3.3 Plasma Membranes
4 min read•august 22, 2024
Plasma membranes are the gatekeepers of cells, controlling what goes in and out. They're made of a phospholipid bilayer with embedded proteins and carbohydrates, forming a fluid mosaic structure that's crucial for cell function.
This topic dives into the membrane's components and how they work together. We'll learn about phospholipids, proteins, and carbohydrates, and how their arrangement affects the membrane's properties and roles in cellular processes.
Plasma Membrane Structure
Phospholipid Bilayer
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- Phospholipids are amphipathic molecules with hydrophilic heads and hydrophobic tails spontaneously form a bilayer in aqueous environments (cell cytoplasm and extracellular fluid)
- The plasma membrane is a selectively permeable phospholipid bilayer separates the interior of the cell from the external environment
- The phospholipid bilayer is fluid, allowing lateral movement of membrane components within the plane of the membrane
Membrane Proteins and Carbohydrates
- The plasma membrane contains various proteins, including:
- Integral proteins span the membrane can move laterally within the membrane (ion channels, receptors)
- Peripheral proteins are attached to the surface of the membrane through interactions with integral proteins or lipids involved in cell signaling, cell adhesion, and providing structural support
- Carbohydrates are attached to some membrane proteins and lipids, forming glycoproteins and glycolipids, respectively
- Glycoproteins and glycolipids on the extracellular face of the membrane participate in cell recognition, cell adhesion, and cell-cell communication (immune recognition, cell-cell adhesion)
- Carbohydrates also contribute to the formation of the glycocalyx, a protective layer provides structural support and helps maintain cell integrity
Cholesterol and Membrane Asymmetry
- Cholesterol is a steroid molecule interspersed between phospholipids helps regulate membrane fluidity
- At higher temperatures, cholesterol reduces membrane fluidity by interfering with phospholipid movement
- At lower temperatures, cholesterol prevents membranes from freezing by maintaining phospholipid spacing
- The distribution of membrane components is asymmetric, with different compositions on the extracellular and intracellular faces of the membrane
- Glycoproteins and glycolipids are primarily found on the extracellular face
- Phosphatidylserine, a negatively charged phospholipid, is primarily located on the intracellular face
Fluid Mosaic Model
Dynamic and Fluid Structure
- The fluid mosaic model describes the plasma membrane as a dynamic, fluid structure with a mosaic of proteins embedded in a phospholipid bilayer
- The phospholipid bilayer is fluid, allowing lateral movement of membrane components within the plane of the membrane
- Integral proteins are embedded in the phospholipid bilayer can move laterally within the membrane
Mosaic of Membrane Components
- The plasma membrane is a mosaic of various components, including phospholipids, proteins, and carbohydrates
- Peripheral proteins are attached to the surface of the membrane through interactions with integral proteins or lipids
- Glycoproteins and glycolipids are distributed on the extracellular face of the membrane
- The distribution of membrane components is asymmetric, with different compositions on the extracellular and intracellular faces of the membrane
Membrane Components and Roles
Phospholipids as Selectively Permeable Barrier
- Phospholipids form the basic structure of the plasma membrane act as a selectively permeable barrier, allowing only certain molecules to pass through
- Hydrophobic core of the phospholipid bilayer prevents passage of polar and charged molecules (ions, glucose)
- Small, non-polar molecules can diffuse through the phospholipid bilayer (oxygen, carbon dioxide, steroids)
- The amphipathic nature of phospholipids, with hydrophilic heads and hydrophobic tails, enables the formation of the bilayer structure in aqueous environments
Protein Functions in the Membrane
- Integral proteins serve various functions in the plasma membrane:
- Transport proteins (channels and carriers) facilitate the movement of specific molecules across the membrane (ion channels, glucose transporters)
- Receptors bind to signaling molecules and initiate intracellular signaling cascades (hormone receptors, neurotransmitter receptors)
- Enzymes catalyze cellular processes and can be involved in membrane synthesis or degradation (adenylate cyclase, phospholipase)
- Peripheral proteins are involved in cell signaling, cell adhesion, and providing structural support to the membrane (cytoskeletal proteins, G proteins)
Carbohydrate Roles in Cell Interactions
- Glycoproteins and glycolipids on the extracellular face of the membrane participate in:
- Cell recognition and adhesion (selectins, integrins)
- Cell-cell communication and signaling (Notch receptors, ephrins)
- The glycocalyx, formed by carbohydrates attached to membrane components:
- Provides structural support and maintains cell integrity
- Acts as a protective barrier against mechanical stress and pathogens
- Participates in cell-cell and cell-matrix interactions (cell migration, tissue organization)
Membrane Fluidity and Function
Importance of Membrane Fluidity
- Membrane fluidity is crucial for maintaining the proper function of membrane proteins and the overall health of the cell
- The fluid nature of the phospholipid bilayer allows for the lateral movement of membrane components, facilitating processes such as:
- Cell signaling (receptor clustering, signal transduction)
- Membrane transport (formation of transport vesicles, fusion events)
- Cell division (cytokinesis, membrane remodeling)
Factors Affecting Membrane Fluidity
- Temperature affects membrane fluidity:
- At lower temperatures, membranes become less fluid due to reduced phospholipid movement
- At higher temperatures, membranes become more fluid as phospholipid movement increases
- The presence of unsaturated fatty acids in phospholipids increases membrane fluidity:
- Unsaturated fatty acids have one or more double bonds introduce kinks in the hydrocarbon tails, disrupting tight packing and increasing fluidity
- Cholesterol helps regulate membrane fluidity:
- At higher temperatures, cholesterol reduces membrane fluidity by interfering with phospholipid movement
- At lower temperatures, cholesterol prevents membranes from freezing by maintaining phospholipid spacing
Consequences of Altered Membrane Fluidity
- Changes in membrane fluidity can affect the function of membrane proteins:
- Transport proteins and receptors may not function properly if membrane fluidity is altered (reduced nutrient uptake, impaired signal transduction)
- Enzyme activity may be affected by changes in membrane fluidity (altered reaction rates, reduced specificity)
- Alterations in membrane fluidity are associated with various diseases:
- Cardiovascular disorders (atherosclerosis, hypertension)
- Neurodegenerative diseases (Alzheimer's, Parkinson's)
- Cancers (altered cell signaling, increased invasiveness)
- Maintaining proper membrane fluidity is essential for cellular health and homeostasis