4.3 Active Transport and Bulk Transport

Active Transport

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ATP-Driven Pumps

Active transport moves molecules against their concentration gradient, from low to high concentration. Unlike passive transport, this process requires energy because it's working against the natural flow of diffusion.

The energy comes from ATP hydrolysis. When ATP is broken down, the released energy causes the transport protein to change shape (a conformational change). This shape change alternately exposes binding sites on opposite sides of the membrane, physically shuttling the molecule across.

• These pumps are essential for maintaining ion gradients and membrane potential
• Cells that rely heavily on active transport include neurons (for nerve impulses) and muscle cells (for contraction)

Sodium-Potassium Pump

The sodium-potassium pump ($\text{Na}^+/\text{K}^+$-ATPase) is the most well-known example of active transport. It's an antiporter, meaning it moves two different ions in opposite directions.

For every one ATP molecule hydrolyzed, the pump moves:

• 3 $\text{Na}^+$ ions out of the cell
• 2 $\text{K}^+$ ions into the cell

Because the pump moves unequal charges (3 positive ions out, 2 positive ions in), it's electrogenic, meaning it contributes directly to the electrical charge difference across the membrane. This creates and maintains the resting membrane potential, which is critical for nerve signaling and muscle contraction.

The pump also helps regulate cell volume. By controlling ion concentrations on each side of the membrane, it manages osmotic balance and prevents cells from swelling or shrinking. Red blood cells depend on this to maintain their shape.

Cotransport

Cotransport (also called secondary active transport) doesn't use ATP directly. Instead, it harnesses the concentration gradient that was already built by a primary active pump like the sodium-potassium pump.

Two solutes cross the membrane at the same time using the same carrier protein. One solute moves down its gradient (releasing energy), and that energy drives the other solute against its gradient.

There are two types:

• Symport: Both solutes move in the same direction. In intestinal epithelial cells, $\text{Na}^+$ flowing down its gradient pulls glucose into the cell alongside it.
• Antiport: The solutes move in opposite directions. For example, $\text{H}^+$ and $\text{Ca}^{2+}$ exchange across mitochondrial membranes.

The key idea is that the cell spends ATP once (to build the ion gradient) and then gets multiple transport events out of it.

Endocytosis

Endocytosis is a form of bulk transport where the cell membrane folds inward to bring large molecules or particles into the cell inside a vesicle. This is necessary for substances too large to pass through membrane proteins.

Phagocytosis

Phagocytosis ("cell eating") is how cells engulf large solid particles like bacteria or dead cells.

1. The cell extends pseudopodia (temporary projections of the membrane) around the target particle.
2. The pseudopodia wrap around and fuse, forming a phagosome (a membrane-bound compartment containing the particle).
3. The phagosome fuses with a lysosome, and digestive enzymes break down the contents.

Specialized immune cells carry out most phagocytosis. Macrophages and neutrophils use this process to destroy invading pathogens and clear cellular debris like apoptotic (dying) cells.

Pinocytosis

Pinocytosis ("cell drinking") is the uptake of extracellular fluid and whatever solutes happen to be dissolved in it. Unlike phagocytosis, pinocytosis is non-specific: the cell isn't targeting anything in particular.

• Small portions of the plasma membrane pinch inward to form tiny vesicles filled with fluid
• This happens continuously in most cell types as a way to sample the surrounding environment
• Cells pick up dissolved nutrients like amino acids and signaling molecules like growth factors through this process

Pinocytosis can be further classified by vesicle size: macropinocytosis produces larger vesicles, while micropinocytosis produces smaller ones.

Receptor-Mediated Endocytosis

Receptor-mediated endocytosis is the most selective form of endocytosis. It allows cells to take in specific molecules based on receptor-ligand binding.

1. Ligands (target molecules) bind to specific receptors on the cell surface.
2. The receptor-ligand complexes cluster together in a region of the membrane called a coated pit, lined on its cytoplasmic side by the protein clathrin.
3. Clathrin forms a lattice-like cage that helps shape the pit into a vesicle as the membrane pinches inward.
4. The coated vesicle pinches off from the membrane, then sheds its clathrin coat to become an endosome.
5. The endosome delivers its contents for processing inside the cell.

This mechanism is how cells take up LDL (low-density lipoprotein, which carries cholesterol) and transferrin (which carries iron). A defect in LDL receptor-mediated endocytosis causes familial hypercholesterolemia, where cholesterol builds up in the blood.

Exocytosis

Exocytosis is the reverse of endocytosis. Instead of bringing material in, the cell releases material out by fusing a vesicle with the plasma membrane.

1. A secretory vesicle containing the molecules to be released moves toward the plasma membrane.
2. A rise in intracellular $\text{Ca}^{2+}$ concentration triggers the process.
3. SNARE proteins on the vesicle and the plasma membrane interact, pulling the two membranes together until they fuse.
4. The vesicle's contents are released into the extracellular space.

Exocytosis serves several functions:

• Cell signaling: Neurons release neurotransmitters into the synapse via exocytosis
• Hormone secretion: Pancreatic beta cells secrete insulin this way
• Structural support: Fibroblasts secrete collagen and other extracellular matrix components
• Membrane remodeling: The vesicle membrane becomes part of the plasma membrane when it fuses, which is how cells add new membrane material

Together, endocytosis and exocytosis allow cells to move bulk quantities of material across the membrane and continuously recycle membrane components.