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BIOL 101 – Membranes and Transport (Lecture Notes Review)

Membrane Structure and Components

  • Membranes define a cell’s boundary and organize cellular processes.

  • Major components visible in the slides:

    • Extracellular matrix protein

    • Glycoprotein

    • Glycolipid

    • Cholesterol

    • Peripheral proteins

    • Integral membrane proteins

    • Actin filaments of the cytoskeleton

    • Intermediate filaments

    • Overall architecture: membranes interface with the cytoskeleton and extracellular space.

  • Key idea: membranes are dynamic, with components that determine permeability, signaling, and mechanical properties.

Cytoskeleton and Filament Types

  • Three main types of cytoskeletal fibers:

    • Microfilaments (actin filaments)

    • Composed of two loosely twined protein chains.

    • Roles: contraction, crawling, pinching during cell movement.

    • Microtubules

    • Largest cytoskeletal elements.

    • Composed of α- and β-tubulin dimers.

    • Roles: facilitate movement of the cell and movement of materials within the cell.

    • Intermediate filaments

    • Intermediate in size between actin filaments and microtubules.

    • Very stable; usually not broken down.

  • Visual aids (slide references): arrangement of actin filaments, microtubules, and intermediate filaments in relation to the cell membrane and cytoplasm.

Centrosomes, Centrioles, and Microtubule Organization

  • Centrosomes: region surrounding centrioles; main microtubule-organizing center in many animal cells.

  • Centrioles: occur in pairs in animal cells and most protists; usually absent in plants and fungi.

  • Centrosomes can nucleate microtubule assembly.

  • Note: in some contexts (e.g., meiosis, mitosis) centrosomes/centrioles organize spindle formation.

Cell Movement and Cilia/Flagella

  • Cell movement driven by reorganization of actin filaments, microtubules, or both.

  • Eukaryotic flagella and cilia have a 9 + 2 arrangement of microtubules:

    • 9 microtubule doublets arranged around a central pair of microtubules.

    • Function: locomotion for cells (flagella) and movement of fluids around the cell surface (cilia).

  • Cilia are shorter and more numerous than flagella.

Internal Structure: Flagella and Cilia

  • Detailed structure: inner components and arrangement support movement; attributed to contributions by researchers such as Dr. William Dentler (Kansas) and images in the slides.

  • Concept: specialized motor proteins (dynein arms) generate bending motions that drive movement.

Concept Check: Nucleus Function

  • Question: Which statement best describes the nucleus?

    • A. Stores nutrients

    • B. Generates energy via respiration

    • C. Controls cell activities and contains DNA

    • D. Digests waste products

  • Answer: C — The nucleus controls cellular activities and houses DNA.

Eukaryotic Cell Walls and Plant vs. Animal Cells

  • Eukaryotic cell walls are present in plants, fungi, and some protists; they differ chemically and structurally from prokaryotic walls.

  • Plant cell walls: cellulose

  • Fungi cell walls: chitin

  • Plant cells may have primary walls and secondary cell walls; fungi/plants have cell walls, animals lack them.

Extracellular Matrix (ECM) and Cell–ECM Interactions

  • Animal cells lack cell walls but secrete an elaborate mixture of glycoproteins into the extracellular space.

  • Collagen can be abundant in the ECM.

  • Integrins connect the ECM to the cell’s cytoskeleton; ECM influences cell behavior.

Plant vs. Animal Cells: Structure and Markers

  • Concept Check: Which structure is found in plant cells but not in animal cells?

    • A. Mitochondria

    • B. Ribosomes

    • C. Cell wall

    • D. Endoplasmic reticulum

  • Answer: C — Cell wall.

Table: Prokaryotic, Animal, and Plant Cells - Exterior and Interior Structures

  • Exterior structures:

    • Cell wall: Prokaryote present; Animal absent; Plant present (cellulose);

    • Cell membrane: Present in all three categories;

    • Flagella/cilia: Prokaryotes may have flagella; Animals may have (9 + 2 structure); Plants usually absent except in a few sperm species.

  • Interior structures (examples): ER, ribosomes, microtubules, centrioles, Golgi, nucleus, mitochondria, chloroplasts, chromosomes, lysosomes, vacuoles.

  • Key contrasts:

    • Mitochondria and chloroplasts: present in animal and plant cells; chloroplasts present only in plants.

    • Nucleus and Golgi: present in animal and plant; absent in prokaryotes.

    • Vacuoles: large central vacuole in plants; usually small or absent in animal cells.

  • Takeaway: plants and animals share many organelles, but plants uniquely have chloroplasts, cell walls, large central vacuoles; prokaryotes lack a defined nucleus and many membrane-bound organelles.

