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Ch 4: Cell Structure and Membranes - Vocabulary Flashcards

4.1 The Cell Membrane Separates the Interior of the Cell from Its Environment

  • The cell membrane defines the boundary between the cell’s interior (cytoplasm) and the external environment.

  • Structure and unifying principle:

    • Plasma membranes are phospholipid bilayers with embedded or associated proteins (fluid mosaic model).

    • Lipids and proteins move laterally; the membrane is a dynamic, not a rigid, structure.

  • Fluid Mosaic Model (Key concepts):

    • Lipids form a hydrophobic core; phospholipids are amphipathic with hydrophilic heads and hydrophobic tails.

    • The bilayer is about 8 nm thick; hydrophilic heads face aqueous environments, hydrophobic tails face the interior.

    • The interior hydrophobic region acts as a barrier to polar molecules and ions.

    • Membrane proteins are embedded or attached on either side of the bilayer; they can be integral, anchored, or peripheral.

    • Lipids and proteins diffuse laterally; flip-flop (transbilayer movement) is rare.

  • Lipid composition and fluidity:

    • Phospholipids vary in fatty acid chain length (common 16–18 C) and degree of unsaturation (0–2 double bonds).

    • Saturated chains pack tightly, reducing fluidity; unsaturated chains (kinks) increase fluidity.

    • Cholesterol modulates membrane fluidity by interacting with fatty acid chains; in animals, cholesterol can constitute ~20–50% of lipid molecules and is intercalated among phospholipids.

    • In plants, phytosterols play a similar role to cholesterol.

  • Lipid diversity and membrane variation:

    • Other lipids and glycolipids contribute to membrane properties.

    • The outer surface often carries carbohydrates (glycolipids, glycoproteins) important for cell–cell recognition.

  • Proteins in membranes:

    • About 1 protein per ~25 phospholipids in many membranes; inner mitochondrial membranes can have ~1 protein per 5 lipids; myelin membranes have ~1 protein per 70 lipids.

    • Membrane proteins are categorized by how they associate with the bilayer:

    • Integral membrane proteins: span the bilayer; have hydrophobic regions inside and hydrophilic regions exposed to aqueous environments.

    • Anchored membrane proteins: covalently bonded to lipids embedded in the bilayer; do not have exposed hydrophobic regions.

    • Peripheral membrane proteins: lack exposed hydrophobic regions; interact with polar/charged regions of other proteins or phospholipid heads.

    • Some integral proteins are transmembrane (extend through the membrane); others are partially integrated.

  • Asymmetry of the membrane surfaces:

    • Inner and outer surfaces have different compositions and properties due to asymmetric distribution of proteins and lipids.

    • This asymmetry is important for functions (e.g., signaling, transport, cell–cell interactions).

  • Mobility and organization:

    • Proteins can migrate laterally; some are organized into lipid rafts or constrained by cytoskeletal attachments.

    • Fusions of cells show that membrane proteins can move, but signaling proteins may be clustered to preserve function.

  • Carbohydrates on the membrane:

    • Glycolipids and glycoproteins are located on the outside and contribute to cell recognition, adhesion, and signaling.

  • Temperature and membrane composition:

    • Environmental temperature affects lipid composition and membrane fluidity; organisms adapt by altering saturation, chain length, and cholesterol/phytosterol content.

    • Example of cold adaptation: increasing unsaturation and shortening chains to maintain fluidity in cold environments.

  • Key concepts and implications:

    • Fluidity is crucial for proper membrane function and protein mobility.

    • Membranes are selective barriers: permeable to small nonpolar molecules and gases; polar, charged, and large molecules require transport proteins.

  • Review & Apply (4.1) representative questions:

    • 1) Explain why fluidity allows integral proteins to move laterally but not flip-flop across the bilayer.

    • 2) For an integral membrane protein, which amino acids are typically located in the interior membrane-spanning region? Give examples.

    • 3) What is the evidence for membrane fluidity?

    • 4) What lipid characteristics would you predict for membranes in ducks’ feet exposed to cold water?

4.2 Passive and Active Transport Are Used by Small Molecules to Cross Membranes

  • Membrane transport concepts:

    • A selectively permeable membrane allows some substances to cross while restricting others.

    • Movement can be passive (no energy input) or active (requires energy and proteins).

  • Passive transport and diffusion:

    • Simple diffusion: small, nonpolar, or slightly polar molecules diffuse directly through the phospholipid bilayer down their concentration gradient.

    • Facilitated diffusion: requires integral membrane proteins (channel or carrier proteins) but does not require energy input; moves with the gradient.

    • Channel proteins: form pores for specific ions or water (e.g., aquaporins for water).

    • Carrier proteins: undergo conformational changes to shuttle specific solutes (e.g., glucose transporters).

