AP Biology
Advanced Placement Biology aligned with the College Board CED: chemistry of life, cell structure and function, cellular energetics, cell communication and the cell cycle, heredity, gene expression and regulation, natural selection, and ecology.
Ämne: Biologi · Nivå: Gymnasium (16–19) · 401 kort
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- Water (H₂O) is a polar molecule because oxygen attracts electrons more strongly than hydrogen, creating partial negative charge on O and partial positive on H. This polarity drives hydrogen bonding, which underlies most of water's biologically important properties.
- Hydrogen bonds form between the partial positive H of one water molecule and the partial negative O of another. They are individually weak (~5% of a covalent bond) but collectively give water cohesion, adhesion, and high specific heat.
- Cohesion (water-water attraction) supports continuous water columns in xylem during transpiration. Adhesion (water-other attraction) helps water cling to vessel walls and produces capillary action.
- Water's high specific heat (4.18 J/g·°C) buffers temperature change in cells, organisms, and aquatic environments. The high heat of vaporization makes evaporative cooling (e.g., sweating, transpiration) energetically powerful.
- Solid water (ice) is less dense than liquid water because hydrogen bonds lock molecules into a crystalline lattice with more empty space. Ice floats and insulates the water below, allowing aquatic life to survive winter.
- The four major classes of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids. All but lipids are true polymers built from monomers through dehydration synthesis and broken by hydrolysis.
- Dehydration synthesis (condensation) joins two monomers by removing one water molecule and forming a covalent bond. Hydrolysis is the reverse: water is added to break the bond and separate the monomers.
- Carbohydrates consist of monosaccharides (glucose, fructose, galactose) joined by glycosidic bonds. Disaccharides include sucrose, lactose, maltose. Polysaccharides include starch (plant storage), glycogen (animal storage), cellulose (plant structure), and chitin (fungal/arthropod structure).
- Cellulose and starch are both polymers of glucose, but cellulose is built from β-glucose (alternating monomer orientation produces straight, hydrogen-bonded fibers) while starch is built from α-glucose (helical, easily digested).
- Lipids are hydrophobic and not true polymers. Triglycerides = glycerol + 3 fatty acids (energy storage). Phospholipids = glycerol + 2 fatty acids + phosphate group (membranes). Steroids = four fused rings (cholesterol, hormones).
- Saturated fatty acids have no C=C double bonds in their tails (solid at room temperature, e.g., animal fats). Unsaturated fatty acids contain one or more C=C double bonds that introduce kinks, preventing tight packing (liquid oils).
- Phospholipids have a hydrophilic head (phosphate + glycerol) and hydrophobic tails (fatty acids). In water they self-assemble into bilayers with tails facing inward — the structural basis of every cellular membrane.
- Proteins are polymers of amino acids joined by peptide bonds. Each amino acid has an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable R-group (side chain) attached to a central alpha carbon. There are 20 standard amino acids.
- Protein structure has four levels: primary (amino-acid sequence), secondary (α-helix and β-sheet from backbone hydrogen bonds), tertiary (overall 3-D shape from R-group interactions), and quaternary (multiple subunits, as in hemoglobin).
- Denaturation unfolds a protein when heat, pH change, or chemicals disrupt non-covalent interactions — primary structure remains intact, but secondary/tertiary/quaternary collapse. A denatured protein loses function because shape determines function.
- Nucleic acids (DNA, RNA) are polymers of nucleotides. Each nucleotide has a pentose sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (A, T/U, G, C).
- DNA strands are antiparallel: one runs 5′→3′, the other 3′→5′. Base pairing is complementary — A pairs with T (two hydrogen bonds), G pairs with C (three hydrogen bonds). The sugar-phosphate backbone forms the helix's outside.
- Purines (A, G) have two fused rings; pyrimidines (C, T, U) have one. A purine always pairs with a pyrimidine, keeping the DNA helix a constant width (≈2 nm).
- The directionality of nucleic acids is set by the sugar: the 5′ carbon carries a phosphate, the 3′ carbon a hydroxyl. New nucleotides can only be added to a free 3′–OH — the reason DNA polymerase synthesizes strictly 5′→3′.
- Prokaryotic cells (bacteria, archaea) lack a membrane-bound nucleus and other membrane-bound organelles. DNA sits in a region called the nucleoid; ribosomes are 70S; a peptidoglycan cell wall surrounds bacteria.
- Eukaryotic cells have a true nucleus and membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts in plants), 80S cytoplasmic ribosomes, and a cytoskeleton. They are generally 10–100 µm — about 10× larger in linear dimension than prokaryotes.
- The surface-area-to-volume ratio limits cell size. As a cell grows, volume increases as r³ while surface area increases as r², so SA/V falls and the membrane can no longer exchange materials fast enough. Cells solve this with division, folded membranes (microvilli), or flattened shape.
- The nucleus stores DNA, controls gene expression, and assembles ribosomes in the nucleolus. Nuclear pores in the double-membrane envelope regulate movement of mRNA, ribosomal subunits, and regulatory proteins.
- Rough ER (studded with ribosomes) synthesizes and modifies proteins destined for membranes, secretion, or lysosomes. Smooth ER lacks ribosomes and handles lipid synthesis, detoxification (especially in liver), and Ca²⁺ storage (muscle).
- The Golgi apparatus receives vesicles from the rough ER on its cis face, modifies cargo (glycosylation, sorting), and ships finished products from its trans face in vesicles to the membrane, lysosomes, or secretion.
- Mitochondria carry out aerobic respiration. They have a smooth outer membrane and a folded inner membrane (cristae) that holds the electron transport chain and ATP synthase. Their matrix contains the Krebs cycle enzymes, mitochondrial DNA, and 70S ribosomes.