Cell-to-Cell Interactions and Identity Markers

  • Surface proteins provide cellular identity; cells read and react to each other.

  • Glycolipids act as tissue-specific cell surface markers.

  • Major histocompatibility complex (MHC) proteins help the immune system distinguish self from non-self.

Cell Connections and Junctions

  • Adhesive junctions mechanically attach cytoskeletons of neighboring cells or cells to the ECM (examples: adherens junctions, desmosomes, hemidesmosomes).

  • Septate or tight junctions connect plasma membranes of adjacent cells in a sheet and prevent leakage.

  • Communicating junctions include gap junctions and plasmodesmata (in plants).

  • Plant cells feature plasmodesmata: cytoplasm of adjoining cells connected through openings in cell walls; functionally analogous to gap junctions in animals.

Plasmodesmata (Plants) vs. Gap Junctions (Animals)

  • Plasmodesmata: openings in plant cell walls; cytoplasm of neighboring cells connected; allow transport and signaling.

  • Gap junctions: protein channels that allow direct chemical or electrical signal passage between adjacent animal cells.

Basic Membrane Structure

  • Phospholipids are arranged in a bilayer with:

    • Hydrophilic heads facing outward (toward aqueous environments).

    • Hydrophobic tails forming the interior of the bilayer.

Fluid Mosaic Model (Singer & Nicolson, 1972)

  • Concept: a fluid lipid bilayer with a mosaic of proteins floating within or on the bilayer.

  • Membrane protein types:

    • Integral proteins: embedded in the membrane; possess hydrophobic regions.

    • Peripheral proteins: on the membrane surface; have polar regions.

    • Membrane-associated proteins: not part of the membrane but interact with it.

Five Components of Cellular Membranes

1) Phospholipid bilayer – flexible matrix; barrier to permeability
2) Sterols – nonpolar with a hydroxyl group; cholesterol is primary in animals; nonpolar except -OH
3) Integral membrane proteins – embedded and some span the membrane (transmembrane domains)
4) Interior protein network – provides structural support and shape; a mix of peripheral and membrane-associated proteins
5) Cell-surface markers – glycoproteins and glycolipids added by the ER; act as cell identity markers

Studying the Membrane

  • Techniques: TEM (transmission electron microscopy) and SEM (scanning electron microscopy) are used to study membranes.

  • Sample prep: embedding specimens in epoxy; thin sections (< 1 μm) can be imaged.

  • Freeze-fracture microscopy reveals the inside of the membrane.

Phospholipids: The Membrane’s Foundation

  • Distinct lipid classes (about 1000 lipids in cells) are divided into three classes:
    1) Glycerol phospholipids – head groups can be zwitterionic or anionic
    2) Sphingolipids – contain saturated hydrocarbon chains; important to vertebrate nervous system
    3) Sterols – cholesterol is primary in animals; nonpolar except for the -OH group

Lipid Structure and Lipid Molecules (Illustrative Details)

  • Fatty acids and head groups form triglycerides or phospholipids; the glycerol backbone links two fatty acids and a phosphate group in phospholipids.

  • Cholesterol sits between phospholipids and modulates membrane properties.

  • Hydrophilic heads face aqueous environments; hydrophobic tails face inward.

  • Transports and interactions among lipids influence membrane properties.

Phospholipids: Spontaneous Bilayer Formation and Amphipathicity

  • Amphipathic structure drives bilayer formation:

    • Polar head group is hydrophilic; phosphate group attaches to head.

    • Two fatty acids are roughly parallel and are nonpolar/hydrophobic.

  • Result: spontaneous formation of a bilayer with a hydrophobic interior and hydrophilic exterior surfaces.

Membrane Fluidity and Phase States

  • Membranes are fluid; lipids and unanchored proteins can move laterally within the bilayer.

  • Factors affecting fluidity:

    • Lipid structure: tail length, degree of saturation, and double bonds

    • Temperature

    • Cholesterol acts as a buffer, stabilizing fluidity across temperature variations

  • Phases: GEL vs LIQUID ORDERED vs LIQUID DISORDERED

    • High saturation and long tails favor GEL state; unsaturated tails and cholesterol favor LIQUID ORDERED/LIQUID DISORDERED depending on conditions

    • Transition temperature: denoted as Tm

  • Mechanisms of fluidity control: fatty acid composition and cholesterol help maintain appropriate membrane fluidity across temperatures

Membrane Fluidity in Detail

  • Conditions that reduce fluidity: saturated fats, long tails, and low temperatures

  • Bacteria can adjust fluidity at low temperatures by desaturating fatty acids (adding double bonds)

  • Visualization: membranes support lateral diffusion and rotation of lipids and proteins; diffusion is in-plane and does not require flips unless special conditions occur

Phospholipid Composition Across Membrane Compartments

  • ER membrane: mainly unsaturated lipids; little or no cholesterol; more fluid; thinner; shorter transmembrane domains (TMDs approx. 20 amino acids)

  • Plasma membrane: mix of saturated and unsaturated lipids with cholesterol; less fluid and thicker (TMDs approx. 25 amino acids)

Endoplasmic Reticulum vs. Plasma Membrane

  • Distinct lipid compositions contribute to different fluidities and protein content.

