Cell and Molecular Biology Guide for Exam 2

Chapter 4: The Structure and Function of the Plasma Membrane

• Use this guide to direct you to specific sections in your book for exam preparation.
• Utilize slides, class notes, and applicable textbook chapters to fully prepare for the exam.
• Information in blue rectangles: Used for multiple choice or essay-type questions.
• Information in red rectangles: Used for multiple choice questions only.

Plasma Membrane

• Cells are enclosed by the plasma membrane, a thin and fragile structure ranging from 5 to 10 nm in thickness.
• John D. Robertson (Duke University):
  • Portrayed plasma membrane as a three-layered structure:
  • Darkly staining inner layer.
  • Darkly staining outer layer.
  • Lightly staining middle layer.
• All membranes, including plasma, nuclear, or cytoplasmic membranes from various organisms (plants, animals, microorganisms), exhibit the same ultrastructure.

Summary of Membrane Functions and Overview of Membrane Structure

I. Compartmentalization

• Membranes encapsulate entire cells and facilitate specialized intracellular activities with minimal external interference, enabling independent regulation.

II. Scaffold for Biochemical Activities

• Membranes provide distinct compartments for biochemical processes.

III. Selectively Permeable Barrier

• Membranes act like a moat around a castle, providing a general barrier with gated "bridges" for controlled entry and exit of materials.

IV. Transporting Solutes

• Membranes contain transport machinery enabling the movement of ions, sugars, and amino acids.
  • Can establish ionic gradients across the membrane.

V. Response to External Signals (Signal Transduction)

• Membranes are critical for responses to external stimuli, such as hormones, growth factors, and neurotransmitters.

VI. Intercellular Interactions

• Membranes facilitate recognition and signaling between adjacent cells.

VII. Energy Transduction

• Membranes are involved in converting one type of energy to another, as seen in chloroplasts and mitochondria.

Chemical Composition of Membranes

I. Membrane Structure

• Membranes are lipid-protein assemblies held together in a thin sheet via non-covalent bonds.

II. Lipid:Protein Ratio

• The lipid to protein ratio varies significantly based on
  • Type of membrane (cell membrane vs. endoplasmic reticulum vs. Golgi apparatus).
  • Organism type (prokaryote vs. plant vs. animal).
  • Cell type (cartilage vs. muscle vs. liver).
• Example: The inner mitochondrial membrane possesses a high protein-lipid ratio compared to red blood cells (RBCs), which in turn is higher than that of the myelin sheath.

III. Membrane Lipids

• Membranes consist of diverse amphipathic lipids containing both hydrophobic and hydrophilic components.
  • Three types of membrane lipids are:
  1. Phosphoglycerides
  2. Sphingolipids
  3. Cholesterol

IV. Phosphoglycerides

• Membrane phospholipids typically are built on a glycerol backbone and referred to as phosphoglycerides (diglycerides).
• Recent interests highlighted health benefits of two polyunsaturated fatty acids:
  • EPA (Eicosapentaenoic acid)
  • DHA (Docosahexaenoic acid)
  • Both are termed omega-3 fatty acids due to the positioning of their last double bond being three carbons from the omega (CH3) end of the fatty acid chain.

V. Sphingolipids

• Less abundant class of membrane lipids derived from sphingosine:
  • Sphingosine: an amino alcohol with a long hydrocarbon chain.
  • A sphingosine combined with a fatty acid and amino group forms a ceramide.
• Varieties of sphingosine-based lipids possess additional groups esterified to the terminal alcohol.

VI. Cholesterol

• A sterol, cholesterol can account for up to 50% of animal membrane lipids, absent from most plants and all bacterial membranes.
  • Contains a small hydrophilic hydroxyl group oriented toward the membrane surface, with the remaining structure embedded in the lipid bilayer.
  • Cholesterol's flat and rigid ring structure interferes with the movement of phospholipid fatty acid tails.

Membrane Carbohydrates

I. Eukaryotic Cell Plasma Membranes

• Comprise carbohydrates, amounting to 2 - 10% of plasma membrane weight depending on species and cell type.
• Less than 10% of membrane carbohydrates are covalently bound to lipids as glycolipids; over 90% are associated with proteins as glycoproteins.

II. Orientation

• All membrane carbohydrates face the extracellular space or the organelle interior, avoiding exposure to the cytosol.

Glycolipids

• Short branched oligosaccharide chains found on RBCs, determining ABO blood types due to variations in carbohydrate chains:
  • Blood Type A: Enzyme adds N-acetylgalactosamine to the chain terminus.
  • Blood Type B: Enzyme adds galactose at the end of the chain.
  • Blood Type AB: Possess both enzymes.
  • Blood Type O: Lacks enzymes for adding terminal sugars.
• Glycolipids play roles in infectious diseases, with cholera toxin and influenza virus binding to gangliosides, indicating they likely serve as receptors in normal cellular functions.

