Plasma Membrane Structure and Function: Study Notes for General Biology

Plasma Membrane

Overview and Functions

The plasma membrane is the outer boundary of every cell, providing a selectively permeable barrier that maintains a constant internal environment. Its structure and fluidity are essential for cell function and are influenced by temperature and cholesterol content.

• Selective Permeability: Allows certain substances to pass while restricting others.

• Cell Communication: Contains proteins for binding and adhering to adjacent cells.

• Dynamic Structure: Membranes are continuously forming, moving, fusing, and breaking down.

Plasma Membrane Structure

Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a mosaic of proteins floating in or on a fluid bilayer of phospholipids. This model explains the flexibility and dynamic nature of the membrane.

• Phospholipid Bilayer: Forms the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.

• Embedded Proteins: Proteins are interspersed throughout the bilayer, allowing movement and interaction.

• Continuous Remodeling: Membranes are not static; they are constantly changing shape and composition.

Factors Affecting Membrane Fluidity

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

• Temperature:

  • Low temperatures decrease fluidity.

  • High temperatures increase fluidity.

• Lipid Composition:

  • Short, unsaturated fatty acids increase fluidity.

  • Long, saturated fatty acids decrease fluidity.

• Cholesterol:

  • Decreases fluidity and prevents crystallization of fatty acids.

Cell Membrane Components

Macromolecules in Membranes

All cell membranes are composed of three main types of macromolecules: lipids, proteins, and carbohydrates. The type and percentage of each component vary among different cell types.

• Lipids: Form the majority of the membrane structure and separate two aqueous environments.

• Proteins: Serve as receptors, channels, and enzymes.

• Carbohydrates: Involved in cell recognition and adhesion.

Lipids in Membranes

• Phospholipids:

  • Amphipathic: Contain both hydrophilic and hydrophobic regions.

  • Form bilayers in aqueous solutions, stabilizing the membrane while allowing flexibility.

• Cholesterol:

  • Abundant in animal cell membranes, rare in plants.

  • Decreases membrane permeability to water.

  • Involved in cell communication and influences membrane fluidity.

Types of Membrane Proteins

Membrane proteins are produced by ribosomes bound to the rough endoplasmic reticulum (ER) and are classified based on their position and function:

• Integral Proteins:

  • Penetrate the phospholipid bilayer.

  • Amphipathic (contain both hydrophilic and hydrophobic regions).

  • Majority of membrane proteins.

• Transmembrane Proteins:

  • Span both sides of the membrane.

• Peripheral Proteins:

  • Do not penetrate the bilayer.

  • Located on the inner or outer surface.

  • Hydrophilic.

Carbohydrates (Oligosaccharides)

• Located outside the plasma membrane.

• Involved in cell-cell recognition.

• Covalently bonded to membrane proteins (glycoproteins, proteoglycans).

Cell Recognition and Adhesion

Mechanisms and Structures

Cells of multicellular organisms exhibit species-specific recognition and adhesion, mediated by glycoproteins and glycolipids. Integral membrane proteins also play a role in adhesion.

• Tight Junctions: Seal intercellular spaces, restrict membrane protein mobility, and prevent movement of materials between cells.

• Desmosomes: Bind cells together, anchoring cytoskeletons and providing mechanical stability.

• Gap Junctions: Create tunnels for communication and sharing between adjacent cells via protein channels (connexons).

Passive Membrane Transport

Diffusion

Diffusion is the random movement of molecules towards equilibrium, driven by the concentration gradient. No energy input is required.

• Substances move from high concentration to low concentration.

• Speed of movement is influenced by:

  • Size and mass of molecules/ions

  • Temperature

  • Density of the solution

Simple Diffusion

• Molecules move through the phospholipid bilayer.

• Small uncharged molecules (e.g., glycerol, ethanol) diffuse easily.

Osmosis

Osmosis is the passive movement of water ($H_2O$) across a selectively permeable membrane. It does not require metabolic energy and depends on the relative concentration of water molecules on either side of the membrane.

• Water always moves from a higher water concentration to a lower water concentration.

Terms Used to Compare Solutions

Term	Definition	Effect on Cell
Hypotonic	Lower solute concentration (higher water concentration)	Cell swells
Hypertonic	Higher solute concentration (lower water concentration)	Cell shrivels (loses water)
Isotonic	Equal solute and water concentration	No net change in cell volume

Facilitated Diffusion

Facilitated diffusion is the passive movement of small molecules through specific channel proteins or carrier proteins. It does not require energy and occurs according to concentration gradients.

• Examples: Polar molecules (water, amino acids, sugars) and ions.

• Channel proteins (integral membrane proteins) form channels across the membrane, allowing substances to pass through.

• Carrier proteins speed up diffusion through the phospholipid bilayer.

Example Application

• If a cucumber is placed in a hypertonic solution (e.g., salty water), water will leave the cucumber cells, causing them to shrink.

• If placed in a hypotonic solution (e.g., pure water), water will enter the cells, causing them to swell.

Additional info: The notes have been expanded to include definitions, examples, and a table for solution comparison, as well as clarification of membrane protein types and transport mechanisms.