B. Biochemistry · BiochemistryActive Transport, Endocytosis and Exocytosis
B3.6 B2.1 B3.1 A1.12About 70 minutes
Learning goals- I can distinguish active transport from passive transport and explain why active transport needs energy.
- I can describe how the sodium-potassium pump works and why it matters.
- I can explain secondary active transport (cotransport) using the sodium-glucose symporter.
- I can describe phagocytosis, pinocytosis, receptor-mediated endocytosis and exocytosis, and give examples of each.
Words to know
active transport
Movement of a substance across a membrane against its concentration gradient (from low to high), using energy, usually from ATP, and a carrier protein (pump).
sodium-potassium pump (Na⁺/K⁺-ATPase)
A carrier protein that uses one ATP to pump 3 Na⁺ out of the cell and 2 K⁺ in.
electrochemical gradient
The combined effect of a concentration difference and an electrical charge difference across a membrane.
secondary active transport (cotransport)
Using the energy stored in one substance's gradient (often Na⁺ or H⁺), made by a pump, to move another substance against its gradient.
symport / antiport
Transport of two substances in the same direction (symport) or in opposite directions (antiport) by one protein.
endocytosis
Taking material into the cell by folding the cell membrane around it to form a vesicle.
phagocytosis / pinocytosis
Endocytosis of large particles such as bacteria ("cell eating") or of droplets of fluid ("cell drinking").
receptor-mediated endocytosis
Endocytosis in which specific molecules bind receptor proteins in a coated pit, which then pinches off as a vesicle.
exocytosis
Release of materials from a vesicle when it fuses with the cell membrane.
Swimming upstream
The fluid inside your nerve cells has roughly 30 times more potassium and over 10 times less sodium than the fluid outside them. Left alone, diffusion would even those out within hours. It doesn't happen, because every one of your cells is constantly pumping ions against their gradients. That costs energy, but without it, your nerves couldn't fire and your cells would swell with water.
This lesson covers transport that needs energy: active transport, and the movement of large particles in vesicles (endocytosis and exocytosis).
Active transport
In active transport, a carrier protein (often called a pump) moves a substance against its concentration gradient, from lower to higher concentration. That requires energy, usually from the hydrolysis of ATP (a reaction from the Four Reactions lesson: ATP + H₂O → ADP + Pᵢ).
A quick rule: passive = downhill, no ATP; active = uphill, ATP.
The sodium-potassium pump
The best-studied pump is the Na⁺/K⁺-ATPase, found in the membranes of animal cells. In each cycle:
- 3 Na⁺ ions from the cytoplasm bind to the pump.
- ATP is hydrolyzed and a phosphate group attaches to the pump (phosphorylation), which changes its shape. The pump opens to the outside and releases the 3 Na⁺.
- 2 K⁺ ions from outside bind. The phosphate is released, the pump returns to its original shape, and the 2 K⁺ are released inside.
So each ATP moves 3 Na⁺ out and 2 K⁺ in. Because three positive charges leave and only two enter, the pump also makes the inside of the cell more negative, contributing to the membrane potential that nerve and muscle cells use (Homeostasis). The pump also controls cell volume: by keeping Na⁺ out, it stops water from flooding in by osmosis.
Other pumps include proton pumps (H⁺ pumps). In the stomach lining, an H⁺/K⁺ pump makes stomach acid; common heartburn medicines called proton pump inhibitors block it. Plant cells use H⁺ pumps to power the uptake of nutrients from the soil.
Secondary active transport (cotransport)
A gradient is stored energy, like water behind a dam. Cells use the Na⁺ gradient made by the pump to drive other transport. In the lining of your small intestine, a sodium-glucose symporter lets Na⁺ flow down its gradient into the cell, and it can only do so if a glucose molecule comes along. Glucose is dragged in against its gradient. No ATP is used by the symporter itself, but the Na⁺ gradient was made by ATP-using pumps, so this is secondary active transport.
This is the science behind oral rehydration solutions, the sugar-and-salt drinks used to treat dehydration from severe diarrhea: glucose and sodium are absorbed together, and water follows by osmosis.
Transport proteins are classified by direction: uniport (one substance), symport (two substances, same direction) and antiport (two substances, opposite directions; the Na⁺/K⁺ pump is an antiporter).
Bulk transport: endocytosis and exocytosis
Some things are far too big for any channel or carrier: a whole bacterium, a protein, a droplet of fluid. Cells move them in vesicles, using the fluidity of the membrane. Forming, moving and fusing vesicles uses energy (ATP).
Endocytosis (into the cell):
- Phagocytosis ("cell eating"): the membrane sends out extensions that surround a large particle such as a bacterium and form a food vacuole (phagosome). White blood cells such as macrophages and neutrophils do this; then lysosomes fuse with the vacuole and digest the contents (you saw this in Organelles at Work). Amoebas feed the same way.
- Pinocytosis ("cell drinking"): the membrane folds in to take up small droplets of extracellular fluid and whatever is dissolved in it. It is not selective.
