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Chapter 5: Structure and Function of Plasma Membranes

5.1 Components and Structure

  • Plasma membrane basics

    • The fluid mosaic model describes a dynamic, mixed composition of lipids, proteins, and carbohydrates that gives the membrane a fluid character.

    • Primary role: define borders, interact with environment, and regulate substance movement and signaling.

    • The membrane is selectively permeable: some substances cross freely, others require specialized structures or energy input.

    • Surface markers (glycoproteins, glycolipids) enable cell recognition, tissue formation, and immune self/non-self discrimination.

    • Complex, integral receptors transmit signals by binding extracellular effectors (hormones, growth factors) and triggering intracellular cascades.

    • Viruses can hijack receptors to enter cells; receptor mutations can disrupt signal transduction and cause disease.

  • Historical context of membrane models

    • 1890s–1915: identification of lipids and proteins; 1935 Davson–Danielli sandwich model (proteins like bread, lipids like filling).

    • 1950s: TEM reveals a double-layer core rather than a single layer.

    • 1972: Singer and Nicolson propose the Fluid Mosaic Model.

    • Model evolves but remains the best explanation for structure and function today.

  • Principal components and their roles

    • Lipids (phospholipids, cholesterol): form the bilayer core; provide fluidity and barrier.

    • Proteins: integral (transmembrane) and peripheral (surface-associated); enable transport, signaling, and linkage to cytoskeleton.

    • Carbohydrates: attached to lipids (glycolipids) or proteins (glycoproteins); form the glycocalyx, aid in cell recognition and interactions.

  • Membrane thickness and scale

    • Typical plasma membrane thickness: 5–10 nm.

    • Human red blood cells (RBCs) are ~8 μm wide, about 1000× wider than the membrane thickness.

  • Key components (Table 5.1 concepts)

    • Phospholipid: main membrane fabric; amphiphilic with a hydrophilic head and hydrophobic tails.

    • Cholesterol: located between phospholipids; buffers temperature effects on fluidity; helps organize protein clusters into lipid rafts.

    • Integral proteins (e.g., integrins): embedded in the bilayer; hydrophobic regions interact with lipid tails; some span the membrane entirely.

    • Peripheral proteins: bound to membrane surfaces or to integral proteins; can act as enzymes or cytoskeletal anchors.

    • Carbohydrates (glycoproteins and glycolipids): exterior surface; provide cell recognition sites; contribute to glycocalyx.

  • Membrane composition by mass (typical human cell)

    • Protein ≈ 50%

    • Lipids ≈ 40%

    • Carbohydrates ≈ 10%

    • Note: proportions vary by cell type (e.g., myelin, mitochondria inner membrane, RBC membranes).

  • Glycocalyx and cell recognition

    • Glycocalyx = carbohydrate components on exterior surface of membranes (glycoproteins and glycolipids).

    • Functions: cell recognition, self/non-self discrimination, embryonic development, cell adhesion.

    • Highly hydrophilic, attracts water; aids in interaction with aqueous environments and nutrient uptake.

  • Evolution and infection context

    • Glycoprotein/glycolipid patterns influence which cells viruses can infect (e.g., HIV targets T-helper cells via CD4 receptor).

    • Viral surface variability complicates immune recognition and vaccine development due to rapid mutation of surface patterns.

  • Phospholipids and bilayer formation

    • Phospholipids are amphiphilic: hydrophilic head (polar) and hydrophobic tails (non-polar).

    • In water, lipids orient with heads outward and tails inward, forming a bilayer that separates intra- and extracellular fluids.

    • In aqueous solutions, phospholipids can form micelles or liposomes with hydrophilic heads on the exterior.

    • A phospholipid molecule structure: ext{Glycerol backbone}
      ightarrow 2 imes ext{Fatty acids}
      ightarrow ext{Phosphate head group}

    • A phospholipid generally has a polar head and two nonpolar tails; amphiphilic nature drives bilayer organization.

  • Integral and peripheral proteins: structure and function

    • Integral proteins: span the membrane via hydrophobic regions; can form single-pass or multi-pass arrangements.

