Plasma Membrane and Cellular Processes
PLASMA MEMBRANE & MEMBRANE TRANSPORT (Week 5, Mon)
Description of Plasma Membrane:
The plasma membrane is described as a dynamic, fluid structure rather than a static barrier.
It follows the fluid mosaic model, indicating that it is not rigid or fixed.
Composed of a phospholipid bilayer:
Hydrophilic heads face outwards (towards water).
Hydrophobic tails face inwards (away from water).
Fluidity:
Phospholipids can move within the bilayer, contributing to the membrane's fluidity.
Key Features:
Not held together by covalent bonds, allowing flexibility.
Some small molecules can pass through the membrane.
Membrane-spanning proteins regulate what enters and exits the cell.
Membrane proteins can move laterally and change positions.
Transport and Selectivity
Definitions:
Transport: Movement of substances across the plasma membrane.
Selectivity: Ability of the membrane to allow some substances to move across more easily than others.
Selective Permeability: The membrane selectively allows certain molecules (e.g., small nonpolar or uncharged molecules) to pass while restricting others (e.g., large polar or charged molecules).
Importance for Cell Survival:
Without selective permeability, harmful substances could easily enter and essential molecules could exit freely.
It is crucial for:
Removal of wastes.
Intake of nutrients.
Maintenance of ion balance and internal homeostasis.
Diffusion and Osmosis
Diffusion:
Movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached.
No energy is required (passive process).
Concentration Gradient: Molecules move down their concentration gradient.
Example: Perfume dispersing in air.
Small nonpolar molecules (e.g., O2, CO2) diffuse directly through the lipid bilayer.
Osmosis:
Diffusion of water across a selectively permeable membrane due to concentration differences.
Water moves towards the area with a higher solute concentration.
Types of solutions:
Hypertonic: Higher solute concentration (causes cells to lose water).
Isotonic: Equal solute concentration (no net movement of water).
Hypotonic: Lower solute concentration (causes cells to gain water).
Factors Influencing Rate:
Temperature: Higher temperature increases net movement.
Pressure.
Electrical currents.
Molecular size: Smaller molecules diffuse faster.
Passive vs. Active Transport
Passive Transport:
Achieved by being in equilibrium with the environment.
Moves substances down their concentration gradient (high to low concentration).
No energy required.
Diffusion: Movement from high to low concentration (no carrier protein).
Facilitated Diffusion: Movement through a membrane down a gradient with the help of proteins (carrier/channel proteins).
Active Transport:
Involves movement of molecules against their concentration gradient (low to high concentration).
Requires energy (ATP).
Specific carrier proteins (pumps) are used, such as the sodium-potassium pump.
Endocytosis: Taking substances into the cell by forming a vesicle.
Exocytosis: Process by which an intracellular vesicle fuses with the plasma membrane to release its contents outside.
Key Differences:
Direction relative to gradient (down vs. up).
Energy use (passive vs. active).
Predicting Molecular Movement
Diffusion: Molecules move from high to low concentration.
Osmosis: Water moves towards higher solute concentration.
Types of Solutions
Hypotonic:
Lower solute concentration inside the cell compared to outside.
Cells gain water by osmosis (may cause hemolysis).
Hypertonic:
Higher solute concentration inside the cell compared to outside.
Cells lose water by osmosis (may cause crenation).
Isotonic:
Equal solute concentration inside and outside of the cell.
Cells maintain their state (no net movement of water).
Fluid-Mosaic Model of Membrane Structure
Structure:
Phospholipid bilayer embedded with proteins.
Phospholipids form a flexible, fluid base.
Proteins scattered throughout in a mosaic pattern.
Cholesterol:
Found in animal cells.
Helps maintain stability and fluidity of the membrane.
Carbohydrates:
Attached to proteins or lipids for cellular recognition.
Roles of Membrane Proteins
Peripheral Proteins:
Provide structural stability and maintain membrane shape.
Function in signaling pathways.
Integral Proteins:
Primarily determine membrane function.
