2.4 - 7.5 (photorespiration & CO2 concentrating mechanisms)

What is the efficiency of Rubisco fixingCO2? Why?

Most important enzyme on planet but inefficient at fixing CO2

* because the active site of Rubisco is not specific to CO2 —a molecule of O2 can also bind to the active site and react with RuBP

What happens when O2 binds instead? Process name?

One of the products is a 2-C compound that is exported from the chloroplast and actually requires the cell to consume ATP to convert it into carbon dioxide, which is simply lost

* wasteful process celled photorespiration

What is photorespiration? Why is it called photorespiration?

A process that metabolizes a by-product of photosynthesis.

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Because it occurs in the light and is similar to cellular respiration in that it consumes O2 and releases CO2 .

Is Rubisco an ancient enzyme that is inhibited by oxygen?

Not entirely correct.

Rubisco still functions as an enzyme under those conditions, however, its function switches from a carboxylase to an oxygenase.

Why would natural selec-tion have led to the evolution of an enzyme that accepts a second substrate molecule that produces a wasteful product?

Rubisco and Rubisco-like proteins evolved at least 3 billion years ago as the primary enzyme in the biosphere for reducing carbon dioxide into organic form.

* the atmosphere 3 billion years ago contained only trace amounts of O2 and much higher levels of CO2 than today
* in these conditions, early form of Rubisco that could bind to both would be detrimental
* photorespiration became a problem when levels of O2 increased

What supports evidence that Rubisco-like proteins evolved at least 3 billion years ago as the primary enzyme in the biosphere?

Support for this comes, in part, from Rubisco being found in a huge diversity of organisms, including many bacteria and archaea (while they don’t carry out photosynthesis, some archaea do have Rubisco).

How has Rubsico evolved?

Slowly evolved to be more specific for CO2 , but the inhibition by O2 remains.

What is O2 competing with CO2 for the active site of Rubisco an example of?

Excellent example of a competitive inhibitor of enzyme function

What does Rubisco act as when oxygen binds to active site? Products created? Carbon gained?

The enzyme acts as an oxygenase instead of a carboxylase

* creates 3-phosphoglycerate & phosphoglycolate which loses Pi to form glycolate
* glycolate leads to reactions releasing CO2
* no carbon gain - 5 C out, 5 in

Why is photorespiration more detrimental?

Photoautotrophs can’t use phosphoglycolate

* In the process of breaking it down to salvage the carbon, a toxic compound called glycolate is produced.
* The elimination of glycolate through its oxidation results in the release of carbon dioxide.
* Thus, whereas the carboxylation reaction leads to carbon gain, the oxygenation reaction actually results in the plant losing carbon

What does carboxylation lead to in terms of carbon gain/loss?

Net increase in carbon by producing two molecules of the three-carbon compound 3-phosphoglycerate.

Does carboxylation or oxygenation occur in equal concentrations of CO2 and O2 in lab environment? What about in normal atmospheric conditions?

Carboxylation reaction will dominate because the active site of Rubisco has a greater affinity for CO2 than O2.

* In fact, the carboxylation reaction will occur about 80 times as fast as the oxygenation reaction.

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21% O2 and only about 0.04% CO2 in atmosphere

* oxygenation reaction can occur about once for every three times the carboxylation reaction occurs.
* means that 25% of the time, the wasteful oxygenation reaction occurs, which results in net carbon loss.

To counter the extent to which the oxygenation reaction occurs, what have species done?

Have evolved mechanisms to try to decrease the prevalence of the oxygenation reaction.

* the strategies involve using mechanisms that increase the CO2 /O2 ratio at the site of Rubisco.

What is the presence of CO2 in water? How does algae respond to addition/removing CO2?

CO2 concentration is low, not enough to saturate active site

* adding CO2 to a culture of algae doesn't result in an increase in the rate of photosynthesis, which isn’t what you’d expect

What explains the lack of response to additional CO2? What does it do? What does this mean?

Explained by the presence of a carbon-concentrating mechanism

* this pumps inorganic carbon into algal cells
* means that even when the concentration of CO2 in the water is low, the amount that is actually within the cells is kept very high by this active pumping mechanism
* ensures that the algae have a sufficient supply of CO2 for photosynthesis

What is the dominant form of inorganic carbon in aquatic environments?

Bicarbonate anion (HCO3-) not CO2

How is bicarbonate pumped into cells? What does bicarbonate do in cell? What does it result in?

By the action of an ATP-dependent transporter on the plasma membrane.

Within the cytosol, the bicarbonate is rapidly converted into CO2 by the enzyme carbonic anhydrase.

* the CO2 then diffuses into the chloroplast to the site of Rubisco

This system results in a concentration of CO2 at the site of Rubisco that is sufficiently high to out-compete the O2 present for the active site of Rubisco

Besides algae, what else face problem of photorespiration? Additional problem? Connection to addition problem?

Land plants

* face the additional problem of trying to conserve water
* photorespiration + ^ are linked

What prevent evaporation of water into air for a plant leaf? What else is prevented? What’s on the surface of plant leaf to allow movement?

The surface of it consist of a waxy cuticle

* this waxy cuticle also inhibits the flow of CO2
* the leaf surface is covered by small pores called stomata (singular, stoma) that facilitate the movement of gases into and out of the leaf

What governs the direction a gas moves though the stomata?

Diffusion (high to low concentration)

Where does CO2 diffuse? Why? Where do O2 & H2O move? Why?

Carbon dioxide diffuses into plant leaves, its concentration is higher outside the leaf than it is inside because CO2 is being consumed within the lead by photosynthesis

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Both O2 H2O (water vapour) diffuse out of the leaf because their concentrations are higher in the leaf than in the outside air;

* O2 is being made during photosynthesis by photosystem II, and water is moving through the plant following uptake by the roots.

