Chapter 7: Membranes – Structure and Function

Chapter 7: Membranes

Membrane Components

The plasma membrane is a fundamental structure in all cells, providing a boundary and regulating the movement of substances. Its unique composition allows it to perform a variety of essential functions.

• 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. Phospholipids are amphipathic molecules, meaning they have both hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads.

• Phospholipid Bilayer: Phospholipids arrange themselves in a double layer, with hydrophobic tails facing inward and hydrophilic heads facing outward, creating a semi-permeable barrier.

Example: The arrangement of phospholipids in water forms a bilayer, as shown in the diagram, with heads interacting with water and tails shielded inside.

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 mainly by weak hydrophobic interactions, allowing most lipids and some proteins to move laterally within the layer. Rarely, lipids may flip-flop between layers.

• Factors Affecting Fluidity:

  • Membranes rich in unsaturated fatty acids are more fluid than those with saturated fatty acids.

  • Cholesterol modulates fluidity: at warm temperatures, it restrains phospholipid movement; at cool temperatures, it prevents tight packing.

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

Membrane Proteins

Proteins embedded in the membrane determine most of its specific functions.

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

• Integral Proteins: Penetrate the hydrophobic core; those that span 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 and Synthesis

Membranes have distinct inside and outside faces, with specific distribution of proteins, lipids, and carbohydrates. This orientation is established during membrane synthesis in the endoplasmic reticulum (ER) and Golgi apparatus.

Cell Recognition and Membrane Carbohydrates

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

• Glycolipids: Carbohydrates covalently bonded to lipids.

• Glycoproteins: Carbohydrates covalently bonded to proteins (more common).

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

Example: The immune system uses cell-surface proteins for recognition; HIV must bind to specific proteins (CD4 and CCR5) to infect cells.

Selective Permeability of the Membrane

The plasma membrane controls the entry and exit of substances.

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

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

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

Transport across membranes is classified based on energy requirements.

• 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; net movement until 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 (channels or carriers); no energy required.

Osmosis and Tonicity

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

• Isotonic Solution: Solute concentration is the same inside and outside the cell; no net water movement.

• Hypertonic Solution: Solute concentration is higher outside the cell; cell loses water.

• Hypotonic Solution: Solute concentration is lower outside the cell; cell gains water.

Solution Type	Animal Cell	Plant Cell
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed
Hypotonic	Lysed	Turgid (normal)

Facilitated Diffusion

• Channel Proteins: Aquaporins facilitate water movement; ion channels allow ions to pass.

• Gated Channels: Open or close in response to stimuli.

• Carrier Proteins: Undergo shape change to move substances; triggered by binding and release of the molecule.

Active Transport

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

• Allows cells to maintain internal concentrations different from the 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.

• The 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 Gradient

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

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

• Electrogenic Pump: Transport protein that generates voltage; sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria.

Co-Transport

• Energy from active transport of one solute drives transport of another against its gradient.

Bulk Transport

Large molecules (e.g., polysaccharides, proteins) cross the membrane in bulk via vesicles, requiring energy.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell; used by 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 takes in extracellular fluid and dissolved solutes.

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

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

Key Terms

• 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.

• Describe mechanisms of cell-cell recognition.

• Explain selective permeability and the roles of transport proteins.

• Compare passive and active transport.

• Distinguish between exocytosis and endocytosis.