Transport Proteins: Facilitate passage across the membrane.
Channel Proteins:
Involved in passive transport of molecules.
Example: Allows hydrogen ions to flow across mitochondrial membranes for ATP production.
May have gates that are opened by specific molecules.
Carrier Proteins:
Bind with substances to help them cross the membrane.
Example: Transports sodium and potassium ions across nerve cell membranes for conduction.
Cell Recognition Proteins:
Glycoproteins that help recognize pathogens for immune response.
Receptor Proteins:
Bind to specific molecules to trigger cellular responses.
Essential for organ coordination (e.g., insulin signaling liver to store glucose).
Enzymatic Proteins:
Catalyze metabolic reactions directly.
Nexus metabolism (e.g., integral membrane proteins of the electron transport chain).
Selective Permeability of Membranes
Definition:
Membranes selectively allow certain molecules through:
Small nonpolar molecules pass easily.
Polar and charged molecules require transport proteins.
Hydrophobic interior of the phospholipid bilayer blocks large and charged molecules.
Molecule Movement Predictions:
Diffusion: Movement from higher concentration to lower concentration until equilibrium.
Osmosis: Water moves towards higher solute concentration:
Hypotonic: Cell gains water (hemolysis).
Hypertonic: Cell loses water (crenation).
Isotonic: Cell remains the same (no net water movement).
Proteins in Movement Across Membranes
Channel Proteins: Provide pathways for ions/molecules.
Carrier Proteins: Bind and change shape to transport molecules.
Pumps: Use ATP to move molecules against gradients.
Role of the Extracellular Matrix in Animal Cells
Components:
Collagen and elastin: Structural support.
Fibronectins: Bind to integrin to facilitate communication between the ECM and the cytoskeleton.
Junctions in Cells
Adhesion Junction:
Function: Mechanically attaches adjacent cells.
Structure: Flexible sheet of cells.
Gap Junction:
Function: Allows direct communication between cells.
Structure: Two identical plasma membrane channels joined.
Tight Junction:
Function: Creates a tight barrier between cells.
Structure: Zipperlike fastening.
CELL COMMUNICATION (NERVOUS SYSTEM AS A MODEL) (Week 5, Wed)
Basic Components of Cell Signaling
Signaling Cells: Release signaling molecules.
Receptors: Proteins (typically on the plasma membrane) that bind to signaling molecules.
Signal Transduction Pathways: Series of intracellular events triggered when receptors change shape upon binding.
Target Responses: Specific reactions in the cell (e.g., opening ion channels, activating enzymes, gene expression).
Structure of a Neuron
Parts of a Neuron:
Soma (Cell Body):
Contains nucleus.
Integrates incoming signals.
Dendrites:
Receive incoming signals.
Axon:
Carries impulses away from the cell body.
Myelin in the Nervous System
Myelin Sheath:
Function: Increases speed of impulse transmission.
Location: Encases some axons.
Structure: Fatty layers that cover nerve fibers.
Membrane Potential
Definition:
The electrical charge difference across the plasma membrane.
Typical value is 0 mV.
Maintenance:
Maintained by the sodium-potassium pump.
Separates different ion concentrations across the membrane.
Requires ATP to change shape and move ions:
Results in 3 Na⁺ outside and 2 K⁺ inside, creating positive charge outside and negative charge inside.
Resting vs. Action Potentials
Resting Potential:
Neuron is prepared to fire (-55 mV to -70 mV).
Inside is negative; outside is positive due to the activity of sodium-potassium pump and more K⁺ leaking out than Na⁺ leaking in.
Action Potential:
Sodium flows into the cell following its concentration gradient.
Depolarization:
Neuron fires, Na⁺ channels open and Na⁺ rushes in, making the interior positive.
Repolarization:
Sodium channels close; K⁺ channels open; K⁺ leaves, restoring negative charge.
Movement of Na⁺ and K⁺ during each phase details the generation and propagation of action potentials:
Resting State:
Voltage-gated Na⁺ and K⁺ channels are closed.
