1. Passive Transport: This type of transport requires no energy from the cell and relies on the concentration gradient or electrochemical gradient of the molecule.
* Simple Diffusion: Molecules move from an area of high concentration to an area of low concentration across the cell membrane. This movement is driven by the random motion of molecules. Examples include oxygen, carbon dioxide, and some lipids.
* Facilitated Diffusion: This process uses membrane proteins to help move molecules across the cell membrane, but still down the concentration gradient. These proteins act as carriers or channels that bind to specific molecules and facilitate their passage. Examples include glucose, amino acids, and some ions.
* Osmosis: The movement of water across a semi-permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration). This movement is driven by the difference in water potential across the membrane.
2. Active Transport: This type of transport requires energy from the cell to move molecules against their concentration gradient (from low to high concentration).
* Primary Active Transport: Uses energy directly from ATP hydrolysis to move molecules across the membrane. Examples include the sodium-potassium pump which maintains the electrochemical gradient across the cell membrane.
* Secondary Active Transport: Uses the energy stored in the electrochemical gradient of one molecule to move another molecule across the membrane. This usually involves coupling the movement of two molecules, one moving down its concentration gradient and the other moving against its gradient. Examples include glucose uptake in the intestines.
The specific process used for a particular small molecule depends on factors like the size and charge of the molecule, the concentration gradient, and the presence of specific transport proteins in the cell membrane.
