Test: Transport across a Cell Membrane - MCAT MCQ

The sodium/glucose transporter found in the nephron is a symporter. A symporter is a type of transporter protein that simultaneously moves two different molecules across a cell membrane in the same direction. In this case, the transporter moves both sodium ions and glucose molecules from the tubular fluid in the nephron into the cells of the renal tubules. This process is known as co-transport or secondary active transport, as it utilizes the energy stored in the electrochemical gradient of sodium ions to drive the uphill movement of glucose against its concentration gradient. The symporter binds both sodium and glucose on one side of the membrane and undergoes a conformational change to transport them together into the cell. This allows for the efficient reabsorption of glucose from the filtrate back into the bloodstream, helping to maintain glucose homeostasis in the body.

Fenestrations are small pores or openings in the endothelial cells lining the capillaries of the nephron. These fenestrations allow small molecules to pass through the capillary walls and enter the surrounding tissues. The presence of fenestrations increases the permeability of the capillaries, allowing for the filtration and exchange of small molecules, such as water, ions, and nutrients, between the blood and the surrounding tissues. This property is important in the function of the nephron, as it allows for the filtration of waste products and the reabsorption of essential substances during urine formation.

Cholera toxin binds to and activates the chloride transporter in the cells lining the small intestine. When the transporter is activated, it leads to increased secretion of chloride ions from the cells into the lumen of the intestine. As the concentration of chloride ions increases in the lumen, it creates an osmotic gradient. This osmotic gradient causes water to move from the surrounding cells and tissues into the lumen of the intestine through osmosis.

The net effect is that water follows the movement of chloride ions, resulting in excessive water loss into the intestinal lumen. This leads to the watery diarrhea characteristic of cholera. The loss of water and electrolytes through diarrhea can quickly lead to dehydration, electrolyte imbalances, and potentially life-threatening complications if not treated effectively.

Potassium ions (K⁺) move into the cell by a process known as facilitated diffusion, which involves the movement of ions from an area of higher concentration to an area of lower concentration. In the case of potassium ions, they move from the extracellular fluid, where their concentration is higher, to the intracellular fluid, where their concentration is lower.

The movement of potassium ions is also influenced by the membrane potential, which is the difference in electrical charge across the cell membrane. The inside of the cell is negatively charged compared to the outside. This electrical gradient tends to attract positively charged potassium ions into the cell.

Therefore, the overall movement of potassium ions involves moving up the concentration gradient (from high to low concentration) and down the membrane potential (from a more positive to a less positive or negative potential) to enter the cell.

Selective serotonin reuptake inhibitors (SSRIs) work by blocking the reuptake of serotonin, a neurotransmitter, in the pre-synaptic neuron. They specifically target and block the serotonin transporter protein, which is a sodium-neurotransmitter symporter. This protein is responsible for the reabsorption of serotonin from the synaptic cleft back into the pre-synaptic neuron. By blocking this transporter, SSRIs increase the concentration of serotonin in the synaptic cleft, allowing it to interact with and stimulate the post-synaptic receptors for a longer period of time. This helps alleviate symptoms of depression and improve mood.

• The apical side faces the lumen, while the basolateral side of the cell is the one closer to the capillary system.
• There is effectively no glucose inside the cell.
• To bring glucose into the cell requires some amount of energy.
• To bring glucose from the blood to the cell, a basolateral glucose channel will allow glucose to move down its gradient. An apical sodium/glucose antiporter will move glucose against its gradient by using the energy of sodium following its gradient. A sodium/potassium pump maintains sodium concentration.

In the context of the immune system, phagocytosis is the process by which immune cells engulf and internalize foreign invaders such as microbes. During phagocytosis, the immune cell surrounds the invader and forms a vesicle called a phagosome. The phagosome then fuses with lysosomes, forming a phagolysosome, where damaging proteases and reactive oxygen species are released to destroy the invaders.

So, the invaders reach the vesicles through phagocytosis. The immune cells recognize the presence of foreign substances and actively engulf them by extending pseudopods to surround the invaders and internalize them into vesicles for destruction. This sequestration in vesicles allows the immune system to isolate and neutralize the invaders while minimizing harm to the surrounding cells.

Pulmonary edema is the accumulation of fluid in the lungs, specifically in the interstitium and alveoli. To decrease the risk of pulmonary edema, it is necessary to address the factors that contribute to the movement of fluid into the interstitium and alveoli.

Hydrostatic pressure refers to the pressure exerted by fluid within blood vessels, and it tends to push fluid out of the vessels and into the surrounding tissues. To reduce the risk of pulmonary edema, it is important to decrease hydrostatic pressure, which can be achieved by improving heart function, reducing fluid volume, or relieving any obstructions or pressures on blood vessels in the lungs.

Osmotic pressure, on the other hand, is the pressure exerted by solutes (such as proteins) that draw water into the blood vessels. By increasing osmotic pressure, more fluid can be retained within the blood vessels, reducing the likelihood of it leaking into the interstitium and alveoli. This can be accomplished by maintaining adequate levels of proteins, particularly albumin, in the bloodstream.

Diffusion is the process responsible for the movement of gases such as oxygen and carbon dioxide across the cell membrane, as they passively move from areas of high concentration to low concentration.

Exocytosis is the process by which cells release large particles or substances from the cell by fusing vesicles containing the material with the cell membrane and expelling the contents outside the cell.