Depolarization:
Threshold stimulus opens Na⁺ channels; Na⁺ enters and membrane becomes positive.
Repolarization:
Na⁺ channels close; K⁺ channels open; K⁺ exits and membrane becomes negative again.
Hyperpolarization:
K⁺ channels remain open longer, making the membrane potential more negative than the resting level.
Return to Resting:
Sodium-potassium pump restores ion balance.
Structure of a Synapse
Components:
Presynaptic Neuron (axon terminal): Releases neurotransmitters.
Synaptic Vesicles: Encloses neurotransmitters.
Synaptic Cleft: Gap between cells.
Postsynaptic Cell: Target cell membrane.
Signal Transmission Across a Synapse
Occurs when an action potential causes neurotransmitter release at presynaptic membrane.
Neurotransmitter diffuses across synaptic cleft and binds to receptors on the postsynaptic membrane.
ENERGY AND ENZYMES (Week 5, Fri)
Energy Forms
Potential Energy:
Stored energy (e.g., energy in food, water behind a dam).
Kinetic Energy:
Energy of motion (e.g., a falling ball, muscle contractions).
Laws of Thermodynamics
First Law: Energy cannot be created or destroyed; only transformed.
Example: Energy in food (potential) converted to movement, heat, or chemical work (kinetic) in cells.
Second Law: When energy is transformed, entropy increases.
Energy cannot be changed from one form to another without a loss of usable energy.
Example: During glucose conversion to ATP, some energy is released as heat.
Exergonic vs. Endergonic Reactions
Exergonic Reactions:
Release energy; G(products) > G(reactants); ΔG is positive.
Products have less free energy than reactants (e.g., cellular respiration occurs spontaneously).
Endergonic Reactions:
Require energy absorption; G(products) < G(reactants); ΔG is negative.
Products have more free energy than reactants (e.g., photosynthesis).
ATP Structure and Function
ATP: Adenosine triphosphate
Structure: Nucleotide with nitrogenous base (adenine), 5 carbon sugar (ribose), and three phosphate groups.
Functions:
Chemical work: Energy for macromolecule synthesis.
Transport work: Energy to pump substances across plasma membrane.
Mechanical work: Energy for muscle contraction, cilia, and flagella movements.
Primary Energy Currency:
ATP is considered the primary energy currency of the cell due to the energy stored in the bonds between phosphates, sufficient for powering various cell functions.
Coupled Reactions & ATP Cycle
Coupled Reactions: Occur simultaneously; one exergonic and one endergonic.
The energy from the exergonic reaction drives the endergonic reaction.
ATP Hydrolysis (exergonic) releases energy, driving endergonic processes (e.g., protein synthesis):
ATP → ADP + P + energy.
ADP is recycled back to ATP using energy from food (ATP cycle).
Activation Energy and Enzymes
Activation Energy: Minimum energy needed to start a reaction.
Enzymes lower activation energy, increasing reaction rates without being consumed (reusable).
Enzyme Specificity
Enzymes have specific active sites matching substrates; binding forms enzyme-substrate complex.
Specificity is explained by “lock-and-key” and “induced fit” models.
Most enzymes only bind one or a few closely related substrates.
Regulation of Enzyme Activity
Inhibitors:
Substrates unable to bind due to blockage or binding to another site altering enzyme shape.
Competitive: Compete for active site.
Noncompetitive: Bind allosteric site, altering enzyme shape.
Environmental Conditions:
Temperature: Increases rate until optimal point; too high = denaturation.
pH: Each enzyme has optimal pH; extremes alter shape.
Substrate Concentration: Rate increases until saturation.
Feedback Inhibition
Occurs when the final product of a metabolic pathway inhibits early enzymes, preventing overproduction and conserving energy.
Examples of inhibitors:
Penicillin: Blocks bacterial cell wall formation.
Pravastatin: Inhibits lipid production, reducing stroke risk.
Cyanide: Inhibits ATP formation (fatal).