Structure and Function of Membrane Proteins: Overview

I. Classification of Membrane Proteins

• Three classes based on how intimately they associate with the lipid bilayer:

  • A. Integral Proteins:

  • Penetrate into the lipid bilayer and can be transmembrane.

  • Domains extend outward on both the extracellular and cytoplasmic sides of the membrane.

  • Can have one or multiple membrane-spanning segments.

  • B. Peripheral Proteins:

  • Reside entirely outside the bilayer, either on the extracellular or cytoplasmic side.

  • C. Lipid-Anchored Proteins:

  • Outside the bilayer but covalently linked to a membrane lipid within the bilayer.

II. Functions of Integral Membrane Proteins

• Integral membrane proteins engage in roles such as:
  • A. Receptors: Bind substances at the membrane surface.
  • B. Channels or Transporters: Assist in moving ions and solutes across the membrane.
  • C. Agents for Electron Transfer: Participate in photosynthesis and respiration processes.

III. Peripheral Membrane Proteins

• Bind via non-covalent (weak electrostatic) interactions to hydrophilic heads of lipids or hydrophilic portions of integral proteins.
  • Typically soluble in aqueous solutions.
  • Can provide mechanical membrane support, act as enzymes, or transmit transmembrane signals.

IV. Lipid-Anchored Proteins

• Two main types determined by lipid anchors and exposure surface:
  • A. GPI-Anchored Proteins:
  • Located on the external face of the plasma membrane, linked through oligosaccharides to glycophosphatidylinositol (GPI).
  • Include various receptors, enzymes, and cell adhesion proteins. Mutations affecting GPI synthesis may lead to diseases such as paroxysmal nocturnal hemoglobinuria.

Movement of Substances Across Cell Membranes: Diffusion and Osmosis

I. Membrane Functionality

• Membranes serve a dual role:
  • Retain dissolved materials within cells.
  • Facilitate exchanges of materials into and out of the cell.
• Movement of substances occurs by:
  • A. Passive processes (Diffusion)
  • B. Active processes (Energy-coupled transport)

II. Methods of Transport

1. Simple diffusion through lipid bilayer.
2. Simple diffusion through an aqueous protein-lined channel.
3. Facilitated diffusion via protein transporters.
4. Active transport using energy-driven protein pumps which move substances against their concentration gradient.

III. Passive and Active Transport

IV. Osmosis

• Membranes are termed semipermeable, allowing water to move faster than solutes.
• Osmosis: The movement of water through a semipermeable membrane from low solute concentration (high water concentration) to high solute concentration (low water concentration).
  • Hypertonic: Higher solute concentration (hyperosmotic) leads water to move towards it.
  • Hypotonic: Lower solute concentration (hypoosmotic) causes water to move away from it.
  • Isotonic: Solutions with equal solute concentrations result in no net movement of water (isoosmotic).

Movement of Substances Across Cell Membranes: Facilitated Diffusion

I. Facilitated Diffusion Explained

• Involves selective binding to a membrane-spanning protein (facilitative transporter) that promotes diffusion.
  • This differs from active transporters involved in energy release processes.
• Defining characteristics of facilitated transporters include:
  • Bind solute from one membrane side at a time.
  • Mechanism requires a conformational change in the protein to transport the solute across the membrane.

Movement of Substances Across Cell Membranes: Active Transport

I. Understand Active Transport Mechanisms

• Active transport creates strong ion gradients through energy expenditure:
  • Potassium (K+): Inside mammalian cells ~100 mM; outside ~5 mM.
  • Sodium (Na+): Outside ~150 mM; inside ~10-20 mM.
  • Calcium (Ca²+): Cytosolic concentration is 10^-7 M; external concentration is 10,000 times higher.
• Conformational changes powered by energy sources like ATP hydrolysis facilitate substance movement.

II. Na+/K+ Pump

• Exclusive to animals; principal mechanism for maintaining cell volume and electrochemical gradients.
• Pumps 3 Na+ ions out of the cell for every 2 K+ ions entering, creating an electrogenic effect that enhances charge separation across the membrane.
• Operates via P-type ion pumps requiring phosphorylation during cycling.

III. Mechanism of Ion Affinity

• Ion binding requires high affinity in low concentration zones and lower affinity for discharge into higher concentration areas.
• Changes of conformation due to phosphorylation modulate binding site exposure to different membrane sides.

