Lesson 3 of 6 / Photosynthesis and respiration
Begin respiration in the cytoplasm
What happens before pyruvate reaches a mitochondrion?
I can identify glycolysis location, initial substrate and final products under aerobic conditions.
I can identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.Syllabus
Syllabus 8876, Core 3(d). Identify inputs and outputs of glycolysis, link reaction, Krebs cycle and oxidative phosphorylation.
I can locate glycolysis in cytoplasm and other aerobic stages in mitochondria.Syllabus
Syllabus 8876, Core 3(e). Locate glycolysis in cytoplasm and other aerobic stages in mitochondria.
About 5 min
Make a guess. You are not marked.
Does all of respiration take place inside mitochondria?
Show the answer
Glycolysis happens in the cytoplasm.
Glucose is split to pyruvate in the cytoplasm. Later aerobic stages happen in mitochondria.
The key idea
Glycolysis occurs in the cytoplasm and converts glucose to pyruvate, with a net ATP gain and reduction of NAD.
Explore the idea
Keep track of the carbon atoms
The circles track carbon count, not the full molecular structure. Glycolysis happens outside mitochondria and does not directly use oxygen.
Original models of required stage inputs and outputs. Shapes are not full chemical formulae; exact pathway ATP yields are not required here.
Explanation
Respiration transfers energy from organic molecules into forms cells can use, especially ATP. ATP can be hydrolysed to couple energy release to processes such as active transport and synthesis. Respiration is not the same as breathing, which supplies and removes gases.
Glycolysis occurs in the cytoplasm. A six-carbon glucose molecule is converted into two three-carbon pyruvate molecules through a sequence of reactions. Some ATP is used initially, but the stage gives a net ATP gain overall. Exact ATP-yield calculations are not required in this H1 syllabus.
NAD accepts hydrogen and electrons and becomes reduced NAD, often written NADH. This carrier can later supply electrons to aerobic energy-transfer processes. Glycolysis does not itself use molecular oxygen, so it can also continue when anaerobic pathways regenerate NAD.
Under aerobic conditions, pyruvate enters the mitochondrion for the link reaction. Carbon dioxide is not released during glycolysis in this glucose-to-pyruvate stage; carbon remains in the two three-carbon products.
Step by step
- 1
Locate the stage
Glycolysis happens in the cytoplasm, outside the mitochondrion.
- 2
Track carbon
Six carbons become two three-carbon pyruvates.
- 3
Track energy and carrier
There is a net ATP gain and NAD is reduced.
Worked example
Count carbon without counting ATP
One glucose enters glycolysis. How many pyruvate molecules form and where are the carbon atoms?
One way to explain it
Two pyruvate molecules form, each with three carbons. Together they retain the six carbon atoms from glucose; this stage does not release carbon dioxide.
Why this answer works
- Conserve carbon atoms.
- Do not borrow carbon-dioxide release from later stages.
Watch out for this
A student says: "All respiration takes place inside mitochondria." What is wrong with this?
Show the answer
Glycolysis occurs in the cytoplasm. The link reaction, Krebs cycle and oxidative phosphorylation occur in mitochondria under aerobic conditions.
Check your understanding
Which is a glycolysis output?
Show the answer
Pyruvate, with net ATP production and reduced NAD.
This captures the required stage-level products.
Quick recall
What happens before pyruvate reaches a mitochondrion?