We’ve already learned that plants need to leave their stomata open during the daytime, for the light dependent reactions. While it’s true, plants don’t need to take in CO 2 for the light dependent reactions, one of the by-products of noncyclic photophosphorylation needs to be emitted from the leaf, and it does so through its stomata. (O 2 from photolysis)  Oxygen build-up in plants is bad! It is a waste material that leads to photorespiration  At the same time, plants lose water through open stomata in the process of transpiration Plants also must take in CO 2 in order for the light independent reactions to occur. This CO 2 goes on to help create glucose. How can plants in hot, and dry biomes possibly allow their stomata to remain open, if it also promotes desiccation?

The most common enzyme on Earth, and indeed protein, is rubisco. It is this enzyme that participates in the C-B cycle and catalyzes the fixation of CO 2 (rubisco catalyzes the merging of CO 2 and RuBP to produce 12 PGA) Rubisco isn’t a particularly efficient molecule, because it also wants to “fix” oxygen. When oxygen is present, it becomes “fixed”, more readily than CO2 in a process known as photorespiration. The products of fixed O 2 are not particularly healthy for the plant, and most assuredly don’t lead to the formation of glucose, like the fixation of CO 2 does.  Instead, the products of RuBP and O 2 are stored in peroxisomes, which are found near the chloroplast. The peroxisome breaks down the products of photorespiration, and gets rid of the by-products.  Over Earth’s vast geologic history, the atmosphere has contained both very high (time of dinosaurs >50%), and very low (when life was still evolving-0%) levels of O 2.  Because of the very low O 2 levels when life was first evolving, plants using rubisco didn’t struggle as much with the issue of photorespiration.

C 4 plants are so named because they form a four-carbon compound as the first product of the dark reactions of photosynthesis (OAA), instead of the 3- carbon PGA formed in typical C 3 photosynthesis. Several thousand species in at least 19 families use the C 4 pathway. Agriculturally important C 4 plants are sugarcane, corn, and members of the grass family. Instead of being fixed by rubisco to form PGA, CO 2 combines with PEP to form OAA (oxaloacetic acid), using the fixing enzyme PEP carboxylase. OAA is then converted to malate, and that is shuttled to the bundle sheath cells. Here, malate is converted to pyruvate and CO 2. The pyruvate moves back to the mesophyll cells where one ATP is broken down to form AMP (not ADP) which is required to convert the pyruvate back to PEP (to help continue the cycle) The overall effect of this process is to move CO 2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient. Since bundle sheath cells rarely make contact with intercellular spaces, very little oxygen reaches them, decreasing the likelihood of photorespriation

As soon as the CO 2 is delivered to the bundle sheath cells, rubisco begins the C-B cycle (C 3 Photosynthesis). Because so little O 2 is present, little photorespiration takes place, and photosynthesis is more efficient. Because it is more efficient, the stomata don’t have to remain open as long, which decreases transpiration of H 2 O… C 4 plants are found in hot, dry places where they possess an advantage over typical C 3 plants, and are able to out-compete them. The advantage it gives them more than makes up for the added energy required (1 ATP to AMP)…two bonds are broken

In ordinary C-3 plants which form a 3-carbon compound (PGA) during the initial steps of the light independent reactions, photosynthesis in the leaf shuts down without a sufficient supply of CO 2. C-4 plants have a competitive advantage during hot summer days because they are able to carry on photosynthesis in the bundle sheaths where CO 2 levels are concentrated. Weedy C-4 plants such as Bermuda grass, spurges and purslane grow rapidly during hot summer days, while photosynthesis and growth in C-3 plants shuts down. Close-up view of a purslane leaf showing the prominent green veins. The chloroplasts are concentrated in bundle sheath cells surrounding the veins. Both C 4 and CAM are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days. However, it should be noted, that in all plants, the Calvin cycle is used to make sugar from carbon dioxide.

