Chapter 7: Membranes – Structure and Function

Chapter 7: Membranes

Introduction to Biological Membranes

Biological membranes are essential structures that define the boundaries of cells and organelles, regulate the passage of substances, and facilitate communication and recognition. The plasma membrane, in particular, is a dynamic and complex structure critical for maintaining cellular homeostasis.

Membrane Components

Plasma Membrane Structure

• Selective Permeability: The plasma membrane allows only certain substances to cross, maintaining internal conditions distinct from the external environment.

• Phospholipids: The most abundant component of membranes, forming a bilayer that serves as the fundamental structure.

• Amphipathic Nature: Phospholipids have both hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads, enabling the formation of bilayers in aqueous environments.

Definition: Amphipathic molecules contain both hydrophobic and hydrophilic regions.

Example: The phospholipid bilayer forms spontaneously in water due to the amphipathic nature of phospholipids.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of protein molecules drifting in a fluid bilayer of phospholipids.

• Fluidity: Membranes are held together by weak hydrophobic interactions, allowing most lipids and some proteins to move laterally within the layer.

• Rare Movements: Lipids may occasionally flip-flop between layers, but this is uncommon.

• Cholesterol: In animal cells, cholesterol modulates membrane fluidity depending on temperature—restraining movement at high temperatures and preventing tight packing at low temperatures.

Additional info: Membrane fluidity is essential for proper function, including protein mobility and cell signaling.

Membrane Proteins

Types and Functions

• Peripheral Proteins: Bound to the surface of the membrane, either internally or externally.

• Integral Proteins: Penetrate the hydrophobic core; those spanning the membrane are called transmembrane proteins.

• Structure: Hydrophobic regions of integral proteins often consist of nonpolar amino acids arranged in alpha helices.

Functions of Membrane Proteins

• Transport: Move substances across the membrane.

• Enzymatic Activity: Catalyze specific reactions at the membrane surface.

• Signal Transduction: Relay signals from outside to inside the cell.

• Cell-Cell Recognition: Allow cells to identify each other.

• Intercellular Joining: Connect adjacent cells.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

Membrane Orientation

• Membranes have distinct inside and outside faces, with specific distribution of proteins, lipids, and carbohydrates determined during assembly in the endoplasmic reticulum (ER) and Golgi apparatus.

Cell Recognition and Membrane Carbohydrates

Mechanisms of Cell-Cell Recognition

• Cells recognize each other by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane.

• Carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to proteins (forming glycoproteins).

• Carbohydrate composition varies among species, individuals, and cell types.

Example: The immune system uses cell-surface proteins for recognition; for instance, HIV requires the CD4 and CCR5 proteins to infect cells.

Selective Permeability and Transport Proteins

Selective Permeability

• Hydrophobic molecules (e.g., O2, CO2) can dissolve in the lipid bilayer and pass through rapidly.

• Hydrophilic molecules (ions, polar molecules) do not cross easily and require transport proteins.

Types of Transport Proteins

• Channel Proteins: Provide hydrophilic channels for specific molecules or ions (e.g., aquaporins for water).

• Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane; highly specific for their substrate.

Categories of Membrane Transport

Passive vs. Active Transport

• Passive Transport: No energy required; substances move down their concentration or pressure gradients.

• Active Transport: Requires energy (usually ATP); substances move against their gradients or are very large.

Types of Passive Transport

• Filtration: Uses pressure gradients.

• Simple Diffusion: Molecules spread out evenly; movement continues until dynamic equilibrium is reached.

• Osmosis: Diffusion of water across a selectively permeable membrane from low to high solute concentration.

• Facilitated Diffusion: Passive movement aided by transport proteins (channel or carrier proteins).

Tonicity and Effects on Cells

• Tonicity: The ability of a solution to cause a cell to gain or lose water, depending on solute concentration.

Solution Type	Solute Concentration	Effect on Animal Cell	Effect on Plant Cell
Isotonic	Same as inside cell	Normal	Flaccid
Hypertonic	Greater than inside cell	Shriveled	Plasmolyzed
Hypotonic	Less than inside cell	Lysed (bursts)	Turgid (normal)

Facilitated Diffusion Details

• Channel Proteins: Aquaporins for water; ion channels for ions; gated channels open/close in response to stimuli.

• Carrier Proteins: Undergo shape change upon binding and releasing the transported molecule.

Active Transport

Mechanisms and Examples

• Requires energy (usually from ATP hydrolysis) to move substances against their concentration gradients.

• Allows cells to maintain internal concentration gradients distinct from their environment.

• Transport proteins involved are always carrier proteins.

Sodium-Potassium Pump

• Animal cells maintain high K+ and low Na+ inside the cell using the sodium-potassium pump.

• Pump is energized by transfer of a phosphate group from ATP.

Equation:

$3\,\mathrm{Na}^+_{\text{in}} + 2\,\mathrm{K}^+_{\text{out}} + \mathrm{ATP} \rightarrow 3\,\mathrm{Na}^+_{\text{out}} + 2\,\mathrm{K}^+_{\text{in}} + \mathrm{ADP} + \mathrm{P}_i$

Membrane Potential and Electrochemical Gradients

• Membrane Potential: Voltage across a membrane due to differences in ion distribution; cytoplasmic side is usually negative.

• Electrochemical Gradient: Combination of chemical (concentration) and electrical (voltage) forces driving ion diffusion.

• Electrogenic Pumps: Transport proteins that generate voltage; sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria.

Co-Transport

• Energy from active transport of one solute is used to drive transport of another solute against its gradient.

Bulk Transport

Exocytosis and Endocytosis

• Bulk Transport: Large molecules (e.g., polysaccharides, proteins) cross the membrane via vesicles; requires energy.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell; common in secretory cells.

• Endocytosis: Cell takes in macromolecules by forming vesicles from the plasma membrane.

Types of Endocytosis

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

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

• Receptor-Mediated Endocytosis: Specific solutes bind to receptors, triggering vesicle formation; allows selective uptake.

Type	Main Feature	Example
Phagocytosis	Engulfs large particles	White blood cell ingesting bacteria
Pinocytosis	Engulfs extracellular fluid	Uptake of nutrients by intestinal cells
Receptor-Mediated	Specific molecules bind to receptors	Cholesterol uptake via LDL receptors

Key Terms and Concepts

• Plasma membrane

• Phospholipid

• Amphipathic

• Cholesterol

• Tonicity

• Isotonic, Hypertonic, Hypotonic

• Membrane potential

• Electrochemical gradient

Summary of Learning Objectives

• Describe the fluid mosaic model of cell membranes.

• Explain the functions of membrane proteins.

• Discuss mechanisms of cell-cell recognition.

• Describe selective permeability and the roles of transport proteins.

• Compare passive and active transport mechanisms.

• Contrast exocytosis and endocytosis.