BIOL 101 Notes: Membranes, Transport, and Cell Structure

Membrane Structure and Function

  • Basic idea: cell membranes are phospholipid bilayers with embedded proteins; hydrophilic heads face outward, hydrophobic tails form the interior. The bilayer forms a flexible, selective barrier that regulates what enters and leaves the cell.
  • Fluid Mosaic Model (1960s–1970s): membranes consist of a fluid lipid bilayer with proteins floating in or on the surface; proteins can move within the layer. Proposes distinction between integral (embedded) and peripheral (surface) proteins; membrane-associated proteins interact with the bilayer but are not permanently part of it.
  • Five components of cellular membranes:
    • Phospholipid bilayer: flexible matrix, barrier to permeability.
    • Sterols: nonpolar with a hydroxyl group; cholesterol is primary in animals; modulate fluidity.
    • Integral membrane proteins: embedded in the bilayer; some have hydrophobic transmembrane domains (TMDs) that span the membrane.
    • Interior protein network: peripheral and membrane-associated proteins that provide structural support and shape.
    • Cell-surface markers: glycoproteins and glycolipids added to the membrane by the endoplasmic reticulum (ER), acting as identity markers.
  • Phospholipids and lipid classes:
    • 3 major classes: glycerol phospholipids, sphingolipids, sterols.
    • Cholesterol acts as a cholesterol molecule between phospholipids, modulating liquidity and stability.
  • Lipid arrangement and amphipathicity:
    • Phospholipids form a bilayer with polar hydrophilic heads facing the aqueous environment and hydrophobic tails forming the interior.
    • Bilayers spontaneously form due to amphipathic structure; heads are polar and hydrophilic; tails are nonpolar and hydrophobic.
  • Membrane components and their functions:
    • Integral proteins: embedded, often with hydrophobic transmembrane domains; some span the membrane multiple times.
    • Peripheral proteins: attached to surface; can interact with integral proteins or be membrane-associated.
    • Anchoring molecules: attach membrane proteins to the membrane surface; modified lipids with nonpolar inserts and covalent bonding domains.
    • Pores and channels: extensive nonpolar regions can create pores (e.g., β-barrel pores) allowing polar molecules to cross.
  • Membrane microstructure and movement:
    • Lipids and many proteins can diffuse laterally; the membrane is dynamic and fluid.
    • Membrane movement is influenced by lipid tail length, degree of saturation, presence of double bonds, and temperature.
    • Cholesterol buffers membrane fluidity, resisting changes across small temperature ranges.
  • ER vs plasma membrane lipid composition:
    • ER membrane: mainly unsaturated lipids, little cholesterol; more fluid and thinner; typically shorter transmembrane domains (TMDs, ~20 amino acids).
    • Plasma membrane: mix of saturated and unsaturated lipids, cholesterol; less fluid and thicker (longer TMDs, ~25 amino acids).
  • Membrane studies and visualization:
    • Techniques include transmission electron microscopy (TEM), scanning electron microscopy (SEM), and freeze-fracture microscopy to visualize membrane structure.
  • Membrane proteins: structure–function relationships:
    • Functions include transporters, enzymes, cell-surface receptors, identity markers, cell-to-cell adhesion, and attachments to the cytoskeleton.
    • The diversity of membrane protein structures underlies diverse functions; proteins show common structural features related to their roles as membrane proteins.

Cytoskeleton: Types and Roles

  • Three main fiber systems:
    • Microfilaments (actin filaments): composed of two loosely twisted protein chains; functions include contraction, crawling, and pinching during cell movement.
    • Microtubules: largest cytoskeletal elements; composed of α- and β-tubulin dimers; facilitate movement of the cell and internal cargo via motor proteins; form spindle apparatus during division.
    • Intermediate filaments: middle-sized; very stable and not readily broken down; provide structural support.
  • Cross-talk and coordination:
    • Cytoskeleton elements interact to drive cell shape changes, intracellular transport, and locomotion.

Centrosomes and Centrioles

  • Centrosomes are the microtubule-organizing centers (MTOCs) in almost all animal cells.
  • Centrioles: typically occur in pairs in animal cells and most protists; nucleate and organize microtubules.
  • Plants and fungi usually lack centrioles.

