learning concepts
### Cell Membrane and Transport
1. Composition of the lipid bilayer
Made of *phospholipids** arranged in two layers.
Each phospholipid has a *hydrophilic (water-loving) head** and hydrophobic (water-fearing) tails.
Embedded with *proteins**, cholesterol, and carbohydrates for structure, signaling, and transport.
2. Why the lipid bilayer is fluid
Phospholipids and proteins can move *laterally** within the layer.
* Unsaturated fatty acids (double bonds) and cholesterol maintain flexibility and prevent solidification.
3. Osmosis vs Diffusion
* Both: Passive transport (no energy required), move from high → low concentration.
* Diffusion: Movement of solutes across a membrane.
* Osmosis: Movement of water across a semipermeable membrane.
4. Concentration gradient and flow of molecules
Molecules move *down** their concentration gradient (high → low).
If energy is used, molecules can move *against** the gradient (low → high).
5. Types of passive transport
* Simple diffusion: Direct movement across the membrane (small, nonpolar molecules).
* Facilitated diffusion: Uses transport proteins for larger or polar molecules.
* Osmosis: Diffusion of water.
6. Hypertonic, Hypotonic, Isotonic
* Hypertonic: Higher solute outside → cell loses water (shrinks).
* Hypotonic: Lower solute outside → cell gains water (swells).
* Isotonic: Equal solute → no net water movement.
7. Endocytosis, Exocytosis, Phagocytosis, Pinocytosis
* Endocytosis: Cell takes in materials via vesicles.
* Exocytosis: Cell expels materials via vesicles.
* Phagocytosis: “Cell eating” – engulfing solids.
* Pinocytosis: “Cell drinking” – engulfing liquids.
8. Na⁺/K⁺ Pump (Sodium-Potassium Pump)
* Active transport (requires ATP).
Pumps *3 Na⁺ out, 2 K⁺ in**, maintaining electrochemical gradient.
Essential for *nerve signaling** and cell volume control.
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### Energy and Enzymes
9. Energy and energy transformations
Energy exists as *kinetic (motion)** or potential (stored).
* Transformations: Energy changes forms but isn’t lost.
10. First Law of Thermodynamics
* Energy cannot be created or destroyed, only transformed.
11. Endergonic vs Exergonic / Endothermic vs Exothermic
* Endergonic: Requires energy (ΔG > 0).
* Exergonic: Releases energy (ΔG < 0).
* Endothermic: Absorbs heat.
* Exothermic: Releases heat.
12. ΔG and spontaneity
* Negative ΔG: Spontaneous reaction (energy released).
* Positive ΔG: Nonspontaneous (requires energy input).
13. Enzymes and catalysts
* Enzymes: Biological catalysts that speed up reactions without being consumed.
Lower *activation energy** required.
14. Activation energy and enzymes
* Activation energy: Minimum energy needed for a reaction.
* Enzymes lower it by stabilizing the transition state.
15. Reactant vs Product
* Reactants: Starting materials.
* Products: Resulting substances after reaction.
16. Gibbs Free Energy (ΔG)
* Measures usable energy in a system.
Tells whether a reaction is *spontaneous (ΔG < 0)** or nonspontaneous (ΔG > 0).
17. Relating ΔG to spontaneity
* ΔG < 0: Spontaneous.
* ΔG > 0: Nonspontaneous.
18. Activation energy & inhibition types
* Competitive inhibition: Inhibitor binds to active site.
* Non-competitive inhibition: Inhibitor binds elsewhere, changes enzyme shape.
* Allosteric inhibition: Regulates enzyme by changing shape at a site other than the active site.
19. Factors influencing enzyme rate
* Temperature, pH, substrate concentration, enzyme concentration, inhibitors.
20. Enzyme saturation (plateau)
When all enzymes are bound to substrates, rate levels off—*maximum velocity (Vmax)** reached.
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### Cellular Respiration
21. Purpose of cellular respiration
Convert *chemical energy (glucose)** into ATP (usable energy).
22. Oxidation and reduction
* Oxidation: Loss of electrons (or H).
* Reduction: Gain of electrons (or H).
* Important for transferring energy via electron carriers.
23. Potential vs Kinetic energy
* Potential: Stored energy (chemical bonds).
* Kinetic: Energy of motion.
* Chemical energy = a type of potential energy.
24. Substrate-level vs Oxidative phosphorylation
* Substrate-level: Direct transfer of phosphate to ADP (in glycolysis & Krebs).
* Oxidative: Uses electron transport chain and ATP synthase (in mitochondria).
25. Glycolysis phases
* Energy investment: 2 ATP used.
* Energy payoff: 4 ATP made (net 2), 2 NADH produced.
26. Glycolysis (inputs, outputs, location)
* Location: Cytoplasm.
* Inputs: Glucose, 2 ATP, 2 NAD⁺.
* Outputs: 2 pyruvate, 4 ATP (net 2), 2 NADH.
* Goal: Split glucose to extract energy.
27. Pyruvate oxidation
* Location: Mitochondrial matrix.
* Inputs: 2 pyruvate.
* Outputs: 2 acetyl-CoA, 2 CO₂, 2 NADH.
* Goal: Prepare acetyl-CoA for Krebs cycle.
28. Krebs (Citric Acid) Cycle
* Location: Mitochondrial matrix.
* Inputs: 2 acetyl-CoA.
* Outputs: 6 NADH, 2 FADH₂, 4 CO₂, 2 ATP.
* Goal: Complete oxidation of glucose derivatives.
29. Electron Transport Chain (ETC) & Chemiosmosis
* Location: Inner mitochondrial membrane.
* Inputs: NADH, FADH₂, O₂.
* Outputs: H₂O, ~32–34 ATP.
* Goal: Use electrons to create a proton gradient driving ATP synthesis.
30. Chemiosmosis and ATP synthesis
H⁺ ions flow through *ATP synthase** (down gradient), powering ATP production.
31. Fermentation
Occurs *without oxygen** after glycolysis.
* Produces: 2 ATP (from glycolysis).
* By-products: Lactic acid or ethanol + CO₂.
* Keeps glycolysis running by regenerating NAD⁺.
32. After glycolysis without oxygen
Cells perform *fermentation** instead of pyruvate oxidation.
33. Pyruvate oxidation vs fermentation
* Pyruvate oxidation: Aerobic, produces acetyl-CoA and NADH.
* Fermentation: Anaerobic, regenerates NAD⁺, no extra ATP made.
34. Citric Acid Cycle vs ETC
* Krebs Cycle: Makes NADH, FADH₂, CO₂, ATP.
* ETC: Uses NADH & FADH₂ to make large amounts of ATP and H₂O.
35. Chemiosmosis and proton gradient
ETC pumps protons into intermembrane space → *proton gradient** forms.
Flow of protons back drives *ATP synthase** to generate ATP.
36. NAD⁺/NADH role
* NAD⁺: Electron carrier; picks up electrons (becomes NADH).
* NADH: Donates electrons to ETC, helping produce ATP.