Question

1.

a) Consider a symporter protein that functions to bring in a specific amino acid into the cell against its gradient. Which of the following ion gradients could this protein use as an energy source?

b) Consider an antiporter protein that functions to transport a molecule out of the cell against its gradient. Which of the following ion gradients could this protein use as an energy source?

c) Consider a symporter protein that functions to transport a molecule out of the cell against its gradient. Which of the following ion gradients could this protein use as an energy source?

Answer choices are: K+, Na+, H+, Ca2+, or Cl- (there can be multiple answers for each part)

Answer 1

Symporter and Antiporter proteins are types of Cotransporters. Cotransporters can be defined as membrane transport proteins (transporters) that allow favorable movement of one molecule with its concentration gradient as well as the unfavorable movement of another molecule against its concentration gradient.

a) A symporter protein is a type of membrane transport protein involved in the transport of various molecules across the cell membrane. The transporter is called a symporter because the molecules will travel in the same direction in relation to each other. Example - Marine invertebrates like arthropods, jellyfish etc use symporters to transport amino acids and sugars present in the seawater in their cells, in the presence of extracellular sodium(against the concentration gradient). This symporter works with the help of NA⁺/K⁺-ATPase pump or it uses Na⁺ and K⁺ ions as its energy source.

b) The antiporter protein is a type of cotransport protein, that moves the ions and molecule in opposite directions or one of the ions will move from the exoplasmic space into the cytoplasmic space and vice versa. Example - The sodium-calcium exchanger removes the excess calcium from the cytoplasmic space into the exoplasmic space against its concentration gradient, while the sodium ions are transported into the cytoplasmic space, down its concentration gradient aided by the active transport of sodium out of the cell by the NA⁺/K⁺ pump. It is a cation antiporter which transports 3 Na⁺ ions for 1 Ca²⁺ ion. An anion antiporter protein like the Band 3 or Anion Exchanger 1 is anion transport protein found in mammalian erythrocytes and moves chloride ion and bicarbonate ion(1:1) across the plasma membrane. This antiporter removes the carbon dioxide waste which is converted to bicarbonate inside the erythrocyte.

c) Example - Sodium-dependent Glucose cotransporters(SGLT1) are found in the intestinal epithelial cells that transport Na⁺ions and glucose across the luminal membrane of the epithelial cells and they are absorbed by the blood. This is the basis of oral rehydration therapy. Without this specific symporter, individual sodium (Na⁺) channels and glucose uniporters couldn't be transported against the concentration gradient into the bloodstream, transferring sugar across the epithelial cells of renal proximal tubules and of the small intestine. The Na-K-Cl cotransporter (NKCC) is a symporter protein, part of the active transport of sodium, potassium, and chloride ions into cells. NKCC1 is widely distributed throughout the human body; NKCC2 is found specifically in the kidney, where it serves to extract sodium, potassium, and chloride from the urine so that they can be reabsorbed into the blood against the concentration gradient. Urine has a high concentration of Sodium in the thick ascending limb of the loop of Henle, but this symporter works against the concentration gradient to transport sodium back into the bloodstream.