HD is called an autosomal dominant disorder because only one copy of the defective gene, inherited from one parent, is necessary to produce the disease. The genetic defect responsible for HD is a small sequence of DNA on chromosome 4 in which several base pairs are repeated many, many times.
Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata.
The hydrogen is provided by the NADPH, which is produced in the Light Rx and carried to the Dark Rx; the stored chemical energy as part of the glucose molecule is provided by the ATP (from the Light Rx). The overall photosynthesis process, from start to finish is summarized in the text on page 116 and in Figure 7.10.