Movement of substances across membrane：練習問題

Nerve cells, or neurons, maintain an electrochemical gradient across their membrane, crucial for nerve impulse transmission. This involves continually pumping sodium ions ($$Na^+$$) out of the cell and potassium ions ($$K^+$$) into the cell, both against their respective concentration gradients. What is the primary transport mechanism involved in this process?

Substance P is a small, uncharged molecule. When cells are incubated with Substance P, its rate of entry into the cells initially increases proportionally with the external concentration of Substance P. However, at very high external concentrations, the rate of entry reaches a maximum and does not increase further. Additionally, the transport of Substance P is found to be unaffected by the presence of a metabolic poison that inhibits ATP production.

Based on this information, which of the following is the most likely mechanism for the movement of Substance P across the cell membrane?

A group of students investigated the uptake of two substances, \( P \) and \( Q \), by human intestinal cells. The experimental results are summarized below:

1. For both substances, the rate of uptake increases with the external concentration initially but reaches a maximum value (plateau) at high concentrations.
2. When the cells are treated with a metabolic inhibitor that prevents the production of \( ATP \), the rate of uptake for substance \( Q \) drops to nearly zero, while the rate of uptake for substance \( P \) is not significantly affected.

Which of the following conclusions can be drawn from these results?

In the human small intestine, epithelial cells absorb glucose. After a large meal, glucose concentration in the intestinal lumen is high, allowing for facilitated diffusion. However, to ensure maximum absorption and to move the remaining glucose from a low concentration in the lumen into the cells, what transport mechanism is primarily employed?

An animal cell maintains a concentration of potassium ions ( \(\text{K}^+\)) that is significantly higher inside the cell than outside. If the cell membrane contains leak channels that allow \(\text{K}^+\) to diffuse out passively, which mechanism must be continuously active to maintain this steep concentration gradient?

Identify the solvent and the type of membrane required for the process of osmosis to occur in biological systems.

Preserving fruit by adding a high concentration of sugar creates a concentrated syrup around the fruit. Explain how this environment inhibits the growth of spoilage bacteria, with reference to water potential.

A researcher applies a metabolic poison that specifically inhibits mitochondrial ATP synthase to a layer of intestinal epithelial cells. Predict the effect on the rate of glucose uptake from the lumen and the subsequent movement of water, and justify your answer based on transport mechanisms.

A student sets up an experiment using a piece of dialysis tubing (Visking tubing) tied at both ends to form a bag. The dialysis tubing acts as a partially permeable membrane. The bag is filled with an aqueous solution containing 5% glucose (a small sugar molecule) and 2% starch (a large polysaccharide molecule). The bag is then submerged in a beaker of distilled water.

(a) State the major process responsible for the net movement of water across the dialysis tubing, and explain the direction of this net movement immediately after setup. (2 points)

(b) After several hours, the student tests the water outside the bag and finds that it contains glucose but no starch. Explain this observation by referring to the relevant mechanism(s) of substance movement and the properties of the membrane. (2 points)

A series of experiments was conducted to study the movement of two different small molecules, Substance X and Substance Y, across the cell membrane of isolated mammalian liver cells. In all experiments, the external concentration of both substances was kept high and constant, initially far exceeding the internal concentration for X, and actively maintained against the concentration gradient for Y.

(a) Temperature Dependence
When the experiment temperature was lowered from \(37^\circ \text{C}\) to \(4^\circ \text{C}\):
i) The rate of uptake of Substance X decreased slightly.
ii) The rate of uptake of Substance Y decreased drastically (by over 90%).
Explain the primary mechanisms of transport for Substance X and Substance Y that account for this difference in temperature dependence. (3 points)

(b) Energy Dependence
A chemical respiratory inhibitor, which prevents ATP production, was added to a separate culture of liver cells maintained at \(37^\circ \text{C}\).
i) It was observed that the net uptake rate of Substance Y immediately dropped to zero, and net movement reversed, causing Substance Y to leak out of the cell. Identify the initial mechanism responsible for the high net uptake rate of Substance Y before the inhibitor was added, and explain the observed change in net movement upon inhibitor addition. (3 points)

(c) Saturation Kinetics
If the external concentration of Substance X was doubled, its uptake rate approximately doubled. However, if the external concentration of Substance Y was doubled (above a certain threshold), its uptake rate increased by only a small amount.
Explain the underlying reason for the difference in the relationship between external concentration and uptake rate between Substance X and Substance Y. (2 points)