Membrane Transport and Cell Signaling: Structure and Function of the Plasma Membrane (chapter 5)

Membrane Transport and Cell Signaling

Learning Objectives

• Describe the structure of the plasma membrane.

• Explain the fluid mosaic model and how cells maintain membrane fluidity.

• Identify the components of the cell membrane: phospholipid bilayer, glycoprotein, peripheral membrane protein, integral membrane protein, cholesterol.

• Describe the functions of membrane proteins.

• Distinguish the two faces of the cell membrane and the process by which cells retain membrane orientation.

• Explain selective permeability and predict whether a molecule will cross the membrane unaided.

• Describe osmosis and predict cell response in different solutions.

• Compare water balance in animal and plant cells.

Structure of the Plasma Membrane

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms the fundamental structure separating the cell from its environment.

• Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

• The hydrophilic heads face outward toward the aqueous environment, while the hydrophobic tails face inward, away from water.

Example: The arrangement of phospholipids creates a semi-permeable barrier, essential for cellular homeostasis.

Membrane Fluidity

Factors Affecting Fluidity

Membrane fluidity is crucial for proper membrane function and is influenced by several factors:

• Temperature: Higher temperatures increase fluidity, while lower temperatures decrease it.

• Fatty Acid Structure:

  • Saturated fatty acids have no double bonds, leading to tightly packed, less fluid membranes.

  • Unsaturated fatty acids have one or more double bonds, introducing kinks that prevent tight packing and increase fluidity.

  • Membranes rich in unsaturated fatty acids are more fluid.

• Cholesterol (or other sterols):

  • At moderate temperatures, cholesterol reduces membrane fluidity by restricting phospholipid movement.

  • At low temperatures, cholesterol prevents solidification by disrupting regular packing of phospholipids.

Additional info: Organisms can adjust membrane lipid composition in response to environmental temperature, a process called thermal adaptation.

Thermal Adaptation

Cells must adapt their membrane composition to maintain fluidity under changing temperatures.

• In cold environments, cells increase the proportion of unsaturated fatty acids to prevent membranes from becoming too rigid.

• In warm environments, cells may increase saturated fatty acids to prevent membranes from becoming too fluid.

Example: Fish living in cold water have membranes with more unsaturated fatty acids compared to those in warm water.