Glycolysis splits one glucose molecule into two molecules of pyruvic acid in ten reactions in the cytoplasm. Two ATP are spent at the start and four are made later by substrate level phosphorylation, so the net gain is 2 ATP. Two molecules of NADH are formed where PGAL is oxidised.
about 25 min1 simulation1 practice question1 Python program5 questions to check yourself
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Splitting sugar
The word glycolysis comes from two Greek words: glycos, sugar, and lysis, splitting. The steps were worked out by Gustav Embden, Otto Meyerhof and J. Parnas, so the pathway is also called the EMP pathway.
Glycolysis happens in the cytoplasm of every living cell. In a cell that lives without oxygen it is the only stage of respiration. In the glycolysis of glucose there are ten reactions, each controlled by its own enzyme. One glucose molecule, with six carbon atoms, becomes two molecules of pyruvic acid, with three carbon atoms each.
Where the glucose comes from
In a plant, the glucose usually comes from sucrose, which is the end product of photosynthesis. It can also come from stored carbohydrates.
The enzyme invertase splits sucrose into glucose and fructose.
Both sugars go into glycolysis. The enzyme hexokinase adds a phosphate group to each, and the phosphate comes from ATP. Glucose becomes glucose-6-phosphate. This changes into fructose-6-phosphate.
After this, the steps for glucose and fructose are the same.
The ten reactions in three parts
A flow chart of glycolysis, from top to bottom. Glucose with six carbons changes to glucose-6-phosphate, spending one ATP. Then fructose-6-phosphate, then fructose-1,6-bisphosphate, spending a second ATP. This splits into two three-carbon compounds, DHAP and PGAL, joined by a double-headed arrow. PGAL changes to 1,3-bisphosphoglyceric acid, with NAD plus becoming NADH and phosphate added. Then 3-phosphoglyceric acid, with ADP becoming ATP. Then 2-phosphoglyceric acid, then phosphoenolpyruvate, then pyruvic acid, with a second ADP becoming ATP. A purple bracket on the right says that the steps below PGAL happen twice for each glucose.
Part 1. Spending ATP. The cell spends ATP twice at the start, like an investment.
Glucose becomes glucose-6-phosphate, and one ATP is used.
Fructose-6-phosphate becomes fructose-1,6-bisphosphate, and a second ATP is used.
Part 2. Splitting. Fructose-1,6-bisphosphate, with six carbon atoms, is split into two compounds with three carbon atoms each: dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL). DHAP changes to PGAL. So one glucose gives two PGAL, and every step below this point happens twice.
Part 3. Earning.
PGAL is oxidised. Two hydrogen atoms are taken from it and passed to NAD+, which becomes NADH + H+. At the same time inorganic phosphate is added, and PGAL becomes 1,3-bisphosphoglyceric acid (BPGA). This is the only step with NADH in glycolysis.
BPGA becomes 3-phosphoglyceric acid (PGA), and ATP is made.
PGA changes to 2-phosphoglyceric acid, and then to phosphoenolpyruvate (PEP).
PEP becomes pyruvic acid, and a second ATP is made.
ATP made directly from a substrate, as here, is made by substrate level phosphorylation. No electron carrier is needed.
The balance for one glucose
Per PGAL
Per glucose
ATP spent
0
2
ATP made
2
4
NADH formed
1
2
Pyruvic acid formed
1
2
Net gain of ATP: 4−2=2 ATP. The cell also holds 2 NADH, which still carry energy.
What next for pyruvic acid?
Pyruvic acid is the key product of glycolysis. What happens next depends on what the cell needs and on whether oxygen is present. There are three ways.
Lactic acid fermentation.
Alcoholic fermentation.
Aerobic respiration, in the mitochondria.
The first two do not need oxygen. The third does.
You should find that the ATP made is always twice the ATP used, so the net gain is the same as the ATP used. Every glucose adds 2 net ATP and 2 NADH.
Try it in Python
The program lists the steps that change ATP or NADH, and the number of times each step runs for one glucose.
Practise with new numbers
Common mistakes
Saying that glycolysis makes 4 ATP. It makes 4 ATP and uses 2. The net gain is 2.
Saying that glycolysis needs oxygen. It does not. It runs with or without oxygen.
Putting glycolysis in the mitochondria. It is in the cytoplasm.
Forgetting that the steps below PGAL run twice. One glucose gives two PGAL.
Saying that the sucrose enters glycolysis as it is. Invertase splits it first into glucose and fructose.
Remember these
Glycolysis: ten reactions, in the cytoplasm, of all living cells. Glucose gives 2 pyruvic acid.
ATP is used at two steps: glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose-1,6-bisphosphate.
NADH is formed when PGAL becomes BPGA. ATP is made when BPGA becomes PGA and when PEP becomes pyruvic acid.
Net gain: 2 ATP and 2 NADH for each glucose. Hexokinase starts it. Invertase splits sucrose.
Check yourself
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
Name the two sugars that sucrose gives, and the enzyme that does it.
At which two steps is ATP used in glycolysis?
How many ATP are made, and how many are used, for each glucose? What is the net gain?
At which step is NADH formed in glycolysis?
Give the three possible fates of pyruvic acid.
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