Cell Membrane Structure, Composition, and Dynamics Study Guide

Introduction to Cell Membranes

Conceptual Overview: * The primary role of the plasma membrane is to act as the "Gateway to the Cell." * It is described by the quote from Robert Frost in Mending Wall (1914): ‐‐Good fences make good neighbors.‐‐ * The membrane provides protection from the outside environment and determines what enters and leaves the cell, making it semi-permeable.
General Organization: * Membranes organize cells into functionally distinct compartments. * Each membrane possesses a unique function and consists of specific protein and lipid components. * Membranes control the chemical composition of the internal compartments (lumens) by regulating the movement of molecules IN and OUT.

Membrane Composition and Physical Properties

Structural Dimensions: * The biological membrane system is two layers thick, forming a sheath-like structure. * Thickness: Ranges from $60\,\text{Å}$ ($6\,\text{nm}$) to $100\,\text{Å}$ ($10\,\text{nm}$).
Chemical Interactions: * Membranes are formed primarily through non-covalent interactions between lipids and proteins. * The lipid component acts as a barrier to the passage of polar molecules and ions.
Main Components: 1. Lipids: Provide the structural framework. 2. Proteins: Facilitate specific functions (transport, signaling, etc.). 3. Carbohydrates: Attached to lipids or proteins, primarily on the extracellular surface.

Membrane Lipids: Classification and Amphipathic Nature

Major Classes: * Phospholipids: The most abundant class; responsible for the lipid bilayer structure. * Glycolipids: Lipids conjugated with sugar residues (typically for recognition). * Sterols: Hydrocarbon tails with fused ring structures (e.g., cholesterol).
Amphipathic Character: * All membrane lipids are amphipathic, meaning they possess one hydrophilic (polar) end and one hydrophobic (non-polar) end. * Hydrophilic End: Interacts with the aqueous environment (inside and outside the cell) via hydrogen bonds. * Hydrophobic End: Contains hydrocarbon chains that interact with each other to pack the membrane, avoiding water.

Chemical Structure of Phospholipids

Structure Components: 1. Polar Head Group: Variable; determines the specific name of the phospholipid. 2. Phosphate: Linked to the glycerol backbone via a phosphodiester bond; common to all phospholipids. 3. Glycerol Backbone: A three-carbon ($C_1, C_2, C_3$) central linking point. $C_3$ is linked to the phosphate, while $C_1$ and $C_2$ are esterified by fatty acids. 4. Fatty Acid Tails: Contain hydrocarbon chains which provide hydrophobic properties.
Fatty Acid Tail Morphology: * Saturated: No double bonds between carbons; the chain is saturated with hydrogen and remains straight. * Unsaturated: Contains some double bonds between carbons, preventing a maximal number of hydrogens. This creates a "kink" or bend in the tail, which is a key regulator of structure and mobility.

Phospholipid Polar Head Groups and Diversity

Naming Conventions: Phospholipids are named based on the "X" group (alcohol) attached to the phosphate. * Phosphatidic acid: Head group is hydrogen ($-H$). * Phosphatidylethanolamine: Head group is ethanolamine ($-CH_2CH_2NH_3^+$). * Phosphatidylcholine (Lecithin): Most abundant phospholipid; head group is choline ($-CH_2CH_2N(CH_3)_3^+$). * Phosphatidylserine: Head group is serine ($-CH_2CH(NH_3)COO^-$). * Phosphatidylglycerol: Head group is glycerol ($-CH_2CH(OH)CH_2OH$). * Diphosphatidylglycerol (Cardiolipin): Complex structure involving two phosphatidylethanolamine groups. * Phosphatidylinositol: Head group is Myo-inositol (a sugar alcohol ring).
Electrochemical Charge: * No Net Charge: Phosphatidylcholine and Phosphatidylethanolamine. * Negative Charge: Phosphatidylserine and Phosphatidylinositol.

Glycolipids and Sterols

Glycolipids: * Composed of sugar residues (Glucose or Galactose) connected by a glycosidic bond to the $C_3$ of glycerol. * Polar head/phosphate groups are replaced by monosaccharide or oligosaccharide (linear or branched) chains. * Functions: Cell recognition, cell adhesion, and membrane stability. Usually found on the extracellular side.
Sterols (Cholesterol): * Structure consists of four fused carbon rings (labeled A, B, C, D) forming a steroid nucleus derived from cyclopentanoperhydrophenanthrene. * Amphipathic: Has a polar hydroxyl ($-OH$) head, a non-polar steroid nucleus, and a non-polar hydrocarbon tail. * Constitutes $30 – 40\%$ of membrane lipids in animal cells.

The Fluid Mosaic Model

Origin: Developed by Singer and Nicolson in 1972.
Definitions: * Fluid: Implies movement within the plane of the membrane. * Mosaic: Highlights that the membrane consists of many different molecules (lipids, proteins, carbohydrates).
Core Characteristics: 1. Bilayer: Two layers (leaflets) with hydrophobic tails facing inward to form a hydrophobic core and hydrophilic heads facing outward. 2. Fluidity: Dynamic movement of components. 3. Asymmetry: Different compositions for intracellular vs. extracellular leaflets. 4. Selective Permeability: Regulates molecular traffic.

