Untitled Notes

Understanding the transformation of food into a usable chemical form for cellular energy involves several key steps:

The first step is digestion, which breaks down food into its component chemical compounds.
Following digestion, these molecules must enter the cells where cellular respiration takes place, converting them into energy.

Digestion results in the extraction of carbohydrates and other nutrients from food, which are then transported into the bloodstream.
Nutrients, especially glucose (a carbohydrate), exit the bloodstream via capillary walls and enter tissue cells.
The primary aim of the entire process is to synthesize ATP (adenosine triphosphate), the energy storage molecule utilized by most cells.

Cellular respiration is a four-stage process aimed at energy production. Each stage plays a vital role in converting glucose into ATP.

Glycolysis, meaning "splitting sugars," is the first stage of cellular respiration, occurring in the cytoplasm.
This stage has two distinct phases:
- Energy Investment Phase:
- Here, two ATP molecules provide energy to convert glucose into a six-carbon sugar diphosphate molecule.
- Energy Harvesting Phase:
- The six-carbon sugar molecule splits into two three-carbon molecules, which are further converted into pyruvate, generating ATP in the process.
Glycolysis consists of ten enzyme-mediated steps.
Output:
- A net of two ATP molecules
- Two pyruvate molecules
- Two high-energy electron-carrying NADH molecules
When oxygen is available, NADH and pyruvate of glycolysis move into the mitochondria for the next phase of cellular respiration.

In this stage, pyruvate moves into the mitochondria, where it is oxidized to form acetyl CoA.
The oxidation process involves:
- Transfer of electrons to NAD, forming more NADH.
- Loss of a carbon atom from pyruvate, resulting in carbon dioxide (CO2).

The acetyl CoA generated from pyruvate oxidation enters the Krebs cycle.
It binds with oxaloacetate, resulting in a series of enzymatic redox reactions.
The cycle processes every carbon, hydrogen, and oxygen from pyruvate into carbon dioxide and water.
Key Characteristics:
- It is called a cycle because oxaloacetate acts both as an initial and final compound.
- For each glucose molecule fully metabolized, the cycle runs twice (once for each pyruvate).
Output:
- A net of eight NADH
- Two FADH2
- Two ATP
- Six CO2

The ETC consists of membrane-bound carriers in the mitochondria that accept and transfer electrons.
As electrons pass through these membrane proteins, they create energy that is captured to synthesize ATP.
The flow of hydrogen ions (H+ ions) pumped across the membrane generates potential energy.
When these hydrogen ions return through ATP synthase, ATP is synthesized.
Oxygen Functionality:
- Oxygen serves as the terminal electron acceptor in this chain, getting reduced to form water (H2O) as a byproduct.
Energy Yield:
- The majority of ATP, usually 32 to 36 molecules, is produced from aerobic respiration in this stage.

The four stages of cellular respiration effectively convert energy stored in glucose into ATP, which powers cellular activities.
An average of thirty-six ATP molecules is produced for each glucose molecule utilized in the process.
Oxygen is vital as it is the final electron acceptor in the electron transport chain, with carbon dioxide being released as a byproduct of metabolic processes.
The entire energy conversion journey starts from ingesting food and culminates in the cellular utilization of ATP, following the complete breakdown of nutrients.