Glycolysis: 1 glucose → 2 pyruvate, net 2 ATP, 2 NADH. In cytoplasm. No oxygen.
Glycolysis
Ten steps of glycolysis — inputs and outputs
Invest 2 ATP early, gain 4 → net 2 ATP. Also produces 2 NADH. Occurs in cytoplasm, no oxygen needed. With oxygen → pyruvate enters mitochondria. Without → fermentation.
What one complete Krebs cycle produces per glucose
Runs twice per glucose (one turn per pyruvate). Carbon atoms leave as CO₂. NADH and FADH₂ carry electrons to ETC where most ATP is made. Occurs in mitochondrial matrix.
Electron Transport Chain
ETC: electrons flow NADH → Complex I → CoQ → III → Cyt c → IV → O₂. H⁺ pumped out → ATP synthase.
Electron Transport Chain
The electron highway producing most of cellular ATP
Electrons from NADH and FADH₂ move through protein complexes. Energy pumps H⁺ into intermembrane space. H⁺ flows back through ATP synthase → ~32 ATP. O₂ is final electron acceptor → becomes H₂O.
ATP Yield
Total ATP per glucose: ~36-38 (2 glycolysis + 2 Krebs + ~32-34 ETC)
ATP Yield
The total energy harvest from complete glucose oxidation
Glycolysis: 2 net ATP. Krebs: 2 ATP. ETC: ~32-34 ATP. Total theoretical maximum: ~36-38 ATP per glucose. Actual yield slightly lower due to membrane proton leakage.
Gluconeogenesis
Gluconeogenesis: makes glucose from non-carbohydrate sources (amino acids, lactate, glycerol) — liver
Gluconeogenesis
The liver makes new glucose when blood sugar is low
Occurs in fasting, starvation, and prolonged exercise. Substrates: amino acids (from muscle breakdown), lactate (from RBCs and anaerobic muscle), glycerol (from fat breakdown). Mostly in liver, some in kidney.
Anabolism vs Catabolism
Anabolism: building molecules (requires energy). Catabolism: breaking molecules (releases energy).
Anabolism vs Catabolism
Two directions of metabolism — building and breaking down
Anabolism: synthesis reactions — build complex molecules from simple ones. Requires ATP. Protein synthesis, fatty acid synthesis, gluconeogenesis. Catabolism: degradation reactions — break complex molecules into simple ones. Releases ATP. Glycolysis, beta-oxidation, protein digestion. Metabolism = anabolism + catabolism.
Pyruvate Oxidation
Pyruvate dehydrogenase complex: pyruvate → acetyl-CoA + CO₂ + NADH. Bridge between glycolysis and Krebs.
Pyruvate Oxidation
The link between glycolysis and the Krebs cycle
Pyruvate (3 carbons) enters mitochondria → pyruvate dehydrogenase complex removes one carbon as CO₂ → acetyl-CoA (2 carbons) + NADH. This step is irreversible — acetyl-CoA cannot be converted back to glucose (unlike pyruvate). Why alcohol can't be converted to glucose: ethanol → acetaldehyde → acetyl-CoA (irreversible).
Fatty Acid Synthesis vs Oxidation
Fatty acid synthesis: in cytoplasm. Beta-oxidation: in mitochondria. Opposite processes, opposite compartments.
Fatty Acid Synthesis vs Oxidation
Two opposing pathways in different cellular compartments
Beta-oxidation (catabolism): mitochondria, uses FAD and NAD⁺, produces acetyl-CoA and NADH. Fatty acid synthesis (anabolism): cytoplasm, uses NADPH, uses malonyl-CoA building blocks, requires biotin. They cannot run simultaneously — when one is active, the other is inhibited. Malonyl-CoA inhibits carnitine shuttle (blocks beta-oxidation).
Pentose Phosphate Pathway
Pentose phosphate pathway: produces NADPH (for biosynthesis and antioxidant) and ribose-5-phosphate (for nucleotides)
Pentose Phosphate Pathway
An alternative glucose pathway producing NADPH and nucleotide precursors
Runs parallel to glycolysis. Two products: NADPH (reduced glutathione, fatty acid synthesis, keeps RBCs from oxidative damage) and ribose-5-phosphate (nucleotide synthesis). G6PD deficiency: can't make NADPH → RBCs vulnerable to oxidative stress → hemolytic anemia triggered by certain foods (fava beans) and drugs.
Glycogen Metabolism
Glycogen synthesis: in liver (blood glucose buffer) and muscle (local energy). Glycogen phosphorylase breaks it down.
Glycogen Metabolism
How the body stores and mobilizes glucose
Glycogen: branched polymer of glucose — rapid glucose storage. Liver glycogen: maintains blood glucose levels during fasting. Muscle glycogen: fuel for muscle contraction only. Glycogen synthase: builds glycogen. Glycogen phosphorylase: breaks it down, regulated by glucagon (liver), epinephrine (muscle and liver).
The two opposing hormones that regulate blood glucose
Fed state (glucose high): insulin released from beta cells. Promotes: GLUT4 insertion (glucose uptake in muscle/fat), glycolysis, glycogen synthesis, fatty acid synthesis. Inhibits gluconeogenesis. Fasting (glucose low): glucagon released from alpha cells. Promotes glycogenolysis and gluconeogenesis in liver.