OSAT Biological Sciences (110)

Metabolism encompasses all chemical reactions in living organisms. Catabolism breaks down complex molecules, releasing energy. Anabolism builds complex molecules, requiring energy input.

Free energy (G) is energy available to do work. Exergonic reactions release free energy (negative ΔG) and proceed spontaneously. Endergonic reactions require free energy input (positive ΔG). ATP hydrolysis is exergonic and powers endergonic cellular reactions through energy coupling.

Enzymes lower activation energy without altering reaction equilibrium. They are not consumed in reactions and can be reused. Substrate specificity results from active site shape and chemistry. Cofactors are non-protein helpers including metal ions. Coenzymes are organic cofactors, often derived from vitamins. NAD+ and FAD are important coenzymes in cellular respiration.

Enzyme regulation occurs through several mechanisms. Competitive inhibitors bind active sites and compete with substrates. Noncompetitive inhibitors bind elsewhere, altering enzyme shape. Allosteric regulation involves binding at sites other than the active site, changing enzyme activity. Feedback inhibition occurs when end products inhibit earlier enzymes in pathways.

Cellular respiration is the process of converting glucose and oxygen into carbon dioxide, water, and ATP. The overall equation is: C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. Cellular respiration occurs in three stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.

Glycolysis occurs in the cytoplasm and does not require oxygen. A six-carbon glucose molecule is converted into two three-carbon pyruvate molecules. The process involves ten enzyme-catalyzed reactions.

The energy investment phase consumes two ATP molecules to phosphorylate glucose. The energy payoff phase produces four ATP through substrate-level phosphorylation and reduces two NAD+ to NADH. Net yield per glucose molecule is two ATP and two NADH.

Key enzymes include hexokinase and phosphofructokinase, which is the primary regulatory enzyme responding to ATP and AMP levels.

Under aerobic conditions, pyruvate enters mitochondria and undergoes oxidative decarboxylation. Pyruvate dehydrogenase complex removes one carbon as CO2 and transfers electrons to NAD+. The remaining two-carbon acetyl group attaches to coenzyme A, forming acetyl-CoA. Each pyruvate yields one acetyl-CoA, one NADH, and one CO2.

The citric acid cycle, also called the Krebs cycle, occurs in the mitochondrial matrix. Acetyl-CoA (two carbons) combines with oxaloacetate (four carbons) to form citrate (six carbons). Through eight enzyme-catalyzed reactions, two carbons are released as CO2, and oxaloacetate is regenerated.

Each turn of the cycle produces three NADH, one FADH2, one GTP (equivalent to ATP), and two CO2. Since each glucose produces two acetyl-CoA, the cycle turns twice per glucose. The cycle is regulated by substrate availability and allosteric control of key enzymes.

Oxidative phosphorylation occurs at the inner mitochondrial membrane and produces most ATP. It involves the electron transport chain and chemiosmosis.

The electron transport chain consists of protein complexes (I, II, III, IV) and mobile carriers (ubiquinone, cytochrome c). Electrons from NADH enter at complex I; electrons from FADH2 enter at complex II. Electrons flow through carriers in a series of redox reactions, releasing energy. The final electron acceptor is oxygen, which combines with electrons and hydrogen ions to form water.

Energy released during electron transport pumps protons from the matrix to the intermembrane space, creating an electrochemical gradient. ATP synthase uses this proton-motive force to phosphorylate ADP. Protons flow through ATP synthase, causing conformational changes that catalyze ATP synthesis—a process called chemiosmosis.

Theoretical maximum yield is approximately 30–32 ATP per glucose molecule. Actual yield varies due to proton leakage and the cost of transporting ATP and pyruvate across membranes.

When oxygen is unavailable, cells use fermentation to regenerate NAD+ and continue glycolysis. Fermentation does not produce additional ATP beyond glycolysis.

Lactic acid fermentation converts pyruvate to lactate using NADH. It occurs in muscle cells during intense exercise and in certain bacteria used in dairy production.

Alcoholic fermentation converts pyruvate to ethanol and CO2 using NADH. It occurs in yeast and some bacteria. Applications include bread making and alcohol production.

Photosynthesis converts light energy into chemical energy stored in glucose. The overall equation is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. Photosynthesis occurs in chloroplasts in two stages: light-dependent reactions and light-independent reactions.

Light-dependent reactions occur in thylakoid membranes. Chlorophyll and accessory pigments absorb light energy. Chlorophyll a is the primary pigment. Chlorophyll b and carotenoids are accessory pigments expanding the absorption spectrum.

Photosystem II absorbs light at 680 nm. Excited electrons leave the reaction center and enter the electron transport chain. Water molecules are split (photolysis), releasing oxygen, protons, and electrons that replace those lost from photosystem II.

Electrons flow through plastoquinone, cytochrome complex, and plastocyanin. Energy released pumps protons into the thylakoid lumen, creating a gradient.

Photosystem I absorbs light at 700 nm. Electrons are re-energized and passed through ferredoxin to NADP+ reductase, which produces NADPH.

