Metabolism and Metabolic Pathways

Metabolism

Living things obtain and use energy, like trees using sunlight and animals consuming food, for growth and reproduction.

All chemical reactions in an organism that obtain or use energy constitute its metabolism.

Metabolic Pathways

A metabolic pathway starts with specific molecules and ends with one or more products. The product of one reaction becomes the reactant for the next step, with each step catalyzed by a specific enzyme.

Enzymes are critical; without them, metabolic pathways would be extremely slow.

Metabolic Pathway Example

Starting Molecule (A) --enzyme 1--> B --enzyme 2--> C --enzyme 3--> Product (D)

Two Types of Metabolic Pathways

Anabolic Pathways

Combine building blocks to create larger molecules.

Also known as endergonic reactions because they consume energy.

• Require an input of energy (non-spontaneous).
• Products are more complex than reactants.
• Decrease the entropy of life.
• Increase the entropy of the surroundings, increasing the universe's total entropy.

Photosynthesis Example

CO2 and H2O combine to form C6H{12}O_6. This process requires energy from sunlight and decreases the entropy of life, but increases the entropy of the environment.

Forming Water Example (Not Anabolic)

The formation of H2O from H2 and O_2 is a spontaneous process that releases energy. While the entropy of water is lower than that of separate hydrogen and oxygen, the heat released increases the entropy of the environment.

Catabolic Pathways

Break down large molecules into smaller building blocks.

Also known as exergonic reactions because they release energy.

• Release energy.
• Products are less complex than reactants.
• Increase the entropy of life.
• Increase the entropy of the environment.

Glucose Breakdown Example

Breaking down glucose to release energy is catabolic, which occurs during cellular respiration.

C6H{12}O6 is broken down into CO2 and H_2O, providing energy to cells.

This process increases the number of particles, thus increasing the entropy of life and the environment.

Digestion Example

When you eat candy, sugar (C6H{12}O_6) is digested by enzymes.

C6H{12}O6 + 6O2 + Enzymes \rightarrow 6CO2 + 6H2O + Enzymes + Energy

This catabolic pathway extracts energy from the candy and releases carbon dioxide and water.

Anabolic Pathways and Life

Anabolic pathways use energy from food to construct molecules necessary for life, like bones, muscles, and brain connections.

Organisms use energy to build larger molecules, decreasing their internal entropy while increasing the entropy of their environment.

Anabolic pathways are characteristic of living things.

Entropy Discussion

Anabolic pathways decrease the entropy of life. However, the universe is always increasing in entropy. This is possible because the processes in life increase entropy in the surroundings to a greater extent.

Enzymes at Work

Enzymes are crucial for various biological processes:

• Hydrolases – cleave molecules by adding -H and -OH.
• Nucleases – break down nucleic acids, releasing nucleotides.
• Proteases – break down proteins, releasing amino acids.
• Lipases – break down lipids, releasing smaller lipids.
• Synthases – carry out anabolic reactions, combining building blocks.
• Polymerases – synthesize DNA and RNA.
• Kinases – add phosphate ions to molecules, changing their shape.
• Phosphatases – remove phosphate ions from molecules.
• Oxidoreductases – oxidize one reactant, reduce the other.
• ATPases – power coupled reactions by breaking down ATP.
• Motor proteins – use cytoskeleton to move molecules and organelles.