Book cover for Biology

Biology

Sylvia S. Mader, Michael Windelspecht

ISBN #9780078024269

12th Edition

687 Questions

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153,501 Students Helped

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter provides an in-depth examination of the plasma membrane's structure and function, highlighting the fluid-mosaic model that illustrates its dynamic and flexible nature. It explains the varied transport mechanisms—passive processes like diffusion and facilitated transport, as well as active transport exemplified by the sodium-potassium pump—that are essential for maintaining cellular homeostasis. Additionally, the chapter explores the role of the extracellular matrix and cell junctions in facilitating cell communication and support, and how cells respond to different tonicities through surface modifications.

Learning Objectives

1

Describe the structure of the plasma membrane and explain the fluid-mosaic model.

2

Explain the mechanisms of passive transport (diffusion and facilitated transport) and active transport (e.g., sodium-potassium pump).

3

Analyze how membrane components such as proteins, cholesterol, and carbohydrate chains contribute to selective permeability and cellular homeostasis.

4

Discuss the role of the extracellular matrix and cell junctions in cell communication, structural support, and responses to varying tonicities.

Key Concepts

CONCEPT

DEFINITION

Plasma Membrane

A flexible, dynamic barrier composed primarily of a phospholipid bilayer interspersed with proteins, cholesterol, and carbohydrates that controls the movement of substances into and out of the cell.

Fluid-Mosaic Model

A model describing the plasma membrane as a dynamic, flexible bilayer in which various proteins and lipids move laterally, creating a 'mosaic' appearance.

Selective Permeability

The property of the plasma membrane that allows it to control the passage of molecules, permitting some substances to pass while restricting others.

Diffusion

A passive transport process by which molecules move from an area of higher concentration to an area of lower concentration.

Facilitated Transport

A form of passive transport in which carrier proteins or channels assist molecules in moving across the membrane down their concentration gradient.

Active Transport

The energy-dependent process by which molecules are transported across the membrane against their concentration gradient, often exemplified by pumps such as the sodium-potassium pump.

Extracellular Matrix (ECM)

A network of proteins and carbohydrates located outside the cell that provides structural support and mediates cell communication.

Cell Junctions

Structures that connect cells to one another, facilitating communication and maintaining the structural integrity of tissues.

Tonicity

The relative concentration of solutes in the solution surrounding the cell, which affects water movement and, subsequently, cell volume and shape.

Example Problems

Example 1

In the fluid-mosaic model, the fluid properties are associated with the nature of the ______ and the mosaic pattern is established by the _______. a. nucleic acids; phospholipids b. phospholipids; embedded proteins c. embedded proteins; cholesterol d. phospholipids; nucleic acids

Example 2

Which of the following is not a function of proteins present in the plasma membrane? a. Proteins assist the passage of materials into the cell. b. Proteins interact with and recognize other cells. c. Proteins bind with specific hormones. d. Proteins carry out specific metabolic reactions. e. Proteins produce lipid molecules.

Example 3

The carbohydrate chains projecting from the plasma membrane are involved in a. adhesion between cells. $\quad$ c. cell-to-cell recognition. b. reception of molecules. $\quad$ d. All of these are correct.

Example 4

When a cell is placed in a hypotonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water enters the cell toward the area of higher solute concentration. d. there is no net movement of water or solutes.

Example 5

When a cell is placed in a hypertonic solution, a. solute exits the cell to equalize the concentration on both sides of the membrane. b. water exits the cell toward the area of lower solute concentration. c. water exits the cell toward the area of higher solute concentration. d. there is no net movement of water or solute.
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Step-by-Step Explanations

QUESTION

How does diffusion facilitate the passive transport of molecules across the plasma membrane?

STEP-BY-STEP ANSWER:

Step 1: Identify the molecule and its concentration gradient; recognize that molecules move from an area of high concentration to an area of low concentration.
Step 2: Understand that no energy is required for diffusion because it is a passive process relying on kinetic energy of molecules.
Step 3: Recognize that the plasma membrane, although selectively permeable, allows small nonpolar molecules like oxygen and carbon dioxide to diffuse through the lipid bilayer.
Step 4: Conclude that diffusion plays a key role in balancing concentrations across the membrane without the need for additional cellular energy.
Final Answer: Diffusion is a passive process that moves molecules down their concentration gradient through the plasma membrane.

Diffusion

QUESTION

How does facilitated transport differ from simple diffusion in moving molecules across the plasma membrane?

STEP-BY-STEP ANSWER:

Step 1: Begin by recognizing that facilitated transport also moves substances down their concentration gradient but requires specific transport proteins.
Step 2: Understand that these transport proteins provide channels or carriers that allow larger or charged molecules to pass through the hydrophobic core of the membrane.
Step 3: Contrast with simple diffusion as facilitated transport involves a protein-mediated process, even though it still remains passive (no energy expenditure).
Step 4: Summarize that facilitated transport is crucial for molecules that cannot easily diffuse through the lipid bilayer.
Final Answer: Facilitated transport uses transport proteins to move molecules down their concentration gradient, offering a route for molecules that cannot diffuse directly through the lipid membrane.

Facilitated Transport

QUESTION

Explain how the sodium-potassium pump functions as an example of active transport across the plasma membrane.

STEP-BY-STEP ANSWER:

Step 1: Recognize that active transport requires cellular energy, usually in the form of ATP.
Step 2: Understand that the sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients.
Step 3: Detail that for each cycle, the pump typically exports three sodium ions and imports two potassium ions, creating and maintaining an electrochemical gradient.
Step 4: Emphasize that this gradient is essential for many cellular processes, including nerve impulse transmission and muscle contraction.
Final Answer: The sodium-potassium pump is an ATP-dependent mechanism that actively transports sodium and potassium ions across the membrane to maintain essential cellular electrochemical gradients.

Active Transport

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Common Mistakes

  • Confusing the dynamic nature of the fluid-mosaic model with a static membrane structure.
  • Overlooking the role of cholesterol and carbohydrate chains in maintaining membrane stability and functionality.
  • Mixing up passive transport mechanisms such as diffusion and facilitated transport, despite their different structural requirements.
  • Assuming that active transport processes do not require direct cellular energy, such as ATP.
  • Neglecting the importance of cell junctions and the extracellular matrix in cell communication and structural integrity.