Chemical reactions are important to living organisms because

1.they alter the activation energies necessary to initiate life processes in an organism.
2.they provide a purpose for the enzymes made by an organism.
3.they allow the organism to use matter and energy from the environment for life processes.
4.they enable the organism to decompose after it dies.

The correct answer for the fill in the blank is 3) They allow the organism to use matter and energy from the environment for life processes.

Chemical reactions form the basis of all life forms on earth. They are involved in all the essential metabolic processes, which are important for the survival of the living organism.

The chemical reactions use matter and energy from the environment in the form of food, which is then metabolized through a series of chemical reactions. This is essential for the survival of the living organism as it helps in the processes like growth, digestion, and respiration.

Thus, option 3) is the right answer.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

explanation:

substitution is the answer in this case