RLO: Concentration Gradients


Active v Passive Transport

By default gradients equalise, moving from greater to lesser - this is often called passive transport because no energy is required by the process. To go in the reverse direction requires energy input, as any cyclist knows when climbing up a hill - this is known as active transport. In the case of cells in the body, substances are often transported up a chemical gradient from lesser to greater concentrations, and this takes energy that's supplied by a source within the cell known as ATP. Examples of active transport in the body are the transport of chemical ions (Na+, K+, Ca++ etc) in the kidney tubules, and the transport of glucose and sodium across the gut epithelium, which is fundamental to nutrient ingestion into the body