Final answer:
Humans must obtain linoleic and alpha-linolenic acid from their diet as they lack the necessary enzymes to synthesize these essential fatty acids.
Step-by-step explanation:
Humans cannot synthesize linoleic and alpha-linolenic acid, which are essential fatty acids, because of a lack of specific enzyme systems needed for their production. These fatty acids are vital for normal growth, the development of the body, and the metabolism of cholesterol. The human body lacks the necessary desaturase enzymes to introduce double bonds into fatty acids beyond the omega-9 and omega-6 positions, which is why these polyunsaturated fatty acids must be obtained through dietary sources, such as plant oils, nuts, flaxseeds, whole grains, and dark green leafy vegetables. Linoleic acid is used within the body to synthesize other important fatty acids, such as arachidonic acid, and alpha-linolenic acid can be converted to longer-chain omega-3 fatty acids, although this conversion process is inefficient, further underlying the importance of dietary intake.