What it does
Every living cell releases energy from food by respiration. It always starts the same way: in the cytoplasm, one glucose molecule (6 carbons) is split into two pyruvate molecules (3 carbons each), which gives a little energy as 2 ATP. What happens next depends on oxygen. With oxygen (aerobic respiration), pyruvate goes into the mitochondria and is broken down completely to carbon dioxide and water, giving much more energy: about 38 ATP per glucose in all. Without oxygen (anaerobic respiration), the pyruvate is changed in the cytoplasm instead: yeast makes ethanol and carbon dioxide (fermentation), and your muscles, when short of oxygen, make lactic acid. Either way only the first 2 ATP are gained. Photosynthesis makes the glucose and oxygen that respiration uses, so it is worth a look in the photosynthesis lab; the glucose itself comes from food broken down on the digestion journey.
Using it
- Pick a Level. Class 10 uses word equations. Class 11 adds the chemical equations, the name glycolysis, and a mass check.
- Pick the cell: yeast or your muscle. It decides what the anaerobic way makes.
- Move the oxygen supply. At 100 % every pyruvate goes the aerobic way; at 0 % none does. In between, both happen, and the arrows’ widths show the share.
- Set the glucose in moles to see the products in moles and grams in the table.
Worked example
Take 1 mol of glucose (180.16 g) in yeast with plenty of oxygen. It uses 6 mol of oxygen (192 g) and gives 6 mol of carbon dioxide (264.06 g) and 6 mol of water (108.12 g): 372.16 g in and 372.18 g out, equal to rounding. Each glucose gives 38 ATP. Now slide the oxygen to 0 %: the yeast ferments, making 2 mol of ethanol (92.14 g) and 2 mol of carbon dioxide (88.02 g), and only 2 ATP per glucose. To make 76 ATP, the aerobic way needs 2 glucose molecules, the anaerobic way 38. That is why yeast without oxygen gets through sugar so fast.
Where this fits in school
Class 10 science (life processes) covers respiration, the breakdown of glucose by different pathways, and lactic acid in muscles and fermentation in yeast. Class 11 biology (respiration in plants) adds glycolysis, fermentation and aerobic respiration in detail with the ATP count. This page is an independent learning aid. It is not made by or connected with NCERT, and it does not copy any textbook.
Questions
Is it 38 ATP or 30? School books count 38 (36 in some). Careful measurements in real cells give about 30 to 32, because some energy is used moving molecules across the mitochondrial membranes. The idea is the same: aerobic gives many times more energy.
Do plants respire? Yes, all the time, in every living cell, day and night.
Is anything saved? No. Nothing you choose is stored or sent anywhere.
Where the numbers come from
- Respiration pathways and their word and chemical equations: standard school biology and chemistry, written by fibub in its own words.
- ATP per glucose: 2 from glycolysis and 38 in all for aerobic respiration, the figure used in school books; the page notes that newer measurements give about 30 to 32.
- Molar masses from IUPAC standard atomic weights (C 12.011, H 1.008, O 15.999). Mass balance and the ATP counts are checked in the tests.
- Drawings are fibub's own simple shapes. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.