Chapter 5: The Working Cell

Membrane Structure and Function

The plasma membrane is a selectively permeable barrier that regulates the movement of substances into and out of the cell. Its structure is described by the Fluid-Mosaic Model.

• Fluid-Mosaic Model: The membrane consists of a phospholipid bilayer with embedded proteins. The hydrophobic (water-fearing) tails face inward, while the hydrophilic (water-loving) heads face outward.

• Selectively Permeable: Only certain molecules can cross freely; others require transport proteins.

• Diagram: Label hydrophobic tails, hydrophilic heads, integral and peripheral proteins, cholesterol, and carbohydrate chains.

Transport Mechanisms Across Membranes

Cells use various mechanisms to move molecules across membranes, depending on the molecule's properties and the cell's needs.

• Simple Diffusion: Movement of molecules from high to low concentration without energy input (e.g., O2, CO2).

• Facilitated Diffusion: Movement via transport proteins; still passive (e.g., glucose transporters).

• Active Transport: Movement against the concentration gradient using energy (ATP). Includes primary and secondary active transport.

• Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

• Endocytosis: Cell engulfs material by forming vesicles from the membrane.

Osmosis and Tonicity

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

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Hypotonic Solution: Lower solute concentration outside; water enters cell (cell may burst).

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves cell (cell shrinks).

Energy and Thermodynamics in Biology

Living organisms obey the laws of physics when acquiring and using energy.

• Energy: The capacity to do work. Essential for all cellular processes.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

• Potential vs. Kinetic Energy: Potential energy is stored (e.g., chemical bonds); kinetic energy is energy of motion.

• Endergonic vs. Exergonic Reactions: Endergonic reactions absorb energy; exergonic reactions release energy.

ATP and Energy Coupling

• ATP (Adenosine Triphosphate): The main energy currency of the cell. Hydrolysis of ATP releases energy for cellular work.

• ATP/ADP Cycle: ATP is converted to ADP (adenosine diphosphate) and inorganic phosphate, releasing energy. ADP is regenerated to ATP using energy from food or light.

• Energy Coupling: The use of exergonic processes to drive endergonic ones, often mediated by ATP.

Enzymes and Metabolic Reactions

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Enzyme Structure: Most are proteins with a specific active site for substrate binding.

• Key Terms:

  • Catalyst: Substance that speeds up a reaction without being consumed.

  • Substrate: The reactant an enzyme acts on.

  • Active Site: Region on enzyme where substrate binds.

  • Induced Fit: Enzyme changes shape to fit substrate.

  • Cofactor/Coenzyme: Non-protein helpers (e.g., vitamins, metal ions).

  • Inhibitors: Competitive (bind active site) or noncompetitive (bind elsewhere, change shape).

  • Feedback Inhibition: End product inhibits an earlier step, regulating pathway activity.

• Factors Affecting Enzyme Activity: Temperature, pH, enzyme concentration, substrate concentration.

• Allosteric Inhibition: Regulation by binding at a site other than the active site, altering enzyme activity.

Chapter 6: Harvesting Chemical Energy

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that convert biochemical energy from nutrients into ATP, releasing waste products.

• Metabolic Intermediates: Compounds formed between the initial substrate and final product.

• Substrate-Level Phosphorylation: Direct transfer of phosphate to ADP to form ATP.

• Oxidative Phosphorylation: ATP formation powered by redox reactions in the electron transport chain (ETC).

• Chemiosmosis: Movement of H+ ions across a membrane, driving ATP synthesis via ATP synthase.

Redox Reactions and Electron Carriers

• Oxidation-Reduction (Redox) Reactions: Transfer of electrons from one molecule to another. Oxidation = loss of electrons; reduction = gain of electrons.

• Electron Carriers: Molecules like NAD+ and FAD that transport electrons during cellular respiration.

Summary Equation of Cellular Respiration

• Equation:

• Reactants: Glucose and oxygen

• Products: Carbon dioxide, water, ATP

• Oxidized: Glucose

• Reduced: Oxygen

Stages of Cellular Respiration

• Glycolysis: Occurs in cytoplasm; glucose → pyruvate; produces ATP and NADH.

• Pyruvate Oxidation: Pyruvate → acetyl-CoA; produces NADH and CO2.

• Krebs (Citric Acid) Cycle: Occurs in mitochondrial matrix; acetyl-CoA → CO2; produces ATP, NADH, FADH2.

• Oxidative Phosphorylation: Occurs in inner mitochondrial membrane; uses NADH, FADH2, and O2 to produce ATP and H2O.

ATP Yield Table

Fermentation

• Fermentation: Anaerobic process; regenerates NAD+ for glycolysis; produces lactic acid or ethanol.

• Occurs in: Cytoplasm

• ATP Yield: 2 ATP per glucose

Regulation and Alternative Fuels

• Feedback Inhibition: End products inhibit enzymes in earlier steps (e.g., ATP inhibits phosphofructokinase).

• Alternative Fuels: Polysaccharides, lipids, and proteins can enter cellular respiration at various points.

• Biosynthesis: Intermediates from respiration can be used to build macromolecules.

Chapter 7: Photosynthesis

Introduction to Photosynthesis

Photosynthesis is the process by which photoautotrophs convert light energy into chemical energy, producing organic molecules and oxygen.

• Photoautotroph: Organism that uses light energy to synthesize organic compounds.

• Electromagnetic Spectrum: Range of all wavelengths of light; visible light is used in photosynthesis.

• Photon: A quantum of light energy.

• Pigment: Molecule that absorbs specific wavelengths of light (e.g., chlorophyll).

Photosynthesis Equation and Redox

• Equation:

• Reactants: CO2, H2O

• Products: Glucose, O2

• CO2 is reduced; H2O is oxidized.

Chloroplast Structure

• Stroma: Fluid-filled space inside chloroplast.

• Grana: Stacks of thylakoids.

• Thylakoids: Membranous sacs containing chlorophyll.

• Thylakoid Space: Interior of thylakoid sacs.

Light Reactions and Calvin Cycle

• Light Reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP, NADPH); split water (photolysis) to release O2.

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

• Photophosphorylation: ATP synthesis using light energy.

• Carbon Fixation: Incorporation of CO2 into organic molecules.

• Carbon Reduction: Conversion of fixed carbon to carbohydrate.

Photosystems and Electron Flow

• Photosystem II: Absorbs light, splits water, transfers electrons to ETC.

• Photosystem I: Absorbs light, transfers electrons to NADP+ to form NADPH.

• ATP Synthesis: Occurs via chemiosmosis and photophosphorylation in thylakoid membrane.

Comparison: Photosynthesis vs. Cellular Respiration

Calvin Cycle Details

• Key Enzyme: Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase)

• Reactants: CO2, ATP, NADPH

• Products: G3P (glyceraldehyde-3-phosphate), which is used to form glucose and other carbohydrates

Integration of Light Reactions and Calvin Cycle

• ATP and NADPH produced in the light reactions are used in the Calvin cycle.

• The Calvin cycle regenerates ADP and NADP+ for use in the light reactions.

Importance of Photoautotrophs

• Photoautotrophs are the primary producers in ecosystems, supporting all other life forms by providing organic molecules and oxygen.

Additional info: Where the original notes listed only terms or objectives, academic context and definitions have been added for clarity and completeness.