3bio4

Question 1

Function of spaces in spongy mesophyll layer

Answer 1

Help gas exchange

Question 2

photorespiration
-stomata closure
-process
-C4 and cam photosynthesis
-Efficiency

Answer 2

Plants must close automata on hot days because water evaporates faster. When stomata are closed, no CO2 enters and O2 cannot leave. Rubisco has a high affinity for both, so it fixes oxygen onto the RuBPs instead. This is photorespiration. The result is a 3 phosphoglycerate and a phosphoglycolate. The phosphoglycolate is broken down into CO2 with an apt. This is where an apt is used that otherwise wouldn’t have been. It also yeilds no sugar. Some plants in warm and arid environments evolved more efficient carbon fixation processes. C3 photosynthesis is the most common, but 60 percent of grass species use C4 photosynthesis. Many plants that do it live very warm climates. they have a higher saturation point than C3 plants. In C4 photosynthesis, carbon fixation happens in mesophyll cells, which are much more numerous than in C3 plants. An enzyme with a higher affinity for CO2, PEP carboxylase, puts a CO2 onto PEP, making oxaloacetate that is converted into malate. The malate splits into puruvate and CO2 once it is sent to bundle sheath cells, which are larger than in C3 plants, where the calvin cycle occurs. The seperates the cycle from the oxygen buildup. The pyruvate is sent back to the mesophyll. This process takes two atp. Another solution is temporal variations, done by cam photosynthesis. CO2 is taken at night when stomata open and fixed. The resulting molecule is stored for the day, when the stomata close and the Calvin cycle occurs. This allows plants to live in drier climates. Its done by cacti. C3 plants are more efficient at low temperatures and high CO2. C4 plants can more easily fix carbon in high temperatures, but high CO2 causes them to lose the efficiency advantage.