Some plants adapted to hot, arid regions have a different photosynthetic mechanism called CAM photosynthesis. CAM (Crassulacean Acid Metabolism) photosynthesis is found in cacti and succulents, including the crassula family. During the hot daylight hours their stomata are tightly closed; however they still carry on vital photosynthesis as carbon dioxide gas is converted into simple sugars. How do they do it? During the cooler hours of darkness their stomata are open and CO 2 enters the leaf cells where it combines with PEP (phosphoenolpyruvate) to form 4- carbon organic acids (malic and isocitric acids). The 4-carbon acids are stored in the vacuoles of photosynthetic cells in the leaf. During the daylight hours the 4-carbon acids break down releasing CO 2 for the dark reactions (Calvin cycle) of photosynthesis inside the stroma of chloroplasts.

The adaptive advantage of CAM photosynthesis is that plants in arid regions can keep their stomata closed during the daytime, thereby reducing water loss from the leaves through transpiration; however, they can still carry on photosynthesis with a reserve supply of CO 2 that was trapped during the hours of darkness when the stomata were open. The CO 2 is converted into glucose through the Calvin-Benson cycle with the help of ATP and NADPH, which were synthesized during the light reactions of daylight in the grana of chloroplasts. The tropical strangler Clusia rosea exhibits CAM photosynthesis. This unusual tree starts out as an epiphyte on other trees and then completely envelops and shades out its host.

Typical, or C 3 photosynthesis is carried out by most plants growing in areas with sufficient water. In this type of photosynthesis, an enzyme called RuBP carboxylase grabs CO 2 in the light independent reactions of photosynthesis. This works fine as long as there is plenty of carbon dioxide and relatively little oxygen. If there is too much oxygen, RuBP carboxylase will grab that instead of the CO 2, and a process called photorespiration will occur. Photorespiration does not help build up any sugars, so if photorespiration occurs, growth stops. Normally, oxygen (produced in photosynthesis) exits the plant through the stomata; however, if there isn't enough water available (as would happen under bright, hot, sunny conditions), excess oxygen may build up and trigger photorespiration, because the stomata close to conserve water. If water is present, however, this process is very efficient because both the light reactions and Calvin Cycle can occur simultaneously in the same cell, and almost all of the cells in the leaf will be producing sugars.

C 4 photosynthesis differs from C 3 in 2 key ways. First, instead of RuBP carboxylase, a different enzyme, PEP carboxylase, is used to grab CO 2. The PEP carboxylase is less likely to bind to oxygen, thus photorespiration is less likely to occur, a decided advantage under hot, dry conditions where water may be scarce and the stomata remain closed for long periods, trapping oxygen in the plant. This process is relatively inefficient, but if water is in short supply the inefficient C 4 route is still better than the C 3 route with photorespiration. Also, since there are fewer cells involved in making sugars, fewer sugars can be made. Thus desert plants can survive the dry conditions, but at the cost of rapid growth. Desert plants are often very slow to grow, and this is one of the reasons they invest so much energy in defensive structures (spines) and chemicals C 3 is best under moist conditions, C 4 under warm, sunny, dry conditions, CAM under desert conditions

Characteristics of Photosynthesis in C 3, C 4 and CAM plants CharacteristicC 3 PlantC 4 PlantCAM Plant PhotorespirationYesLittlenone Lower temp limit for photorespiration o C30-40 o C Rubisco presentYes PEP Carboxylase presentNoYes Initial CO 2 fixation directly into Calvin Cycle via Rubisco into OAA via PEP carboxylase, then to malic acid which moves from mesophyll cell to bundle sheath cell and then releases CO 2. into OAA via PEP carboxylase, then to malic acid which moves into vacuole (during night). CO 2 released from malate during day. Secondary CO 2 fixation In bundle sheath cell using Rubisco In “mesophyll”* cell using Rubisco – in morning Site of Calvin cyclemesophyll cellsbundle sheath cells“mesophyll” cells Site of Light Reactionsmesophyll cells mostly in mesophyll cells “mesophyll” cells