Organic compounds
11. American scientists Stanley Miller and Harold Urey used an apparatus similar to
the one in the diagram to simulate how life could have formed on Earth. The apparatus
contained hydrogen gas, water vapor, ammonia, and methane. These gases were subjected
to a spark, to simulate lightning. They found that organic molecules could form from the
inorganic gases. Their theory of how life began, however, did not hold up under further
testing. What crucial fact did they not have when they conducted their experiment?
A There was no lightning in Earth's early atmosphere.
B Ammonia and methane were not in the early atmosphere.
C The chemicals used in the experiment were contaminated,
D Organic molecules already existed before the atmosphere formed.
12

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:

haploid cells;