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Chapter 5: Problem 13

Why must active transport of molecules across plasma membranes function continuously? a. Diffusion cannot occur in certain cells. b. Diffusion is constantly moving solutes in opposite directions. c. Facilitated diffusion works in the same direction as active transport. d. Not all membranes are amphiphilic.

Short Answer

Expert verified
b. Diffusion is constantly moving solutes in opposite directions.

Step by step solution

01

Identify the Function of Active Transport

Active transport moves molecules against their concentration gradient, from areas of low concentration to areas of high concentration, using energy typically in the form of ATP.
02

Understand Diffusion

Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, without the use of energy.
03

Recognize the Opposing Actions

Active transport and diffusion fundamentally oppose each other. While diffusion tends to equalize concentrations by moving solutes in one direction, active transport must counteract this by continuously moving molecules in the opposite direction to maintain concentration gradients.
04

Evaluate the Options

Option (a) is incorrect because diffusion occurs in all cells. Option (c) is incorrect because facilitated diffusion also moves substances down their concentration gradient. Option (d) is irrelevant because amphiphilic property of membranes does not affect the need for continuous active transport.
05

Determine the Correct Answer

Therefore, the correct answer is (b). Active transport must function continuously because diffusion is constantly moving solutes in the opposite direction.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

diffusion in biology
Diffusion is a fundamental process in biology, where molecules move from an area of high concentration to an area of low concentration. This movement happens naturally due to random molecular motion and does not require energy. For example, oxygen entering cells and carbon dioxide exiting cells both occur via diffusion. This process is crucial for the survival of cells, ensuring they receive nutrients and remove waste products efficiently.

However, diffusion alone cannot always meet a cell's needs. That's where other types of transport, like active transport, come into play. Diffusion aims to equalize concentrations on both sides of a membrane, but cells often need to maintain specific concentration gradients. This is essential for functions like nerve signal transmission and muscle contraction.
plasma membrane transport
The plasma membrane is a selectively permeable barrier that surrounds the cell. It controls what enters and exits the cell, maintaining the internal environment (homeostasis). The membrane is composed of a lipid bilayer with embedded proteins that act as channels or carriers.

There are several types of transport mechanisms across this membrane:
  • Simple Diffusion: Movement of small or nonpolar molecules like oxygen and carbon dioxide directly through the lipid bilayer.
  • Facilitated Diffusion: Utilizes channel proteins or carrier proteins to help larger or polar molecules move down their concentration gradient.
  • Active Transport: Moves molecules against their concentration gradient using energy, often from ATP. This is crucial for maintaining concentrations of ions like sodium and potassium that are essential for cellular functions.
ATP energy usage
ATP (Adenosine Triphosphate) is the primary energy currency of the cell. It powers various cellular activities, including active transport, cell division, and muscle contractions. In the context of membrane transport, ATP is essential for moving molecules against their concentration gradient. This type of transport is known as active transport.

Active transport requires energy because it moves substances from areas of low concentration to areas of high concentration, which is against their natural flow. This process is like pushing a ball uphill, requiring continuous energy input. This is why active transport must operate continuously, especially when diffusion works to move substances in the opposite direction. Without this continuous function, cells could not maintain their necessary concentration gradients, leading to dysfunction.

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Most popular questions from this chapter

In addition to a plasma membrane, eukaryotic cell organelles, such as mitochondria, also have membranes. In which way would these membranes differ? a. The proportion of phosphate within the phospholipids will vary. b. Only certain membranes contain phospholipids. c. Only certain membranes are selectively permeable. d. The proportions of proteins, lipids, and carbohydrates will vary.

Choose the statement that describes processes of receptor-mediated endocytosis, exocytosis, and the changes in the membrane organization. a. Endocytosis involves the opsonization of a receptor and its ligand in clathrin-coated vesicles, along with the inward budding of the plasma membrane. In exocytosis, waste material is enveloped in a membrane that fuses with the interior of the plasma membrane via attachment proterins. b. In endocytosis, waste material is enveloped in a membrane that fuses with the interior of the plasma membrane via attachment proteins. Exocytosis involves the opsonization of the receptor and its ligand in a clathrin-coated vesicles. c. In endocytosis, waste material is enveloped in a membrane that fuses with the interior of the plasma membrane via attachment proteins. Exocytosis involves the opsonization of the receptor and its ligand in caveolae-coated vesicles. d. Endocytosis involves the opsonization of the receptor and its ligand in clathrin-coated vesicles. In exocytosis, waste material is enveloped in a membrane that fuses with the exterior of the plasma membrane via attachment proteins.

In what important way does receptor-mediated endocytosis differ from phagocytosis? a. It transports only small amounts of fluid. b. It does not involve the pinching off of membrane. c. It brings in only a specifically targeted substance. d. It brings substances into the cell, while phagocytosis removes substances.

How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell? a. The sodium-potassium pump forces out three (positive) \(\mathrm{Na}^{+}\) ions for every two (positive) \(\mathrm{K}^{+}\) ions it pumps in, thus the cell loses a net positive charge of one at every cycle of the pump. b. The sodium-potassium pump expels three ions \(\mathrm{K}^{+}\) for every two \(\mathrm{Na}^{+}\) inside the cells, creating a net positive charge outside the cell and a net negative charge inside the cell. c. The sodium-potassium pump helps the development of negative charge inside the cell by making the membrane more permeable to negatively charged proteins. d. The sodium-potassium pump helps in the development of negative charge inside the cell by making the membrane impermeable to positively charged ions.

An experiment was set up to determine the movement of molecules through a dialysis-tubing bag into water. A dialysis-tubing bag containing 5\(\%\) lactose and 5\(\%\) fructose was placed in a beaker of distilled water, as illustrated. After four hours, fructose is detected in the distilled water outside of the dialysis-tubing bag, but lactose is not. What conclusions can be made about the movement of molecules in this experiment? a. Fructose, being a monosaccharide, diffused through the dialysis bag into the distilled water. However, lactose, being a disaccharide, could not diffuse through the dialysis bag. b. Fructose was homogenized by lactose, allowing and into the distilled water. Lactose is not homogenized, so it could not pass through the dialysis bag. c. Fructose and lactose are oppositely charged and separated out due to the force of repulsion. d. Fructose diffused because of the pore specificity of the semipermeable membrane, not because of its concentration gradient.
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