---
title: "The Citric Acid Cycle as a Hub, and Refilling It"
description: "The cycle both oxidises acetyl groups and supplies carbon skeletons for biosynthesis, so intermediates are constantly withdrawn. Because it is a cycle, those withdrawals stop it unless separate reacti"
canonical: https://lightmysky.com/learn/science/the-citric-acid-cycle-as-a-hub-and-refilling-it-mt_BPPfI-vnab
source: https://lightmysky.com/learn/science/the-citric-acid-cycle-as-a-hub-and-refilling-it-mt_BPPfI-vnab.md
retrieved: 2026-09-12
---

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# The Citric Acid Cycle as a Hub, and Refilling It

The cycle both oxidises acetyl groups and supplies carbon skeletons for biosynthesis, so intermediates are constantly withdrawn. Because it is a cycle, those withdrawals stop it unless separate reactions top the intermediates back up.

Subject: Science · Area: Biochemistry & Molecular Biology · Ages 20 to 21
Page: https://lightmysky.com/learn/science/the-citric-acid-cycle-as-a-hub-and-refilling-it-mt_BPPfI-vnab

## Ready when they can

- Names two intermediates withdrawn for biosynthesis and what they become.
- Explains why removing an intermediate slows the whole cycle.
- Identifies a reaction that replenishes an intermediate and says when it is needed.

## Lesson: The wheel that must stay full

Pyruvate enters the mitochondrion, loses carbon dioxide, and becomes acetyl CoA, which joins oxaloacetate to form citrate. The cycle then turns twice per glucose, releasing carbon dioxide and loading NADH and FADH2 for the electron transport chain. Oxaloacetate is regenerated at the end, so the cycle is catalytic in that intermediate.

**Example.** The cycle is also a supply hub: alpha-ketoglutarate leaves to build amino acids, and oxaloacetate leaves to start glucose making and other precursors. Each withdrawal removes a catalyst the wheel needs to keep turning. Cells match withdrawal with refilling through anaplerotic reactions, such as pyruvate carboxylase making oxaloacetate directly.

Count one turn as one acetyl CoA in, two carbon dioxide plus carriers out, with oxaloacetate regenerated. Fatty acid oxidation supplies acetyl CoA, while amino acid breakdown feeds in at several intermediates. Refilling runs hottest in growing cells with heavy biosynthetic demand, where drains would otherwise grind respiration down.

**Tip.** Apply the drain test to any scenario: if an intermediate leaves for biosynthesis, the wheel slows unless a refilling reaction tops it back up. Name the leaver, name the refiller, and state when the refill matters most.

**Recap.** The cycle burns acetyl groups yet constantly loses intermediates to biosynthesis, so anaplerotic refilling keeps the wheel turning.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Gluconeogenesis and the Reciprocal Regulation of Opposing Pathways](https://lightmysky.com/learn/science/gluconeogenesis-and-the-reciprocal-regulation-of-opposing-pathways-mt_D1MbKcb6Le)
- [The Krebs Cycle and the Reduced Carriers](https://lightmysky.com/learn/science/the-krebs-cycle-and-the-reduced-carriers-mt_eviWcVke2x)

## Opens up

- [Redox Potentials and the Proton-Motive Force](https://lightmysky.com/learn/science/redox-potentials-and-the-proton-motive-force-mt_lU8kgepFRN)