C 3, C 4 and CAM plants

Alternative Methods of Carbon Fixation Rubisco – the double agent! Normally rubisco adds CO 2 to RuBP but when O 2 is very plentiful it adds it to RuBP This is called Photorespiration – it occurs in light! Produces phosphoglycolate (a 2C compound) & one PGA Ie. It removes PGA molecules from the Calvin cycleDecrease CO 2 fixation and less sugar formed Reduces the efficiency of photosynthesis

Rubisco - The Double Agent! HELPFUL! ANNOYING!! 3

Photorespiration Oxygen acts as a competitive inhibitor. Rubisco is an evolutionary remnant (from a time in earth’s history when O 2 was not as prevalent Some plants have adapted strategies to work around this!

Introduction C 3, C 4 and CAM photosynthesis are the three types of photosynthesis in green plants. C 3 photosynthesis is the photosynthesis we have learned about in class. C 4 and CAM photosynthesis are adaptations to arid conditions.

C 3 Photosynthesis C 3 is so called because the first compound made when carbon dioxide is fixed from the atmosphere has three carbons (PGA). Stomata are open during the day. RuBisco is the enzyme involved in carbon fixation.

C 3 Photosynthesis Photosynthesis takes place throughout the leaf. Most plants are C3 because it requires fewer enzymes and less specialized machinery than C4 and CAM photosynthesis. It is the most efficient form of photosynthesis under normal light intensity, lower temperature and normal moisture.

C 4 Photosynthesis Called C 4 because carbon dioxide is fixed into a four carbon compound in the mesophyll cells Stomata open during the day. Dark Reactions take place in inner cells called Kranz Anatomy – Bundle-sheath cells.

C 4 Photosynthesis Uses the enzyme PEP carboxylase to fix carbon dioxide into a four carbon compound called Oxaloacetate (OAA) Converted to 4C malate which then crosses the membrane into the bundle-sheath cells. Malate is decarboxylated resulting in pyruvate. CO2 produced goes to the Calvin Cycle – much like in C3 plants. Pyruvate crosses the membrane back into mesophyll cells and converted into PEP.

C 4 Photosynthesis This works faster than C 3 photosynthesis under high light intensity and high temperature because it delivers CO 2 directly to RuBisCO thereby maximizing carbohydrate formation and preventing product loss due to photorespiration.

C 4 Photosynthesis It is also more efficient in terms of water use because PEP carboxylase brings in CO 2 faster and therefore does not need to keep the stomata open for as long, thereby minimizing water loss. C 4 photosynthesis is common in plants that grow mainly during the intense heat of summer in North America (i.e. Corn).

CAM Photosynthesis Crassulacean acid metabolism (CAM) photosynthesis is another adaptation for plants that are in arid conditions. Stomata are closed during the day and open at night to reduce water loss through transpiration.

CAM Photosynthesis These plants fix carbon dioxide during the night, going to a similar process as C4 pathway until creates malate Malate is stored in a vacuole until daytime. During the day, once the light dependent reactions have created enough energy to fuel them, the malate transfers out of the vacuole and is decarboxylated to produce carbon dioxide The CO2 is ready then to be fixed by RuBisCO and enter the Calvin cycle to create carbohydrate.

CAM Photosynthesis CAM plants often have thick, reduced leaves with a low SA to V ratio; thick cuticle; and stomata sunken into pits. Some store water in vacuoles (succulent plants). CAM plants can also be recognized as plants whose leaves have an increasing sour taste during the night yet become sweeter-tasting during the day. This is due to malic acid stored in the vacuoles of the plants' cells during the night and then used up during the day. That is why we let pineapple ripen before eating!

CAM Photosynthesis CAM plants can “CAM-idle” which allows them to keep their stomata closed at all times during extremely arid conditions and therefore any oxygen they give off in photosynthesis is immediately used in cellular respiration and vice versa. This allows the plant to survive dry periods and recover very quickly when water returns.

CAM vs. C 4 Photosynthesis Similarities Differences Both are a response to arid conditions. Both use water more efficiently than C3 plants. Both minimize the amount of photorespiration by proving carbon dioxide directly to RuBisCO. CAM plants provide CO 2 temporally (stockpile it at night and provide it during the day) whereas C4 plants provide CO 2 spatially (take it from the outer cells and provide it to the inner Kranz Anatomy). C4 plants require special Krantz Anatomy. C4 plants have stomata open during the day while CAM plants have stomata open at night.