3 Cellular Transport

Passive movement across membranes

Cell membranes are selectively permeable: some substances cross the lipid bilayer directly, while others need proteins or membrane-enclosed vesicles. Which route a substance takes depends on its properties and on concentration and electrical gradients.

Passive transport moves a substance downhill along its concentration or electrochemical gradient without the cell directly spending energy to move it.

• Simple diffusion allows small nonpolar molecules, such as oxygen and carbon dioxide, to pass through the lipid bilayer from higher to lower concentration.

• Facilitated diffusion uses channels or carrier proteins to help specific ions or polar molecules move downhill. For example, glucose can enter some cells through glucose carriers. The protein helps the movement, but the process does not itself require ATP.

• Osmosis is the movement of water across a selectively permeable membrane toward the side with the higher effective concentration of solutes that cannot cross. Water can pass through the bilayer or through aquaporin channels.

Transport proteins are selective. Carriers bind their substrates and change shape, and they may become saturated when all available carriers are occupied. Channels provide a passage for particular ions and can permit rapid movement.

Takeaway: Passive transport moves substances downhill; a membrane protein can assist this movement without making it active transport.

Energy-powered transport

Active transport moves substances uphill, against an electrochemical gradient, by coupling their movement to an energy source. It differs from facilitated diffusion even though both processes can use membrane proteins: facilitated diffusion is downhill, whereas active transport requires energy to move a substance uphill.

• Primary active transport uses energy directly, commonly from ATP hydrolysis. The sodium–potassium pump uses one ATP to move three Na⁺ ions out of the cell and two K⁺ ions in. This helps maintain ion gradients and contributes to the cell’s electrical conditions.

• Secondary active transport uses energy stored in an ion gradient, usually one established by primary active transport. In symport, two substances move in the same direction; in antiport, they move in opposite directions. A sodium gradient, for example, can power glucose uptake even when glucose moves against its own gradient.

Takeaway: To identify active transport, look for uphill movement coupled to an energy source, either directly or through an ion gradient.

Ion gradients and membrane potential

The membrane potential is the voltage difference across a membrane, usually described in cells as the interior’s electrical potential relative to the exterior. Unequal ion distributions and selective permeability to those ions create this difference. Potassium leak channels and ion pumps help establish and maintain it in many cells.

An ion’s electrochemical gradient combines two influences: its concentration gradient and the electrical attraction or repulsion across the membrane. The net effect determines the direction of passive ion movement. As a result, an ion can move against its concentration gradient if the electrical force makes the overall movement downhill. When open, ion channels allow ions to move passively; the sodium–potassium pump helps sustain the gradients that underlie membrane potential.

Takeaway: An ion’s concentration gradient alone does not determine its movement; the electrical component also matters.

Bulk movement by vesicles

Vesicular transport moves large particles and many macromolecules across the plasma membrane. The membrane bends to form a vesicle, or a vesicle fuses with the membrane; the cargo does not pass directly through the lipid bilayer. These processes require cellular energy and redistribute membrane material.

Endocytosis brings material into the cell in vesicles:

• Phagocytosis engulfs large particles, such as cellular debris or microbes.

• Pinocytosis takes up extracellular fluid and its dissolved substances.

• Receptor-mediated endocytosis selectively internalizes cargo that binds particular surface receptors.

Exocytosis releases vesicle contents outside the cell when a vesicle fuses with the plasma membrane. It can secrete substances such as hormones or neurotransmitters and deliver membrane proteins and lipids to the cell surface. Some exocytosis is continuous; regulated exocytosis occurs in response to a signal. Together, endocytosis and exocytosis help balance the cell’s surface membrane.

Takeaway: Vesicular transport handles bulk cargo through membrane budding and fusion, rather than movement through the lipid bilayer.