    • Transmembrane segments: typically 20–25 amino acids; single-pass proteins may span all or part of the membrane; multi-pass proteins can have multiple hydrophobic and hydrophilic regions.

    • Alpha-helices and beta-sheets: integral proteins may include one or more alpha-helices or beta-sheets spanning the membrane.

    • Peripheral proteins: attached to inner or outer surfaces; can be enzymes or cytoskeletal anchors; contribute to cell-specific functions.

  • Carbohydrates and recognition on the exterior surface

    • Carbohydrates are attached to proteins (glycoproteins) or lipids (glycolipids).

    • Glycocalyx patterns provide cell identity cues and are involved in tissue formation and immune interactions.

  • Practical implications and applications

    • Immunology: membrane composition affects antigen presentation and immune recognition; immunologists study membrane components for vaccines and transplantation compatibility.

    • Cell signaling: receptors and signaling cascades are fundamental to hormonal responses and growth factor signaling.

    • Medical relevance: receptor mutations or altered receptor expression can disrupt signaling and immune responses.

5.2 Passive Transport

  • Core concept: passive transport moves substances down their concentration gradients without cellular energy expenditure.

  • Selective permeability and membrane asymmetry

    • Membrane interior and exterior are not identical; proteins and carbohydrates create binding sites and influence selective transport.

    • Amphiphilic nature allows some molecules to diffuse while others require channels or carriers.

  • Non-polar, small, and lipid-soluble molecules

    • Diffuse easily through the lipid core (e.g., fat-soluble vitamins A, D, E, K; oxygen, CO2).

    • Diffusion through the membrane is energy-free and driven by concentration gradients.

  • Diffusion and diffusion rate factors

    • Diffusion: movement from high to low concentration until dynamic equilibrium (no net movement).

    • Factors affecting diffusion rate:

    • Extent of concentration gradient: larger gradient → faster diffusion

    • Mass of diffusing molecules: heavier molecules diffuse more slowly

    • Temperature: higher temperature → faster diffusion

    • Solvent density: higher density → slower diffusion

    • Channel vs. carrier-mediated diffusion (facilitated diffusion) adds specificity and can saturate.

  • Channel proteins

    • Some channels are always open; others are gated and respond to voltage, ligands, mechanical forces, etc.

    • Specific to ions (e.g., Na+, K+, Ca2+, Cl−) and tissue type (kidney tubules, neurons, muscles).

    • Critical for electrical signaling and action potentials in nerves and muscles.

  • Carrier proteins

    • Bind a substance and change conformation to shuttle it across the membrane.

    • Highly specific for a single substrate; saturable when all carriers are bound.

    • Glucose transport proteins (GLUTs) illustrate facilitated diffusion; can become saturated (e.g., glucose spillage in diabetes when supply exceeds transport capacity).

  • Transport rates

    • Channel-mediated diffusion: tens of millions of molecules per second.

    • Carrier-mediated diffusion: ~10^3 to 10^6 molecules per second.

  • Osmosis and water movement

    • Osmosis = movement of water through a semipermeable membrane along its concentration gradient.

    • Aquaporins greatly facilitate water diffusion in cells (e.g., RBCs, kidney tubules).

    • Osmosis proceeds until the water concentration gradient is balanced or hydrostatic/osmotic pressures balance.

  • Tonicity and osmolarity

    • Osmolarity: total solute concentration in a solution; can be different from visible color or turbidity.

    • Tonicity describes how extracellular solutions affect cell volume via osmosis:

    • Hypotonic: extracellular fluid has lower osmolarity than the cell cytoplasm → water enters the cell.

    • Hypertonic: extracellular fluid has higher osmolarity → water leaves the cell.

    • Isotonic: extracellular fluid has the same osmolarity as the cell → no net water movement.

  • Practical terms for living systems

    • Hypotonic environments cause swelling; hypertonic environments cause shrinking; isotonic conditions maintain cell size.