Cell Movement: Flagella and Cilia

  • Eukaryotic flagella and cilia share a 9 + 2 arrangement of microtubules: 9 outer doublet microtubules surrounding 2 central single microtubules.
  • Cilia are shorter and more numerous; flagella are longer and typically fewer in number.
  • Movement arises from coordinated dynein motor activity along microtubules driving bending motions.
  • Internal structure of flagella and cilia includes a complex arrangement of microtubules and associated proteins.

Concept Check: Nucleus

  • Question: Which statement best describes the nucleus? A) Stores nutrients. B) Generates energy through respiration. C) Controls the cell's activities and contains DNA. D) Digests waste.
  • Correct answer: C

Eukaryotic Cell Walls and ECM

  • Eukaryotic cell walls:
    • Plants and many protists have cell walls made of cellulose.
    • Fungi have cell walls made of chitin.
    • Animal cells lack cell walls.
    • Plant cells may have a primary cell wall and, in some cases, secondary cell walls.
  • Extracellular matrix (ECM):
    • Animal cells secrete an elaborate mixture of glycoproteins into the extracellular space; collagen may be abundant.
    • ECM forms a protective layer around cells and can influence cell behavior.
    • Integrins link the ECM to the cell’s cytoskeleton.

Plant vs Animal Cell Walls and Junctions

  • Plant cells: plasmodesmata—specialized openings in cell walls that connect cytoplasm of neighboring cells; function similarly to gap junctions in animal cells.
  • Animal cell junctions:
    • Adhesive junctions attach cytoskeletons of neighboring cells or to the ECM (adherens junctions, desmosomes, hemidesmosomes).
    • Septate (tight) junctions connect plasma membranes of adjacent cells in a sheet with no leakage.
    • Communicating junctions (gap junctions) allow chemical or electrical signals to pass directly between adjacent cells.

Basic Membrane Structure and Components

  • Phospholipid bilayer as the foundational structure with hydrophilic heads and hydrophobic tails.
  • Hydrophobic interior provides the main permeability barrier; small nonpolar molecules cross readily, polar/charged species cross poorly without help.
  • Integral and peripheral proteins: various roles including transport, signaling, and cell-to-cell communication.
  • Lipid mobility and membrane phases:
    • Lipids can rotate, diffuse laterally, and flip-flop across the bilayer (less frequent).
    • Membrane phases include gel and liquid-disordered/liquid-ordered states depending on temperature and lipid composition.
    • Cholesterol helps stabilize membranes by resisting changes in fluidity.

Phospholipids: The Membrane’s Foundation

  • Three classes of membrane lipids:
    1) Glycerol phospholipids: head groups can be zwitterionic or anionic.
    2) Sphingolipids: often have saturated hydrocarbon chains; important in the vertebrate nervous system.
    3) Sterols: cholesterol in animals is a primary sterol; nonpolar except for the hydroxyl group.
  • Amphipathic nature drives bilayer formation; polar heads face water, nonpolar tails face inward.
  • Cholesterol fits between phospholipids, modulating fluidity and stability in animal membranes.
  • Phospholipid bilayer properties:
    • Polar hydrophilic head groups; phosphate group attached to the head; two fatty acids form nonpolar tails.
    • Bilayers are fluid, allowing lateral movement of lipids and proteins.

Membrane Fluidity and Lipid Composition

  • Factors affecting fluidity:
    • Degree of tail overlap, tail length, degree of unsaturation (double bonds), and temperature.
    • Cholesterol acts as a temperature buffer, stabilizing membranes by resisting changes in fluidity.
  • Membrane fluidity varies by membrane region:
    • ER membrane: mostly unsaturated lipids and little cholesterol; more fluid and thinner; shorter TMDs (~20 amino acids).
    • Plasma membrane: mix of saturated and unsaturated lipids with cholesterol; less fluid and thicker (longer TMDs, ~25 amino acids).
  • Bacteria synthesize fatty acid desaturases to introduce double bonds, increasing fluidity at lower temperatures.

