Membranes and Molecular Transport — Study Notes (Transcript-Based)
New Topic: Membranes and Molecular Transport
This is a foundational topic for the semester and will connect to digestion and the urinary system later in April.
Core idea: membranes are a key concept; understanding structure and function is critical.
The Composite Cell and the Cell Boundary
The cell is shown as a composite cell with various organelles (cilia, endoplasmic reticulum, nucleus, Golgi, mitochondria, flagella).
Not every cell contains all of these components (e.g., erythrocytes lack a nucleus).
The boundary that all cells share is the cell membrane.
Membranes enable compartmentalization and regulate what goes in and out (selectively permeable).
Nuclear membrane is another example of a boundary that creates compartments (DNA is not free-floating).
Phospholipid Bilayer: Structure and Function
The membrane is built from phospholipids; the term phospholipid arises from one of the fatty acid chains being replaced by a phosphate group.
Basic lipid scaffold: a glycerol backbone with two fatty acid tails and a phosphate-containing head group.
The fatty acid tails are hydrophobic (water-fearing); the phosphate head is hydrophilic (water-loving).
This arrangement creates a phospholipid bilayer where heads face water on both sides and tails face inward away from water.
Key terms:
Hydrophilic heads (water-loving) face the aqueous environments.
Hydrophobic tails (water-fearing) tuck inside the bilayer.
Polar vs nonpolar: polar = hydrophilic heads; nonpolar = hydrophobic tails.
The term triglyceride is mentioned as a precursor to phospholipids; a triglyceride has three fatty acid chains attached to glycerol; phospholipids replace one fatty acid with a phosphate group to become the membrane-forming molecule.
The role of polarity in membrane organization:
Polar (hydrophilic) heads interact with water in and around the cell.
Nonpolar (hydrophobic) tails avoid water and stay in the interior of the bilayer.
Lipids and their Properties
Lipids (fats) generally do not mix with water (oil and water do not mix).
Hydrophobic and hydrophilic properties determine membrane orientation and interactions with water.
Saturated vs unsaturated fatty acids:
Saturated: all carbon bonds are single bonds; chains are straight and pack tightly.
Unsaturated: contain one or more C=C double bonds; chains have kinks (cis double bonds) that prevent tight packing.
Saturated fatty acids: straight chains; tendency to pack tightly; generally less fluid.
Unsaturated fatty acids: bent chains due to double bonds; create more space and increase membrane fluidity.
The phospholipid bilayer is typically composed of two fatty acid tails per phospholipid; each tail can be saturated or unsaturated, influencing overall bilayer fluidity.
A special case mentioned: the surface of the eye has a single layer of phospholipids because tails are fatty acids that interact with oils.
Membrane Organization: Bilayer, Polarity, and the Fluid Mosaic Model
The bilayer is often described using the fluid mosaic model: lipids and proteins move laterally within the layer, giving a dynamic structure.
Temperature affects membrane fluidity:
Higher temperature → molecules move more → potential gaps in the membrane; increased fluidity.
Lower temperature → molecules move less; membranes become more rigid and tightly packed.
Cholesterol’s role:
Cholesterol sits within the bilayer and helps regulate fluidity.
At high temperatures, cholesterol fills gaps to stabilize the membrane and reduce excessive fluidity.
At low temperatures, cholesterol keeps membranes from becoming too rigid by preventing tight packing.
Fluidity must be balanced to maintain integrity and selective permeability; too fluid or too rigid compromises function.
Real-world note: fish in cold waters increase unsaturated fatty acids in membranes to maintain fluidity in cold environments.“Fun fact” about unsaturated fats increasing membrane fluidity was mentioned as a non-test note, but the concept is important for understanding adaptation.
Cholesterol: A Multifaceted Membrane Component
Cholesterol is a lipid, not a protein, and acts as a membrane component.
It is a precursor for steroid hormones (e.g., testosterone, estrogen, progesterone, cortisol) and helps regulate membrane fluidity.
It is not the same as simply “good” or “bad” cholesterol; its membrane role is essential for stability and function.
The speaker notes it as a building block for steroid hormones and emphasizes its membrane role rather than hormone biosynthesis exclusively.
Membrane Proteins: Location and Function
Proteins are embedded in or associated with the membrane; their placement determines classification:
Integral proteins: embedded in the membrane; may span the entire bilayer.
Peripheral proteins: attached to the membrane surface; not embedded.
Transmembrane proteins: a subset of integral proteins that span the entire membrane from inside to outside.
All transmembrane proteins are integral, but not all integral proteins are transmembrane.
Visual cues from the image (not just reading): some proteins span the membrane (purple/blue) while others sit on the surface (green).
Membrane proteins’ functions (two broad ways to classify): by location and by function.
Location-based categories:
Integral proteins (including transmembrane proteins)
Peripheral proteins
Function-based categories:
Receptor proteins: receive signals (chemical signals like hormones or ligands) and transduce information.
Enzymes: catalyze chemical reactions at the membrane (bind substrate, convert it to product).
Channel proteins: create selective passageways for specific ions or molecules; inherently transmembrane.
Cell surface proteins: often involved in cell identification and interactions; may have carbohydrate chains (glycoproteins) attached; important for self vs nonself recognition (immune recognition).
Cell adhesion molecules (CAMs): mediate adhesion between cells and extracellular matrix; integrins are a key example and help anchor cytoskeleton connections.
Receptors can be integral or peripheral; they respond to extracellular signals and trigger intracellular responses.
Enzymes can be integral or peripheral; they may bind substrates at the membrane and catalyze reactions affecting downstream processes.
Channel proteins are generally transmembrane and move specific ions or molecules across membranes.