    • Red blood cells can lyse in hypotonic solutions; plants maintain turgor pressure via cell walls.

  • Osmoregulation and organismal adaptations

    • Plants: cell walls prevent lysis in hypotonic environments; turgor pressure stiffens cells; plasmolysis occurs if water is unavailable.

    • Protists (paramecia, amoebas): contractile vacuoles pump out excess water to prevent lysis.

    • Marine vs freshwater animals:

    • Freshwater fish: osmotic environment is hypotonic; gain water, lose salts; gills uptake salts, dilute urine.

    • Saltwater fish: hypertonic environment; lose water, excrete concentrated urine; salts expelled via gills.

    • Vertebrates: kidneys regulate body water; osmoreceptors monitor solute levels in blood; albumin helps maintain osmotic pressure.

  • Visual aids and learning links

    • Visual connections illustrate osmosis and tonicity effects on red blood cells and plant cells.

5.3 Active Transport

  • Electrochemical gradients and ion transport

    • Ions experience both chemical (concentration) gradients and electrical gradients across membranes.

    • The electrochemical gradient for an ion i combines chemical and electrical components: ext{G}i = RT \, \ln\left(\frac{[i]{in}}{[i]{out}}\right) + zi F \Delta\psi where z_i is the ion charge, F is Faraday’s constant, and \Delta\psi is the membrane potential.

  • Moving against gradients requires energy

    • Primary active transport uses ATP directly to move substances against their gradient.

    • Secondary (co-) transport uses the energy from an established gradient (usually from primary transport) to move another substance against its gradient.

  • Carrier proteins for active transport

    • Uniporters: move one substance in one direction.

    • Symporters: move two substances in the same direction.

    • Antiporters: move two substances in opposite directions.

    • Many carriers can also function in facilitated diffusion (no ATP required) when gradients exist.

  • Major pumps and examples

    • Na+/K+-ATPase (Na+-K+ pump): uses ATP to move Na+ out and K+ in, maintaining electrochemical gradients.

    • Mechanism (overview):
      1) Three Na+ ions bind inside the cell.
      2) ATP is hydrolyzed; a phosphate group is transferred to the pump.
      3) Pump changes shape and releases Na+ outside.
      4) Pump affinity shifts to K+; two K+ ions bind outside.
      5) phosphate is released; pump changes back to pull K+ in.
      6) K+ is released inside; cycle restarts.

    • Stoichiometry: 3\ Na^+ \,\text{out} \quad\text{and}\quad 2\ K^+ \,\text{in} per ATP hydrolyzed.

    • Result: interior becomes more negative; generates membrane potential (electrogenic pump).

    • H+/K+ ATPase, Ca2+ ATPase, H+ ATPase: other pumps that move H+, K+, Ca2+; examples of antiporters or pumps.

  • Secondary active transport (co-transport)

    • Uses established electrochemical gradient to move substances against their gradient.

    • Commonly transports amino acids and glucose in conjunction with Na+ movement.

    • Stores energy in mitochondrial H+ gradient to drive ATP synthesis via ATP synthase.

    • Example transporter: Na+/glucose cotransport via symporters in intestinal epithelia and kidney tubules.

  • Functional implications and limitations

    • Pumps and transporters set intracellular ion and solute concentrations essential for cell function.

    • Transport rate depends on transporter availability and ATP supply; poisoning metabolic pathways can disrupt active transport.

  • Secondary transport example: NCX transporter

    • Na+/Ca2+ exchanger (NCX) operates as secondary active transport in cardiac muscle, driven by Na+ gradient to move Ca2+.

  • Visual summaries

    • Figures illustrate uniporters, symporters, antiporters, and the Na+/K+-ATPase cycle, highlighting electrogenic effects and gradient formation.

5.4 Bulk Transport

  • Endocytosis overview

    • Active transport process where the cell ingests large particles, macromolecules, or whole cells.

    • The plasma membrane invaginates to form a pocket, which pinches off into an intracellular vesicle.