Sarin: Cholinesterase inhibitor affecting nerve transmission.
Different Forms of Energy
Chemical Energy: Energy in covalent bonds (crucial for cells).
Mechanical Energy: Energy associated with movement.
Two Laws of Thermodynamics Applied to Cells
First Law (Conservation of Energy):
Energy in food is transformed into ATP for cellular work.
Second Law (Entropy):
Energy transfer increases entropy; reactions release heat.
Metabolic Reaction Relationships
Anabolic: Endergonic; builds larger molecules from smaller ones.
Catabolic: Exergonic; breaks down larger molecules into smaller ones.
Summary of ATP Cycle and Metabolic Pathways
ATP is regenerated continually; powers both exergonic and endergonic reactions.
A metabolic pathway consists of linked reactions controlled by specific enzymes for efficient energy release.
AEROBIC AND ANAEROBIC RESPIRATION (Week 6)
Cellular Respiration Overview
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP.
Process:
Glucose is broken down; oxygen is used; CO₂ and H₂O are produced; energy stored in glucose is transferred to ATP.
Glucose is oxidized, oxygen is reduced.
Energy Harvest in Cellular Respiration
Stages of Harvesting Energy:
Glycolysis: Breaks glucose into two pyruvate in the cytosol.
Produces 2 ATP and NADH.
Citric Acid Cycle: Breaks down pyruvate into CO₂ and energy-carrying molecules in the mitochondrial matrix.
Produces NADH, FADH₂, and ATP.
Oxidative Phosphorylation: Produces ATP via ion gradient transformation across mitochondria.
Creates the majority of ATP (32 to 34).
Without Oxygen: Glycolysis leads to fermentation (producing lactic acid or ethanol).
Summary of Cellular Respiration Phases
Glycolysis (Anaerobic): Occurs in cytoplasm—2 ATP (net) produced.
Preparatory Reaction (Aerobic): Converts pyruvate to acetyl-CoA, occurs in mitochondria.
Citric Acid Cycle (Aerobic): Fully oxidizes Acetyl-CoA; occurs in mitochondria; produces CO₂, NADH, FADH₂, and ATP.
Electron Transport Chain (Aerobic): Main ATP production through electron carriers in mitochondrial membranes.
Distinct Phases of Glycolysis
Glycolysis has two phases:
Energy Investment Phase:
Two ATP used to initiate glucose breakdown.
Energy Harvesting Phase:
Produces 4 ATP (net gain of 2 after investment).
High-Energy Molecules of Glycolysis
ATP and NADH produced.
Glycolysis produces only 2 net ATP per glucose; more energy remains in pyruvate, requiring further processing.
Role of Electron Carriers
Roles:
NADH and FADH₂ carry high-energy electrons to the electron transport chain.
These donors are crucial for ATP generation as they release energy during electron transport.
Role of Oxygen in Cellular Respiration
Oxygen acts as the final electron acceptor in the electron transport chain; essential for lowering potential energy and maintaining ATP production.
When missing, NADH cannot unload electrons, halting cellular respiration.
Electron Transport Chain and Chemiosmosis
Mechanism:
H⁺ ions are pumped creating a proton gradient; protons then flow back through ATP synthase, producing ATP.
Oxidative Phosphorylation: Involves coupled oxidation-reduction reactions generating ATP in the process, linking electron transfer and phosphorylation.
Function of Fermentation
Occurs When: No oxygen is present; generates NAD+ allowing glycolysis to continue.
Types:
Lactic Acid Fermentation: Pyruvate converts to lactic acid; utilized in muscle cells; used in yogurt and cheese production.
Alcohol Fermentation: Pyruvate converts to ethanol and CO₂; utilized in yeast; used in bread making and alcohol production.
Relationship Between Chloroplasts and Mitochondria
Chloroplasts: Perform photosynthesis; store energy in carbohydrates.
Mitochondria: Perform cellular respiration to release energy from carbohydrates.
Products of one are reactants for the other, highlighting their interdependent roles in energy conversion within cells.