Membrane Structure and Function: Study Notes for General Biology

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Membrane Structure and Function

Introduction

The plasma membrane is a fundamental structure that separates the living cell from its environment. It exhibits selective permeability, allowing some substances to cross more easily than others, and plays a critical role in maintaining cellular homeostasis.

Concept 7.1: Cellular Membranes as Fluid Mosaics of Lipids and Proteins

Phospholipids and Membrane Structure

Phospholipids are amphipathic molecules containing hydrophobic and hydrophilic regions.
The hydrophobic tails are shielded inside the membrane, while the hydrophilic heads are exposed to water on either side.
In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.
Proteins are not randomly distributed in the plasma membrane.

The Fluidity of Membranes

Membranes are held together mainly by weak hydrophobic interactions.
Most lipids and some proteins can move sideways within the membrane; rarely, a lipid may flip-flop across the membrane.
Membranes remain fluid as temperature decreases until a critical point, which depends on lipid composition.
Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids.
Cholesterol acts as a fluidity buffer in animal cells, resisting changes in membrane fluidity caused by temperature shifts.

Evolution of Differences in Membrane Lipid Composition

Variations in lipid composition of cell membranes among species are adaptations to specific environmental conditions.

Membrane Proteins and Their Functions

Types of Membrane Proteins

Integral proteins penetrate the hydrophobic core of the lipid bilayer; many are transmembrane proteins.
Peripheral proteins are loosely bound to the surface of the membrane.

Functions of Membrane Proteins

Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)

Medical Relevance

Cell-surface proteins are important in immune responses (e.g., HIV must bind to CD4 and CCR5 to infect a cell).

Role of Membrane Carbohydrates in Cell-Cell Recognition

Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.
Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or proteins (glycoproteins).
The diversity of carbohydrate chains allows for cell type specificity.

Synthesis and Sidedness of Membranes

Membranes have distinct inside and outside faces.
The asymmetrical distribution of proteins, lipids, and carbohydrates is determined during membrane synthesis in the ER and Golgi apparatus.

Concept 7.2: Membrane Structure Results in Selective Permeability

Permeability of the Lipid Bilayer

Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through rapidly.
Hydrophilic molecules, including ions and polar molecules, do not cross the membrane easily.
Proteins built into the membrane play key roles in regulating transport.

Transport Proteins

Allow passage of hydrophilic substances across the membrane.
Channel proteins (e.g., aquaporins) facilitate the transport of water.
Carrier proteins bind to molecules and change shape to shuttle them across the membrane.
Each transport protein is specific for the substance it moves.

Concept 7.3: Passive Transport Is Diffusion of a Substance Across a Membrane with No Energy Investment

Diffusion

Diffusion is the tendency of molecules to spread out evenly into the available space.
At dynamic equilibrium, as many molecules cross one way as cross in the other direction.
Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases.
No energy is expended by the cell to make diffusion happen.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.
Water diffuses from the region of lower solute concentration to the region of higher solute concentration.

Tonicity and Water Balance

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water.
Isotonic solution: Solute concentration is the same as inside the cell; no net water movement.
Hypertonic solution: Solute concentration is greater than inside the cell; cell loses water.
Hypotonic solution: Solute concentration is less than inside the cell; cell gains water.

Osmoregulation

Osmoregulation is the control of solute concentrations and water balance, necessary for life in hypertonic or hypotonic environments.

Water Balance of Cells with and without Cell Walls

Cell walls help maintain water balance.
In a hypotonic solution, plant cells become turgid (firm); in an isotonic solution, they become flaccid (limp); in a hypertonic environment, they lose water and the membrane pulls away from the wall (plasmolysis).

Facilitated Diffusion: Passive Transport Aided by Proteins

Transport proteins speed the passive movement of molecules across the plasma membrane.
Channel proteins provide corridors for specific molecules or ions.
Aquaporins facilitate the diffusion of water.
Ion channels open or close in response to a stimulus (gated channels).
Carrier proteins undergo a subtle change in shape to translocate the solute-binding site across the membrane.

Concept 7.4: Active Transport Uses Energy to Move Solutes Against Their Gradients

Active Transport

Active transport moves substances against their concentration gradients using energy, usually in the form of ATP.
All proteins involved in active transport are carrier proteins.
Allows cells to maintain concentration gradients that differ from their surroundings.

Sodium-Potassium Pump

One of the most important active transport systems in animal cells.
Pumps three sodium ions out of the cell and two potassium ions into the cell, using ATP.

Membrane Potential

Voltage across a membrane due to differences in the distribution of positive and negative ions.
The cytoplasmic side is usually negative relative to the extracellular side.
The electrochemical gradient drives the diffusion of ions across a membrane.
Electrogenic pumps generate voltage across membranes (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

Cotransport

Cotransport occurs when active transport of a solute indirectly drives transport of another solute.
Example: Plant cells use the gradient of hydrogen ions generated by proton pumps to drive the active transport of nutrients into the cell.

Concept 7.5: Bulk Transport Across the Plasma Membrane

Exocytosis

Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell.

Endocytosis

The cell takes in macromolecules by forming vesicles from the plasma membrane.
Three types of endocytosis:
- Phagocytosis (cellular eating): Cell engulfs a particle in a vacuole, which fuses with a lysosome to digest the particle.
- Pinocytosis (cellular drinking): Molecules dissolved in droplets are taken up when extracellular fluid is gulped into tiny vesicles.
- Receptor-mediated endocytosis: Binding of specific solutes to receptors triggers vesicle formation.
Receptor proteins, receptors, and other molecules from the extracellular fluid are transported in the vesicles.
Emptied receptors are recycled to the plasma membrane.
Example: Human cells use receptor-mediated endocytosis to take in cholesterol carried in low-density lipoproteins (LDLs).
Individuals with familial hypercholesterolemia have missing or defective LDL receptor proteins.

Key Terms and Definitions

Phospholipid bilayer: Double layer of phospholipids that forms the core of all cell membranes.
Integral protein: Protein embedded in the membrane, often spanning the bilayer.
Peripheral protein: Protein attached to the membrane surface.
Concentration gradient: Difference in the concentration of a substance from one location to another.
Isotonic: Solution with equal solute concentration as the cell; no net water movement.
Hypertonic: Solution with higher solute concentration than the cell; cell loses water.
Hypotonic: Solution with lower solute concentration than the cell; cell gains water.
Osmoregulation: Regulation of solute concentrations and water balance by a cell or organism.
Turgor pressure: Pressure of the cell contents against the cell wall in plant cells.
Plasmolysis: Shrinking of the cytoplasm away from the cell wall due to water loss.
Facilitated diffusion: Passive transport of molecules across membranes via transport proteins.
Active transport: Movement of substances against their concentration gradient, requiring energy.
Electrochemical gradient: Combined effect of an ion's concentration gradient and electrical gradient across a membrane.
Exocytosis: Process by which cells expel materials in vesicles that fuse with the plasma membrane.
Endocytosis: Process by which cells take in materials by forming vesicles from the plasma membrane.

Table: Comparison of Passive and Active Transport

Feature	Passive Transport	Active Transport
Energy Requirement	No	Yes (usually ATP)
Direction of Movement	Down concentration gradient	Against concentration gradient
Examples	Diffusion, Osmosis, Facilitated diffusion	Sodium-potassium pump, Proton pump
Transport Proteins Involved	Channel and carrier proteins (for facilitated diffusion)	Carrier proteins only