Unit 4: Carbohydrate Metabolism & Carbon Oxidation
1. Synthesis and Catabolism of Starch
Starch is the primary storage polysaccharide in plants, existing as amylose and amylopectin.
- Synthesis: Occurs in plastids (chloroplasts and amyloplasts) using ADP-glucose as the precursor.
- Catabolism: Involves enzymes like amylases and starch phosphorylase to mobilize stored energy during darkness or germination.
2. Glycolysis and its Regulation
Glycolysis is the sequence of reactions that converts glucose into pyruvate while producing energy in the form of ATP and NADH.
[Image of the glycolysis pathway steps]
- Location: Occurs in the cytosol.
- Key Steps: Includes phosphorylation, cleavage, and energy-harvesting phases.
- Regulation: Controlled by key enzymes such as phosphofructokinase (PFK) and hexokinase, which respond to the energy status (ATP/ADP levels) of the cell.
3. Pentose Phosphate Pathway (PPP)
This pathway is an alternative to glycolysis for glucose oxidation.
- Primary Functions: Generates NADPH for biosynthetic reactions and provides pentose sugars (like ribose-5-phosphate) for nucleic acid synthesis.
- Phases: Consists of an oxidative phase (irreversible) and a non-oxidative phase (reversible).
4. TCA Cycle and Krebs Cycle Regulation
The Tricarboxylic Acid (TCA) cycle, or Krebs cycle, is a series of chemical reactions used by all aerobic organisms to generate energy.
- Location: Occurs in the mitochondrial matrix.
- Input: Acetyl-CoA derived from pyruvate.
- Anaplerotic Reactions: Reactions that replenish cycle intermediates to maintain metabolic flux.
- Regulation: Regulated by the availability of substrates and the energy charge of the cell (inhibited by high ATP and NADH).
5. Mitochondrial Electron Transport and Oxidative Phosphorylation
The final stage of aerobic respiration where most of the ATP is produced.
- Electron Transport Chain (ETC): A series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen.
- Proton Gradient: The transfer of electrons drives the pumping of protons across the membrane, creating a gradient.
6. Anaerobic Respiration and Mechanism of ATP Synthesis
In the absence of oxygen, plants can switch to anaerobic respiration to sustain energy production.
- Anaerobic Respiration: Involves fermentation processes (alcoholic or lactic acid fermentation) to regenerate NAD+ for glycolysis.
- ATP Synthesis: Driven by ATP synthase through the process of chemiosmosis, where the flow of protons back across the membrane powers the synthesis of ATP from ADP and Pi.
[Image of ATP synthase mechanism in the mitochondria]
7. Exam Focus: Tips and FAQs
Exam Tip: Be prepared to differentiate between aerobic and anaerobic respiration yields. Aerobic respiration is far more efficient, yielding much more ATP per glucose molecule compared to fermentation.
Common Pitfalls
- Mistake: Thinking the TCA cycle produces the most ATP. Correction: The TCA cycle produces high-energy electron carriers (NADH, FADH2); the ETC/Oxidative Phosphorylation phase produces the most ATP.
- Mistake: Forgetting the location of different processes. Correction: Glycolysis (Cytosol), TCA Cycle (Mitochondrial Matrix), ETC (Inner Mitochondrial Membrane).
Frequently Asked Questions
Q: What are anaplerotic reactions?
A: They are reactions that form intermediates of a metabolic pathway, such as the TCA cycle, to replace those that have been diverted for other biosynthetic purposes.
Q: Why is the Pentose Phosphate Pathway important if it doesn't produce ATP?
A: It is essential for providing NADPH, which is used in reductive biosynthesis like fatty acid synthesis, and ribose sugars for making DNA and RNA.