Question

REFLECT AND APPLY Metabolic cycles are rather common (Calvin cycle, citric acid cycle, urea cycle \() .\) Why are cycles so useful to organisms?

Short answer

Metabolic cycles are efficient, conserve energy, support biosynthesis, and help remove toxins.

Step-by-step solution

Introduction to Metabolic Cycles

Understand that metabolic cycles like the Calvin cycle, citric acid cycle, and urea cycle are sequence of chemical reactions that regenerate their starting components.

Efficiency of Cycles

Realize that cycles are efficient because they can continuously supply intermediate products needed for cellular functions without the need to constantly input new resources.

Energy Conservation

Highlight that metabolic cycles conserve energy. For instance, the citric acid cycle captures high-energy electrons in the form of NADH and FADH2, which are then used to produce ATP, the energy currency of the cell.

Versatility in Biosynthesis

Note that cycles can help create a variety of essential compounds. For example, the Calvin cycle in plants produces glucose, which can then be used to form other organic molecules.

Removal of Toxic Substances

Explain that some cycles help detoxify and remove harmful substances. For instance, the urea cycle helps to convert toxic ammonia into urea, which can then be safely excreted from the body.

Key concepts

Calvin cycle

The Calvin cycle is a series of biochemical reactions that occur in the chloroplasts of plant cells. It is the second stage of photosynthesis, where carbon dioxide is fixed into glucose using the energy derived from ATP and NADPH produced in the light-dependant reactions.
Key points:

• The cycle starts with the fixation of CO_2 by the enzyme RuBisCO.
• It proceeds through a series of intermediate compounds to produce glyceraldehyde-3-phosphate (G3P).
• G3P can then be used to form glucose and other carbohydrates for the plant.

This cycle helps in carbon fixation and is crucial for the biosynthesis of key organic molecules in plants, providing energy and building blocks for growth and development.

Citric acid cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a central metabolic pathway in aerobic organisms. It takes place in the mitochondria of cells and is essential for energy production.
Key points:

• It begins with the oxidation of acetyl-CoA to carbon dioxide.
• The cycle generates high-energy electron carriers NADH and FADH2.
• These carriers then drive the production of ATP in the electron transport chain.

This cycle not only supplies energy but also provides intermediates for the biosynthesis of various biomolecules like amino acids and nucleotides.

Urea cycle

The urea cycle is a series of chemical reactions that take place in the liver to convert ammonia into urea, a less toxic substance that can be excreted through the urine.
Key points:

• The cycle begins with the combination of ammonia and bicarbonate to form carbamoyl phosphate.
• Ornithine transcarbamylase then converts it to citrulline.
• Steps proceed to eventually form urea, which is excreted by the kidneys.

This cycle is crucial for detoxification, helping to remove excess nitrogen from the body safely.

Energy conservation

Energy conservation is a vital concept in metabolic cycles. These cycles allow the cell to efficiently use and recycle energy.
Key points:

• The citric acid cycle captures high-energy electrons in NADH and FADH2.
• These molecules later produce ATP through oxidative phosphorylation.
• Efficient cycling reduces the need for constant input of raw energy sources.

This principle is fundamental for the survival and growth of organisms, as it maximizes the utility of available resources and helps maintain cellular functions.

Biosynthesis

Biosynthesis is the process by which living organisms synthesize complex molecules from simpler ones. Metabolic cycles play a crucial role in providing the necessary intermediates and energy.
Key points:

• The Calvin cycle produces glucose, which can form carbohydrates, fats, and proteins.
• The citric acid cycle generates intermediates used in the synthesis of amino acids and nucleotides.
• Metabolites from these cycles are essential for cell structure and function.

This ensures that organisms can create all the compounds needed for growth, repair, and everyday functions using a few basic substrates.

Detoxification

Detoxification is the process of removing harmful substances from the body, and metabolic cycles are crucial for this.
Key points:

• The urea cycle converts toxic ammonia into urea, which is safely excreted.
• Other cycles can neutralize and remove various metabolic byproducts and toxins.
• Effective detoxification is essential for maintaining homeostasis and preventing damage to cells and tissues.

By understanding these cycles, one can appreciate how the body efficiently manages waste products and prevents the buildup of harmful substances.