    • Permeability depends on molecule properties: small, nonpolar molecules diffuse readily; small polar molecules diffuse with lower efficiency; ions and large polar molecules require transport proteins.

  • Osmosis and osmotic pressure:

    • Osmosis: movement of water across a membrane from low solute concentration to high solute concentration when the membrane is permeable to water but not to solutes.

    • Aquaporins increase water permeability.

    • Osmotic pressure (π) is the pressure needed to prevent water flow across a membrane by osmosis: oxed{ \Pi = CRT } where C is osmolarity, R is the gas constant, and T is absolute temperature.

    • Osmolarity: total number of solute particles per liter of solution; includes permeable and impermeable solutes.

    • Tonicity depends only on membrane-impermeable solutes; permeable solutes equilibrate and do not contribute to net water movement.

  • Isotonic, hypotonic, hypertonic solutions:

    • Isotonic: same osmolarity across membrane; no net water movement if the membrane is impermeable to all solutes.

    • Hypotonic outside (or inside) depends on relative solute concentrations; net water movement toward higher solute concentration.

    • Hypertonic outside: water tends to move out of the cell.

    • Animal cells: swollen and may lyse in hypotonic solutions; plant cells resist lysis via turgor pressure due to cell wall.

  • Key transport mechanisms:

    • Simple diffusion vs. facilitated diffusion (channel vs. carrier proteins).

    • Active transport: moves substances against their concentration gradient, requiring energy.

  • Primary vs. secondary active transport:

    • Primary active transport uses ATP hydrolysis (e.g., Na⁺/K⁺-ATPase) to move ions against gradients.

    • Secondary active transport uses the gradient established by primary active transport to drive transport of another solute (coupled transport).

    • Example: Na⁺-K⁺ pump establishes Na⁺ gradient; Na⁺ moving back in drives glucose uptake via a Na⁺/glucose co-transporter.

  • Comparative summary (Table concept):

    • Facilitated diffusion (channel or carrier): down the gradient; requires membrane protein; specific but not energy-requiring.

    • Active transport: up gradient; requires energy (ATP or ion gradients); specific; can be primary or secondary.

  • Review & Apply (4.2) representative questions:

    • 1) What properties determine diffusion rate across a membrane?

    • 2) Compare channel-mediated vs. carrier-mediated facilitated diffusion; which might be faster and why?

    • 3) Why do cut flowers wilt in salt water but not in distilled water?

    • 4) Identify the transport mechanism for bacteriorhodopsin (light-driven proton pumping).

    • 5) Why is brown fat heat generation associated with uncoupled diffusion from active transport?

4.3 Vesicles Are Used to Transport Large Molecules across Membranes in Eukaryotes

  • Large molecules and particles cannot efficiently cross membranes by diffusion.

  • Vesicular transport concepts:

    • Exocytosis: vesicles fuse with the plasma membrane to release contents outside the cell.

    • Endocytosis: plasma membrane invaginates to form vesicles that bring materials into the cell.

    • Endocytosis types:

    • Phagocytosis (cellular eating): engulfs large particles or cells; forms a phagosome; often receptor-mediated to initiate engulfment.

    • Pinocytosis (cellular drinking): nonspecific uptake of fluids and dissolved solutes.

    • Receptor-mediated endocytosis: highly specific uptake of macromolecules; receptors cluster in coated pits (often clathrin-coated).

  • LDL uptake as a classic example of receptor-mediated endocytosis.

  • Endomembrane system and vesicle trafficking:

    • Vesicles shuttle between ER, Golgi, lysosomes, plasma membrane.

    • Clathrin-coated pits support vesicle formation in receptor-mediated endocytosis.

  • Lysosomes and autophagy:

    • Primary lysosomes contain hydrolytic enzymes; fuse with endocytosed vesicles to form secondary lysosomes where digestion occurs.

    • Autophagy: degradation of cellular components via lysosomes.

  • Review & Apply (4.3) representative questions:

    • 1) Why is vesicle transport feasible for large molecules?

    • 2) What is the difference between phagocytosis and pinocytosis?

    • 3) Would amino acids enter a cell by receptor-mediated endocytosis? Why or why not?

4.4 Cell Size, Shape, and Ability to Move Are Determined by Internal and External Structures

  • Why cells are small:

    • Surface area-to-volume ratio decreases as size increases for objects of the same shape; limits diffusion of resources and waste removal.

    • For a sphere, SA = 4πr^2, Volume = (4/3)πr^3, so SA/V = 3/r; increasing r reduces SA/V.

  • Exceptions: some cells (e.g., neurons) are elongated to maintain adequate SA/V despite large volume.

  • Cytoskeleton and cell shape/movement:

    • Cytoskeleton: network of protein filaments providing shape, organization, and transport pathways.