- Receptor-mediated endocytosis: specific molecules (ligands) bind receptor proteins clustered in coated pits on the membrane. The pit pinches off as a coated vesicle, bringing in large amounts of one particular substance. Cells take up cholesterol this way, packaged in LDL particles that bind LDL receptors. In the inherited condition familial hypercholesterolemia, LDL receptors are missing or faulty, so LDL builds up in the blood and raises the risk of heart disease. Michael Brown and Joseph Goldstein shared the 1985 Nobel Prize in Physiology or Medicine for discovering this.
Exocytosis (out of the cell): a vesicle moves to the cell membrane and fuses with it, releasing its contents outside. Examples: pancreas cells secreting insulin and digestive enzymes, nerve cells releasing neurotransmitters, gland cells releasing mucus, and plant cells exporting cell-wall materials. Each fusion also adds membrane to the cell surface, while endocytosis removes some, so the two processes balance.
| Process | Direction relative to gradient | Energy (ATP)? | Protein or vesicle? | Example |
|---|
| Simple diffusion | Down | No | Neither | O₂ into cells |
| Facilitated diffusion | Down | No | Channel or carrier | Glucose into red blood cells |
| Osmosis | Water toward higher solute | No | Bilayer or aquaporins | Water into root cells |
| Primary active transport | Up (against) | Yes, directly | Pump | Na⁺/K⁺ pump |
| Secondary active transport | Up for one substance | Indirectly (uses a gradient) | Cotransporter | Na⁺-glucose symport |
| Endocytosis / exocytosis | Not gradient-based | Yes | Vesicles | Phagocytosis; insulin secretion |
Worked exampleExample: Counting with the sodium-potassium pump
A nerve cell membrane contains pumps that each complete 100 cycles per second. (a) For one pump, how many Na⁺ ions are moved out, K⁺ ions moved in and ATP molecules used per second? (b) What is the net movement of positive charge per second, and in which direction? (c) If a poison stopped ATP production, predict two effects on the cell.
- (a) Each cycle moves 3 Na⁺ out and 2 K⁺ in using 1 ATP. Per second: 100 × 3 = 300 Na⁺ out, 100 × 2 = 200 K⁺ in, 100 ATP used.
- (b) Charges: 300 positive charges leave and 200 enter, so a net 100 positive charges move out each second. This makes the inside more negative.
- (c) Without ATP, the pump stops. Na⁺ and K⁺ slowly leak down their gradients, so the gradients run down and the membrane potential is lost (the nerve can't send impulses).
- Also, Na⁺ builds up inside, so water enters by osmosis: the cell swells, and may burst.
(a) 300 Na⁺ out, 200 K⁺ in, 100 ATP; (b) net 100 positive charges out per second; (c) gradients and membrane potential are lost, and the cell swells.
Just for funYour brain uses about 20% of your body's energy at rest, and a large share of that runs sodium-potassium pumps. So even when you're "doing nothing", your neurons are hard at work pumping ions.
Hands-on activityModel the pump with coins and buttons
You need: 2 bowls (label one OUTSIDE and one INSIDE), 40 coins or small paper squares marked Na (Na⁺), 30 buttons or dried beans (K⁺), 10 paper clips (ATP), 6 small paper squares marked G (glucose), and paper for a tally table.
- Start with 20 coins and 15 buttons in each bowl. Put the 10 paper clips beside the INSIDE bowl.
- One pump cycle: use (remove) 1 paper clip; move 3 coins INSIDE → OUTSIDE and 2 buttons OUTSIDE → INSIDE. Record the counts in each bowl.
- Do 4 cycles. Then add a leak: after each cycle, move 1 button INSIDE → OUTSIDE (K⁺ leak channels). Do 2 more cycles with the leak.
- Cotransport: put the 6 G squares: 1 INSIDE, 5 OUTSIDE. A G square may move inside only if a coin moves inside with it (no paper clip needed). Move G squares until you run out of coins outside or G squares outside. Record.
- Answer: How many "ATP" did you use? Which bowl now has more Na⁺? More K⁺? More glucose? Was the glucose moved up or down its gradient?
What to look for: each cycle shifts 3 coins out and 2 buttons in, so after 4 cycles OUTSIDE has 20 + 12 = 32 coins (INSIDE 8) and INSIDE has 15 + 8 = 23 buttons (OUTSIDE 7), using 4 paper clips. After the 2 leaky cycles, INSIDE has 2 coins and 25 buttons. Gradients build with each ATP. In step 4, glucose ends up concentrated inside using the Na⁺ gradient, not a paper clip: that is secondary active transport. The leak shows why the pump must run constantly.
Your turn
1What two features distinguish active transport from facilitated diffusion?
Active transport moves substances against their gradient (low to high) and requires energy (usually ATP). Facilitated diffusion moves them down their gradient with no ATP. (Both use specific membrane proteins.)
2How many Na⁺ and K⁺ ions does the sodium-potassium pump move per ATP, and in which directions?
3 Na⁺ out of the cell and 2 K⁺ in.
3Explain how the sodium-glucose symporter can move glucose against its gradient without using ATP directly.
