Active transport, the sodium–potassium pump and co-transport – Transport Across the Cell Membrane (Senior 5 Biology) | Ganzaa

Transport Across the Cell Membrane

Lessons

Senior 5 · Biology

Transport Across the Cell Membrane

5 lessons

Lesson 4 of 5

Active transport, the sodium–potassium pump and co-transport

25 min

Uphill against the gradient

The cells of a root hair hold potassium ions at a concentration hundreds of times higher than in the soil water around them. A red blood cell keeps about thirty times more K⁺ inside than the plasma outside, and far less Na⁺. Diffusion could never build gradients like these; left alone, it would erase them. The cell is moving ions uphill, and that costs energy. If you poison a root with cyanide, which stops aerobic respiration, potassium uptake almost stops. This is active transport.

Definition

Active transport: the movement of molecules or ions across a membrane against their concentration gradient (from lower to higher concentration), through specific carrier proteins, using energy from ATP made in respiration.

Pump: a carrier protein that uses ATP to move ions or molecules against their gradient. Pumps are ATPases: they hydrolyse ATP themselves.

Co-transport (secondary active transport): movement of one substance against its gradient, coupled to the movement of another substance, usually Na⁺ or H⁺, down its gradient through the same carrier protein.

Features of active transport

It moves substances against a concentration gradient.
It needs carrier proteins, each specific to particular ions or molecules.
It needs ATP, so it depends on respiration: it slows or stops when oxygen is lacking or respiration is poisoned.
It can be selective: a cell can take up K⁺ while keeping Na⁺ out, although both are small positive ions.
Cells that do much active transport, such as root hair cells and the cells of kidney tubules, have many mitochondria.

The sodium–potassium pump

Almost every animal cell has many copies of the Na⁺/K⁺ pump in its membrane. In each cycle it moves 3 Na⁺ out and 2 K⁺ in for each molecule of ATP hydrolysed:

The phosphate attached in step 2 changes the shape of the protein, so its binding sites face outwards and lose their attraction for Na⁺; releasing the phosphate in step 3 changes the shape back. Because more positive charge leaves than enters, the pump helps make the inside of the cell negative: this is part of the resting potential of nerve cells. The pump also keeps cell volume steady by controlling the water potential of the cytoplasm. In a resting human it uses a large share of all the ATP made.

Worked example 1: evidence of accumulation

Illustrative data: the concentration of K⁺ is 0.05 mmol dm⁻³ in pond water and 54 mmol dm⁻³ in the cell sap of an alga living in it. (a) Calculate how many times more concentrated K⁺ is inside the cells. (b) Explain why this is evidence of active transport.

(a) $\frac{54}{0.05} = 1\,080$ times.
(b) Diffusion can only move ions from higher to lower concentration, so it could at most make the concentrations equal. A 1 080-fold accumulation means the ions were moved against their gradient, which needs energy: active transport.

Worked example 2: what stops the pump?

Discs of carrot root were kept in a K⁺ solution under different conditions, and their uptake of K⁺ was measured (illustrative data, arbitrary units per hour).

Condition	K⁺ uptake
Air bubbled through, 20 °C	120
Nitrogen bubbled through (no oxygen), 20 °C	18
Air, 20 °C, with potassium cyanide	12
Air bubbled through, 5 °C	30

(a) Calculate the percentage reduction in uptake caused by removing oxygen. (b) Explain each result.

(a) $\frac{120 - 18}{120} \times 100 = 85\,\%$ .
(b) Without oxygen, aerobic respiration stops and little ATP is made, so the pumps slow. Cyanide inhibits the electron transport chain, so it has the same effect. At 5 °C, respiration enzymes and the pump proteins work slowly, so less ATP is made and used. The small uptake that remains is mostly diffusion and anaerobic ATP.

Co-transport: absorbing glucose in the small intestine

Some carriers do not use ATP directly. Instead they use a gradient of Na⁺ that a pump has built. In the epithelial cells of the small intestine (Figure 2):

Na⁺/K⁺ pumps in the basal membrane, next to the blood, pump Na⁺ out of the cell, keeping the Na⁺ concentration inside low.
On the side facing the gut lumen, a co-transporter protein lets Na⁺ diffuse into the cell down its gradient, and it carries a glucose molecule in with each Na⁺, even when glucose is more concentrated inside the cell.
Glucose leaves the cell by facilitated diffusion through a different carrier and enters the blood.

The same principle saves lives. In cholera and other severe diarrhoea, the gut loses water and salts fast. Oral rehydration solution (ORS), recommended by the World Health Organization, contains both glucose and sodium. The co-transporters still work during diarrhoea, so they pull Na⁺ and glucose into the gut cells, and water follows by osmosis. A drink of plain water, or of very sugary water without salt, does not do this.

Factors affecting the rate of active transport

Oxygen and glucose supply, which control the rate of aerobic respiration and so of ATP production.
Respiratory inhibitors such as cyanide.
Temperature, which affects both respiration and the shape-changes of the carrier proteins; very high temperatures denature them.
The number of carrier proteins: when all are working, the rate reaches a maximum.