Membrane Proteins: Structure and Function

  • Functions of membrane proteins:
    • Transporters: move substances across the membrane.
    • Enzymes: catalyze reactions at the membrane surface or within the bilayer.
    • Cell-surface receptors: receive signals from the environment.
    • Cell-surface identity markers: glycoproteins/glycolipids used in recognition.
    • Cell-to-cell adhesion proteins: link cells together.
    • Attachments to the cytoskeleton: help maintain cell shape and stabilize membranes.
    • Influence membrane structure and dynamics.
  • Transmembrane proteins:
    • Span the lipid bilayer; nonpolar regions embedded in the interior.
    • Often contain α helices; polar regions protrude on both sides of the membrane.
    • Transmembrane domains (TMDs) are hydrophobic regions; proteins may have multiple TMDs.
  • Anchoring molecules:
    • Attach membrane proteins to membrane surfaces; embedded lipid anchors and protein-linking domains.
  • Pores and channels:
    • Nonpolar interior pockets forming pores; β-barrels can create hydrophilic interior channels for water and small polar molecules.

Transport Across Membranes: Passive, Active, and Beyond

  • Passive transport:
    • Movement without energy input; driven by concentration gradients.
    • Diffusion: movement from high to low concentration until equilibrium is reached.
    • Membrane interior hydrophobicity limits diffusion of polar/charged molecules.
  • Facilitated diffusion (through proteins):
    • Channel proteins: form hydrophilic channels; can be gated (open/close in response to stimuli).
    • Carrier proteins: bind specific substrates and undergo conformational changes to shuttle them across the membrane.
    • The membrane is selectively permeable due to channels and carriers; diffusion along a gradient.
  • Channel proteins and ion channels:
    • Ion channels allow specific ions to cross the membrane through a hydrophilic channel.
    • Gated channels: respond to chemical or electrical stimuli; determine direction by gradients and membrane potential.
  • Carrier proteins in facilitated diffusion:
    • Uniporters move one molecule at a time; symporters move two molecules in the same direction; antiporters move two molecules in opposite directions.
    • Saturation occurs when all transporters are bound and working at maximum velocity.
  • Osmosis:
    • Diffusion of water across a semipermeable membrane toward higher solute concentration.
    • Aquaporins are specialized water channels that facilitate rapid water movement.
  • Osmotic concepts:
    • Hypertonic: higher solute concentration outside the cell.
    • Hypotonic: lower solute concentration outside the cell.
    • Isotonic: equal osmotic concentration on both sides.
    • Osmotic pressure: force required to stop osmotic flow; drives water movement and can cause cells to swell or shrink.
  • Osmotic pressure and cell types:
    • In hypotonic solutions, cells may swell; plant cells use turgor pressure to maintain rigidity.
    • Prokaryotes, fungi, plants, and many protists maintain appropriate osmoregulation to avoid lysis.

Active Transport and Energy Use

  • Active transport requires energy (ATP) to move substances against their concentration gradient; uses highly selective carrier proteins.
  • Direct active transport:
    • Primary active transport uses ATP directly to drive transport (e.g., Na+/K+ ATPase).
    • Na+/K+ ATPase pump specifics:
    • Direct use of ATP; antiporter moves 3 Na+ out of the cell and 2 K+ into the cell per ATP hydrolyzed, moving against gradients.
    • Mechanism involves phosphorylation/dephosphorylation cycles that alter protein conformation.
    • Representation: 3\,\mathrm{Na^+}\text{ out},\quad 2\,\mathrm{K^+}\text{ in per ATP hydrolysis}
  • Coupled transport (secondary active transport):
    • Indirect energy use: energy released by diffusion of one molecule (often Na+) drives transport of another molecule against its gradient.
    • Symporters and antiporters can be used for coupled transport (e.g., glucose–Na+ symporter).

Bulk Transport: Endocytosis and Exocytosis

  • Endocytosis: uptake of materials into the cell; energy-dependent.
    • Phagocytosis: cells ingest particulate matter.
    • Pinocytosis: uptake of fluids.
    • Receptor-mediated endocytosis: uptake of specific molecules after binding to receptors; can involve clathrin-coated pits (example: familial hypercholesterolemia elucidates receptor tail function and vesicle formation).
  • Exocytosis: secretion of materials from the cell; energy-dependent.
    • Used by plants to export cell wall material; animals to secrete hormones, neurotransmitters, digestive enzymes.

Cell-Cell Interactions and Junctions

  • Surface proteins confer cell identity; glycolipids and glycoproteins serve as tissue-specific markers.
  • Major junction types:
    • Adhesive junctions (adherens, desmosomes, hemidesmosomes): mechanically attach cytoskeletons of adjacent cells or cells to the ECM.
    • Tight (septate) junctions: connect plasma membranes of adjacent cells in a sheet to prevent leakage.
    • Gap junctions (communicating junctions): allow direct chemical or electrical signaling between adjacent cells.
  • Plant-specific: plasmodesmata—plasmodesmata are cytoplasmic channels through cell walls that connect plant cells; function similarly to gap junctions.

