Passive transport does not require energy input from the cell. It relies on the concentration gradient of a substance and occurs spontaneously down the gradient. Substances move from areas of higher concentration to areas of lower concentration. There are three types of passive transport mechanisms:
* Simple diffusion: Small, nonpolar molecules such as oxygen, carbon dioxide, and lipids can move directly across the cell membrane. The rate of diffusion is proportional to the concentration gradient and the surface area of the membrane.
* Facilitated diffusion: Some substances, such as glucose and amino acids, are too polar or too large to diffuse through the cell membrane on their own. They require the help of transport proteins to move across the membrane. Transport proteins bind to the substance and change their shape, allowing them to move through the membrane.
* Osmosis: Osmosis is the diffusion of water across a semipermeable membrane. Water moves from areas of lower osmotic pressure (higher water concentration) to areas of higher osmotic pressure (lower water concentration). Osmosis is important for maintaining the cell's water balance.
2. Active Transport
Active transport requires energy input from the cell. It occurs against the concentration gradient, from areas of lower concentration to areas of higher concentration. Substances are moved across the cell membrane by transport proteins that use ATP as an energy source. There are two types of active transport mechanisms:
* Primary active transport: Primary active transport proteins use ATP directly to move substances across the membrane. An example of primary active transport is the sodium-potassium pump, which pumps sodium ions out of the cell and potassium ions into the cell.
* Secondary active transport: Secondary active transport proteins use the energy stored in an ion gradient to move other substances across the membrane. For example, the glucose-sodium symport protein uses the sodium gradient to move glucose into the cell.
3. Endocytosis
Endocytosis is the process by which cells take in materials from the outside environment. There are three types of endocytosis:
* Phagocytosis: Phagocytosis is the process by which cells engulf solid particles. The particle is surrounded by the cell membrane and then drawn into the cell by pseudopodia. Once inside the cell, the particle is enclosed in a membrane-bound vesicle called a phagosome. The phagosome then fuses with a lysosome, which contains digestive enzymes that break down the particle.
* Pinocytosis: Pinocytosis is the process by which cells take in fluid and dissolved substances. The cell membrane invaginates and forms a small vesicle called a pinosome. The pinosome then detaches from the membrane and moves into the cell. The contents of the pinosome are released into the cytoplasm.
* Receptor-mediated endocytosis: Receptor-mediated endocytosis is the process by which cells take in specific molecules that bind to receptors on the cell membrane. The molecule-receptor complex is then internalized into a clathrin-coated pit. The pit then pinches off and forms a vesicle. The vesicle then fuses with a lysosome, which breaks down the molecule.