Cell surface proteins and CAMs contribute to self-identification and tissue compatibility, crucial for organ transplants and immune responses; mismatches can trigger rejection and require immunosuppressive therapy.
Integrin example: a CAM that helps anchor the cytoskeleton and mediate adhesion.
Functional Implications: Why These Proteins Matter
Channel proteins are central to neuronal and muscular physiology (ion channels drive action potentials and muscle contraction).
Dysfunctional channels lead to diseases (channelopathies):
Cystic fibrosis: faulty chloride channels.
Long QT syndrome: faulty potassium channels → arrhythmias.
Hyperkalemic periodic paralysis: faulty sodium channels.
Channel function can be targeted by drugs:
Calcium channel blockers: used to lower blood pressure by reducing calcium influx.
Diuretics: promote urine production and fluid loss; impact membrane and channel function indirectly.
Membrane channels and transport processes are essential for homeostasis and signal transduction.
Hormone regulation and systemic effects link back to membrane components: cholesterol as steroid hormone precursor; cholesterol’s role in the membrane supports signaling and transport across tissues.
Special Case: Eye Membrane and Practical Notes
The eye’s surface is a special case with a single layer of phospholipids rather than a full bilayer due to interactions with oils on the surface.
This is a curiosity note rather than a core test item, but it highlights variability in membrane architecture depending on tissue context.
Protein Structure Primer (as context for membrane proteins)
Proteins are built from amino acids; there are ~20 standard amino acids with different R groups attached to a central framework (amine group, carboxyl group, and side chain).
Disulfide bonds can form between sulfur-containing amino acids, stabilizing protein structure.
Protein structure levels:
Primary structure: amino acid sequence (the primary sequence).
Secondary structure: local folding patterns such as beta sheets and alpha helices stabilized by hydrogen bonds.
Tertiary structure: overall 3D folding of a single polypeptide chain.
Quaternary structure: assembly of multiple subunits into a functional protein (example: four-subunit protein shown).
The specific R group chemistry dictates folding and function; changes in structure lead to changes in function (structure → function relationship).
Self vs Nonself and Immunology Concepts in Membranes
Cell surface proteins act as self-identification markers; immune cells (like white blood cells) use these tags to distinguish self from nonself.
Transplant compatibility depends on matching these markers to avoid immune rejection.
Mismatched organ tissue triggers immune attack; immunosuppressive therapy is often required to prevent rejection.
Real-World Relevances and Implications
Homeostasis: membrane fluidity must be balanced for proper diffusion of gases (e.g., oxygen) and ions; cholesterol and fatty acid composition modulate this balance.
Disease connections: channel dysfunctions lead to systemic diseases; understanding membrane proteins is critical for pharmacology and clinical interventions.
Ethical/practical implications: organ transplantation relies on immunological compatibility at the membrane level; advances in this area affect transplant success and patient outcomes.
Connections to Foundational Principles
Structure determines function: molecular arrangement in membranes (phospholipid bilayer, cholesterol, proteins) dictates permeability and signaling.
Compartmentalization enables specialized internal environments and regulated biochemistry within organelles and cells.
Dynamic equilibrium: membrane fluidity and protein mobility support responsive cellular behavior to temperature, chemical signals, and mechanical stresses.
The nervous and muscular systems rely on channel proteins for excitability and contraction, illustrating how membrane components underpin physiology.
Quick Reference: Key Terms to Remember
Phospholipid bilayer, phospholipid, triglyceride, hydrophilic, hydrophobic, polar, nonpolar, saturated fatty acid, unsaturated fatty acid, kink, transmembrane, integral protein, peripheral protein, receptor, enzyme, channel protein, cell surface protein, cell adhesion molecule (CAM), integrin, cholesterol, fluid mosaic model, homeostasis, diffusion, osmosis (implied), channelopathies, cystic fibrosis, Long QT syndrome, hyperkalemic periodic paralysis, diuretics, calcium channel blockers, antidiuretic hormone (ADH).
Summary of Core Takeaways
The cell membrane is a dynamic phospholipid bilayer with hydrophilic heads facing water and hydrophobic tails inside the membrane, with cholesterol modulating fluidity.
Membrane proteins play diverse roles (receptors, enzymes, channels, cell surface markers, CAMs) and are classified by location (integral vs peripheral) and function.
Saturated vs unsaturated fatty acids influence membrane packing and fluidity; unsaturated chains increase fluidity due to kinks in the chains.
Temperature and lipid composition regulate membrane fluidity; cholesterol helps stabilize the membrane across temperature changes.
Membrane structure underpins essential physiological processes, including signaling, transport, ion homeostasis, and immune recognition; dysfunctions in channels and receptors lead to disease and inform pharmacology.
The “structure-function” principle is reinforced throughout: the composition and arrangement of membrane components determine the cell’s physiological behavior.
Equations and Notable Formulas (LaTeX)
Saturated vs unsaturated fatty acids:
ext{Saturated fatty acid: no C=C bonds; all C-C bonds are single bonds (fully saturated).}
ext{Unsaturated fatty acid: contains one or more C=C double bonds; cis configuration introduces a kink.}Transmembrane and integral relationships:
ext{Transmembrane proteins} \subseteq \text{Integral proteins}Polar vs nonpolar (interactions with water):
ext{Polar (hydrophilic) heads} \,\leftrightarrow \,\text{Hydrophilic interactions with water}
ext{Nonpolar (hydrophobic) tails} \,\leftrightarrow \,\text{Hydrophobic interior of the bilayer}Cholesterol as a regulatory component (qualitative):
ext{Cholesterol modulates membrane fluidity: higher temperatures increase fluidity; cholesterol reduces excessive fluidity at high T and prevents tight packing at low T.}