Where active transport matters

Uptake of mineral ions such as nitrate and K⁺ by root hair cells from dilute soil water.
Nerve cells: the Na⁺/K⁺ pump restores the ion gradients after each impulse.
Kidneys: reabsorption of all the glucose and most of the Na⁺ from the filtrate back into the blood.
Digestion: absorption of glucose and amino acids by co-transport.
Phloem loading: companion cells load sucrose into sieve tubes by co-transport with H⁺ ions.

Worked example 3: counting the cost

A nerve cell pumps out $4.5 \times 10^{-6}$ mol of Na⁺ while recovering from activity. How many moles of ATP are used, and how much energy is that? (Hydrolysis of ATP releases 30.5 kJ mol⁻¹.)

Each ATP moves 3 Na⁺: $\frac{4.5 \times 10^{-6}}{3} = 1.5 \times 10^{-6}\,\mathrm{mol}$ ATP.
Energy: $1.5 \times 10^{-6} \times 30.5 = 4.6 \times 10^{-5}\,\mathrm{kJ} = 0.046\,\mathrm{J}$ .

Khan Academy: "Sodium potassium pump". Watch for how phosphorylation changes the shape of the pump, and why 3 Na⁺ leave for every 2 K⁺ that enter.

Warning

Common mistakes

"Active transport makes energy." It uses energy, from ATP made in respiration.
"Co-transport moves glucose down its gradient." The Na⁺ moves down its gradient; glucose is carried against its gradient.
Writing "the pump moves Na⁺ and K⁺ in". It moves 3 Na⁺ out and 2 K⁺ in.

Tip

Spotting active transport in data: look for (1) accumulation against a gradient, (2) a sharp fall when oxygen is removed or a respiratory poison is added, and (3) a fall at low temperature. Any one of these is evidence; all three together are strong evidence.

✦

Practice: active transport

Give three differences between active transport and facilitated diffusion.
Describe the four steps of the sodium–potassium pump.
Explain why root hair cells contain many mitochondria.
Explain why removing oxygen from a solution reduces the uptake of nitrate by roots.
A cell contains 140 mmol dm⁻³ of K⁺ and the fluid around it contains 4 mmol dm⁻³. Calculate the ratio and state what it suggests.
How many moles of ATP are used to pump $6.0 \times 10^{-9}$ mol of Na⁺ out of a cell?
Explain how sodium–glucose co-transport allows glucose to be absorbed even when its concentration in the gut is lower than in the epithelial cells.
Explain why ORS contains salt as well as glucose.

Show answersHide answers

Active transport moves substances against the gradient, uses ATP, and stops when respiration is inhibited; facilitated diffusion moves substances only down the gradient, uses no ATP, and does not depend on respiration. (Both use specific proteins.)
(1) Three Na⁺ bind from the cytoplasm; (2) ATP is hydrolysed and phosphate attaches, the pump changes shape and releases Na⁺ outside; (3) two K⁺ bind from outside and the phosphate is released; (4) the pump returns to its original shape and releases K⁺ inside.
They absorb mineral ions by active transport against a gradient; this needs much ATP, made in mitochondria by aerobic respiration.
Without oxygen, aerobic respiration stops, so little ATP is made and the carrier proteins cannot pump nitrate against its gradient.
$\frac{140}{4} = 35$ ; K⁺ is accumulated against its gradient, which suggests active transport.
$\frac{6.0 \times 10^{-9}}{3} = 2.0 \times 10^{-9}$ mol of ATP.
The Na⁺/K⁺ pump keeps Na⁺ low inside the cell, so Na⁺ diffuses in from the lumen through the co-transporter, and each Na⁺ brings a glucose molecule with it. The energy of the Na⁺ gradient moves glucose against its own gradient.
The co-transporters need Na⁺ to carry glucose in. Absorbing Na⁺ and glucose lowers the water potential of the gut cells, so water follows by osmosis and the body is rehydrated.

Key points

Active transport moves substances against their gradient through specific carriers, using ATP from respiration.
The Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP.
Co-transport uses a Na⁺ gradient to move glucose against its gradient; this is how ORS works.
Oxygen, respiratory poisons, temperature and the number of carriers affect the rate.

Sources and credits

Written by Ganzaa. The account of oral rehydration follows the World Health Organization fact sheet Diarrhoeal disease. No text was copied.
Photo: Lepidium sativum seedlings, Salicyna, CC BY-SA 4.0, via Wikimedia Commons.
Video: Sodium potassium pump, Khan Academy, embedded from YouTube.

Figures 1 and 2, the table, the worked examples and the practice questions are written by Ganzaa. The data in Worked examples 1 and 2 are illustrative.

1Uphill against the gradient

2Features of active transport

3The sodium–potassium pump

4Co-transport: absorbing glucose in the small intestine

5Factors affecting the rate of active transport

6Where active transport matters