The survival of any cell depends on its ability to precisely control the flow of substances across its boundary. This barrier is the plasma membrane, a dynamic structure composed primarily of a phospholipid bilayer that separates the cell’s internal environment from the external surroundings. The membrane’s architecture gives it the property of selective permeability, allowing certain molecules to pass while restricting others. Regulating this molecular traffic is fundamental for maintaining cellular homeostasis, a stable internal state. Cell transport manages the constant movement of nutrients, waste products, and signaling molecules, ensuring the cell’s chemical composition remains within life-sustaining limits.
Movement Through the Lipid Bilayer
The simplest way for a substance to cross the plasma membrane is through simple diffusion, relying on the intrinsic kinetic energy of molecules. This mechanism allows small, uncharged, and nonpolar molecules, such as oxygen (\(\text{O}_2\)) and carbon dioxide (\(\text{CO}_2\)), to slip directly through the hydrophobic core of the phospholipid bilayer. Movement is always down the concentration gradient, proceeding from higher concentration to lower concentration until equilibrium is achieved.
This movement requires no input of metabolic energy, such as adenosine triphosphate (ATP), and does not utilize specialized membrane proteins. The transport rate is proportional to the steepness of the concentration gradient and the molecule’s solubility within the lipid layer.
A specialized type of simple diffusion is osmosis, the net movement of water molecules across a selectively permeable membrane. Water moves toward the area where the solute concentration is higher, effectively diluting it.
Osmotic movement is described by comparing the tonicity of the external solution to the cytoplasm inside the cell. An isotonic solution has an equal solute concentration, resulting in no net water movement and maintaining cell shape. In a hypertonic solution, the external solute concentration is higher, causing water to exit the cell and resulting in shrinkage. Conversely, a hypotonic solution has a lower external solute concentration, causing water to rush into the cell and potentially leading to swelling or lysis.
Protein-Assisted Passive Movement
Molecules that are too large, polar, or electrically charged cannot easily cross the membrane by simple diffusion, even when moving down their concentration gradient. These substances rely on facilitated diffusion, which employs specific transmembrane proteins to assist their passage. This movement is passive because it requires no energy expenditure and follows the concentration gradient. The assisting proteins are categorized as channel proteins and carrier proteins.
Channel proteins form hydrophilic pores that span the membrane, providing a direct, regulated conduit for specific ions or water molecules to pass rapidly. Ion channels are highly selective, allowing only one type of ion, such as potassium or chloride, to cross when open. Aquaporins are channel proteins that dramatically increase the rate of water transport beyond simple osmosis.
Carrier proteins do not form a continuous channel but instead bind the specific molecule, such as glucose or an amino acid, on one side of the membrane. This binding induces a conformational change in the protein’s structure, physically shuttling the molecule across the membrane and releasing it on the other side. Transport mediated by carrier proteins is generally slower than transport through channel proteins because the conformational change takes time.
A distinguishing feature of facilitated diffusion is that it exhibits saturation kinetics. As the concentration of the transported substance increases, the rate of transport increases only up to a maximum velocity. This maximum is reached when all available channel and carrier proteins are occupied or working at capacity, meaning the transport system has become saturated. This behavior differentiates facilitated diffusion from simple diffusion, where the transport rate increases linearly with the concentration gradient.
Energy-Driven Movement Against the Gradient
Cellular processes often require substances to be moved against the concentration gradient, which is an uphill movement from a region of low concentration to high concentration. This process is known as active transport and requires a direct input of metabolic energy, typically supplied by ATP hydrolysis. Active transport is necessary for maintaining the precise concentration gradients required for nerve signaling, muscle contraction, and nutrient absorption.
Primary Active Transport
Primary active transport directly uses the energy from ATP hydrolysis to power the conformational change of a transport protein. The Sodium-Potassium Pump (\(\text{Na}^+/\text{K}^+\)-ATPase) is the most studied example, found in the plasma membrane of nearly all animal cells. In a single cycle, the pump binds three sodium ions (\(\text{Na}^+\)) from the inside and two potassium ions (\(\text{K}^+\)) from the outside.
Binding and ATP hydrolysis cause a phosphorylation event that changes the protein’s shape, releasing the three \(\text{Na}^+\) ions outside the cell. The new conformation binds the two \(\text{K}^+\) ions, and subsequent dephosphorylation returns the pump to its original shape, releasing the \(\text{K}^+\) ions inside. This 3-to-2 ion exchange is electrogenic, moving a net positive charge out of the cell and contributing to the electrical potential difference across the membrane.
Secondary Active Transport
The electrochemical gradient established by primary active transport provides the energy for secondary active transport, also known as cotransport. This mechanism does not directly use ATP but instead harnesses the potential energy stored in the concentration gradient of one substance, such as \(\text{Na}^+\), to move a second substance against its own gradient. A carrier protein couples the downhill movement of the first substance to the uphill movement of the second.
Secondary active transport occurs via two mechanisms: symport and antiport. In symport, both the driving ion (e.g., \(\text{Na}^+\)) and the co-transported solute (e.g., glucose) move across the membrane in the same direction. For example, sodium-glucose linked transporters drag a glucose molecule into the cell against its gradient while \(\text{Na}^+\) moves down its gradient. Antiport moves the driving ion and the co-transported solute in opposite directions, such as exchanging \(\text{Na}^+\) moving inward for a calcium ion (\(\text{Ca}^{2+}\)) moving outward.
Bulk Movement Across the Cell
For very large molecules, substantial quantities of fluid, or entire particles, the cell relies on vesicular transport. This process involves packaging substances within membrane-bound sacs called vesicles. Since it requires significant cellular energy to deform and fuse the plasma membrane, vesicular transport is a form of active transport. It allows for the uptake and release of material that cannot pass through individual transport proteins.
Endocytosis
The process of bringing material into the cell is termed endocytosis, where the plasma membrane invaginates, surrounds the substance, and pinches off to form a vesicle inside the cytoplasm. Endocytosis has three main subtypes:
- Phagocytosis, or “cell eating,” involves the engulfment of large solid particles, such as bacteria or cellular debris, often performed by specialized immune cells.
- Pinocytosis, or “cell drinking,” is a non-specific process where the cell takes up small droplets of extracellular fluid and any dissolved solutes.
- Receptor-mediated endocytosis uses specific receptor proteins on the cell surface to bind target molecules. The receptors cluster, initiating vesicle formation and ensuring only the desired substance, such as cholesterol or certain hormones, is internalized.
Exocytosis
The reverse process, exocytosis, is the mechanism by which the cell expels material into the extracellular space. A membrane-bound vesicle containing substances, such as proteins, neurotransmitters, or waste products, moves to the plasma membrane and fuses with it. This fusion opens the vesicle and releases its contents outside the cell, simultaneously adding the vesicle’s membrane material to the cell surface.