    • Filaments classified into three groups:

    • Microfilaments (actin): ~7 nm diameter; involved in shape, cytoplasmic streaming, cytokinesis, pseudopodia; dynamic assembly/disassembly; often interact with myosin for contraction.

    • Intermediate filaments: 8–12 nm; provide mechanical strength; more stable; anchor organelles and resist tension.

    • Microtubules: ~25 nm; hollow cylinders made of tubulin (α and β subunits); dynamic instability; tracks for motor proteins; spindle components in mitosis; form cilia/flagella cores (axoneme is 9+2 structure).

  • Dynamic instability and motor proteins:

    • Microfilaments and microtubules exhibit dynamic assembly/disassembly; critical for cell movement, shape change, and organelle transport.

    • Motor proteins (e.g., kinesin, dynein) move cargo along cytoskeletal tracks using ATP hydrolysis; directionality along microtubules (minus to plus for kinesin, plus to minus for dynein) and along actin filaments via myosin.

  • Specialized cellular structures:

    • Cilia and flagella: microtubule-based; dynein motors bend microtubule doublets to generate movement; 9+2 arrangement in motile cilia/flagella; primary cilium is non-motile but important for signaling.

  • Extracellular structures and cell adhesion:

    • Plants: cell walls provide structural support; plasmodesmata allow cell-to-cell communication.

    • Animals: extracellular matrix (ECM) composed of collagen, proteoglycans, and linking proteins (e.g., integrin) to connect cytoskeleton to ECM; integrins anchor cells and mediate adhesion and signaling; junctions connect adjacent cells:

    • Tight junctions: seal paracellular spaces to prevent diffusion between cells.

    • Desmosomes: mechanical linkages that resist tension.

    • Gap junctions: channels enabling intercellular communication via ions/small molecules.

  • Tissue-level organization:

    • Plant cell walls vs. animal ECM; plasmodesmata vs. gap junctions.

  • Review & Apply (4.4) representative questions:

    • 1) Compare the three cytoskeletal filament classes in composition, size, and stability.

    • 2) Why are motor proteins used to move cargo even when movement is along a concentration gradient?

    • 3) Which junction types would you expect in digestive tracts, in rapidly signaling tissues, and in mechanically stressed heart tissue?

    • 4) How might integrin–collagen interactions relate to cancer metastasis?

4.5 Compartmentalization Occurs in Prokaryotic Cells and Is Extensive in Eukaryotic Cells

  • Core idea: compartmentalization allows specialized reactions to occur in defined environments, increasing efficiency and regulation.

  • Prokaryotes:

    • Generally lack membrane-enclosed organelles; cytoplasm contains non-membrane compartments (protein-based microcompartments) and a nucleoid containing the circular chromosome.

    • Notable examples of prokaryotic compartments:

    • Carboxysomes (CO₂ fixation) – microcompartments enclosing enzymes.

    • Metabolosomes – sequester toxic molecules and catalyze their breakdown.

    • Encapsulins – protein shells enclosing specific enzymes (e.g., peroxidase in some microcompartments).

    • Gas vesicles – provide buoyancy to position cells in water columns.

    • Cyanobacteria possess internal membranes for photosynthesis, connecting to endosymbiotic theory (link to chloroplasts in eukaryotes).

    • The cytoskeleton and intracellular organization help localize reactions even without membranes.

  • Eukaryotes:

    • Extensive membrane-bound organelles organized into the endomembrane system: nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and more.

    • Endomembrane system components shuttle materials via vesicles; membranes continuously fuse and bud to relocate contents.

  • Organelles and their primary roles:

    • Nucleus: houses most DNA; transcription and replication occur here; nucleolus is the ribosome assembly site; nuclear envelope with pores controls traffic.

    • Endomembrane system:

    • Rough ER (RER): ribosomes on surface; protein synthesis destined for secretion or ER/endomembrane compartments; many proteins are glycosylated in the ER and tagged for destinations.

    • Smooth ER (SER): lipid synthesis, detoxification, Ca²⁺ storage.

    • Golgi apparatus: modifies, sorts, and packages proteins; cis face receives cargo from ER, trans face sends cargo to lysosomes, plasma membrane, or other destinations; cisternal maturation/vesicle transport mechanisms.

    • Lysosomes: digestive organelles containing hydrolases; primary lysosomes fuse with endocytic vesicles to form secondary lysosomes; autophagy recycles cellular components.

    • Energy organelles:

    • Mitochondria: powerhouse; generate ATP via oxidative phosphorylation; inner membrane with cristae; matrix contains enzymes, DNA, ribosomes; two membranes with selective permeability.

    • Chloroplasts (plants/algae): photosynthesis; internal thylakoid membranes; stroma contains enzymes, DNA, ribosomes; two membranes; energy conversion from light to chemical-bond energy.

