In cells, genes are segments of dna that code for proteins; not sugars or complex carbohydrates, or lipids, but proteins. the code in the sequence of nucleotides in a gene specifies the sequence of nucleotides in messenger rna which in turn specifies the sequence of amino acids in a protein. the sequence is variable because of mutations which can change the sequence of amino acids in the protein coded for by that gene. therefore, in evolution, a change in a single code for a single amino acid may have an effect on individual survival. proteins carry out many functions in a cell such as:

Explanation:

Proteins are involved in almost every function of cell and inside the body and each protein is specific for its role. Some of them are  

1)Protein constitute major part of structure of cell. Proteins are major constituent of cell membrane along with lipids.  

2)  Enzymes which are proteins in nature catalyze all the important chemical reactions inside and outside the cell e.g transporting material across the cell, synthesizing and repair of DNA

3)Hormones which are proteins play important role in the regulation of metabolic reactions inside the cell

4)  Regulation of cell division is performed by DNA-associated proteins  

5)  Proteins plays important role in storage of iron inside the cells

6)Proteins can be stored as fats inside the body and used as energy when needed by cells for its metabolic activities.  

cellular respiration oxidizes food into energy in the form of atps.

glycolysis (6 c hexose) > 2x atps are used > 6c hexose biphosphate > 2 x 3c triphosphate >

2 x 3c pyruvate (2x nadh produced from nad) and (4x atp produced)

pyruvate actively pumped mitochondrial matrix.