Overview
This lecture explains glycolysis, the first stage of cellular respiration, detailing how glucose is broken down into pyruvate to produce energy in the form of ATP.
Energy, Glucose, and Cellular Respiration
- Cells require energy for all activities, which ultimately derives from the sun via glucose produced by photosynthesis.
- Cellular respiration is the breakdown of biomolecules like glucose to generate useable cellular energy.
- Aerobic respiration needs oxygen and converts glucose into carbon dioxide, water, and energy.
- The process utilizes NAD+ (nicotinamide adenine dinucleotide) as an electron carrier, which cycles between NAD+ and NADH.
- Cellular respiration consists of three pathways: glycolysis, citric acid cycle, and oxidative phosphorylation.
Glycolysis Overview
- Glycolysis is the first metabolic pathway in cellular respiration and occurs in the cell cytoplasm.
- It is an anaerobic process (does not require oxygen) and is found in all living cells.
- The main function is to split one glucose (6 carbons) into two pyruvate molecules (3 carbons each).
- Ten enzymes catalyze glycolysis, which consists of two phases: preparatory and payoff.
- The net yield from glycolysis is 2 ATP molecules per glucose.
Glycolysis Steps
- Preparatory phase (first 5 steps):
- Hexokinase phosphorylates glucose to glucose-6-phosphate (costs 1 ATP).
- Glucose-6-phosphate isomerized to fructose-6-phosphate.
- Phosphofructokinase 1 phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate (costs 1 ATP).
- Fructose bisphosphate aldolase splits fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate (GADP) and dihydroxyacetone phosphate (DHAP).
- Triosephosphate isomerase converts DHAP into another GADP.
- Payoff phase (last 5 steps):
- Glyceraldehyde phosphate dehydrogenase oxidizes GADP to 1,3-bisphosphoglycerate (requires NAD+ and free phosphate).
- Phosphoglycerate kinase produces one ATP per GADP (total 2 ATP).
- Phosphoglycerate mutase converts 3-phosphoglycerate to 2-phosphoglycerate.
- Enolase dehydrates 2-phosphoglycerate to produce phosphoenolpyruvate.
- Pyruvate kinase transfers the last phosphate to ADP, producing another ATP and pyruvate.
Key Terms & Definitions
- Glycolysis — Metabolic pathway that splits glucose into two pyruvate molecules.
- ATP (Adenosine triphosphate) — Primary energy carrier in cells.
- NAD+/NADH — Electron carrier that cycles between oxidized (NAD+) and reduced (NADH) states.
- Pyruvate — 3-carbon product of glycolysis, substrate for the next stage of respiration.
- Enzyme — Protein that catalyzes biochemical reactions.
Action Items / Next Steps
- Memorize the major steps and enzymes of glycolysis.
- Review a table summarizing each enzyme and input/output if detailed recall is required.
- Prepare for upcoming lectures on the citric acid cycle and oxidative phosphorylation.