Plasma Membrane Structure and Function

Plasma Membrane Structure and Function

• The plasma membrane is a lipid bilayer, a selectively permeable barrier
• It contains phospholipids (polar head groups and nonpolar tails) and cholesterol
• Phospholipids provide fluidity and stability to the membrane
• Cholesterol helps maintain fluidity at different temperatures
• Membrane proteins are embedded in the lipid bilayer, performing a variety of functions
• Proteins can be transmembrane or peripheral (on one surface)
• Membrane proteins include channels, carriers, receptors, enzymes, and cell-adhesion molecules

Functions of Cell Membrane

• Maintains homeostasis and cell survival, coordinating cell activity with other cells
• Acts as a mechanical barrier to separate cells
• Facilitates exchange of nutrients, wastes, and secretions from cell to cell
• Responds to environmental changes and signals
• Maintains ionic gradients for electrical activity

Components of Cell Membrane: Lipids

• Phospholipids and cholesterol form the lipid bilayer, acting as a barrier to water-soluble substances
• It provides fluidity and stability to the membrane
• The phospholipid's hydrophilic head group faces outward, while the hydrophobic tail group faces inward

Components of Cell Membrane: Proteins

• Proteins have both hydrophobic and hydrophilic regions, embedded in the lipid bilayer
• Integral proteins are transmembrane proteins, spanning the bilayer
• Peripheral proteins are on one surface only
• Proteins perform diverse functions, including transport, signaling, and cell adhesion

Components of Cell Membrane: Carbohydrates

• Carbohydrates are present on the outer surface of the membrane only
• Glycolipids and glycoproteins are carbohydrate chains attached to lipids or proteins
• These form glycocalyx, functioning as self-identity markers

Structure of Cell Membrane: Fluid Mosaic Model

• The fluid mosaic model describes the membrane's structure as a mosaic of protein molecules embedded in a fluid bilayer of phospholipids
• The membrane is characterized by its fluidity and the dynamic movement of components

Cell-Cell Adhesions

• Extracellular matrix (ECM) is a biological glue, secreted by cells to adhere cells to each other.
• Specialized cell junctions, such as desmosomes and tight junctions, hold cells together, mediating communication and maintaining tissue integrity
• Cell adhesion molecules (CAMs) mediate cell–cell interactions and cell–extracellular matrix interactions

Extracellular Matrix (ECM)

• The ECM is an area outside the cell, containing interstitial fluid and fibrous proteins
• It acts as a filler to support and maintain the structure of cells and tissues
• Collagen (cable-like) provides tensile strength
• Elastin (rubber-like) allows organs to stretch
• Fibronectin promotes cell adhesion

Transport Across the Membrane: Overview

• Membrane transport is crucial for maintaining homeostasis
• The plasma membrane is selectively permeable
• Permeability factors include lipid solubility and particle size, requiring forces to transport substances

Transport Across the Membrane: Passive Transport

• Passive transport does not require energy input from the cell
• Common types include diffusion, osmosis, and facilitated diffusion
• Diffusion is movement from high to low concentration
• Osmosis is water movement from high to low water concentration

Transport Across the Membrane: Active Transport

• Active transport requires energy expenditure
• This includes primary and secondary active transport
• Primary active transport uses ATP directly
• Secondary active transport uses an electrochemical gradient

Diffusion

• Diffusion is the net movement of a substance from high to low concentration
• Factors influencing the diffusion rate: concentration gradient, temperature, surface area, membrane permeability, and distance

Osmosis

• Osmosis is the net movement of water across a semipermeable membrane from high to low water concentration
• Tonicity describes the concentration of non-penetrating solutes
• Osmolarity describes the total concentration of all solute particles, including penetrating and non-penetrating solutes
• Isotonic solutions have equal solute concentrations inside and outside the cell.
• Hypotonic solutions have a lower solute concentration than inside the cell; water moves into the cell, potentially causing the cell to burst.
• Hypertonic solutions have a higher solute concentration than inside the cell, water moves out of the cell, potentially causing the cell to shrink.

Carrier-Mediated Transport

• Carrier proteins bind to specific solutes (e.g. glucose)
• Binding changes the carrier protein's shape, moving the solute across the membrane
• Uniport, symport, antiport are different types of carrier-mediated transport based on the direction and number of substances being moved

Cell Junctions: Desmosomes, Tight Junctions, and Gap Junctions

• Desmosomes/Adhering junctions are anchoring junctions that facilitate mechanical strength
• Tight junctions create an impermeable barrier to block fluid movement between cells
• Gap junctions allow communication between adjacent cells

Test your knowledge on the structure and function of the plasma membrane. This quiz covers the lipid bilayer, the roles of phospholipids and cholesterol, and the various membrane proteins. Understand how the cell membrane maintains homeostasis and facilitates cellular communication.