Active transport, cotransport and absorption of glucose

Active transport is the movement of substances against their concentration gradient (i.e. from an area of low concentration to an area of higher concentration) and thus requires energy. This process must occur across a membrane because it involves a carrier protein to transport the particles. For this movement to take place, a molecule of ATP must bind to the carrier protein, providing the necessary energy for the transport. When a particle of the substance being transported binds to an activated carrier protein, it causes a change in the shape of the protein. This change causes the ATP to be broken down into ADP and Pi (inorganic phosphate), which are then released. The carrier protein then moves the particle across the membrane. When another ATP molecule binds, the carrier protein returns to its original shape, ready to transport another particle.

Co-transport

Co-transport involves the simultaneous movement of two different particles through the same carrier protein. Symporters move one particle down its concentration gradient and another in the same direction but against its concentration gradient. Antiporters, on the other hand, move particles in opposite directions. An example of a cotransporter is found in the epithelial cells lining the small intestine, where it simultaneously moves a sodium ion (down the concentration gradient) and a glucose or amino acid molecule (against the concentration gradient).

Absorption of glucose and amino acids

Absorption of glucose and amino acids across the epithelial cells lining the ileum (small intestine) ensures that absorption continues even when the concentration is lower in the lumen than in the cell cytoplasm. It occurs in three stages:

1. A Sodium/Potassium Pump: on the capillary side of the epithelial cell transports sodium ions out of the epithelial cell and potassium ions into the cell. This process requires energy from ATP, as both ions are moving against their concentration gradients. This movement results in a lower concentration of sodium ions in the cell than in the small intestine lumen.

2. A Glucose/Sodium Co-Transporter: protein on the lumen side of the epithelial cell brings a molecule of glucose and a sodium ion into the epithelial cell from the gut lumen. Although glucose is moving against its concentration gradient, the sodium ion moves down the concentration gradient, facilitating the 'coupled' transport of glucose without the need for energy from ATP.

3. A Glucose Carrier: allows the facilitated diffusion of glucose, now at a relatively high concentration inside the epithelial cell, into the blood plasma on the capillary side of the epithelial cell.

The absorption of glucose in the ileum.

Adaptations of cells for moving substances

Cells have developed adaptations to increase the efficiency of substance movement. For instance, the epithelial cells lining the small intestine contain thousands of transporter proteins in their cell membranes, providing numerous sites for sodium ions and glucose to move through. Additionally, these cells have hundreds of finger-like projections from the cell membrane, known as microvilli, and the tissue is folded into larger projections called villi, both of which increase the surface area available for transport. The villi also contain capillaries, reducing the distance for diffusion into the blood and maintaining a steep concentration gradient. The epithelial cells have many mitochondria, providing the energy required for active transport, and contain numerous ribosomes and Golgi apparatus to synthesise the necessary channel and carrier proteins.

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