}^{+}$ keeps leaking in, including through the cotransporters themselves, while the stopped pump no longer removes it. As the ${Na}^{+}$ gradient flattens, there is nothing left to drive glucose up its own gradient. Secondary active transport depends on the primary transport that builds its gradient.

Keep exploring

A patch of membrane opens in Membrane transport with the sodium-potassium pump, ATP present and ${Na}^{+}$ $10$ times as concentrated outside as inside. The tool shows the pump moving ${Na}^{+}$ from the inside to the outside, up its gradient.

Return to the concept →

Sources and scope

Authored study material. Tool results depend on the stated inputs and model assumptions.

After reading

Study it, then try it

Practice the key terms as flashcards, then open the example in its tool, change a value and keep a useful result on your board.

Study the 13 key terms Open the pump in Membrane transport Open worked example on a board Cells and membranes in Biology Reference

Your existing work stays on this device. Examples open as editable copies.

O2O2​ entering a muscle cell

Glucose entering a red blood cell

Na+Na+ leaving a nerve cell

Glucose entering a gut cell

A whole bacterium

Count the pump’s cycles

Result

Your turn

1. One like the example

(a) Simple diffusion, no energy. (b) Facilitated diffusion through a channel, no energy. (c) Primary active transport, using ATP.

2. A variation

450 Na+450 Na+ out, 300 K+300 K+ in, 150150 ATP, and a net 150150 positive charges out.

3. Somewhere new

The cotransporters run on the Na+Na+ gradient, and without the pump that gradient runs down.

Keep exploring

Study it, then try it

$O_{2}$ entering a muscle cell

${Na}^{+}$ leaving a nerve cell

$450 {Na}^{+}$ out, $300 K^{+}$ in, $150$ ATP, and a net $150$ positive charges out.

The cotransporters run on the ${Na}^{+}$ gradient, and without the pump that gradient runs down.