Chapter 7 p2: Membrane structure and function – Transport across membranes

Plasma membrane: regulation of inbound and outbound traffic

• The plasma membrane regulates traffic into and out of the cell.
• Transport types:
  • Passive transport of small molecules: does not require energy; may involve transport proteins.
  • Vesicle-based transport: bulk processes.
  • Exocytosis: large molecules are secreted when a vesicle fuses with the plasma membrane.
  • Endocytosis: large molecules are taken in when the plasma membrane pinches inward, forming a vesicle.
  • Active transport: small molecules moved with energy input via a transport protein, against their gradient.
• Bulk transport moves large molecules using vesicles.
• ATP/ADP role: energy currency driving active processes.

Key concepts: transport across membranes

• Passive transport is diffusion of a substance across a membrane with no energy investment.
• The concentration gradient represents the potential energy that drives diffusion.
• At dynamic equilibrium, as many molecules cross in one direction as in the opposite direction.
• Velocity and rate depend on gradient, temperature, and membrane properties.

Membrane proteins: functions in the membrane

• Proteins serve multiple roles in the membrane:
  • Enzymatic activity (enzyme catalysis on membrane surface or within membrane).
  • Transport (carriers and channels).
  • Signal transduction (receptors triggering cascades when ligands bind).
  • Cell–cell recognition (glycoproteins as identifiers).
  • Intercellular joining (tight or gap junction-like associations).
  • Attachment to the cytoskeleton and extracellular matrix (ECM) for stability and shape.
• Glycoproteins often participate in cell recognition and signaling.

Passive transport: diffusion details

• Diffusion = net drift of molecules toward lower concentration.
• The concentration gradient itself provides the driving energy.
• Passive transport = diffusion of a substance across a membrane without energy investment.
• At dynamic equilibrium, net flux is zero (equal rates in both directions).

Facilitated diffusion: carrier proteins

• Facilitated diffusion is passive and uses carrier proteins.
• Carrier proteins undergo subtle conformational changes to move a solute across the membrane.
• Shape change is triggered by binding and release of the transported molecule.
• Substances move down their concentration gradients; no energy input required.
• Facilitated diffusion vs. simple diffusion: both are passive, but facilitated diffusion requires a transport protein.

Gated channels

• Some channels are gated by small-molecule signals.
• Example: acetylcholine binding closes or opens an ion channel; when acetylcholine binds, the channel changes conformation and opens, allowing ion passage.

Active transport: energy use and gradient creation

• Active transport uses energy (usually ATP) to move substances against their concentration gradient.
• Facilitated diffusion is passive and moves substances down their gradient.
• The sodium–potassium pump is a classic example of active transport.
• Key feature: transport protein undergoes conformational changes to move ions against the gradient.

Electrogenic pumps and membrane potential

• Electrochemical gradient drives ion movement across membranes:
  • Chemical force: due to ion concentration gradient.
  • Electrical force: due to membrane potential.
• Proton pumps are the main electrogenic pumps in plants, bacteria, and fungi.
• The Na+/K+ ATPase is electrogenic because it pumps three Na+ out and two K+ in, contributing to membrane potential.
• All cells have a membrane potential (voltage across the membrane) due to charge separation; typical cytoplasmic interior is negative.
• Common membrane potential range: approximately V_m ext{ between } -200\,\mathrm{mV} ext{ and } -50\,\mathrm{mV}. (negative inside)

Distinctions: facilitated diffusion vs. active transport (concept check)

• Answer to a typical exam distinction:
  • Facilitated diffusion depends on a pre-existing favorable energy gradient, while active transport creates or maintains such a gradient.
  • Other distinctions (e.g., requiring integral membrane proteins, energy input) are not correct when comparing these two processes.
  • Correct statement: facilitated diffusion depends on a pre-existing gradient; active transport uses energy to move against it.

Types of transport across membranes

• Uniport: transport of a single substance in one direction.
• Cotransport (secondary active transport): two different substances move together.
  • Symport: both substances move in the same direction.
  • Antiport: substances move in opposite directions.
• Cotransport can occur when active transport of a solute indirectly drives transport of another solute.
• In plants, proton gradients generated by proton pumps drive active transport of sugars (example of symport).

Sodium–glucose cotransporter (SGLT) example

• Symporter: glucose transporter driven by a Na+ gradient.
• The rate of glucose uptake increases with a steeper Na+ gradient; steeper gradient -> faster glucose uptake.
• This demonstrates how an ion gradient powers the transport of another solute.

