Membrane Structure and Function: Study Notes for General Biology

Membrane Structure & Function

Overview: Life at the Edge

The plasma membrane is a fundamental structure that separates the living cell from its environment. It acts as a selective barrier, controlling the movement of substances into and out of the cell.

• Phospholipid Bilayer: The main structural component of the plasma membrane, forming a double layer that is selectively permeable.

• Selective Permeability: Allows some substances to cross more easily than others, maintaining homeostasis.

• Embedded Proteins: Proteins within the bilayer contribute to the membrane's functions, including transport and communication.

• Membrane Formation: The formation of a membrane enclosing a solution different from the surrounding solution is key for nutrient uptake and waste elimination.

Cellular Membranes as “Fluid Mosaics” of Phospholipids and Proteins

Cell membranes are composed of phospholipids and proteins, forming a dynamic and flexible structure known as the fluid mosaic model.

• Amphipathic Molecules: Phospholipids and some proteins have both hydrophilic and hydrophobic regions.

• Fluid Mosaic Model: Describes the membrane as a mosaic of various proteins embedded in or attached to the bilayer of phospholipids.

• Protein Functions: Proteins carry out specialized tasks, such as transport, signaling, and structural support.

• Membrane Fluidity: Most lipids and some proteins drift laterally in the plane of the membrane, contributing to its fluid nature.

• Cholesterol: Acts as a "fluidity buffer," maintaining membrane fluidity at different temperatures.

Membrane Proteins: Structure and Function

Membrane proteins are essential for the diverse functions of the plasma membrane. They are classified based on their position and association with the membrane.

• Integral Proteins: Penetrate the hydrophobic core of the lipid bilayer; many are transmembrane proteins.

• Peripheral Proteins: Bound to the surface of the membrane, often to exposed parts of integral proteins.

• Protein Functions: Include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

Membrane Carbohydrates and Cell Recognition

Carbohydrates attached to proteins and lipids on the extracellular surface of the plasma membrane play a key role in cell-cell recognition.

• Glycoproteins and Glycolipids: Carbohydrates covalently bonded to proteins and lipids, respectively.

• Cell Recognition: Important for sorting cells into tissues and organs, and for immune response.

• Variation: The composition of membrane carbohydrates varies among species, individuals, and cell types.

Chemical Properties & Membrane Structure: Selective Permeability

Selective Permeability

The plasma membrane allows some substances to cross more easily than others, maintaining the internal environment of the cell.

• Small Molecules: Such as water, oxygen, and carbon dioxide, move freely across the membrane.

• Ions and Large Molecules: Require transport proteins to cross the membrane.

• Transport Proteins: Facilitate the movement of specific substances across the membrane.

Movement of Molecules Across Membranes

Molecules move across membranes by diffusion, facilitated diffusion, osmosis, and active transport.

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration.

• Concentration Gradient: The difference in concentration of a substance across a space.

• Passive Transport: Diffusion of substances across a biological membrane without energy input.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Facilitated Diffusion

Transport proteins assist the passive movement of molecules across the membrane.

• Channel Proteins: Provide corridors for specific molecules or ions to cross.

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

Table: Types of Membrane Transport

Type	Energy Required?	Transport Proteins?	Direction
Simple Diffusion	No	No	Down concentration gradient
Facilitated Diffusion	No	Yes	Down concentration gradient
Active Transport	Yes (ATP)	Yes	Against concentration gradient
Osmosis	No	Sometimes (aquaporins)	Down water potential gradient

Osmosis and Water Balance

Osmosis is the diffusion of water across a selectively permeable membrane. Water balance is crucial for cell survival.

• Isotonic Solution: No net movement of water; cell volume remains stable.

• Hypertonic Solution: Water leaves the cell; cell shrinks.

• Hypotonic Solution: Water enters the cell; cell swells and may burst.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually ATP).

• Transport Proteins: Such as pumps, use energy to move ions and molecules.

• Sodium-Potassium Pump: Exchanges Na+ for K+ across the plasma membrane of animal cells.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

$3\,\text{Na}^+_{\text{out}} + 2\,\text{K}^+_{\text{in}}$

Bulk Transport: Exocytosis and Endocytosis

Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles in processes called exocytosis and endocytosis.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell.

• Endocytosis: The cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane.

• Types of Endocytosis:

  • Phagocytosis: "Cellular eating"; cell engulfs large particles.

  • Pinocytosis: "Cellular drinking"; cell engulfs extracellular fluid.

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Table: Types of Endocytosis

Type	Main Function	Specificity
Phagocytosis	Engulfing large particles or cells	Non-specific
Pinocytosis	Engulfing extracellular fluid	Non-specific
Receptor-Mediated Endocytosis	Uptake of specific molecules	Highly specific (receptor binding)

Examples and Applications

• Red Blood Cells: Use membrane proteins for gas exchange and maintaining ion balance.

• Neurons: Rely on membrane transport for nerve impulse transmission.

• Plant Cells: Maintain turgor pressure via osmosis.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Tables have been recreated to summarize key comparisons and classifications.