Review: Prokaryotes, Animal, and Plant Cells

  • Exterior structures: cell wall presence varies by group; prokaryotes have a cell wall (protein-polysaccharide); animal cells lack a cell wall; plant cells have cellulose cell walls.
  • Interior structures: nucleus present in eukaryotes, absent in prokaryotes; mitochondria present in eukaryotes (absent in prokaryotes); chloroplasts present in plants.
  • Golgi apparatus, ER, ribosomes: present in eukaryotes; absent in prokaryotes.
  • Cytoskeletal elements: present in eukaryotes (actin filaments, microtubules, intermediate filaments); prokaryotes lack organized cytoskeletons comparable to eukaryotic ones.

Key Equations and Numerical References

  • Na+/K+ pump (direct active transport):
    • Movement per ATP hydrolyzed: 3\,\mathrm{Na^+}\text{ out},\quad 2\,\mathrm{K^+}\text{ in}
  • Transmembrane protein organization concepts often described as having one or more transmembrane domains (TMDs) that span the lipid bilayer; transmembrane domain length commonly ~20–25 amino acids in mammalian membranes for typical single-span to multi-span receptors.
  • 9 + 2 microtubule arrangement in flagella/cilia: 9+2 arrangement across the axoneme.

Study and Course Information (Contextual References)

  • Course delivery and resources on McGraw Hill Connect; student support and help resources are provided (videos, help pages, live chat/phone support).
  • Chapter focus areas include:
    • 5.1 Structure of membranes
    • 5.2 Phospholipids: foundation
    • 5.3 Proteins: multifunctional components
    • 5.4 Passive transport across membranes
    • 5.5 Active transport across membranes
    • 5.6 Bulk transport by endocytosis and exocytosis
  • Course logistics and announcements emphasize Blackboard LMS usage, weekly assignments, and exam scheduling (Exam 1 dates, project deadlines).

Connections to Foundational Principles and Real-world Relevance

  • The fluid mosaic model explains how membranes remain semi-fluid and selectively permeable, enabling dynamic signaling, transport, and cell communication essential for multicellular life.
  • Transport mechanisms (diffusion, osmosis, channel- and carrier-mediated transport, active transport, and coupled transport) underpin cellular homeostasis, nutrient uptake, neurotransmission, kidney function, and many physiological processes.
  • The extracellular matrix and cell junctions coordinate tissue architecture and communication, with implications for development, cancer metastasis, wound healing, and immune recognition (e.g., MHC markers).
  • Plasmodesmata demonstrate how cell walls in plants are not barriers to intercellular communication, highlighting plant-specific strategies for coordinating physiology.

Ethical, Philosophical, and Practical Implications

  • Understanding membrane transport and cellular communication informs medical therapies (e.g., targeting ion channels, transport pumps, receptor pathways, endocytosis mechanisms).
  • Defects in membrane proteins or transport pathways can lead to diseases (e.g., familial hypercholesterolemia involving receptor-mediated endocytosis defects).
  • The study of membranes intersects with nanotechnology, drug delivery, synthetic biology, and bio-inspired materials, emphasizing the ethical considerations of manipulating cellular interfaces.

Quick Concept Checks and Key Takeaways

  • Nucleus: central control of cell activities and housing of DNA (correct statement: C).
  • Fluid mosaic: membranes are a mosaic of lipids and proteins in a fluid matrix; proteins move within the lipid bilayer.
  • Membrane lipids shape fluidity; cholesterol modulates it; unsaturated vs saturated tails affect phase behavior.
  • Transport types: diffusion (passive), channel-facilitated diffusion, carrier-facilitated diffusion, active transport, and coupled transport.
  • Osmosis: water movement toward higher solute concentration; aquaporins greatly facilitate water movement.
  • Bulk transport: endocytosis and exocytosis enable intake and export of large quantities of materials.
  • Plant vs animal cells: plants have cell walls (cellulose); fungi have chitin; animals lack cell walls; plasmodesmata connect plant cells; tight and adherens junctions help maintain tissue integrity in animals.
  • Receptors, MHC markers, and cell-surface identity markers enable tissue specificity, immune recognition, and intercellular communication.