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The 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.
• 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
Membrane Selectivity
Functions of Membrane Proteins
Figure 5.11
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• 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 Across A Membrane
– Passive transport of small or hydrophobic
molecules
– Facilitated diffusion of larger and polar
molecules
Diffusion
Examples of Passive Transport Across a Phospholipid Bilayer
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• 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)
Terms Used to Compare the Purity of Water Solutions
• A hypertonic solution
– Has a higher concentration of dissolved
substances (solute)
5%
salt
1%
salt
• A hypotonic solution
– Has a lower concentration of dissolved
substances (solute)
1%
salt
5%
salt
• An isotonic solution
– Has an equal concentration of
dissolved substances (solute).
1%
salt
1%
salt
Cells Must Regulate Salt and Water Balance
Turgid Elodea
Plasmolysis
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Active Transport: The Pumping of Molecules Across Membranes
• Active transport is the movement of molecules from low
concentration to high concentration
• Active transport requires energy to pump molecules across
a membrane, into more a more crowded space
Active Transport
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
Bulk Transport: Exocytosis and Endocytosis
• Exocytosis
– Dumping molecules out of the cell (export)
Exocytosis and Endocytosis Introduction
Bulk Transport: Exocytosis and Endocytosis
•
Endocytosis
–
Bringing molecules into the cell (import)
–
Includes pinocytosis, phagocytosis, and receptor-mediated endocytosis
Pinocytosis: cell “drinking”
Phagocytosis: cell “eating”
Receptor-mediated endocytosis
Receptor-Mediated Endocytosis
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling
The Role of Membranes in Cell Signaling
•
Cellular
communication
1.
Begins with the
reception of an
extracellular signal.
2.
The signal is
transduced or
passed across the
membrane
3.
A response is
elicited from the
transduced signal
The Working Cell: Membrane Function
CHAPTER 5
• Membrane Function
• Passive Transport
• Osmosis and Water Balance
• Active Transport
• Bulk Transport
–Exo- and Endocytosis
• Cell Signaling