  • Phagocytosis (cell eating)

    • Engulfs large particles or microbes (e.g., neutrophils ingesting pathogens).

    • Involves clathrin-coated pits that help form vesicles; vesicles become endosomes and fuse with lysosomes for degradation; nutrients released into cytosol or extracellular fluid after processing.

  • Pinocytosis (cell drinking)

    • Engulfs extracellular fluid and dissolved solutes in smaller vesicles than phagocytosis.

    • Does not typically require lysosome fusion; a variant, potocytosis, uses caveolin-coated pits (caveolae).

    • Potocytosis can enable transcytosis, transporting material across the cell.

  • Receptor-mediated endocytosis

    • Highly specific uptake mechanism using receptors that bind particular ligands (e.g., LDL receptor for cholesterol-carrying LDL).

    • Clathrin-coated pits concentrate selected ligands; defective receptors (e.g., familial hypercholesterolemia) lead to disease due to reduced clearance from blood.

    • Some pathogens exploit this pathway via cross-reactive binding to receptors (e.g., flu, diphtheria, cholera toxins).

  • Exocytosis

    • Reverses endocytosis; vesicles fuse with the plasma membrane and release contents to the extracellular space.

    • Roles include neurotransmitter release, secretion of extracellular matrix proteins, and other signaling molecules.

  • Table 5.2 (transport methods, energy use, materials)

    • Summary mapping of different transport types, energy requirements, and typical transported materials.

  • Connections and applications

    • Endocytosis and exocytosis are essential for communication between cells, immune responses, and maintenance of membrane composition.

    • Dysfunctions in receptor-mediated endocytosis can lead to disease; exocytosis underlies neurotransmission and secretory pathways.

Key Terms (selected definitions)

  • amphiphilic: molecule with both polar (hydrophilic) and nonpolar (hydrophobic) regions.

  • antiporter: transporter that moves two or more different ions or molecules in opposite directions.

  • aquaporin: channel protein that enables rapid water movement across membranes.

  • carbohydrate: sugar units attached to lipids or proteins on the membrane surface; part of the glycocalyx.

  • caveolin: coating protein used in potocytosis on the cytoplasmic side of the membrane.

  • cholesterol: lipid that modulates membrane fluidity and organizes lipid rafts.

  • diffusion: passive transport down a concentration gradient without energy expenditure.

  • electrochemical gradient: combined chemical and electrical gradient affecting ion movement; represented by
    \Delta Gi = RT \ln\left(\frac{[i]{in}}{[i]{out}}\right) + zi F \Delta\psi

  • exocytosis: bulk transport that releases vesicle contents to the exterior.

  • facilitated diffusion: passive transport via transport proteins (channels or carriers) down a gradient.

  • fatty acid tail: hydrophobic component of phospholipids, which can be saturated or unsaturated.

  • glycocalyx: sugar-rich coating on the cell surface formed by glycoproteins and glycolipids.

  • glycoprotein: protein with attached carbohydrate chains.

  • glycolipid: lipid with attached carbohydrate chains.

  • glycolysis: (not in this content) — not applicable; included here as a memory cue: focus is on membrane transport, not glycolysis.

  • glycoprotein/glycolipid: carbohydrate groups attached to proteins/lipids, respectively.

  • glycocalyx: see glycocalyx entry above.

  • hypotonic/hypertonic/isotonic: tonicity terms describing how extracellular solutions affect cell volume.

  • integrin: example of an integral membrane protein; participates in signaling and cell adhesion.

  • LDL receptor: receptor involved in receptor-mediated endocytosis of LDL cholesterol; defects cause familial hypercholesterolemia.

  • lipid raft: cholesterol- and sphingolipid-enriched membrane microdomain that organizes signaling molecules.

  • lysosome: organelle that digests endocytosed material.

  • micelle/liposome: self-assembled phospholipid structures in aqueous environments.

  • NAD+/NADH (not relevant here) – not part of this chapter’s focus; omitted.

  • Na+/K+-ATPase: classic primary active transporter that exchanges 3 Na+ out for 2 K+ in per ATP hydrolysis; electrogenic.

