This cheat sheet summarizes the major metabolic pathways that cells use to harvest, store, and redirect chemical energy. It connects carbohydrate, lipid, and amino acid metabolism so students can see how pathways feed into one another. College biochemistry requires tracking carbon flow, redox carriers, ATP yield, and regulation, so a compact pathway overview helps organize many details.
Use it to compare pathway locations, inputs, outputs, and control points.
Key Facts
- Glycolysis converts one glucose to two pyruvate with net products and : .
- Pyruvate dehydrogenase links glycolysis to the TCA cycle by forming acetyl-CoA: .
- Each acetyl-CoA oxidized in the TCA cycle yields , , , and .
- Oxidative phosphorylation uses electrons from and to drive proton pumping, and ATP synthase uses the gradient to make ATP from .
- Approximate ATP yields are per and per under typical mitochondrial conditions.
- Beta-oxidation shortens a saturated fatty acyl-CoA by two carbons per cycle, producing , , and per cycle except for the final cleavage pattern.
- Gluconeogenesis bypasses irreversible glycolysis steps using pyruvate carboxylase, PEP carboxykinase, fructose -bisphosphatase, and glucose -phosphatase.
- Reciprocal regulation prevents futile cycling, so high and citrate generally slow glycolysis while high and fructose -bisphosphate promote glycolysis.
Vocabulary
- Metabolic pathway
- A linked series of enzyme-catalyzed reactions that transforms molecules through defined intermediates.
- Catabolism
- The set of pathways that break down molecules and often capture energy as , , or .
- Anabolism
- The set of pathways that build complex molecules and usually require energy input such as or reducing power such as .
- Acetyl-CoA
- A two-carbon activated carrier that delivers acetyl groups to the TCA cycle, lipid synthesis, ketone body formation, and other pathways.
- Redox carrier
- A molecule such as , , or that transfers electrons during oxidation-reduction reactions.
- Allosteric regulation
- Control of enzyme activity when a molecule binds at a site other than the active site and changes enzyme shape or function.
Common Mistakes to Avoid
- Counting gross ATP instead of net ATP in glycolysis is wrong because the pathway uses early and produces later, giving net per glucose.
- Treating and as interchangeable is wrong because mainly supports ATP production while mainly supports biosynthesis and antioxidant defense.
- Placing all metabolic pathways in the same compartment is wrong because glycolysis occurs in the cytosol, the TCA cycle occurs in the mitochondrial matrix, and electron transport occurs in the inner mitochondrial membrane.
- Assuming gluconeogenesis is simply glycolysis in reverse is wrong because several glycolysis reactions are irreversible and require separate bypass enzymes.
- Forgetting carbon loss as is wrong because carbon accounting is essential for understanding pyruvate oxidation, the TCA cycle, and amino acid entry points.
Practice Questions
- 1 One glucose molecule produces in glycolysis, from pyruvate dehydrogenase, and plus in the TCA cycle. Using per and per , how many ATP equivalents come from these reduced carriers?
- 2 A saturated -carbon fatty acyl-CoA undergoes beta-oxidation. How many acetyl-CoA molecules are produced, and how many beta-oxidation cycles occur?
- 3 If one turn of the TCA cycle produces , , and , estimate the ATP equivalent yield per acetyl-CoA using per , per , and per .
- 4 Explain why a cell should not run glycolysis and gluconeogenesis at high rates at the same time, and identify one regulatory molecule that helps prevent this.
Understanding Biochemistry Metabolic Pathways Overview
Metabolism is easier to understand when you follow both carbon atoms and electrons. Carbon from glucose can be burned for energy, stored as glycogen or fat, or used to build amino acids and other cell materials. Acetyl coenzyme A is a major meeting point because it can come from sugars, fats, and some amino acids.
It can enter the TCA cycle when energy is needed. It can be redirected toward fatty acid synthesis when fuel is abundant. Carbon flow is not completely reversible.
Animals cannot make net glucose from the acetyl coenzyme A produced by fatty acid breakdown. This matters during fasting, when the liver must protect blood glucose for tissues that depend heavily on it.
The electron carriers NADH and FADH two are more than bookkeeping labels. They hold high energy electrons removed during oxidation reactions. In mitochondria, those electrons move through a series of protein complexes in the inner membrane.
The released energy moves protons to one side of that membrane. This creates a difference in proton concentration and electrical charge. ATP synthase provides a route for protons to return.
Its shape changes as protons pass through it, allowing it to join ADP with phosphate. Oxygen is essential at the end of this chain because it accepts electrons and protons to form water. Without enough oxygen, cells must use other ways to regenerate NAD plus, including lactate formation in many animal cells.
Pathways respond to the body’s current needs rather than running at full speed all the time. After a carbohydrate rich meal, insulin favors glucose use and storage. In fasting, glucagon helps the liver release glucose and increase fat use.
Muscle has different priorities from liver. Working muscle can rapidly break down its own glycogen, but it does not release free glucose into the blood in the same way as liver. During prolonged exercise, fat oxidation becomes increasingly important, though its rate depends on oxygen delivery.
The brain normally uses much glucose, yet it can use ketone bodies during a long fast. These examples show why a pathway diagram must be connected to tissues, hormones, and time.
When studying, mark every reaction as one of a few basic jobs. A reaction may transfer electrons, add or remove a carbon group, use ATP, release carbon dioxide, or rearrange atoms. Then identify the enzyme steps that are strongly regulated.
These are often far from equilibrium and act like control valves. Be careful with pathway locations. Glycolysis occurs in the cytosol, while much fatty acid breakdown, the TCA cycle, and electron transport occur in mitochondria.
Also distinguish direct ATP formation from ATP made later through electron transport. ATP totals are estimates because transport costs, cell type, oxygen supply, and shuttle systems can change the final amount. Accurate reasoning matters more than memorizing one fixed total.