Cell Membranes and Their Functions

Plasma Membranes and Other Cell Membranes

• Membranes are found in all cells, including the plasma membrane, nuclear membranes, endoplasmic reticulum membranes, Golgi apparatus membranes, vacuole membranes, plastid membranes, and mitochondrial membranes.
• These membranes (called cytomembranes) form compartments within the cell, separating them from their surroundings.
• Prokaryotic cells have simpler membranes than eukaryotic cells, which have more complex compartmentalization.
• The plasma membrane, also called the plasmalemma or cytoplasmic membrane, is crucial for cell integrity by separating the cytoplasm from the surrounding environment.
• Damage to the plasma membrane can lead to cell death.
• Cell membranes are involved in many biochemical reactions, such as signal transduction, phosphorylation, and cell respiration (in prokaryotes).
• They also facilitate communication and interaction between neighboring cells.
• Cell membranes have selective permeability, regulating the passage of substances between compartments.
• Some molecules readily pass through the lipid bilayer (hydrophobic), while others need channels or transporters (hydrophilic).
• Passage can be active or passive.

The Lipid Bilayer

• Cell membranes primarily consist of a phospholipid bilayer.
• Phospholipids are amphipathic, meaning they have both polar (hydrophilic) heads and nonpolar (hydrophobic) tails.
• They arrange in a bilayer to minimize contact between hydrophobic tails and water.
• Proteins and carbohydrates are also components of cell membranes.
• The proportions of these components vary between different cell types.
• Different proteins have different functions, including transport, signaling, and structure.

The Fluid-Mosaic Model

• The fluid-mosaic model describes the structure of cell membranes.
• The model depicts a phospholipid bilayer with embedded proteins.
• Protein types include peripheral (extrinsic) proteins, loosely bound to the membrane's surface, and integral (intrinsic) proteins, firmly embedded within the bilayer.
• Integral proteins can span the membrane entirely (transmembrane) or only partly.
• Carbohydrates are typically attached to proteins (glycoproteins) or lipids (glycolipids) on the outer surface.

Membrane Asymmetry

• Proteins and lipids are not distributed evenly across the membrane layers (inner and outer leaflets).
• Different types of lipids and proteins are more prevalent in one leaflet than the other.
• This asymmetry is crucial for the function of the cell membrane.

Membrane Fluidity

• Membranes are dynamic, their components are in constant motion within the bilayer.
• Factors influencing fluidity include temperature, fatty acid saturation, and the presence of cholesterol.
• Cholesterol acts as a regulator, decreasing fluidity at high temperatures and increasing it at low temperatures.
• Membrane fluidity is essential for proper function.

Membrane Specializations

• Cell membranes exhibit specialized domains, with variations in protein and lipid compositions.
• Junctions like tight junctions, intermediate junctions (desmosomes), and gap junctions facilitate cell-cell interactions.
• Gap junctions allow communication and the movement of molecules between cells.
• Microvilli and stereocilia increase surface area for absorption or secretion.
• Infolds of the basal membrane enhance absorption or secretion (e.g., in kidney cells).

Specialization of the plasma membrane

• Junctions facilitate cell-cell adhesion, communication, and transport.
• Junctions include tight, gap, and desmosomes, all with unique functions.

Transport of Macromolecules

• Larger molecules use bulk transport (endocytosis and exocytosis) which is a process involving vesicles.
• Endocytosis (inward movement) includes phagocytosis (for large particles) and other types of endocytosis for other molecules.
• Exocytosis (outward movement) releases molecules out of the cell.
• Both processes are essential for maintaining the integrity and function of the membrane.

Plant Cell Walls and Plasmodesmata

• Plant cells have cell walls, aiding structural support and protection.
• Plasmodesmata are channels in the cell wall permitting communication between neighboring cells.
• This structure differs from the animal cell counterparts in having a rigid cell wall.
• The channels enable the transport of water, ions, and small molecules between cells.

Microvilli

• Microvilli increase surface area in parts of cells where absorption or secretion need enhancing.
• The structure includes bundles of actin filaments providing support and possibly movement.

Explore the diverse types of cell membranes, focusing on their structure and function. Learn about the critical role of the plasma membrane, as well as how membranes in prokaryotic and eukaryotic cells differ. This quiz covers essential concepts regarding membrane permeability and cellular communication.