  • nanotropic (not used) – not applicable; omitted.

  • osmosis: diffusion of water across a semipermeable membrane along water’s gradient.

  • osmoregulation: regulatory mechanisms that maintain osmotic balance.

  • osmolality/osmolarity: measure of solute concentration; osmolarity accounts for all solutes, including non-diffusible particles.

  • permeability: ease with which a substance crosses a membrane.

  • phagocytosis: endocytic uptake of large particles by engulfment.

  • pinocytosis: endocytic uptake of extracellular fluid and solutes.

  • receptor-mediated endocytosis: selective endocytosis triggered by ligand–receptor interactions.

  • transcytosis: transport across a cell via endocytosis on one side and exocytosis on the other.

  • vesicle: membrane-bound compartment used in transport.

  • vesicle fusion: joining of a vesicle membrane with a target membrane, releasing contents.

Chapter Summary

  • 5.1 Components and Structure

    • The plasma membrane is best described by the Fluid Mosaic Model.

    • Core components: phospholipids, cholesterol, proteins, and carbohydrates.

    • Membrane fluidity and mosaic nature arise from phospholipid tails, cholesterol buffering, and the mobility of proteins within the bilayer.

    • Glycocalyx and surface carbohydrates enable cell recognition and immune interactions.

  • 5.2 Passive Transport

    • Passive diffusion and osmosis move substances across membranes without direct energy use.

    • Non-polar molecules diffuse readily; polar molecules and ions require channels or carriers.

    • Channel proteins enable rapid diffusion; carrier proteins enable selective, saturable transport.

    • Osmosis and tonicity determine water movement and cell volume; osmoregulation adapts cells to varying environments.

  • 5.3 Active Transport

    • Active transport uses ATP to move substances against gradients; electrochemical gradients combine chemical and electrical forces.

    • Primary active transport uses direct ATP hydrolysis (e.g., Na+/K+-ATPase).

    • Secondary active transport uses the energy stored in gradients to move other substances (e.g., glucose with Na+).

    • Transport proteins include uniporters, symporters, and antiporters; pumps such as Ca2+ ATPase and H+/K+ ATPase.

    • The Na+/K+-ATPase pump creates an electrochemical gradient and contributes to the cell’s resting potential.

  • 5.4 Bulk Transport

    • Endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) internalizes large particles or solutes.

    • Potocytosis uses caveolin-containing vesicles for selective uptake and transcytosis.

    • Receptor-mediated endocytosis is highly specific and can be blocked by receptor defects, leading to disease;
      viruses can exploit this pathway for cell entry.

    • Exocytosis releases vesicle contents to the extracellular space and is essential for secretion and signaling.

  • Overall themes

    • The membrane’s structure supports selective permeability, signaling, cell recognition, and dynamic responses to environmental changes.

    • Proper function depends on a balanced composition of lipids, proteins, and carbohydrates, as well as energy availability for active processes.

    • Understanding membrane transport mechanisms is key to biology, medicine, and physiology (e.g., nerve signaling, kidney function, immune responses).

Visual Connection and Review prompts (highlights)

  • Why is membrane fluidity advantageous for cells?

  • How do phospholipids spontaneously form membranes and why is amphiphilicity critical?

  • How can extracellular peripheral proteins act as receptors to transmit signals into the cell?

  • What factors influence diffusion rates (molecular size, temperature, solution density, travel distance)?

  • Why does water move through a membrane?

  • Why are isotonic intravenous solutions important in clinical settings?

  • How do decreased temperatures affect diffusion rates across membranes?

  • What happens if potassium channels fail to allow ions to leave the cell while aquaporins remain active?

  • Where does the energy for active transport come from? Why is the Na+/K+ pump electrogenic?

  • How does receptor-mediated endocytosis differ from phagocytosis, and why does this matter in disease?

  • What organelles rely on exocytosis for function (e.g., neurotransmitter release, secretory pathways)?

  • How do bulk transport processes contribute to tissue and organismal homeostasis?