In plants, the first product of carbon fixation is the three-carbon compound 3-pga formed in the calvin cycle. 2. when such plants close their stomata on hot, dry days to conserve water, builds up in the leaf and is added to rubp in place of co2. a two-carbon product of this reaction is broken down to co2, consuming atp and generating no sugar. 3. the apparently wasteful process described in sentence 2 is called . 4. in plants, co2 is first fixed into a four-carbon compound, which moves into bundle-sheath cells and releases co2 to the calvin cycle. 5. in plants, stomata are open at night, and co2 is first fixed into a four-carbon compound, which releases co2 to the calvin cycle during the day. 6. in all three types of plants, the enzyme brings co2 into the calvin cycle.