Metabolic pathways and phenotype

• Metabolism is all the different chemicals reactions living organsims carry out to transform the food they produce or eat into all the different chemicals needed by their cells.
• Any series of reactions that changes one chemical into another and then another is called a metabolic pathway.
• Each step in the pathway is controlled by a specific enzyme that is coded for by a specific gene. The expression of each gene, through protein synthesis, produces each of the enzymes needed for the pathway. The product of one enzyme controlled reaction becomes the substrate for the next reaction.
• When there is sufficient end product, a signal stops the pathway (negative feedback). 
• Example: glycolysis is a metabolic pathway where glucose is changed into pyruvate
• Different metabolic pathways are linked through shared substrates. 
• At the end of a metabolic pathway, the end product can be used in the cell eg chlorophyll, used in another pathway, or stored in the cell eg starch.
• Some chemicals produced by the metabolic pathway become part of the phenotype of an organism eg hair or flower colour.
• Phenotype is determined by the absence or presence of specific amounts of end product of the metabolic pathway. eg plants have several pigments produced by metabolic pathways involved in photosynthesis. Leaf colour is due to the presence or absence of these pigments. High concentration of chlorophyll gives green leaves. High concentration of xanthophylls give yellow/green leaves.