Membrane Fluidity and Movement

Types of Movement: * Lateral Diffusion: Rapid movement within the plane of the membrane; occurs approximately $10^7$ times per second. * Axial Rotation: Individual phospholipids rotate on their own axis. * Flip-Flopping: Movement from one leaflet to the other. This is extremely rare because it is energetically unfavorable; regulated by enzymes called flippases with an exchange rate of $6 \rightarrow 20\,\text{h}$.
Fluidity Regulation: * Saturation: Saturated tails result in tighter packing and less movement (Gel-phase membrane). Unsaturated cis-fatty acid tails result in loose packing and more movement (Sol-phase membrane). * Cholesterol (Fluidity Buffer): * In high fluidity areas (unsaturated), it fills gaps and increases packing/stiffness. * In low fluidity areas (saturated), it disrupts tail-to-tail interactions to increase fluidity and prevent brittleness. * Always lowers membrane permeability to water-soluble molecules. * Lipid Rafts: Patches rich in cholesterol and specific proteins that form stabilizing interactions. They lower fluidity in focused areas and are involved in signaling, trafficking, and protein sorting.

Membrane Asymmetry

Leaflet Differences: * Extracellular Leaflet: Enriched in Glycolipids, Sphingomyelin, and Phosphatidylcholine. * Cytosolic Leaflet: Enriched in Phosphatidylinositol, Phosphatidylserine, and Phosphatidylethanolamine. * Equal Distribution: Cholesterol is typically found equally in both leaflets in animals.
Synthesis Sites: * Lipids are predominantly synthesized at the Endoplasmic Reticulum (ER). * Cytosolic Leaflet of ER: Site for Phosphatidylinositol, Phosphatidylserine, and Phosphatidylethanolamine synthesis. * Luminal Leaflet of ER: Site for Glycolipid synthesis.

Selective Permeability and Diffusion

Passive Diffusion: * Fast: Small hydrophobic molecules ($O_2, CO_2, N_2$, benzene). * Slow: Small uncharged polar molecules ($H_2O$, glycerol, ethanol).
Difficult to Cross (Require Transporters): * Larger uncharged polar molecules (glucose, amino acids). * Ions: Charged molecules ($H^+, Na^+, HCO_3^-, K^+, Ca^{2+}, Cl^-, Mg^{2+}$) and nucleotides cannot cross the hydrophobic core without assistance.

Membrane Proteins: Types and Functions

Scale: $20 – 30\%$ of the genome encodes membrane proteins. They occupy approximately $50\%$ of the membrane volume, with a ratio of about $1\,\text{protein} : 50\,\text{lipid molecules}$.
Functional Classes and Examples: * Transporters: e.g., $Na^+$ pump (actively pumps $Na^+$ out and $K^+$ in); Aquaporins (water transport); GLUT Transporter (glucose). * Anchors/Linkers: e.g., Integrins (link intracellular actin filaments to extracellular matrix proteins). * Receptors: e.g., Platelet-derived growth factor (PDGF) receptor (binds extracellular signal to trigger cell growth/division). * Enzymes: e.g., Adenylyl cyclase (catalyzes production of cyclic AMP signaling molecules).
Structural Association: 1. Integral Membrane Proteins: Embedded within the bilayer. Structures include ̑-helical (hydrophobic amino acids interact with lipid tails) and ̒-barrel. 2. Peripheral Membrane Proteins: Associated with the surface of the membrane.
Restriction of Mobility: * While many proteins move laterally (demonstrated by human/mouse cell fusion experiments showing mixing after $40\,\text{minutes}$ at $37^\circ\text{C}$), some are restricted. * Restricting Mechanisms: * Inside tethering (to actin cortex). * Outside tethering (to extracellular matrix). * Cell-cell tethering (e.g., desmosomes). * Diffusion barriers (e.g., tight junctions in gut epithelial cells creating cell polarity).

Macromolecular Complexes

Glycocalyx: * A carbohydrate coat on the extracellular side consisting of glycolipids and glycoproteins (proteins with sugar groups). * Functions: 1. Protection: Cushions the membrane and protects from chemical injury. 2. Immunity: Identification of foreign organisms. 3. Molecular Signature: Cell-type specific sugar combinations. 4. Communication: Aids in cell-cell recognition.
Cell Cortex: * A framework of proteins on the cytosolic side providing mechanical support. * Red Blood Cells: Utilize a spectrin meshwork lattice combined with actin for resilience and flexibility to squeeze through capillaries. * General Cells: Most have an actin-based cell cortex, which is more dynamic than spectrin. It enables cytokinesis (cell division), membrane trafficking, cell migration, and cell polarity.

Questions & Discussion

Q1: What is the outer boundary of the cell? * Answer: Plasma membrane.
Q2: Which is NOT correct about phospholipids? * A: Each phospholipid has four nonpolar tails (Incorrect - they have two).
Q3: Which is NOT a component of the plasma membrane? * Answer: Nucleic acids.
Q4: Bonding between lipids and proteins is…? * Answer: Non-covalent.
Q5: A glycolipid is composed of sugar residues, phosphate, glycerol, and fatty acid tails. (True/False) * Answer: False (They do not contain phosphate).
Q6: What reduces permeability to water-soluble molecules in animals? * Answer: Cholesterol.
Q7: Determining cell functions is largely done by proteins. (True/False) * Answer: True.
Q8: The plasma membrane is… * Answer: Selectively permeable.
Q9: Which statement is NOT correct? * C: Charged molecules and ions readily move across the membrane (Incorrect - they cannot cross the lipid core easily).
Q10: Phosphatidyl serine is enriched on the cytosolic side. (True/False) * Answer: True.
Q11: Which protein has carbohydrate attached for cell distinction? * Answer: Glycoproteins.
Q12: Actin filaments in the cell cortex can be very dynamic. (True/False) * Answer: True.