Proteins don't have a direct "entry point" into cellular respiration in the same way that carbohydrates (glucose) or fats do. Instead, proteins are broken down into their constituent amino acids through a process called protein catabolism.

Here's how proteins enter cellular respiration:

1. Protein breakdown: Proteins are broken down into individual amino acids by enzymes called proteases. This process can occur in the stomach, intestines, and within cells.

2. Amino acid deamination: The amino group (NH2) is removed from amino acids in a process called deamination. This process generates ammonia (NH3), which is toxic to the body and needs to be excreted.

3. Entry of carbon skeletons: The remaining carbon skeletons from amino acids can enter cellular respiration at various points depending on the specific amino acid:

* Glycolysis: Some amino acids can be converted into pyruvate, which enters glycolysis.

* Krebs cycle: Other amino acids can be converted into intermediates of the Krebs cycle, such as α-ketoglutarate or oxaloacetate.

* Acetyl-CoA: Some amino acids can be converted into acetyl-CoA, which enters the Krebs cycle.

Therefore, proteins enter cellular respiration indirectly through the breakdown of amino acids and the subsequent entry of their carbon skeletons into various stages of the process.

It's important to note that protein catabolism is a less efficient energy source compared to carbohydrate or fat metabolism. This is because protein breakdown requires more energy to remove the amino group and convert the carbon skeletons.