The cell is surrounded by a plasma membrane responsible for the selective permeability of the cell, which allows only tiny materials into the cell. Inside the cell, it is portioned into organelles with different structures and functions. Cell transport is how the materials enter into and out of the cell. The cell membrane regulates cell transport and is made of a lipid bilayer. The protein-free lipid bilayer allows molecules to diffuse down their concentration gradient.
Cell transport is divided into passive and active transport. In passive transport, molecules require no energy input to cross the cell membrane. However, for active transport to occur, energy is required. Passive transport is where the specified molecules diffuse across the lipid bilayer on their membrane proteins down their concentration gradient. The molecules diffuse naturally without energy input. Passive transport is classified into; Diffusion, where molecules move from higher to lower concentrations regions until the concentration of molecules on both sides is the same. Osmosis is a special type of diffusion that specifies how molecules move across the cell membrane (Tomkins et al., 2021). The movement of the water particles is affected by the solute concentration. When the cell is placed in a hypertonic solution, the water molecules will flow from the cell, across the cell membrane and into the hypertonic solution leading to the shrinking of the cell. When the cell is placed in a hypotonic solution, the molecules will flow from the solution into the cell leading to the bursting of an animal cell, and a plant cell becomes firm. Lastly, there will be no change when the cell is placed in an isotonic solution because they have the same concentration. The last type of passive transport is facilitated diffusion, where materials move across the cell membrane via transport proteins found in the plasma membrane. The transport proteins help in the movement of large molecules because they cannot cross the semi-permeable pores of the cell membrane.
The second type of cell transport is active transport. It is the movement of molecules across the cell membrane against the concentration gradient. The molecules move from a region of low to a high concentration with the help of energy in terms of ATP. Active transport is divided into two; primary and secondary active transport. The primary active transport includes the sodium-potassium pump, which maintains the cell potential by pumping out sodium ions and pumping in potassium ions. This mode of transport involves the direct incorporation of ATP. However, there is no direct energy incorporation in the secondary active transport. It uses electrochemical potential differences created by pumping ions out of the cell. Secondary active transport includes antiport and symport transport. In antiport transport, two ions or molecules are pumped in opposite directions, while in symport transport, two ions are pumped in the same direction. Larger molecules are transported through active transport by vesicles. This includes endocytosis and exocytosis. Endocytosis is when the cell ingests large substances into the cell by engulfing the substance using a membrane to form a vesicle and then ingesting it into the cell. When the substance ingested is solid, it is called phagocytosis, and when it is a fluid, it is called pinocytosis. In exocytosis, substances are moved from the cell to the cell’s exterior. The vesicle moves from the cytoplasm and fuses with the plasma membrane, allowing substances to be removed from the cell.
Reference
Tomkins, M., Hughes, A., & Morris, R. (2021). An update on passive transport in and out of plant cells. Plant Physiology, 187(4), 1973-1984. https://doi.org/10.1093/plphys/kiab406
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