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The link between glycolysis and the Krebs cycle (also known as the citric acid cycle or TCA cycle) occurs through the conversion of pyruvate, the end product of glycolysis. This process occurs in the mitochondria of eukaryotic cells, and it involves the following key reactions:
### 1. **Transport of Pyruvate**
After glycolysis, which occurs in the cytosol, pyruvate (a 3-carbon molecule) is transported into the mitochondria where it can undergo further processing.
### 2. **Decarboxylation of Pyruvate**
Once inside the mitochondria, pyruvate undergoes a reaction catalysed by the **pyruvate dehydrogenase complex (PDC)**. This series of reactions includes:
– **Decarboxylation:** One carbon atom from pyruvate is released as carbon dioxide (CO₂).
– **Oxidation:** The remaining two-carbon molecule (acetyl group) is oxidised, and electrons are transferred to NAD⁺, forming NADH.
– **Coenzyme A Addition:** The acetyl group is then attached to Coenzyme A (CoA), forming acetyl-CoA.
The overall reaction can be summarised as:
\[
\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2
\]
### 3. **Entry into the Krebs Cycle**
The acetyl-CoA produced from the decarboxylation of pyruvate is the starting substrate for the Krebs cycle. In the Krebs cycle, acetyl-CoA combines with oxaloacetate (a 4-carbon molecule) to form citrate (a 6-carbon molecule).
This reaction is catalysed by the enzyme **citrate synthase**:
\[
\text{Acetyl-CoA} + \text{Oxaloacetate} \rightarrow \text{Citrate} + \text{CoA}
\]
### Summary
These reactions serve as a pivotal link between glycolysis and the Krebs cycle by converting pyruvate to acetyl-CoA, which is then utilised in the Krebs cycle to produce additional energy carriers (NADH and FADH₂) and further carbon dioxide. This connection is crucial for cellular respiration, contributing to the overall process of ATP production.
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