Mitochondrial Structure and Function

I. Introduction to Aerobic Respiration

• Early Earth supported anaerobic organisms that derived energy without oxygen.
• The rise of cyanobacteria increased atmospheric oxygen levels, leading to the evolution of aerobes that utilize oxygen for enhanced energy extraction from organic substrates.
• Aerobic respiration occurs in mitochondria for eukaryotic cells.

II. Mitochondrial Morphology

• Generally, mitochondria are bean-shaped but may display round or threadlike forms. Characteristically have two membranes:
  1. Outer Membrane: The boundary layer.
  2. Inner Membrane: Forms cristae with deep folds.
• Mitochondrial structures create two aqueous compartments within the organelle.

III. Membrane Composition

• The inner membrane contains more than 75% protein, contrasting the outer membrane's 50% protein content.
• The inner membrane includes cardiolipin (absent in cholesterol) functioning efficiently in ATP synthesis.
• Outer membranes contain porin proteins to form large pores, while inner membranes are selectively impermeable even to small molecules.

IV. Mitochondrial Matrix

• Encompasses circular DNA, ribosomes, and metabolic enzymes with a gel-like consistency due to high protein concentration (≤500 mg/ml). RNA and protein synthesis occur here, and it contains Ca²+ transport molecules for heme group synthesis.

Summary of Mitochondrial Transport and Dynamics

I. Mitochondrial Division and Fusion

• Mitochondrial dynamic regulation requires DRP proteins (e.g., DRP1 for division) and membrane-anchoring proteins like MFN1/MFN2 and OPA1 for fusion.

II. Pathways for Oxidative Phosphorylation

• Calculation of ATP yield from glucose requires understanding glycolysis and the TCA cycle.
• Identify the location of glycolysis and the TCA cycle; glycolysis occurs in the cytoplasm, while the TCA cycle takes place in the mitochondrial matrix, producing NADH and FADH₂.

Components of the Electron Transport Chain

I. Electron Carriers

• A. Flavoproteins: Contain riboflavin-derived prosthetic groups (e.g. FAD, FMN); key components include NADH dehydrogenase and succinate dehydrogenase.
• B. Cytochromes: Contain heme prosthetic groups; several species exist within the system.
• C. Copper Atoms: Three located in a single protein in the inner mitochondrial membrane.
• D. Ubiquinone (Coenzyme Q): Soluble in lipid bilayers, facilitating electron transfer.
• E. Iron-Sulfur Proteins: Contain iron which accepts and donates single electrons.

Peroxisomes

I. Definition and Importance

• Peroxisomes, discovered in 1954, are single membrane-bound vesicles with diameters ranging from 0.1 – 1.0 µm that may contain dense cores of oxidative enzymes.

II. Multifunctional Metabolism

• Involved in:
  • Oxidation of very long chain fatty acids (VLCFAs).
  • Synthesis of plasmalogens, unique phospholipids found abundantly in myelin sheaths insulating brain axons.
  • Generate light via luciferase enzyme found in fireflies.

Human Health and Mitochondrial Dysfunction

I. Diseases Linked to Mitochondrial Abnormalities

• Mitochondrial disorders stem from structural or functional mutations in mitochondrial DNA (mtDNA).
• Inherited maternally, these disorders display various clinical manifestations, significantly affecting skeletal muscle.

II. Aging and Mitochondrial Mutations

• Accumulation of mtDNA mutations is proposed as a significant contributor to premature aging.
• Experimental mutations in mice demonstrate aging phenotypes attributing to mtDNA integrity.

Cellular Interactions and Extracellular Matrix

I. Importance of Extracellular Interactions

• Organizing multicellular organisms, the extracellular materials between cells regulate vital functions including cell migration, growth, and differentiation essential for tissue and organ organization during development.

II. Extracellular Matrix (ECM)

• Organized network of extracellular materials crucial for determining cell shape and functionality.
• Influence of ECM is highlighted by experimental evidences where significant cell functions revert upon ECM degradation and restoration.

III. Components of ECM

• Collagens: Fibrous glycoprotein family known for their tensile strength, abundant and vital within various ECMs; three chains form a rod-like triple helix.

• Proteoglycans: Core proteins linked to glycosaminoglycans, contributing to hydrated gel formation crucial for cellular interactions.

Intercellular Junctions: Structure and Function

I. Types of Intercellular Junctions

• Junctions maintain cellular integrity through adhesion processes:
  1. Adherens Junctions: Tightly connect cells via Ca²⁺-dependent cadherins.
  2. Desmosomes: Disk-shaped junctions in tissues subjected to mechanical stress, also utilize cadherins.
  3. Tight Junctions: Ensure barrier integrity at epithelial surfaces; membranes make intermittent contact.
  4. Gap Junctions: Facilitate intercellular communication via connexin-formed channels.