Membrane Proteins: Functions and Diversity

  • Functions include:

    • Transporters

    • Enzymes

    • Cell-surface receptors

    • Cell-surface identity markers

    • Cell-to-cell adhesion proteins

    • Attachments to the cytoskeleton

    • Effects on membrane structure

  • The diversity in structure underpins a wide range of functions.

Structure-Function Relationship in Membrane Proteins

  • Membrane proteins have common structural features linked to their roles; various shapes support specific functions.

Anchoring Molecules and Transmembrane Domains

  • Anchoring molecules attach membrane proteins to the membrane surface.

    • They are modified lipids with nonpolar regions that insert into the bilayer and bonding domains that link to proteins.

  • Transmembrane proteins span the bilayer with nonpolar regions embedded in the interior; may form α-helices or β-pleated sheets; polar regions extend on either side of the membrane.

  • A single transmembrane domain can anchor a protein; some proteins have multiple transmembrane domains.

  • Receptors are sometimes classified by the number of transmembrane domains.

Pores and β-Barrels

  • Some transmembrane proteins create pores through the membrane.

  • Structural motif: a cylinder of β-sheets forming a β-barrel; the interior is polar, allowing water and small polar molecules to pass.

Passive Transport: Diffusion and Permeability

  • Passive transport does not require energy.

  • Diffusion: movement from high to low concentration; continues until uniform concentration is reached.

  • The hydrophobic interior of membranes generally repels polar molecules but allows nonpolar molecules to diffuse.

  • The membrane is selectively permeable: small nonpolar molecules diffuse readily; small polar molecules diffuse slowly; ions have very limited permeability.

Facilitated Diffusion: Proteins Allow Membrane Transport

  • Some molecules cross membranes via transport proteins without energy input, moving down a concentration gradient.

  • Types:

    • Channel proteins: form hydrophilic channels when open; allow specific ions/molecules to pass.

    • Carrier proteins: bind specific molecules and undergo conformational changes to transfer them across.

  • The membrane is selectively permeable due to channels and carriers.

Ion Channels and Gating

  • Ion channels enable selective ion passage through the nonpolar interior of the membrane.

  • Gated channels open or close in response to stimuli (chemical or electrical).

  • Direction of ion movement depends on:

    • Relative concentrations on each side

    • Voltage differences across the membrane

    • Whether the channel is currently open or closed

Facilitated Diffusion through Carriers

  • Carriers can transport ions and other solutes (e.g., sugars, amino acids) via diffusion.

  • Carriers must bind to the molecule they transport.

  • Saturation: transport rate is limited by the number of transporters.

Osmosis and Water Movement

  • Osmosis: net diffusion of water across a semipermeable membrane toward higher solute concentration.

  • Aquaporins are specialized channels facilitating water movement across membranes.

  • Osmotic concentration concepts:

    • Hypertonic: higher solute concentration outside the cell

    • Hypotonic: lower solute concentration outside

    • Isotonic: equal solute concentrations across membrane

Osmotic Pressure and Cell Response

  • Osmotic pressure is the force required to stop osmotic flow.

  • In hypotonic solutions, cells may gain water and swell; plant cells use turgor pressure to maintain rigidity; cell walls resist lysis.

  • Animal cells require isotonic environments to prevent lysis.

Maintaining Osmotic Balance in Organisms

  • Extrusion via contractile vacuoles in some cells helps eject water.

  • Isosmotic regulation strategies:

    • Marine organisms adjust internal osmolarity to sea water.

    • Terrestrial animals maintain isotonic bodily fluids.

    • Plant cells use turgor pressure to keep rigidity.

Active Transport: Energy-Driven Movement

  • Active transport requires energy (direct or indirect) and moves substances against their concentration gradient.

  • Highly selective carrier proteins are involved.

Carrier Proteins in Active Transport

  • Types of carriers:

    • Uniporters: move one molecule at a time

    • Symporters: move two molecules in the same direction

    • Antiporters: move two molecules in opposite directions

  • Note: these terms also apply to transporters used in facilitated diffusion.