    • Other organelles:

    • Peroxisomes: breakdown of toxic peroxides (e.g., H₂O₂).

    • Glyoxysomes: convert stored lipids to carbohydrates (specialized to plants).

    • Vacuoles: storage, structure, pigment sequestration, and digestion (large central vacuole in plant cells contributes to turgor pressure).

    • Plastids: broad class including chloroplasts; contain circular DNA and ribosomes; multiple forms (e.g., chloroplasts, leucoplasts).

  • Endosymbiotic theory and evidence:

    • Mitochondria and chloroplasts resemble bacteria in size and contain their own circular DNA and ribosomes; double membranes suggest engulfment events.

  • Cell walls and extracellular matrices:

    • Plant cell walls: cellulose-based, provide rigidity and protection; plasmodesmata connect plant cells for intercellular communication.

    • Animal ECM: collagen fibers, proteoglycans, and integrin-mediated connections anchor cells and influence signaling, tissue structure, and protective functions.

  • Organellar dysfunction and disease example:

    • I-cell disease: failure to add phosphorylated sugar markers to lysosomal enzymes; enzymes fail to be targeted to lysosomes, causing intracellular accumulation of undigested materials and inclusions.

  • Visual summaries and microscopy:

    • Advances in microscopy (light vs. electron) drastically improved resolution and understanding of cellular organization.

    • Typical light microscope resolution ~0.2 μm (2 × 10⁻⁷ m); electron microscopes reach ~0.0002 μm (2 × 10⁻⁷ m or higher resolution, effectively 10⁶× magnification) with the trade-off that living cells cannot be seen.

    • Cutting-edge EM can reach ~50 pm (5 × 10⁻⁸ μm), enabling atomic-level observation.

  • Practical connections and real-world relevance:

    • Endomembrane dynamics are central to protein processing and trafficking, with implications for human disease and drug targeting.

    • Pathology uses microscopy and targeted stains to diagnose cancers and identify subtypes for targeted therapies (example: estrogen receptor staining in breast cancer).

  • Review & Apply (4.5) representative questions:

    • 1) Why is compartmentalization important for cellular function?

    • 2) How do prokaryotes achieve compartmentalization without lipid-bound organelles?

    • 3) List organelles of the endomembrane system and their primary roles.

    • 4) In I-cell disease, what happens to proteins not properly targeted to lysosomes, and what are the cellular consequences?

Microscopy and Resolution (Think Interdisciplinary Think-Scientist box)

  • Optical resolution as a limiting parameter for viewing cells.

  • Light microscopy: practical resolution around 0.2 μm; magnification sufficient to see cells but not detailed organelles.

  • Electron microscopy (EM): uses electron beams and magnets; resolution ~0.0002 μm (10⁶× magnification) but cannot image living cells.

  • Ongoing advances push resolution to ~50 pm, enabling observation near the atomic scale.

  • Implications: microscopy drove the cell theory and our understanding of organelles, compartments, and cellular architecture.

  • Practical exercise prompts (sample):

    • Calculate magnification limits for 0.2 μm resolution and describe which structures can be observed in prokaryotes vs. eukaryotes at that resolution.

    • Explain how higher resolution EM reveals subcellular architecture not visible with light microscopy.

Notes on formulas and numerical references used in the material

  • Osmotic pressure formula:

    • oxed{ \Pi = CRT }

    • Where $C$ is osmolarity (solute particles per liter), $R$ is the gas constant, and $T$ is absolute temperature.

  • Surface area and volume for a sphere (to illustrate surface area-to-volume constraints):

    • ext{Surface area} = 4\pi r^2,
      \text{Volume} = rac{4}{3}\pi r^3,
      \text{SA:V} = rac{4\pi r^2}{\frac{4}{3}\pi r^3} = rac{3}{r}

    • As radius $r$ increases, SA:V decreases, constraining cell size.

  • Key structural relationships include the bilayer thickness (~8 nm), and the general idea that the hydrophobic interior blocks polar/charged molecules while proteins mediate selective transport.

  • Connections across concepts:

    • The fluidity and asymmetry of membranes (4.1) underpin selective transport (4.2).

    • Vesicle trafficking (4.3) relies on membrane fusion/fission dynamics, powered by cytoskeletal elements and motor proteins (4.4).

    • Compartmentalization (4.5) creates specialized environments for diverse cellular processes and informs organelle function and disease mechanisms.

  • Practical implications and ethics:

    • Microscopy-based pathology informs treatment decisions for cancer and other diseases; personalized medicine relies on detecting receptor status and other molecular markers.

    • Understanding membrane trafficking and organelle dysfunction informs drug targeting and disease mechanisms (e.g., lysosomal storage diseases).