Active transport in plants and animals
Cell biology • Transport in cells
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Definition and energy requirement
Active transport is the net movement of particles from a low concentration to a high concentration across a membrane, against a concentration gradient. Movement against the gradient cannot occur passively and therefore requires metabolic energy. The energy originates from respiration and is supplied in a usable form (ATP) to membrane carrier proteins or pumps. Mitochondria supply ATP locally so membrane transport proteins operate continuously where active transport is required . Limiting factors include the rate of respiration (ATP availability), the number and efficiency of transport proteins in the membrane, and temperature because enzyme-driven processes slow at low temperatures. Poisoning of respiration or damage to mitochondria reduces ATP supply and therefore reduces active transport capacity.
Mineral ion uptake by root hair cells
Soil water often contains mineral ions at very low concentrations. Root hair cells maintain higher internal mineral concentrations than the surrounding soil. Carrier proteins in the root hair cell membrane bind specific mineral ions and use ATP energy from respiration to move those ions into the cell against the concentration gradient. Accumulation of mineral ions inside root cells increases solute concentration in the cell sap, which promotes water uptake by osmosis and supports plant hydration and growth . Root hair cells provide a large surface area and many mitochondria to supply ATP. Root hairs maintain concentration gradients by transporting ions into the plant and then moving them into the xylem for distribution throughout the plant.
Sugar absorption in the small intestine
The lining of the small intestine absorbs sugars into the blood. When sugar concentration in the gut lumen is lower than in the blood, active transport moves sugars into epithelial cells and then into the bloodstream. Intestinal carrier proteins use ATP to transport sugar molecules from the gut into epithelial cells and often couple transport to ion gradients maintained by ATP-driven pumps. Villi and microvilli increase surface area and provide rich blood supply so absorbed sugars enter the circulation rapidly for distribution to tissues and storage organs . Limiting factors include ATP availability, integrity and number of carrier proteins, and blood flow: reduced blood flow raises blood concentration locally and slows net uptake.
Comparisons with diffusion and osmosis; adaptations
Diffusion and osmosis are passive processes that move substances down concentration gradients and do not require metabolic energy. Active transport differs by moving substances up a concentration gradient using energy from respiration. Cells that perform active transport show adaptations: many mitochondria for ATP supply, specific membrane carrier proteins or pumps, thin exchange surfaces and large surface areas (root hairs, villi). These adaptations increase the rate and capacity of active transport where needed .
Key notes
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