ATP synthase uses the proton gradient to produce ATP through chemiosmosis. This process is called photophosphorylation.

The products of light-dependent reactions are ATP, NADPH, and O2. ATP and NADPH power the light-independent reactions.

Light-independent reactions, also called the Calvin cycle, occur in the stroma. They use ATP and NADPH to fix carbon dioxide into organic molecules.

Carbon fixation is catalyzed by rubisco, which combines CO2 with ribulose bisphosphate (RuBP, five carbons) to form an unstable six-carbon compound that splits into two molecules of 3-phosphoglycerate (3-PGA).

Reduction converts 3-PGA to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH. For every three CO2 fixed, six G3P are produced. One G3P exits the cycle for glucose synthesis.

Regeneration uses ATP to convert five G3P back into three RuBP, allowing the cycle to continue.

Three turns of the cycle fix three CO2 and produce one G3P. Six turns produce one glucose molecule. The cycle consumes 18 ATP and 12 NADPH per glucose.

Photorespiration occurs when rubisco binds oxygen instead of CO2, producing a two-carbon compound that must be recycled at energy cost. It is more common at high temperatures when stomata close to conserve water.

C4 plants (corn, sugarcane) separate initial carbon fixation from the Calvin cycle spatially. PEP carboxylase in mesophyll cells fixes CO2 into four-carbon compounds, which release CO2 in bundle sheath cells where rubisco operates at high CO2 concentration.

CAM plants (cacti, succulents) separate carbon fixation temporally. Stomata open at night to fix CO2 into organic acids stored in vacuoles. During the day, stomata close and stored acids release CO2 for the Calvin cycle.

Organisms differ in where their energy comes from. Photoautotrophs (plants, algae, cyanobacteria) use light. Chemoautotrophs, such as nitrifying bacteria and the bacteria at deep-sea hydrothermal vents, oxidize inorganic molecules like ammonia or hydrogen sulfide to make ATP and fix carbon. Heterotrophs, including animals, fungi, and most bacteria, obtain energy and carbon by breaking down organic molecules made by other organisms.

The cell cycle consists of interphase and mitotic phase. Interphase includes G1 (cell growth), S (DNA synthesis), and G2 (preparation for division). The mitotic phase includes mitosis and cytokinesis.

During S phase, each chromosome is replicated, producing sister chromatids joined at the centromere. Cyclin-dependent kinases (CDKs) regulate cell cycle progression. Checkpoints ensure proper completion of each phase before proceeding.

Mitosis produces two genetically identical daughter cells. It occurs in somatic cells for growth, repair, and asexual reproduction.

Prophase involves chromatin condensation into visible chromosomes, nuclear envelope breakdown, centriole migration to opposite poles, and spindle fiber formation.

Metaphase involves chromosome alignment at the cell's equator (metaphase plate) with spindle fibers attached to kinetochores.

Anaphase involves separation of sister chromatids, which move toward opposite poles as spindle fibers shorten.

Telophase involves chromosome decondensation, nuclear envelope reformation, and spindle fiber disassembly.

Cytokinesis divides the cytoplasm. In animal cells, a cleavage furrow forms through actin microfilament contraction. In plant cells, a cell plate forms from vesicles and develops into a new cell wall.

Meiosis produces four genetically diverse haploid cells from one diploid cell. It consists of two divisions: meiosis I (reductional) and meiosis II (equational).

Meiosis I separates homologous chromosomes. Prophase I is extended and includes synapsis (homolog pairing), crossing over (exchange of chromosome segments between nonsister chromatids), and formation of chiasmata. Metaphase I involves homologous pair alignment at the equator with random orientation (independent assortment). Anaphase I separates homologs. Telophase I produces two haploid cells with replicated chromosomes.

Meiosis II resembles mitosis. Sister chromatids separate, producing four haploid cells.

Genetic diversity results from crossing over, independent assortment, and random fertilization.

Nondisjunction is the failure of chromosomes to separate properly. If homologs fail to separate in meiosis I, all four gametes are abnormal (two with an extra chromosome, two missing one). If sister chromatids fail to separate in meiosis II, two gametes are normal and two are abnormal. Fertilization of an abnormal gamete produces a trisomic (2n + 1) or monosomic (2n − 1) zygote.

Errors in mitosis matter too. When spindle checkpoints fail, daughter cells can gain or lose chromosomes, a common feature of cancer cells.

The cell cycle is controlled at checkpoints. The G1 checkpoint assesses cell size, nutrients, and DNA damage. The G2 checkpoint verifies complete DNA replication. The M checkpoint (spindle checkpoint) confirms proper chromosome attachment.

Tumor suppressor genes (p53, Rb) inhibit cell division. Proto-oncogenes promote cell division and become oncogenes when mutated. Cancer results from accumulated mutations disrupting cell cycle control.

Apoptosis is programmed cell death essential for development and eliminating damaged cells. It involves activation of caspases, DNA fragmentation, and cell shrinkage without inflammation.