How can plants regulate the size of their stomata? Example?

From fully closed (to minimize water loss) to fully open (to maximize CO2 uptake)

* needs to be balanced
* perfectly demonstrated by plants in dry climates

How is balancing stomata size made difficult? Why?

Made more difficult in environments that are not only dry but hot as well.

* because photorespiration becomes a bigger problem the warmer the climate.

Why is photorespiration a problem in warmer climate? Aqueous environment? Oxygenation or carboxylation common?

The reason for this relates to the effect of temperature on the solubility of gases in solution.

* the solubility of O2 & CO2 (or all gases) decreases as temp increases
* decreases more rapidly fro CO2
* means that in the aqueous environment of the chloroplast stoma, the CO2 /O2 ratio decreases as the temperature increases, and as a consequence the oxygenation reaction of Rubisco (photorespiration) becomes more common

How do plant species that have adapted to hot, dry climate minimize photorespiration?

Hav evolved a mode of carbon fixation

* addition to Calvin cycle have a second carbon fixation pathway called C4 cycle

What happens in the C4 cycle?

CO2 initially combines with a 3-C molecule phosphoenol-pyruvate (PEP)

* produces oxaloacetate which is reduced to malate

Malate travels to site of Calvin cycle, is oxidized to pyruvate releasing CO2

* pyruvate is converted back into PEP to complete cycle

Why are levels of CO2 high at the site of the Calvin cycle? What does this do?

Because CO2 is generated by the enzymatic conversion of malate to pyruvate

* this inhibits the oxygenation reaction of Rubisco, thereby minimizing photorespiration

Where does C4 cycle get its name from?

Its first product, oxaloacetate, is a four-carbon molecule rather than the three-carbon phosphoglycerate, the first product of the Calvin cycle (C3)

What is the key distinction between C4 & C3 metabolism?

It’s regarding the carboxylation reactions

What is C4 cycle metabolism like?

* initial carboxylation reaction that incorporates CO2 into PEP is catalyzed by the enzyme PEP carboxylase. 
* compared to Rubisco, PEP carboxylase has a rate of catalysis that is much faster, and, O2 can’t compete with CO2 for its active site
* it can efficiently catalyze the carboxylation of PEP regardless of the O2 concentration near the enzyme 

Where is C4 metabolism found? Where does this cycle occur in these species?

Found in many tropical plants and several temperate crop species, including corn and sugar cane.

* in these species, occurs in the mesophyll cells which lie close to the surface of leaves and stems where O2 from air is abundant

What does the malate intermediate of the C4 cycle do?

Diffuses from the mesophyll cells to bundle sheath cells located in deeper tissues where O2 concentrations are lower

* in these cells, in which the Calvin cycle operates, the malate enters chloroplasts and is converted to pyruvate and CO2

Why don’t all plants use C4 metabolism if it’s so good at preventing photorespiration? Hot climates?

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C4 pathways has an additional energy requirement

* for each turn of the C4 cycle, one molecule of ATP is required to regenerate PEP.
* In hot climates, photorespiration can decrease carbon fixation efficiency by over 50%, so the additional ATP requirement is worthwhile. As well, hot climates tend to receive a lot of sunlight, so the requirement for more ATP is easily met by absorbing more light energy and increasing the output of the light reactions.

in what temp is photorespiration not a big problem? ATP?

Temperate climates (like in Canadian), lower ambient temps

* additional ATP requirement is often harder to meet given that these regions, on average, receive less sunlight.

C4 vc C3 used by location?

Hotter temps C4

Cooler temps C3

In what other climate beside hot do C4 plants perform better than C3 plants?

Dry

* Because of the competing oxygenation reaction, C3 plants need to keep their stomata open longer to fix the same number of CO2 molecules as C4 plants
* means C4 plants lose less water and are much better suited to arid conditions

What is spatial and temporal separation? What species do it?

Spatial: running Calvin cycle & C4 cycles simultaneously in different locations

* C4 plants

Temporal: run cycles at different times

* other plants like pineapple

What are CAM plants? What kind of plants?

A C4 plant that runs the Calvin and C4 cycles at different times to circumvent photorespiration.

* CAM stands for crassulacean acid metabolism.
* plants in this group include many with thick, succulent leaves or stems, such cactus

Where do CAM plants live? What kind of leaves?

Live in regions that are hot and dry during the day and cool at night.

* fleshy leaves or stems have a low surface-to-volume ratio, and their stomata are reduced in number

When do CAM plants stomata open? What moves and where?

Only at night, when they release O2 that accumulates from photosynthesis during the day and allow CO2 to enter the leaves

* The entering CO2 is fixed/converted by the C4 to pathway into malate, which accumulates throughout the night and is stored in large cell vacuoles.

What happens to stomata when sun comes up with temp rising?

Stomata close reducing water loss and cutting off the exchange of gases with the atmosphere.

* Malate diffuses from cell vacuoles into the cytosol, where it is oxidized to pyruvate, and CO2 is released in high concentration

What does the high CO2 concentration that favours carboxylase activity of Rubisco allow for?

Allows the Calvin cycle to proceed at maximum efficiency with little loss of organic carbon from photorespiration.

* the pyruvate produced by malate breakdown accumulates during the day; as night falls, it enters the C4 reactions, converting it back to malate. During the night, oxygen is released by the plants, and more CO2 enters.

What makes CAM plants highly resistant to dehydration? Tolerate what?

Reduction of water loss by closure of the stomata during the hot daylight hours has the added benefit of making CAM plants highly resistant to dehydration. As a result, CAM species can tolerate extreme daytime heat and dryness.