The Working Cell: Membrane Function CHAPTER 5

The Working Cell: Membrane Function CHAPTER 5The Working Cell: Membrane Function CHAPTER 5

• Membrane Function

• Passive Transport

• Osmosis and Water Balance

• Active Transport

• Bulk Transport

–Exo- and Endocytosis

• Cell Signaling

Membrane Function

• Working cells must control the flow of materials to and from the environment.

Membrane Selectivity

• A cell membrane or wrapper made of phospholipids is relatively impermeable

• Proteins embedded in the membrane provide for selective permeability and transport of materials into and out of the cell, among other things

Figure 5.11

Functions of Membrane Proteins






• Bulk Transport


• Cell Signaling

Passive Transport: Diffusion Across Membranes

• Molecules contain heat energy.

– They vibrate and wander randomly.

• Diffusion is movement of molecules from regions of high concentration to low concentration

– Molecules tend to spread into the available space.

Diffusion

• Diffusion Across A Membrane

– Passive transport of small or hydrophobic molecules

– Facilitated diffusion of larger and polar molecules

Examples of Passive Transport Across a Phospholipid Bilayer






• Bulk Transport


• Cell Signaling

Osmosis and Water Balance in Cells

• Osmosis is the passive transport of water across a selectively permeable membrane.

• Water moves across a membrane from high concentration (high purity) to low concentration (low purity)

• A hypertonic solution

– Has a higher concentration of dissolved substances (solute)

Terms Used to Compare the Purity of Water Solutions

• A hypotonic solution

– Has a lower concentration of dissolved substances (solute)

• An isotonic solution

– Has an equal concentration of dissolved substances (solute).

5% salt

1% salt

1% salt

1% salt

1% salt

5% salt

Cells Must Regulate Salt and Water Balance

PlasmolysisTurgid Elodea






• Bulk Transport


• Cell Signaling

Active Transport: The Pumping of Molecules Across Membranes

• Active transport requires energy to pump molecules across a membrane, into more a more crowded space

Active Transport

• Active transport is the movement of molecules from low concentration to high concentration






• Bulk Transport


• Cell Signaling

Bulk Transport: Exocytosis and Endocytosis

• Exocytosis

– Dumping molecules out of the cell (export)

Exocytosis and Endocytosis Introduction

• Endocytosis

– Bringing molecules into the cell (import)

– Includes pinocytosis, phagocytosis, and receptor-mediated endocytosis

Bulk Transport: Exocytosis and Endocytosis

Pinocytosis: cell “drinking”

Phagocytosis: cell “eating”

Receptor-mediated endocytosis

Receptor-Mediated Endocytosis






• Bulk Transport


• Cell Signaling

The Role of Membranes in Cell Signaling

• Cellular communication

1. Begins with the reception of an extracellular signal.

3. A response is elicited from the transduced signal

2. The signal is transduced or passed across the membrane






• Bulk Transport


• Cell Signaling

The Working Cell: Membrane Function CHAPTER 5

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