Biology: Cellular Metabolism

Cellular metabolism is the set of chemical reactions that occur in living organisms to maintain life.
It involves complex sequences of controlled biochemical reactions that allow organisms to grow and reproduce, maintain their structures, and respond to environmental changes.

Catabolism: chemical reactions that break down large compounds into smaller units.
Anabolism: chemical reactions that build more complex molecules from smaller ones.

Autotrophs: self-feeders that obtain energy from non-living sources such as sunlight and carbon dioxide.
Autotrophs can be classified into:
- Photoautotrophs: obtain energy from sunlight and convert it into usable energy (sugar) through photosynthesis.
- Chemoautotrophs: obtain energy from chemicals, mainly inorganic substances such as hydrogen sulfide and ammonia.
Heterotrophs: obtain energy by oxidation of organic compounds (carbohydrates, lipids, or protein).

Oxygen:
- Aerobes: live in the presence of oxygen and use oxygen to oxidize organic nutrients.
- Anaerobes: live in the absence of oxygen and catabolize nutrients without molecular oxygen.
- Obligate anaerobes: are poisoned by oxygen.
- Facultative anaerobes: can live in either aerobic or anaerobic conditions.

ATP (Adenosine Triphosphate): a high-energy compound that is the main direct fuel for cellular processes.
GTP (Guanosine Triphosphate): a high-energy compound similar to ATP, but with three phosphate groups linked to guanosine.
NAD+ (Nicotinamide Adenine Dinucleotide): a coenzyme derived from vitamin B (niacin), acts as an electron carrier in cells.
NADP+ (Nicotinamide Adenine Dinucleotide Phosphate): a coenzyme derived from vitamin B (niacin), differs from NAD+ in the presence of an additional phosphate group.
CoA (Coenzyme A): a cofactor derived from vitamin B (pantothenic).

Photosynthesis: a process that takes place in two stages:
- Light reaction (Light-dependent reaction): captures energy from sunlight and converts it into ATP and NADPH.
- Dark reaction (Light-independent reaction) or Calvin cycle: uses ATP and NADPH to power the synthesis of organic molecules.

Four major metabolic pathways:
- Glycolysis: converts glucose to pyruvic acid, producing 2 ATP and 2 NADH molecules.
- Conversion of pyruvic acid to Acetyl CoA: occurs in the mitochondria, producing Acetyl CoA, CO2, and NADH.
- Citric acid cycle (Krebs cycle): a series of chemical reactions that release stored energy through the oxidation of acetyl-CoA.
- Electron transport chain (Oxidation phosphorylation): produces ATP through the oxidation of NADH and FADH2.

Also known as the Krebs cycle or tricarboxylic acid (TCA) cycle.
A series of chemical reactions that release stored energy through the oxidation of acetyl-CoA.
Occurs in the mitochondria.
Produces 2CO2, 3NADH, 1FADH2, and 1GTP (converted to ATP in the electron transport chain).

Produces ATP through the oxidation of NADH and FADH2.
Requires coenzymes and cytochromes located in the inner membrane of mitochondria.
NADH molecules deliver pairs of high-energy electrons to the beginning of the chain.
FADH2 also enters this pathway, producing fewer ATP than electron pairs carried by NADH.

Lipids function primarily as an energy reserve.
Lipids yield 9 kcal of energy per gram, while carbohydrates and proteins yield 4 kcal of energy per gram.
Lipid metabolism involves fatty acid oxidation to produce energy or the synthesis of lipids (Lipogenesis).

Breakdown of proteins into amino acids and simple derivative compounds.
The primary reason for protein catabolism is to convert proteins into a form of energy that can be used or stored.
The first step of protein catabolism is breaking the protein down into amino acids by cleaving their peptide bonds, also known as proteolysis, to convert it to other compounds via the Krebs cycle.