RESPIRATION IN PLANTS - QUICK REVISION.
RESPIRATION BASICS: Respiration is the breaking of C-C bonds of complex compounds by oxidation within cells, releasing energy that is trapped as ATP (the energy currency of the cell). The oxidised compounds are respiratory substrates - usually carbohydrates, but also fats, proteins and organic acids. Energy is released stepwise via enzyme-controlled reactions, not in a single step. Plants have no special respiratory organs; they use stomata and lenticels for gaseous exchange by diffusion, because each part meets its own low gas-exchange needs and cells lie close to the surface.
GLYCOLYSIS (EMP pathway, by Embden, Meyerhof, Parnas): Occurs in the cytoplasm of ALL living organisms; the only respiratory process in anaerobes. Glucose (6C) is partially oxidised to two molecules of pyruvic acid (3C) through ten enzyme-controlled steps. Sucrose is split by invertase into glucose and fructose. Glucose is phosphorylated to glucose-6-phosphate by hexokinase, isomerised to fructose-6-phosphate, then to fructose-1,6-bisphosphate, which splits into PGAL and DHAP. ATP is USED at 2 steps; ATP is MADE at 2 steps (BPGA to PGA, and PEP to pyruvate); one NADH+H+ is formed (PGAL to BPGA). NET gain = 2 ATP and 2 NADH per glucose.
FATE OF PYRUVATE: depends on cellular need - (1) lactic acid fermentation, (2) alcoholic fermentation, (3) aerobic respiration.
FERMENTATION (anaerobic, incomplete oxidation): Alcoholic (yeast) - pyruvic acid to CO2 + ethanol via pyruvic acid decarboxylase and alcohol dehydrogenase. Lactic acid (some bacteria, muscle during exercise) - pyruvic acid to lactic acid via lactate dehydrogenase. In BOTH, NADH+H+ is reoxidised to NAD+. Less than 7 per cent of glucose energy is released; net gain only 2 ATP. Yeast dies at about 13 per cent alcohol.
AEROBIC RESPIRATION (mitochondria, needs O2): Pyruvate enters the matrix and undergoes oxidative decarboxylation by pyruvic dehydrogenase (needs NAD+ and Coenzyme A) to give acetyl CoA + CO2 + NADH (2 NADH per glucose).
KREBS / TCA / CITRIC ACID CYCLE (matrix): Acetyl CoA + oxaloacetic acid (OAA) + water to citric acid (citrate synthase). Citrate to isocitrate to alpha-ketoglutaric acid to succinyl-CoA (two decarboxylations) to succinic acid (with GTP made by substrate level phosphorylation, GTP then forms ATP) to OAA (regenerated). Per turn: 3 NAD+ reduced to 3 NADH+H+, 1 FAD+ reduced to 1 FADH2, 1 ATP. OAA must be continuously replenished.
ELECTRON TRANSPORT SYSTEM (ETS, inner mitochondrial membrane): NADH oxidised by NADH dehydrogenase (complex I) to ubiquinone; FADH2 feeds in via complex II; ubiquinol to cytochrome c via cytochrome bc1 (complex III); cytochrome c (mobile carrier) to cytochrome c oxidase (complex IV, has cytochromes a and a3 and two copper centres) to O2. O2 is the FINAL hydrogen acceptor, reduced to water.
OXIDATIVE PHOSPHORYLATION: As electrons flow through complexes I-IV they are coupled to ATP synthase (complex V) which makes ATP from ADP + Pi. Energy of oxidation-reduction (not light, unlike photophosphorylation) drives the proton gradient. ATP synthase = F1 (peripheral headpiece, site of ATP synthesis) + F0 (integral channel for protons). For each ATP, 4H+ pass through F0 from intermembrane space to matrix. One NADH gives 3 ATP; one FADH2 gives 2 ATP.
RESPIRATORY BALANCE SHEET (theoretical, net 38 ATP per glucose): Assumptions - sequential pathways, glycolytic NADH enters mitochondria, no intermediate withdrawn, only glucose respired. These are not truly valid because pathways run simultaneously.
AMPHIBOLIC PATHWAY: Respiration is both catabolic and anabolic. Substrates enter at different points - fats break to glycerol (enters as PGAL) and fatty acids (enter as acetyl CoA); proteins break to amino acids (enter as pyruvate, acetyl CoA, or Krebs intermediates after deamination). The same intermediates are withdrawn for biosynthesis, so the pathway is amphibolic.
RESPIRATORY QUOTIENT (RQ): RQ = volume of CO2 evolved / volume of O2 consumed. Carbohydrate = 1.0; fat (tripalmitin) = about 0.7; protein = about 0.9. In living organisms substrates are usually mixed.