Alternative Alternative Methods of Carbon Methods of Carbon

FixationFixation Photorespiration & CPhotorespiration & C33 Plants Plants CC4 4 Photosynthesis & PlantsPhotosynthesis & Plants CAM & CAM PlantsCAM & CAM Plants (pages 168-172)(pages 168-172)

Photorespiration & CPhotorespiration & C33 PlantsPlants

Remember the STOMA?Remember the STOMA? Stomata allow for plants to take in COStomata allow for plants to take in CO22, ,

release Orelease O22 and H and H22O.O.

OO22

Under Hot and Dry Conditions….Under Hot and Dry Conditions….

Guard cells close the stomata (or Guard cells close the stomata (or decrease its size) to conserve water.decrease its size) to conserve water.

→→ HH22O can’t get out.O can’t get out.

→ → OO22 can’t get out. can’t get out.

→ → COCO22 can’t get in. can’t get in.

↓↓↓ ↓↓↓ COCO22 and ↑↑↑ O and ↑↑↑ O22

This leads to This leads to PHOTORESPIRATIONPHOTORESPIRATION..

PhotorespirationPhotorespiration CC33 plants (i.e. soybeans, and sunflowers) plants (i.e. soybeans, and sunflowers)

use the Calvin Cycle to fix carbon.use the Calvin Cycle to fix carbon.

COCO22 is required for the Calvin Cycle. is required for the Calvin Cycle.

Photorespiration is a process in which Photorespiration is a process in which OO22 is used to produce is used to produce COCO22. .

HOW???HOW???

Rubisco (the enzyme that binds RuBP to Rubisco (the enzyme that binds RuBP to COCO22 in the Calvin Cycle) can also bind in the Calvin Cycle) can also bind RuBP to RuBP to OO22. .

When RuBP binds to When RuBP binds to OO22 it produces a 3- it produces a 3-carbon PGA molecule and a 2-carbon carbon PGA molecule and a 2-carbon GLYCOLATE molecule.GLYCOLATE molecule.

Some glycolate will leave the chloroplast Some glycolate will leave the chloroplast and go to the mitochondria to yield and go to the mitochondria to yield COCO22..

Some will be returned to the cycle as G3P Some will be returned to the cycle as G3P to regenerate RuBPto regenerate RuBP

Normal ConditionsNormal Conditions

Sugars are producedSugars are produced Cycle regeneratedCycle regenerated

PhotorespirationPhotorespiration

COCO22 is produced is produced No (net) sugar No (net) sugar

producedproduced

Photorespiration & CPhotorespiration & C33 Plants Plants

The The COCO22 produced can be used for produced can be used for

photosynthesis BUT overall, photosynthesis BUT overall, photorespiration decreases photosynthetic photorespiration decreases photosynthetic output.output.

Photorespiration & CPhotorespiration & C33 Plants Plants

Under normal conditions, 20% of fixed Under normal conditions, 20% of fixed carbon is lost to photorespiration.carbon is lost to photorespiration.

The optimum temperature for The optimum temperature for photorespiration is 30photorespiration is 30ººC – 40C – 40ººC.C.

The optimum temperature for The optimum temperature for photosynthesis is 15photosynthesis is 15ººC – 25C – 25ººC.C.

Why do Plants Undergo Why do Plants Undergo Photorespiration?Photorespiration?

Hypothesis: Rubisco evolved when the Hypothesis: Rubisco evolved when the Earth’s atmosphere was rich in COEarth’s atmosphere was rich in CO2 2 and and poor in Opoor in O22, so it did not matter that rubisco , so it did not matter that rubisco also had oxigenase activity.also had oxigenase activity.

Over evolutionary time, as OOver evolutionary time, as O22 levels levels increased, plants did NOT evolve a increased, plants did NOT evolve a modified enzyme that would only bind to modified enzyme that would only bind to COCO22 and not O and not O22..

However, some plant species have However, some plant species have evolved alternative mechanisms of carbon evolved alternative mechanisms of carbon fixation that effectively suppress the rate of fixation that effectively suppress the rate of photorespiration.photorespiration.

1.1. C C44 photosynthesis photosynthesis

2.2. CAM (Crassulacean Acid CAM (Crassulacean Acid

Metabolism)Metabolism)

CC44 Plants Plants

CC44 plants including sugar cane, corn, and many plants including sugar cane, corn, and many grasses, undergo Cgrasses, undergo C44 photosynthesis. photosynthesis.

CC44 plants have a unique leaf anatomy that plants have a unique leaf anatomy that facilitates this form of photosynthesis.facilitates this form of photosynthesis.

CC44 plant leaves contain two types of photosynthetic plant leaves contain two types of photosynthetic cells: bundle sheath cells and mesophyll cells.cells: bundle sheath cells and mesophyll cells.

