Metabolism and Energetics

Individuals inherit varying percentages of slow and fast muscle fibers.
- Slow fibers: generate ATP aerobically (using oxygen).
- Fast fibers: generate ATP anaerobically (without oxygen).

All cells harvest chemical energy (ATP) from food.
Cellular Respiration: Aerobic harvesting of energy from sugar by cells.
Respiration implies:
- Breathing/exchanging gasses.
- Harvesting of energy from food molecules by cells.

Photosynthesis: Uses sunlight energy to make glucose from $H2O$ and $CO2$ , releasing $O_2$ .
Humans: Use energy in sugar and $O2$ , releasing $H2O$ and $CO_2$ .
These processes are responsible for the majority of life on earth.
Breathing and cellular respiration are closely related:
- Breathing supplies $O_2$ to cells for cellular respiration.
- Cellular respiration uses $O_2$ to harvest energy from glucose.
- The process produces $CO_2$ , which is removed by breathing.

Cellular respiration: An exergonic process that transfers energy from glucose bonds to ATP.
Equation: $C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + ATP$
- Glucose + Oxygen -> Carbon dioxide + Water + Energy
 *Exergonic: a metabolic or chemical process accompanied by the release of energy

Cellular respiration produces up to 38 ATP molecules from each glucose molecule (~40% of glucose energy).
The other ~60% of energy is released as heat.
Homeotherms maintain a body temperature of $37°C$ .
Other organic molecules can be used as an energy source.
Cellular respiration occurs in three main steps:
- Glycolysis: 2 ATP
- Krebs cycle: 2 ATP
- Oxidative Phosphorylation (Ox Phos): 34 ATP

Average adult requires ~2,200 kcal of energy per day.
kilocalorie (kcal): the quantity of heat required to raise the temperature of 1 kilogram (kg) of water by $1°C$ .
Energy is used for baseline body maintenance and voluntary activities.

The arrangement of electrons in organic molecules' chemical bonds contains energy.
When carbon-hydrogen bonds of glucose break, electrons transfer to oxygen.
Oxygen has a strong tendency to attract electrons.

Energy can be released from glucose by burning it.
- However, this energy is dissipated as heat and light, not available to living organisms.
Cellular respiration: Controlled breakdown of organic molecules.
- Energy is released in small amounts, captured by a biological system, and stored in ATP.

Glucose loses its H atoms and converts to $CO_2$ .
Oxygen gains H atoms and converts to $H_2O$ .
- Loss of electrons is called oxidation.
- Gain of electrons is called reduction.
OIL RIG (Oxidation Is Loss, Reduction Is Gain)
Equation: $C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + Energy (ATP)$

Enzymes are necessary to oxidize glucose and other foods.
Dehydrogenase: Enzyme that removes H from an organic molecule.
Dehydrogenase requires $NAD^+$ (nicotinamide adenine dinucleotide) as a coenzyme to shuttle electrons.
$NAD^+$ can be reduced when it accepts electrons and oxidized when it gives them up.

The transfer of electrons to $NAD^+$ forms NADH, the reduced form of $NAD^+$ .
- Here, $NAD^+$ is called an electron acceptor (gains an electron).
- Eventually, it becomes oxidized and is then called an electron donor (loses an electron).
There are other electron carrier molecules that function like $NAD^+$ .
They form a staircase where electrons pass from one to the next, down the staircase.
These electron carriers collectively are called the Electron Transport Chain, and as electrons are transported down the chain, ATP is generated.

Step 1: Glycolysis
- Begins cellular respiration by breaking apart glucose (6-carbon molecule) into two molecules of pyruvate (3-carbon compound).
- This occurs in the cytoplasm.

Step 2: Citric Acid Cycle
- aka: TCA or Krebs Cycle
- The citric acid cycle breaks down pyruvate into carbon dioxide ( $CO_2$ ) and supplies