Sodium-Potassium Pump (Na+/K+ ATPase)

  • Direct use of ATP for active transport; uses an antiporter to move 3 Na+ out and 2 K+ in per ATP hydrolyzed.

  • Mechanism:

    • 1) ATP binds to pump; Na+ ions bind from the cytoplasm.

    • 2) ATP phosphorylates the pump; pump undergoes conformational change, reducing Na+ affinity so Na+ is released outside.

    • 3) Extracellular K+ binds; dephosphorylation occurs causing pump to return to original conformation.

    • 4) K+ is released inside; cycle restarts.

  • Overall stoichiometry: 3 \, ext{Na}^+{out} \, + \, 2 \, ext{K}^+{in} \,
    ightarrow \, ext{via ATP hydrolysis}

Coupled Transport (Secondary Active Transport)

  • Indirect use of ATP: energy from diffusion of one molecule powers transport of another via the same protein.

  • Can be via symporters or antiporters.

  • Example: glucose–Na+ symporter uses Na+ diffusion energy to move glucose against its gradient.

Bulk Transport: Endocytosis and Exocytosis

  • Endocytosis: movement of substances into the cell; requires energy.

    • Phagocytosis: uptake of particulate matter.

    • Pinocytosis: uptake of fluid.

    • Receptor-mediated endocytosis: uptake of specific molecules after binding to a receptor.

  • Exocytosis: movement of substances out of the cell; requires energy; used for secretion of hormones, neurotransmitters, digestive enzymes, and exporting cell-wall materials in plants.

Receptor-Mediated Endocytosis and Disease Example

  • In familial hypercholesterolemia, LDL receptors lack tails and cannot be fastened in clathrin-coated pits, failing vesicle formation; cholesterol remains in bloodstream, leading to arterial plaques and heart attacks.

Key Takeaways for Exam Preparation

  • Membrane structure is a dynamic, asymmetric bilayer with diverse proteins that govern transport, signaling, adherence, and cell identity.

  • The Fluid Mosaic Model is a central framework for understanding membrane organization and protein diversity.

  • Transport across membranes includes passive diffusion, facilitated diffusion (channels and carriers), osmosis, active transport (primary and secondary), and bulk transport (endocytosis/exocytosis).

  • The cytoskeleton, centrosomes, centrioles, and extracellular matrices coordinate cellular shape, movement, and tissue organization.

  • Plant and animal cells differ in cell walls, chloroplasts, vacuoles, and junction types (plasmodesmata vs. gap junctions).

  • Ethical/philosophical/practical implications: understanding membrane transport underpins knowledge of physiology, pharmacology (drug delivery), and pathology (e.g., hypercholesterolemia) with real-world relevance to health and disease.

Quick Practice Prompts (to test understanding)

  • Describe how the Na+/K+ ATPase pump maintains cellular ion balance and membrane potential.

  • Compare and contrast tight junctions, adherens junctions, desmosomes, and hemidesmosomes in terms of function and location.

  • Explain how membrane fluidity is affected by cholesterol and temperature, including what happens at the molecular level when the temperature drops.

  • Outline the differences between plasmodesmata and gap junctions in terms of structure and function.

  • Explain why plant cells rely on turgor pressure for rigidity and how osmotic balance contributes to this.

  • Provide a detailed description of receptor-mediated endocytosis and give a real-world example of its importance.

Exam-Focused Takeaways

  • Know the five membrane components and their roles.

  • Be able to describe the Fluid Mosaic Model and distinguish between integral, peripheral, and membrane-associated proteins.

  • Understand the structural basis and functional implications of transmembrane domains and β-barrel pores.

  • Distinguish passive vs. active transport, including channels vs. carriers and the concept of saturation for carriers.

  • Explain osmosis, osmotic pressure, and the conditions of hypertonic, hypotonic, and isotonic solutions, including plant cell responses.

  • Recall the Na+/K+ ATPase mechanism and the concept of coupled transport.

  • Recognize endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis, including their cellular and physiological roles.

  • Connect cell–ECM interactions to integrins and their influence on cell behavior and tissue organization.

References to Figures and Illustrations (for quick lookup)

  • 5.1 Structure of Membranes; 5.2 Phospholipids: The Membrane’s 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.

  • Cytoskeleton drawings: actin filaments, microtubules, intermediate filaments; 9 + 2 arrangement in flagella/cilia.

  • TEM/SEM images; freeze-fracture visuals; plasmodemata diagrams; and membrane protein schematics (integral vs. peripheral vs. membrane-associated).