It couples glucose movement to the movement of Na⁺ down its steep gradient into the cell. The energy stored in the Na⁺ gradient (created by the ATP-powered Na⁺/K⁺ pump) drives glucose uphill. This is secondary active transport.
4Compare phagocytosis and pinocytosis.
Both are endocytosis. Phagocytosis engulfs large solid particles (such as bacteria) into a food vacuole; pinocytosis takes in droplets of fluid with dissolved solutes and is not selective.
5Give two examples of exocytosis in the human body.
Any two: insulin secretion by pancreas cells, release of digestive enzymes, release of neurotransmitters at synapses, mucus secretion.
6Why does receptor-mediated endocytosis bring in specific substances more efficiently than pinocytosis?
Receptor proteins bind the specific molecule and concentrate it in coated pits before the vesicle forms, so each vesicle carries a large amount of that substance instead of whatever happens to be in the fluid.
More practice: warm-up
1What molecule usually supplies the energy for active transport?
2What is "cell eating" called?
3Is exocytosis movement into or out of the cell?
4What does "symport" mean?
Two substances transported in the same direction by one protein.
5Name an organelle that fuses with a food vacuole after phagocytosis.
More practice: on your own
1A cell has a lower concentration of an ion inside than outside, yet the ion keeps moving out. What kind of transport must this be? Explain.
Active transport: the ion is moving from low concentration (inside) to high (outside), against its gradient, which requires energy and a pump.
2Explain why the sodium-potassium pump makes the inside of the cell negative relative to the outside.
Each cycle moves 3 positive charges out but only 2 in, so there is a net loss of 1 positive charge per cycle from the inside. (K⁺ leaking out through channels adds to the effect.)
3Predict what happens to glucose absorption in the small intestine if the Na⁺/K⁺ pumps in the intestinal cells stop working. Explain.
Glucose absorption drops. Without the pumps, Na⁺ is not removed from the cells, so the Na⁺ gradient into the cells disappears. The symporter has no energy source to pull glucose in against its gradient.
4People with familial hypercholesterolemia have high blood LDL. Explain the link to membrane transport.
Their cells have missing or defective LDL receptors, so they can't take up LDL by receptor-mediated endocytosis. LDL stays in the blood instead of being removed by the liver and other cells.
5Is endocytosis passive or active? Explain.
Active (energy-requiring): forming the vesicle, pinching it off and moving it require ATP, even though no gradient is involved.
Challenge
1Proton pump inhibitors reduce stomach acid. Explain how blocking a pump in the stomach lining reduces acidity, and predict the effect on the digestion of protein by pepsin.
The H⁺/K⁺ pump actively moves H⁺ into the stomach against a huge gradient. Blocking it means fewer H⁺ ions are secreted, so the stomach's pH rises (less acidic). Pepsin works best at about pH 2 and is activated from pepsinogen by acid, so protein digestion in the stomach would be slower (though enzymes in the small intestine still digest protein).
2A cell uses 30% of its ATP to run Na⁺/K⁺ pumps. If the cell makes 2.0 × 10⁹ ATP molecules per second, how many Na⁺ ions are pumped out per second?
ATP for pumps = 0.30 × 2.0 × 10⁹ = 6.0 × 10⁸ per second. Each ATP moves 3 Na⁺: 3 × 6.0 × 10⁸ = 1.8 × 10⁹ Na⁺ ions per second.
3Oral rehydration solutions contain both salt and glucose. Using membrane transport, explain why adding glucose helps a dehydrated person absorb water better than salt water alone.
Intestinal cells absorb Na⁺ efficiently through the sodium-glucose symporter, which needs both Na⁺ and glucose. With glucose present, much more Na⁺ (and glucose) is absorbed into the cells and then into the blood. This raises the solute concentration on the body side of the gut lining, so water follows by osmosis. Salt water alone, without glucose, is absorbed less well and can even draw water into the gut.
Quiz
1. Active transport differs from facilitated diffusion because active transport:
2. For each ATP used, the sodium-potassium pump moves:
3. White blood cells engulf bacteria by:
4. The sodium-glucose symporter is an example of:
5. Cells take up cholesterol in LDL particles mainly by:
6. Insulin leaves pancreas cells by:
Big idea: This lesson completes B3.6 (the movement of large particles by endocytosis and exocytosis) and the B2.1 terms active and passive transport. It links to organelles and vesicles (B3.1).
How to help: The coin-and-button model takes about 20 minutes and makes the 3 : 2 ratio stick. Ask your teen to fill in the summary table from memory: for each kind of transport, does it go up or down the gradient, and does it use ATP?
Common difficulties:
- Thinking any transport that uses a protein is active. Facilitated diffusion uses proteins but is passive.
- Reversing the pump: it's 3 Na⁺ OUT, 2 K⁺ IN ("salty outside, like seawater").
- Forgetting that secondary active transport still depends on ATP, indirectly, through the pump that builds the gradient.
- Oral rehydration is a medical treatment; for real illness, follow a health professional's advice.