Chloroplasts in CChloroplasts in C44 plants are concentrated in the plants are concentrated in the bundle sheath cells.bundle sheath cells.

CC44 Plant Leaf Plant LeafSubstances can move from mesophyll to bundle-Substances can move from mesophyll to bundle-sheath cells via plasmodesmata: cell-cell connectionssheath cells via plasmodesmata: cell-cell connections

CC44 Photosynthesis Photosynthesis

In the cytoplasm (NOT the chloroplast) of In the cytoplasm (NOT the chloroplast) of mesophyll cells, the enzyme PEP mesophyll cells, the enzyme PEP carboxylase catalyzes the reaction of COcarboxylase catalyzes the reaction of CO22

and PEP to form the 4-carbon molecule and PEP to form the 4-carbon molecule oxaloacetate (OAA).oxaloacetate (OAA).

OAA is converted into the 4-carbon acid OAA is converted into the 4-carbon acid malate.malate.

Malate diffuses from the mesophyll cells Malate diffuses from the mesophyll cells into bundle-sheath cells through into bundle-sheath cells through plasmodesmata.plasmodesmata.

Malate converts into COMalate converts into CO22 and 3-carbon and 3-carbon pyruvate. pyruvate.

Pyruvate diffuses back into the mesophyll Pyruvate diffuses back into the mesophyll to regenerate PEP, and COto regenerate PEP, and CO22 enters the enters the Calvin cycle to be catalyzed by rubisco Calvin cycle to be catalyzed by rubisco and produce sugar.and produce sugar.

Since the Calvin Cycle is localized to the Since the Calvin Cycle is localized to the bundle-sheath cells, CObundle-sheath cells, CO22 is continuously is continuously pumped into the bundle-sheath pumped into the bundle-sheath chloroplasts from surrounding mesophyll chloroplasts from surrounding mesophyll cells via malate and the Ccells via malate and the C44 pathway. pathway.

The concentration of COThe concentration of CO22 is increased and is increased and rubisco is saturated with COrubisco is saturated with CO22. .

Because there is COBecause there is CO22 available, rubisco available, rubisco won’t bind to Owon’t bind to O22 and photorespiration is and photorespiration is minimized.minimized.

Photorespiration is minimizedPhotorespiration is minimized Sugar production is maximized.Sugar production is maximized.

CC44 photosynthesis uses almost TWICE the photosynthesis uses almost TWICE the amount of ATP (compared to Camount of ATP (compared to C33 photosynthesis) photosynthesis) BUTBUT without it, without it, photorespiration would stress the plant.photorespiration would stress the plant.

The process is called CThe process is called C44 photosynthesis, photosynthesis, because the first product of CObecause the first product of CO22 fixation is fixation is a 4-carbon molecule (OAA)a 4-carbon molecule (OAA)

CAM Plants CAM Plants

CAM plants are water-storing plants CAM plants are water-storing plants (succulents) such cacti and pineapples.(succulents) such cacti and pineapples.

To conserve water, they open their To conserve water, they open their stomata at night and close them during the stomata at night and close them during the day – the REVERSE of other plants.day – the REVERSE of other plants.

Closing the stomata during the day Closing the stomata during the day prevents water loss, but also prevents COprevents water loss, but also prevents CO22

from entering the leaves.from entering the leaves.

At night, the stomata open to allow the At night, the stomata open to allow the intake of COintake of CO22..

COCO22 is converted into C is converted into C44 organic acids organic acids

(such as malate) using PEP carboxylase.(such as malate) using PEP carboxylase.

The 4-carbon organic acids are stored in The 4-carbon organic acids are stored in the vacuole until the morning.the vacuole until the morning.

When the stomata close in the morning, When the stomata close in the morning, the organic acids release COthe organic acids release CO22 molecules molecules to enter the Calvin cycle.to enter the Calvin cycle.

This process is called CAM – This process is called CAM – Crassulacean Acid MetabolismCrassulacean Acid Metabolism because because it was first discovered in the crassulacean it was first discovered in the crassulacean family of plants.family of plants.

In CIn C44 plants, 1 plants, 1stst part of part of

carbon fixation and the carbon fixation and the Calvin cycle occur in Calvin cycle occur in different compartments. different compartments.

In CAM plants, the steps In CAM plants, the steps occur in same occur in same compartment, but at compartment, but at different times of the day. different times of the day.

CC4 4 and CAMand CAM

The CThe C44 and CAM pathways present and CAM pathways present

evolutionary solutions to the problem of evolutionary solutions to the problem of maintaining photosynthesis when stomata maintaining photosynthesis when stomata close on sunny, hot, dry, days.close on sunny, hot, dry, days.

Both methods produce organic acids that Both methods produce organic acids that eventually transfer COeventually transfer CO22 to the Calvin to the Calvin

Cycle.Cycle.