Active transport and filtration

Active transport uses energy from ATP

Active transport moves a substance across a membrane against its concentration gradient, from where it is less concentrated to where it is more concentrated. Because this cannot happen by diffusion, the cell supplies energy, usually by splitting ATP into ADP and phosphate. The membrane protein that does the work is called a pump. Pumps are specific: each one accepts particular substances, which is why a cell can accumulate potassium while keeping sodium low. There are two broad forms. Primary active transport uses ATP directly to drive the pump. Secondary active transport uses the gradient of one substance, often sodium, to drag another substance along, so ATP is spent indirectly on building the first gradient. Both forms let the body store energy in gradients and use it later.

For example: The lining of the intestine absorbs glucose even when its concentration inside the cell is already higher than outside, because active transport moves it against the gradient.

The sodium–potassium pump

The sodium–potassium pump is the most important active transporter in the body. For each ATP it uses, it moves three sodium ions out of the cell and two potassium ions into the cell. This does two things. First, it keeps sodium high outside and potassium high inside, which is the basis of the electrical signals of nerve and muscle cells. Second, because it exports three positive charges for every two it imports, it removes net positive charge and helps keep the inside of the cell electrically negative. The pump also helps control cell volume: by keeping ions outside, it keeps water from entering by osmosis. Because it consumes a large share of the body’s resting energy, any condition that reduces ATP production quickly affects it, and the cell begins to swell as sodium and water accumulate inside.

For example: In a cell deprived of oxygen, ATP falls, the sodium–potassium pump slows, sodium and water enter, and the cell swells. This is one way ischaemic tissue is injured.

Common misconception: The pump moves sodium and potassium in opposite directions at the same time; it is not two separate one-way channels.

Secondary active transport

Secondary active transport does not split ATP itself. Instead, it uses a gradient that another pump has already built. The commonest example uses sodium. Because the sodium–potassium pump keeps sodium high outside the cell, sodium tends to flow back in. A cotransporter protein harnesses that inward flow to carry a second substance with it, often against the second substance’s own gradient. In the intestine and kidney, sodium and glucose enter together this way, which is how glucose can be absorbed even when its concentration inside is higher than outside. When both substances move in the same direction the protein is a symporter; when they move in opposite directions it is an antiporter. The energy still comes from ATP, but indirectly, through the sodium gradient.

For example: Oral rehydration solutions contain both salt and glucose because sodium and glucose are absorbed together; the glucose helps the sodium, and the sodium helps the water.

Filtration is pressure-driven

Filtration is the movement of water and small dissolved particles through a membrane or a porous barrier because of a pressure difference. The driving force is pressure, not the concentration gradient of the filtered substance, which is what makes filtration different from diffusion and osmosis. In capillaries, blood pressure pushes water and small solutes out through the vessel wall into the tissue fluid, while larger plasma proteins are held back; this is how tissue fluid is formed. In the kidney, pressure in the glomerulus pushes water, ions and small molecules out of the blood into the first part of the nephron, while cells and large proteins remain in the blood. The rate of filtration depends on the pressure difference and on how permeable the barrier is. If pressure falls, filtration falls; if the barrier becomes leaky, substances that should be retained appear in the filtrate.

For example: In dehydration, blood pressure and blood volume fall, so capillary and kidney filtration fall; less urine is produced and tissue fluid is reduced.

Comparing passive, active and filtration

Three questions separate these processes. Does the substance move with or against its gradient? With, for passive; against, for active. Is energy spent? No for passive and filtration; yes, directly or indirectly, for active transport. What provides the driving force? A concentration gradient for diffusion, osmosis and facilitated diffusion; a pressure difference for filtration. Filtration also differs in what it filters: it separates substances mainly by size, allowing water and small solutes through while holding back cells and large proteins. Clinically, the distinction matters. A drug that inhibits the sodium–potassium pump, such as digoxin, changes the ion gradients of heart muscle and thereby its electrical behaviour. A fall in blood pressure reduces filtration and urine output. Knowing which mechanism is at work tells you what will change the patient’s condition and what to monitor.

For example: Digoxin slows the sodium–potassium pump in heart muscle, raising intracellular sodium and, indirectly, calcium, which strengthens contraction.