Bulk transport across the plasma membrane

• Bulk transport occurs via exocytosis and endocytosis.
• Endocytosis brings material into the cell; exocytosis exports material.
• Endocytosis pathway: endosome formation, trafficking, and fusion with the plasma membrane; Golgi apparatus involved in processing cargo prior to export.
• Endosomes are intracellular vesicles that sort contents and route them to destinations.

Exocytosis

• Exocytosis secretes large macromolecules by fusing vesicles with the plasma membrane.
• Transport vesicles migrate to the membrane, fuse, and release their contents.
• Secretory cells (e.g., cells producing hormones or antibodies) rely on exocytosis to export products.

Endocytosis: three animal-cell types

• Phagocytosis: cellular ingestion of a particle by extending pseudopodia, forming a food vacuole; vacuole fuses with a lysosome for digestion.
• Pinocytosis: nonspecific uptake where extracellular fluid is gulped into tiny vesicles; solutes in the fluid are taken up; coat proteins line vesicles' inner surface forming coated vesicles.
• Receptor-mediated endocytosis: vesicle formation is triggered when solutes bind to receptors; receptors clustered in coated pits form coated vesicles; emptied receptors are recycled back to the plasma membrane via the same vesicle.

Receptor-mediated endocytosis: details

• Specific solutes bind to receptors on the cell surface.
• Receptors and bound ligands cluster within coated pits that invaginate to form coated vesicles.
• After internalization, receptors are often recycled to the plasma membrane for reuse.

Animal cell membrane structure (overview)

• Phospholipid bilayer includes sterols (e.g., cholesterol in animal membranes).
• The bilayer separates two aqueous regions: extracellular and intracellular.
• Proteins are embedded in the bilayer (integral and peripheral proteins).
• Carbohydrates are attached to the exterior of the membrane (glycoproteins and glycolipids) and contribute to cell recognition.
• These structural features underpin transport, signaling, and interactions with the environment.

Connections and implications

• These processes illustrate how cells regulate internal conditions, communicate, and respond to changes in the environment.
• Energy coupling (ATP hydrolysis, ion gradients) is central to moving substances against gradients and maintaining homeostasis.
• Understanding electrogenic pumps and membrane potential is crucial for nerve signaling and muscle contraction.
• Bulk transport mechanisms are essential for secretion of hormones, antibodies, and extracellular matrix components, as well as for nutrient uptake in multicellular organisms.

Foundational principles and practical relevance

• Transport across membranes exemplifies core thermodynamic and kinetic principles: gradients drive diffusion; energy input drives active transport; conformational changes in proteins enable selective movement.
• The electrochemical gradient combines chemical and electrical forces, linking thermodynamics to membrane biophysics.
• Transport proteins provide specificity and regulation; malfunctions can underlie diseases or influence pharmacokinetics of drugs.

Key terms and symbols

• Diffusion, concentration gradient, dynamic equilibrium
• Facilitated diffusion, carrier proteins, conformational change
• Gated channels, neurotransmitter signaling
• Active transport, ATP, transport proteins
• Electrochemical gradient, chemical force, electrical force
• Proton pumps, Na+/K+ ATPase, membrane potential, V_m
• Uniport, symport, antiport, cotransport
• Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis)
• Exocytosis, endosome, Golgi apparatus
• Phospholipid bilayer, sterols, cholesterol, glycoproteins, glycolipids

Summary of numerical references and formulas

• Membrane potential range: Vm ext{ is typically } ext{negative inside}, ext{ approximately } -200\,\mathrm{mV} \le Vm \le -50\,\mathrm{mV}.
• Na+/K+ pump stoichiometry: pumps out 3\,\mathrm{Na}^+ for every 2\,\mathrm{K}^+ it brings in, contributing to membrane potential.
• Electrochemical gradient for an ion moving across a membrane can be described by the generalized form: \Delta \mu = RT \ln\left(\frac{[\mathrm{S}]{in}}{[\mathrm{S}]{out}}\right) + zF\Delta \psi, where z is the ion charge, F is Faraday's constant, and \Delta \psi is the membrane potential difference.
• Proton pumps and Na+/K+ ATPase create and maintain chemical and electrical gradients essential for transport and signaling.

Connections to foundational principles

• Diffusion and gradient-driven movement reflect Fick’s laws and thermodynamics.
• Energy coupling (ATP hydrolysis) links biochemical reactions to transport work.
• Membrane potential is a basic electrical property arising from selective ion transport and charge separation, paralleling principles in electrostatics and physiology.