Chapter 7: Membranes – Structure, Function, and Transport in Cells

Membranes: Structure and Function

Introduction to Plasma Membranes

The plasma (cell) membrane is a fundamental structure in all living cells, separating the internal environment from the external surroundings. It plays a crucial role in maintaining homeostasis by regulating the movement of substances in and out of the cell.

• Definition: The plasma membrane is a selectively permeable barrier composed primarily of lipids and proteins.

• Key Functions: Protection, communication, transport, and cell recognition.

• Homeostasis: The membrane helps maintain a stable internal environment.

Phospholipid Bilayer

General Membrane Structure

The basic structure of the plasma membrane is the phospholipid bilayer, which forms the foundation for all biological membranes.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Bilayer Arrangement: Hydrophobic tails face inward, shielded from water, while hydrophilic heads face outward toward the aqueous environment.

• Fluid Mosaic Model: The membrane is a dynamic structure with proteins and other molecules embedded within the lipid bilayer.

Example: Lipid-bilayer spheres and sheets are common arrangements in cell membranes.

Phospholipid Chemical Bonds and Structure

• Phosphate Group: Forms the polar head of the molecule.

• Glycerol Backbone: Connects the head to the fatty acid tails.

• Fatty Acid Chains: Nonpolar tails that determine membrane fluidity.

Selective Permeability of the Plasma Membrane

Transport Across Membranes

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others. This property is essential for cellular function and survival.

• Factors Affecting Permeability: Size, polarity, and charge of molecules.

• Hydrophobic (nonpolar) molecules: Pass through the membrane rapidly (e.g., hydrocarbons, O2, CO2).

• Hydrophilic (polar) molecules and ions: Do not pass easily and often require transport proteins.

Examples of Selective Permeability

• Gases: CO2, N2, O2 diffuse freely.

• Small uncharged polar molecules: Ethanol, water, urea can cross but less efficiently.

• Ions and large polar molecules: Glucose, Na+, K+, Cl-, proteins, and nucleic acids require specialized transport mechanisms.

Membrane Fluidity

Factors Influencing Fluidity

Membrane fluidity is vital for proper membrane function, affecting the movement of proteins and lipids within the bilayer.

• Unsaturated Fatty Acids: Increase fluidity due to kinks in the tails.

• Saturated Fatty Acids: Decrease fluidity, making the membrane more viscous.

• Temperature: Lower temperatures can cause membranes to solidify; higher temperatures increase fluidity.

Membrane Proteins

Types and Functions of Membrane Proteins

Proteins embedded in the plasma membrane perform a variety of essential functions.

• Peripheral Proteins: Bound to the surface of the membrane.

• Integral Proteins: Penetrate the hydrophobic core; some span the membrane (transmembrane proteins).

• Functions:

  • Transport of molecules

  • Enzymatic activity

  • Signal transduction

  • Cell-cell recognition

  • Attachment to cytoskeleton and extracellular matrix

Example: Glycoproteins and glycolipids are involved in cell recognition and immune responses.

Transport Processes Across Membranes

Passive Transport

Passive transport is the movement of substances across the membrane without energy input, driven by concentration gradients.

• Simple Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Requires transport proteins but no energy; includes channel and carrier proteins.

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

Equation for Diffusion Rate:

$J = -D \frac{dC}{dx}$

Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

Osmosis and Tonicity

Osmosis is critical for maintaining water balance in cells. Tonicity describes the effect of a solution on cell volume.

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

• Hypertonic Solution: Cell loses water and shrivels.

• Hypotonic Solution: Cell gains water and may burst (lyse).

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

Active Transport

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

• Example: Sodium-potassium pump ($Na^+/K^+$-ATPase) maintains electrochemical gradients.

• Electrochemical Gradient: Combination of concentration and electrical gradients that drive ion movement.

Equation for Membrane Potential:

$V_m = \frac{RT}{F} \ln \frac{[K^+]_{out}}{[K^+]_{in}}$

Where $V_m$ is membrane potential, $R$ is the gas constant, $T$ is temperature, $F$ is Faraday's constant, and $[K^+]_{out}$ and $[K^+]_{in}$ are potassium concentrations outside and inside the cell.

Coupled Transport (Co-transport)

Co-transport occurs when the active transport of one solute drives the transport of another solute.

• Symporter: Both solutes move in the same direction.

• Antiporter: Solutes move in opposite directions.

Transport Type	Number of Solutes	Direction
Uniporter	One	One direction
Symporter	Two	Same direction
Antiporter	Two	Opposite directions

Bulk Transport: Exocytosis and Endocytosis

Exocytosis

Exocytosis is the process by which cells export large molecules, such as proteins and polysaccharides, via vesicles that fuse with the plasma membrane.

• Example: Release of neurotransmitters from nerve cells.

Endocytosis

Endocytosis allows cells to import large molecules or particles by engulfing them in vesicles formed from the plasma membrane.

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

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

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors.

Example: White blood cells use phagocytosis to consume bacteria.

Viruses and Endocytosis

Some viruses exploit receptor-mediated endocytosis to enter host cells by mimicking signal molecules.

Summary Table: Methods of Membrane Transport

Method	Type	Examples
Simple Diffusion	Passive	O2, CO2
Osmosis	Passive	Water
Facilitated Diffusion	Passive	Ions, glucose
Active Transport	Active	Na+, K+ pump
Phagocytosis	Active	Bacteria, food particles
Pinocytosis	Active	Extracellular fluid
Receptor-mediated Endocytosis	Active	Cholesterol, iron
Exocytosis	Active	Neurotransmitters, hormones

Additional info:

• Diseases such as cystic fibrosis and certain neurological disorders are associated with defects in membrane transport proteins.

• Membrane transport is essential for nerve impulse transmission, muscle contraction, and cellular signaling.