- Chloroplasts perform photosynthesis. The double membrane encloses the stroma, where the Calvin cycle takes place. Inside the stroma are stacked thylakoids (grana) whose membranes hold chlorophyll, photosystems, and ATP synthase for the light reactions.
- Endosymbiotic theory (Margulis) proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence: double membranes, circular DNA, 70S ribosomes, and independent binary-fission-like division.
- Lysosomes are membrane-bound bags of hydrolytic enzymes (active at pH ≈4.5) that digest worn organelles (autophagy), engulfed material (phagocytosis), and recycle macromolecules. Tay-Sachs and other lysosomal storage disorders result from missing enzymes.
- The cytoskeleton consists of microfilaments (actin — contraction, shape), intermediate filaments (mechanical strength), and microtubules (tubulin — cell shape, organelle transport, spindle, cilia/flagella).
- The plasma membrane is a phospholipid bilayer with embedded proteins (fluid mosaic model, Singer & Nicolson 1972). Cholesterol modulates fluidity: at high T it stiffens; at low T it prevents tight packing.
- Membrane transport: small nonpolar molecules (O₂, CO₂) cross by simple diffusion. Ions and polar molecules need protein channels or carriers (facilitated diffusion — no ATP). Active transport pumps solutes against the gradient using ATP.
- Osmosis is the diffusion of water across a selectively permeable membrane from a region of higher water potential (lower solute) to one of lower water potential (higher solute). In animal cells: hypotonic = lyse, hypertonic = shrivel, isotonic = stable.
- Water potential Ψ = Ψₛ (solute potential) + Ψₚ (pressure potential). Ψₛ is always ≤0 (more solute lowers potential). Water moves from higher Ψ to lower Ψ. In a turgid plant cell, positive Ψₚ offsets negative Ψₛ.
- The Na⁺/K⁺-pump uses one ATP to expel 3 Na⁺ and import 2 K⁺ per cycle, creating both a chemical gradient and a net negative interior charge (≈–70 mV). This electrochemical gradient powers secondary active transport and excitable-cell signaling.
- Bulk transport across membranes: endocytosis (cell engulfs material — phagocytosis for solids, pinocytosis for fluids, receptor-mediated for specific molecules) and exocytosis (vesicles fuse with membrane to release contents).
- Enzymes are biological catalysts (almost always proteins) that lower the activation energy of a reaction without being consumed. They are highly specific because the active site fits substrates by induced fit.
- Enzyme activity depends on temperature, pH, substrate concentration, and the presence of cofactors/coenzymes. Each enzyme has an optimum range; outside it, enzymes denature or work poorly.
- Competitive inhibitors bind the active site, blocking substrate — effect can be overcome with more substrate. Noncompetitive (allosteric) inhibitors bind elsewhere, changing the enzyme's shape — cannot be overcome by more substrate.
- Feedback inhibition: a metabolic pathway's end product binds an upstream enzyme allosterically and slows production when product levels are high. Example: isoleucine inhibits threonine deaminase, the first enzyme of its synthesis pathway.
- First law of thermodynamics: energy is neither created nor destroyed. Second law: every energy transformation increases the total entropy of the universe — cells maintain order locally by importing free energy and exporting heat and waste.
- Exergonic reactions release free energy (ΔG<0, spontaneous, e.g., ATP hydrolysis). Endergonic reactions absorb free energy (ΔG>0, nonspontaneous, e.g., protein synthesis). Cells couple the two so an exergonic reaction drives an endergonic one.
- ATP (adenosine triphosphate) = adenine + ribose + 3 phosphates. Hydrolysis of the terminal phosphate (ATP → ADP + Pᵢ) releases ≈30 kJ/mol of usable free energy.
- Photosynthesis overall: 6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂. The light reactions (thylakoid membranes) produce ATP, NADPH, and O₂. The Calvin cycle (stroma) uses them to fix CO₂ into G3P, which builds glucose.
- Light reactions: Photosystem II splits water (releasing O₂) and excites electrons that travel down the ETC to Photosystem I, pumping H⁺ into the thylakoid lumen. Chemiosmosis through ATP synthase makes ATP, and PSI passes electrons to NADP⁺ → NADPH.
- The Calvin cycle has three phases: (1) carbon fixation — RuBisCO attaches CO₂ to RuBP forming 3-PGA; (2) reduction — ATP and NADPH convert 3-PGA to G3P; (3) regeneration — most G3P regenerates RuBP. Three CO₂ = one net G3P. Two G3P make glucose.
- C3 plants fix CO₂ directly with RuBisCO and suffer photorespiration in hot/dry conditions. C4 plants spatially separate fixation (PEP carboxylase in mesophyll) from the Calvin cycle (bundle-sheath). CAM plants temporally separate fixation (night) and Calvin cycle (day) — stomata closed by day.
- Cellular respiration overall: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ≈30–32 ATP. Four stages: glycolysis (cytosol), pyruvate oxidation (matrix), Krebs cycle (matrix), oxidative phosphorylation (inner membrane).
- Glycolysis (cytosol, anaerobic) splits one glucose into two pyruvate, with a net yield of 2 ATP (substrate-level phosphorylation) and 2 NADH. It is the only ATP-producing step that does not require oxygen or organelles.
- The Krebs (citric acid) cycle starts when acetyl-CoA joins oxaloacetate to form citrate. Per acetyl-CoA the cycle releases 2 CO₂ and makes 3 NADH, 1 FADH₂, and 1 GTP/ATP. Each glucose feeds two acetyl-CoA, so the cycle turns twice.