BIOL161 Exam 2 Study Guide: Cell Membranes, Energy, Enzymes, and Cellular Processes

Cell Membranes and Transport

Phospholipid Structure and Bilayer

The cell membrane is primarily composed of phospholipids arranged in a bilayer, which is essential for cellular compartmentalization and function.

• Phospholipid Structure: Each phospholipid molecule consists of a hydrophilic (water-attracting) phosphate head and two hydrophobic (water-repelling) fatty acid tails.

• Phospholipid Bilayer: The bilayer forms as phospholipids arrange themselves with heads facing outward toward water and tails facing inward, away from water.

• Layer Cohesion: Hydrophobic interactions between fatty acid tails and hydrophilic interactions with water stabilize the bilayer.

• Components of Cellular Membranes: The four main components are phospholipids, proteins, carbohydrates, and cholesterol.

  • Phospholipids: Form the basic structure.

  • Proteins: Serve as channels, carriers, receptors, and enzymes.

  • Carbohydrates: Attach to proteins/lipids for cell recognition.

  • Cholesterol: Modulates fluidity and stability.

• Fluidity Factors: Membrane fluidity is influenced by fatty acid saturation, temperature, and cholesterol content.

Example: Unsaturated fatty acids increase membrane fluidity, while cholesterol can stabilize membranes at high temperatures and prevent rigidity at low temperatures.

Membrane Proteins and Transport Mechanisms

Proteins embedded in the membrane perform various functions essential for cell survival and communication.

• Functions of Membrane Proteins: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton.

• Anchoring Molecules: Anchor proteins to the membrane or cytoskeleton, maintaining cell shape and stability.

• Transmembrane Domain: Hydrophobic region of an integral protein that spans the lipid bilayer.

• Pores: Channel-forming proteins with hydrophilic interiors allowing passage of specific molecules.

• Passive Transport: Movement of substances across the membrane without energy input, including diffusion and facilitated diffusion.

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Transport aided by channel or carrier proteins.

• Proteins in Facilitated Diffusion: Channel proteins and carrier proteins.

Example: Glucose transport into cells via GLUT transporters is an example of facilitated diffusion.

Ion Channels, Osmosis, and Membrane Transport

Specialized proteins and processes regulate the movement of ions and water across membranes.

• Ion Channels: Proteins that allow specific ions to pass through the membrane.

• Direction of Ion Movement: Determined by concentration gradient, charge (electrochemical gradient), and whether the channel is open or closed.

• Saturation: Occurs when all transport proteins are occupied, limiting the rate of transport.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Osmotic Concentration: Measure of solute concentration in a solution.

• States of Osmotic Concentration:

  • Isotonic: Equal solute concentration inside and outside the cell.

  • Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

  • Hypotonic: Lower solute concentration outside; water enters the cell.

Example: Red blood cells in a hypotonic solution swell and may burst (lyse).

Osmotic Pressure and Active Transport

Cells must regulate water and solute movement to maintain homeostasis.

• Osmotic Pressure: Pressure required to prevent water movement across a membrane due to osmosis.

• Cellular Effects: Excessive osmotic pressure can cause cell swelling or shrinkage.

• Osmotic Balance: Cells use mechanisms like contractile vacuoles or ion pumps to maintain balance.

• Active Transport: Movement of substances against their concentration gradient using energy (usually ATP).

• Active Transport System: Sodium-potassium pump ($Na^+/K^+$ ATPase) is a classic example.

• Coupled Transport: Transport of one molecule is linked to the movement of another (symport or antiport).

Example: The sodium-glucose symporter uses the sodium gradient to transport glucose into cells.

Cellular Processes: Endocytosis, Exocytosis, and Energy

Endocytosis and Exocytosis

Cells exchange materials with their environment through vesicular transport mechanisms.

• Endocytosis: Uptake of materials into the cell via vesicles.

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of fluids and small molecules.

• Exocytosis: Release of materials from the cell via vesicle fusion with the membrane.

Example: Neurotransmitter release at synapses is an example of exocytosis.

Thermodynamics and Energy in Biology

Biological systems obey the laws of thermodynamics, which govern energy transformations.

• Thermodynamics: Study of energy and its transformations.

• Energy Definition: Capacity to do work.

• Potential vs. Kinetic Energy:

  • Potential: Stored energy (e.g., chemical bonds).

  • Kinetic: Energy of motion.

• Measurement: Energy is most conveniently measured in units of heat (calories or joules).

• First Law of Thermodynamics:

  • Energy cannot be created or destroyed.

  • Energy can be transformed from one form to another.

  • Total energy in a closed system remains constant.

  • Organisms must obtain energy from their environment.

Example: Photosynthesis converts light energy to chemical energy in glucose.

Entropy, Free Energy, and Catalysis

Energy transformations are associated with changes in disorder and free energy.

• Entropy: Measure of disorder; increases in spontaneous processes.

• Second Law of Thermodynamics: Entropy of the universe tends to increase.

• Free Energy Equation: $\Delta G = \Delta H - T\Delta S$

  • $\Delta G$: Change in free energy

  • $\Delta H$: Change in enthalpy

  • $T$: Temperature (Kelvin)

  • $\Delta S$: Change in entropy

• Interpretation:

  • $\Delta G > 0$: Non-spontaneous (requires energy)

  • $\Delta G < 0$: Spontaneous (releases energy)

• Activation Energy: Energy required to initiate a reaction.

• Catalysts: Substances that lower activation energy, increasing reaction rate.

Example: Enzymes are biological catalysts that speed up cellular reactions.

ATP Structure and Function

ATP (adenosine triphosphate) is the primary energy currency of the cell.

• Structure: Composed of adenine, ribose, and three phosphate groups.

• Energy Storage: Energy is stored in the high-energy phosphate bonds, especially the terminal phosphate.

Example: Hydrolysis of ATP to ADP releases energy for cellular work.

Enzymes and Metabolic Regulation

Enzyme Structure and Function

Enzymes are proteins that catalyze biochemical reactions, increasing their speed and specificity.

• Active Site: Region on the enzyme where substrate binds and reaction occurs.

• Location: Enzymes are found throughout the cell, including cytoplasm, membranes, and organelles.

• Multi-Enzyme Complex: Group of enzymes physically associated to carry out sequential reactions.

• Ribozymes: RNA molecules with catalytic activity; types include self-splicing introns and ribosomal RNA.

• Factors Affecting Enzyme Function: Temperature, pH, substrate concentration, inhibitors, and cofactors.

Example: Digestive enzymes in the stomach function optimally at low pH.

Enzyme Regulation and Inhibition

Enzyme activity is tightly regulated to maintain metabolic balance.

• Inhibitors: Molecules that decrease enzyme activity.

  • Competitive: Bind to active site, blocking substrate.

  • Non-competitive: Bind elsewhere, altering enzyme shape.

• Allosteric Enzymes: Enzymes regulated by molecules binding at sites other than the active site.

• Allosteric Activators/Inhibitors: Increase or decrease enzyme activity by changing enzyme conformation.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, preventing overproduction.

• Anabolism vs. Catabolism:

  • Anabolism: Building complex molecules from simpler ones (requires energy).

  • Catabolism: Breaking down complex molecules into simpler ones (releases energy).

Example: ATP inhibits phosphofructokinase in glycolysis via feedback inhibition.

*Additional info: Academic context and definitions have been expanded for clarity and completeness. Further topics from the original file (e.g., cellular respiration, photosynthesis, cell cycle, and cancer genetics) are not included in this excerpt but are relevant for a full study guide.*