Metabolic pathways are the step by step biochemical reactions involved in synthesis or break down of substances in the living beings.
These pathways occur in plants, animals and also in microorganisms.
They occur in almost every cell of the body and few organs are distinctly meant for these pathways.
In animals, the liver is the critical organ for metabolism. Whereas in the plants, leaves are the organs with high metabolic activity.
Metabolic pathways exist from birth to death of any living organism.
Examples of Metabolic Pathways
1. Respiration
2. Photosynthesis
3. DNA synthesis
4. RNA synthesis
5. Protein synthesis
6. Glycogenesis
7. Glycogenolysis
8. Gluconeogenesis
9. Mevalonic acid pathway
10. Sedoheptulose pathway
Respiratory pathway: This is a pathway present in almost all the living beings as it generates essential life energy in the form of ATP. In most cases, this is an aerobic pathway which means it happens in the presence of air, i.e., by use of oxygen gas.
In the absence of air few organisms and cells can go for anaerobic respiration for a short time. Here glucose is broken down to lactic acid and thereby release required ATP as energy. But the difference is aerobic respiration releases more energy per the amount of carbohydrate metabolized then in anaerobic respiration.
In cells, this pathway occurs mainly in the mitochondria of cells.
This pathway has other pathways within it like
1. Glycolysis
2. Pyruvate pathway
3. Tricarboxylic acid cycle(TCA cycle) or CREB cycle.
Photosynthesis: This is the pathway which contributes to the production of food for all the animals on the earth. This pathway occurs primarily in plants and few microbes like Euglena, algae, etc. In plants, this pathway occurs in the leaves.
The light energy from the sun is trapped in the greenish pigment namely chlorophyll in the leaves and used to form carbohydrates.
This is an anabolic reaction wherein carbon-dioxide and water are combined to form glucose like larger molecules. The energy is entrapped in the bonds of carbohydrates. These when subjected to the respiratory pathway, breakdown to release energy.
DNA synthesis: This pathway is a multifaceted one. First purines and pyrimidines are synthesized and ribose sugars are synthesized by sedoheptulose pathways.
Once the bases and sugar are ready, DNA undergoes replication to form a new set of DNA by use of the bases and sugars.
RNA synthesis: In scientific terms, it is also called as transcription. Here mRNA (messenger RNA) is formed from DNA. This RNA is used for protein synthesis.
Protein synthesis: This pathway is called translation. Here, amino acids chains are formed on the surface of mRNA. These amino-acids chains further polymerize and aggregate to form proteins.
Glycogenesis: This is a process of conversion of glucose into glycogen in the presence of insulin. This occurs predominantly in the lever and formed glycogen is stored there for future use.
Glycogenolysis: Here the formed glycogen in the lever is broken down to release glucose in times of starvation and need.
Gluconeogenesis: This pathway is one where glucose is synthesized from non carbohydrate sources like the proteins, amino acids, and lipids. 
This predominantly occurs in case of starvation. Hence the body mass goes down due to starvation as the protein and fat stored in the body is used up to make glucose.
Mevalonic acid pathway: This is a pathway which produced five-member carbon molecules namely isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate which are used in the synthesis of cholesterol,vitamin-K, steroid hormones, etc.
Sedoheptulose pathway: This is a pathway wherein seven carbon sugar namely sedoheptulose is formed. From this sedoheptulose, ribose sugars required for DNA and RNA synthesis are obtained.