Biology Chapter 7

Active transport by the sodium-potassium pump follows this cycle

1. corresponding to step one in a diagram below the text. Three Na+ ions from the cytosol bind to the pump. 2. corresponding to step two in the diagram below the text. The binding of Na+ stimulates the phosphorylation of the pump protein by ATP. 3. corresponding to step three in the diagram below the text. Phosphorylation causes a conformational change in the pump that moves the three Na+ ions against their concentration gradient and releases them outside the cell. 4. corresponding to step four in the diagram below the text. The release of the Na+ ions permits two K+ ions from outside the cell to bind to the pump, and the phosphate group is released. 5. corresponding to step five in the diagram below the text. Release of the phosphate group causes another conformational change in the pump. 6. corresponding to step six in the diagram below the text. The conformational change in the pump moves the two K+ ions against their concentration gradient and releases them into the cytosol.

Which of the following statements concerning carbohydrates associated with the plasma membrane is correct?

Membrane carbohydrates function primarily in cell-cell recognition. Variations in carbohydrate structure distinguish one species from another, one individual from another, and even one cell type from another.

A selectively permeable membrane separates two solutions. Water is able to pass through this membrane; however, sucrose (a disaccharide) and glucose (a monosaccharide) cannot pass. The membrane separates a 0.2-molar sucrose solution from a 0.2-molar glucose solution. With time, how will the solutions change?

Osmotic pressure is produced by the concentration of dissolved substances and is not influenced by the relative sizes of the solutes.

the driving forces for diffusion of and K+ ions through their respective channels

The concentration gradient of K+ ions across the membrane (higher K+ concentration inside) facilitates the diffusion of K+ out of the cell. However, the electrical gradient across the membrane (excess positive charge outside) impedes the diffusion of K+ out of the cell. The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and the electrical gradient (charge difference) across the membrane. For Na+ ions, diffusion through the Na+ channel is driven by both the concentration gradient and the electrical gradient. But for K+ ions, the electrical gradient opposes the concentration gradient. Therefore, the electrochemical gradient for Na+ is greater than the electrochemical gradient for K+.

the driving forces for diffusion of Na+ through their respective channels

The concentration gradient of Na+ ions across the membrane (higher Na+ concentration outside) facilitates the diffusion of Na+ into the cell. At the same time, the electrical gradient across the membrane (excess positive charge outside) drives Na+ into the cell. The electrochemical gradient for an ion is the sum of the concentration (chemical) gradient and the electrical gradient (charge difference) across the membrane. For Na+ ions, diffusion through the Na+ channel is driven by both the concentration gradient and the electrical gradient. But for K+ ions, the electrical gradient opposes the concentration gradient. Therefore, the electrochemical gradient for Na+ is greater than the electrochemical gradient for K+.

Consider the transport of protons and sucrose into a plant cell by the sucrose-proton cotransport protein. Plant cells continuously produce a proton gradient by using the energy of ATP hydrolysis to pump protons out of the cell. Why, in the absence of sucrose, do protons not move back into the cell through the sucrose-proton cotransport protein?

The movement of protons through the cotransport protein cannot occur unless sucrose moves at the same time. The obligate coupling of proton movement to sucrose movement prevents the energy of the proton gradient from being lost if sucrose is not present.

Which of the following statements about the sodium-potassium pump is correct?

The sodium-potassium pump moves Na+ and K+ in opposite directions, resulting in a net negative charge inside the cell. This is a true statement. An electrogenic pump creates a net movement of charge across a membrane. The sodium-potassium pump moves three Na+ out and two K+ in for a net transport of one positive charge out of the cell.

A single plant cell is placed in an isotonic solution. Salt is then added to the solution. Which of the following would occur as a result of the salt addition?

Water would leave the cell by osmosis, causing the volume of the cytoplasm to decrease. The added salt makes the solution hypertonic compared to the cell. Water will leave the cell by osmosis.

Which of the following processes and organelles account for the replacement of lipids and proteins lost from the plasma membrane?

exocytosis and smooth ER and rough ER In exocytosis, vesicles derived from the endomembrane system fuse with the plasma membrane, thus increasing the number of phospholipids in the plasma membrane and increasing its surface area. The smooth ER is largely responsible for production of lipids destined for the membrane, and the rough ER produces proteins destined for the plasma membrane.

endocytosis

forms vesicles from inward folding of the plasma membranedecreases the surface area of the plasma membrane substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle. Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell.

Which of the following pairs correctly matches a membrane transport process to its primary function?

pinocytosis: the uptake of water and small solutes into the cell by formation of vesicles at the plasma membrane Pinocytosis is the uptake of liquid and the solutes dissolved in the liquid.

exocytosis

secretes large molecules out of the cellrequires fusion of vesicles with the plasma membraneincreases the surface area of the plasma membrane substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell.