CHAPTER 5

Membrane Transport and Cell Signaling

Overview of Membrane Transport

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Its structure and associated proteins enable a variety of transport mechanisms, from passive diffusion to energy-dependent processes and vesicle-mediated trafficking.

• Membrane fluidity is influenced by lipid composition and cholesterol content.

• Membrane proteins serve as channels, carriers, receptors, and enzymes.

• Transport mechanisms include passive transport, active transport, co-transport, endocytosis, and exocytosis.

Membrane Fluidity

Lipid Composition and Fluidity

Membrane fluidity is essential for proper cell function, affecting permeability, protein mobility, and cell signaling.

• Unsaturated fatty acids (with double bonds) increase fluidity by preventing tight packing of phospholipids, especially at low temperatures.

• Cholesterol acts as a fluidity buffer:

  • At moderate/warm temperatures, cholesterol reduces phospholipid movement, making the membrane less fluid.

  • At low temperatures, cholesterol prevents solidification by disrupting regular packing.

• Viscosity refers to a more solid-like state ("hard animal fat"), while fluidity refers to a more liquid-like state ("olive oil").

Condition	Predominant Lipid	Effect on Fluidity
Cold (e.g., winter wheat)	High unsaturated fats	Membrane stays fluid, prevents cracking
Warm	Cholesterol present	Membrane becomes less fluid
Extreme cold	Cholesterol + unsaturated fats	Prevents solidification

Membrane Proteins

Types and Functions

Membrane proteins are critical for transport, signaling, and structural support.

• Integral (Intrinsic) Proteins:

  • Span the lipid bilayer, penetrating the hydrophobic core.

  • Functions: transport (channels, pumps), enzymatic activity, signal transduction (receptors).

• Peripheral (Extrinsic) Proteins:

  • Loosely attached to the membrane surface; do not penetrate the hydrophobic core.

  • Functions: support, cytoskeletal anchoring, signaling assistance.

Protein Type	Location	Embedded?	Typical Roles
Integral	Across bilayer	Yes	Channels, pumps, receptors, enzymes
Peripheral	Surface (inner or outer leaflet)	No	Cytoskeletal anchoring, signaling assistance

Transport Mechanisms

Passive Transport

Passive transport moves substances down their concentration gradients without energy input.

• Simple diffusion: Small, nonpolar molecules (e.g., O2, CO2) cross the lipid bilayer directly.

• Facilitated diffusion: Uses channel or carrier proteins for ions and polar molecules (e.g., Na+, K+, glucose).

• Osmosis: Diffusion of water across a selectively permeable membrane, often via aquaporins.

Active Transport

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

• Sodium–Potassium Pump (Na+/K+ ATPase):

  • Exports 3 Na+ ions and imports 2 K+ ions per cycle.

  • Mechanism:

    1. Na+ binds to the pump.

    2. ATP hydrolysis phosphorylates the protein, causing a conformational change and releasing Na+ outside.

    3. K+ binds; phosphate is released, protein returns to original shape, releasing K+ inside.

• Co-transport (Secondary Active Transport):

  • Couples the movement of one solute down its gradient to drive another solute against its gradient.

  • Example: Proton-sucrose symporter in plants.

    1. Proton pump creates H+ gradient (active, uses ATP).

    2. H+ flows back through a co-transporter, carrying sucrose into the cell (no direct ATP use).

Transport	Energy Required?	Protein Involved?	Example Molecule
Simple diffusion	No	No	O2, CO2
Osmosis	No	Aquaporin (channel)	H2O
Facilitated diffusion (channel)	No	Channel protein	Ions (Na+, K+)
Facilitated diffusion (carrier)	No	Carrier protein	Glucose
Active transport	Yes (ATP)	Pump (e.g., Na+/K+ ATPase)	Na+, K+
Co-transport	No (uses gradient)	Symporter (e.g., H+/sucrose)	Sucrose
Endocytosis	No (uses membrane remodeling)	Membrane vesicles	Particulate nutrients

Tonicity in Cells

Osmotic Balance

• Animal cells: Prefer isotonic conditions (no net water movement).

• Plant cells: Prefer hypotonic environments, which generate turgor pressure (cell becomes firm).

• Plasmolysis: Loss of water in a plant cell due to a hypertonic exterior, causing the cell membrane to pull away from the wall.

• Turgidity: Water influx in a hypotonic environment, making the cell rigid.

Endocytosis

Types of Endocytosis

Endocytosis is the process by which cells internalize extracellular material via vesicle formation.

Type	Typical Cargo	Size	Key Feature
Phagocytosis	Large particles (e.g., bacteria, dead cells)	> 0.5 µm	Whole-particle engulfment; vesicle fuses with lysosomes
Pinocytosis	Fluid & dissolved solutes	< 0.1 µm	Continuous "drinking" of extracellular fluid; non-specific
Receptor-Mediated Endocytosis	Specific ligands (e.g., hormones, nutrients)	Variable (generally small)	Ligand binding triggers vesicle formation; highly selective

• Phagocytosis ("cellular eating"):

  • Cytoplasmic extensions (pseudopodia) surround the target particle.

  • Membrane encloses the particle in a vesicle (phagosome), which fuses with lysosomes for digestion.

  • Example: Amoebae and macrophages use phagocytosis to clear pathogens.

• Pinocytosis ("cellular drinking"):

  • Continuous uptake of extracellular fluid and solutes in small vesicles.

  • Non-specific; does not require receptors.

  • Example: Kidney tubule cells reabsorb filtrate via pinocytosis.

• Receptor-Mediated Endocytosis:

  • Specific ligands bind to cell-surface receptors, triggering vesicle formation (often involving clathrin).

  • Allows regulated uptake of essential molecules (e.g., cholesterol via LDL receptors).

  • Example: Placental cells use this pathway for developmental signaling.

Feature	Phagocytosis	Pinocytosis	Receptor-Mediated
Cargo specificity	Non-selective (size-based)	Non-selective	Highly selective (receptor-ligand)
Energy requirement	ATP for actin remodeling	Minimal ATP (membrane curvature)	ATP for coat protein assembly
Typical outcome	Degradation in lysosomes	Nutrient harvesting	Targeted delivery (often recycled)
Example	Macrophage engulfing bacteria	Kidney tubule reabsorbing filtrate	LDL uptake by liver cells

Exocytosis

Mechanism and Biological Roles

Exocytosis is the process by which cells expel materials via vesicle fusion with the plasma membrane.

1. Synthesis & packaging: Proteins are synthesized in the endoplasmic reticulum and processed in the Golgi apparatus.

2. Vesicle targeting: Vesicles travel along the cytoskeleton to the plasma membrane.

3. Docking & fusion: SNARE proteins mediate fusion, forming a channel for release.

4. Release: Vesicle contents (e.g., hormones, neurotransmitters) are expelled into the extracellular space.

• Physiological examples:

  • Pancreatic β-cells secrete insulin to regulate blood glucose.

  • Pituitary gland releases growth hormone.

  • Adrenal medulla releases adrenaline during stress response.

Process	Speed	Typical Cargo	Biological Role
Exocytosis (bulk)	Very fast (seconds)	Large quantities of hormones or enzymes	Rapid systemic signaling
Single-molecule secretion	Slower, continuous	Small peptides, neurotransmitters	Fine-tuned local signaling

Summary of Key Terms

• Membrane fluidity: The ease with which phospholipids move within the bilayer, influenced by fatty-acid saturation and cholesterol.

• Integral protein: Spans the lipid bilayer; forms channels or receptors.

• Peripheral protein: Attached to the membrane surface; aids structure or signaling.

• Passive transport: Movement down a concentration gradient without energy input.

• Facilitated diffusion: Passive transport requiring a specific protein (channel or carrier).

• Active transport: Energy-dependent movement against a concentration gradient, typically via ATP-driven pumps.

• Co-transport: Coupling the movement of one solute down its gradient to transport another against its gradient.

• Endocytosis: Vesicle-mediated internalization of extracellular material.

• Exocytosis: Vesicle-mediated export of cellular material.

Integrated View of Membrane Transport

The plasma membrane employs a spectrum of transport strategies to maintain homeostasis and respond to environmental cues:

• Passive mechanisms (diffusion, facilitated diffusion) move small, non-charged molecules down gradients without energy.

• Active mechanisms (pumps, co-transport) consume ATP or exploit existing gradients.

• Endocytosis and exocytosis handle large particles and bulk macromolecules, linking membrane dynamics to metabolism and intercellular communication.

Key takeaway: The cell’s plasma membrane is a versatile gatekeeper, integrating multiple transport strategies to support life.