3.4 Alternative Pathways of Photosynthesis

Alternative Pathways of Photosynthesis

C3, C4, CAM Plants

All plants capture sunlight for energy

But not all plants immediately proceeds with the Calvin Cycle because the nature of RuBisCO is just in quirky. In this Q.Lesson, you are expected that you have already watched the recorded lecture on photorespiration and alternative pathways to Photosynthesis.

The Problem with RuBisCO

When light reaction is done, Calvin cycle usually follows wherein carbon dioxide is fixed to a 5-carbon compound called Ribulose 1,5 Bisphosphate (RuBP) to form 2 molecules of PGA. The enzyme responsible for this carbon fixation is called Ribulose 1,5 Bisphosphate Carboxylase / Oxygenase (RuBisCO).

But RuBisCO is not very good at its job. It sometimes binds RuBP with OXYGEN instead of CO2.

Multiple Choice

The enzyme that binds the CO2 with a 5C compound in carbon fixation is:

• NADH dehydrogenase

• RuBisCO

• ATP Synthase

Multiple Select

Select all that applies:

RuBisCO can bind RuBP with

• oxygen

• carbon dioxide

• acetyl CoA

• pyruvate

Multiple Choice

When carbon dioxide binds with RuBP it creates a 6 Carbon compound that is broken apart into

• two pyruvates

• two acetyl CoA

• two PGA

• two G3P

When Oxygen is chosen over CO2

As RuBisCO can be inefficient at its job, it sometimes pick oxygen to bind with the RuBP resulting into a 5 carbon compound that breaks into one molecule of PGA (3 carbons) and one toxic molecule of phosphoglycolate (2 carbons) which is then repurposed and detoxified with the help of other organelles (peroxisome and mitochondrion) to be converted eventually into PGA.

Phosphorespiration vs Calvin Cycle

In ideal scenarios, carbon dioxide binds with RUBP and makes PGA which is either recycled into RUBP or used to produce sugars in CALVIN CYCLE.

Unfortunately, oxygen can also bind with RUBP sometimes. When this happens, it results into an inefficient process where no sugar is made. This process is called PHOTORESPIRATION.

When RuBP binds to CO2, Calvin Cycle happens. When RuBP binds to Oxygen, Photorespiration happens.

the (5C) RuBP turns into (3C) PGA and (2C) Phosphoglycolate

Multiple Choice

Plants would much rather have _________ in order to survive.

• Calvin Cycle

• Photorespiration

Multiple Choice

The first event that happens to CO2 in Calvin Cycle turns it into

• 2 carbon compound (phosphoglycolate)

• 3 carbon compound (PGA)

• 4 carbon compound (malate)

Multiple Choice

The fate of RuBP in Photorespiration is to form

• phosphoglycolate and PGA

• two PGA molecules

Multiple Choice

Photorespiration involves a long, inefficient, and tedious process of converting phosphoglycolate back into PGA with the help of

• chloroplast and ER

• peroxisomes and mitochondria

• nucleus and ribosomes

Plants have Specializations

With various adaptations to different kind of climates, plants have tried to avoid several constraints to photosynthesis. These include abilities to avoid photorespiration due to limitations in CO2 levels and little water supply due to arid conditions.

Stomata

Plants have these guard cells in their stomata that can either open or close to avoid too much transpiration (loss of water vapor from the leaves) and to control the entrance and exit of gases.

Mesophyll Cells

Light reaction and Calvin cycle typically happens in the mesophyll cells. And when this happens, oxygen concentration increases in the spongy mesophyll so the stomata needs to let it out and allow CO2 to enter.

C3 Pathyway

Some plants are just this simple. They use the C3 pathway, which means that they have ample supply of CO2 and they use that to form 3 carbon compounds - PGA. 3 carbons so its called C3 pathway.

Multiple Choice

Where does C3 Pathway occur?

• bundle sheath cells

• guard cells

• mesophyll cells

Multiple Choice

When should stomata open?

• When not enough oxygen is available

• When there is not enough water

• When there is not enough carbon dioxide

Multiple Choice

Why should stomata close?

• to avoid water loss

• to avoid oxygen loss

• to replenish lost CO2

CO2 and Water

Plants that do not have ample source of water and lives in dry conditions also face the problem of having depleted CO2 levels. They need to close their stomata to avoid water loss, they also avoid exchange of gases. For this reason, CO2 cannot easily enter.

C4 plants

Some plants are adapted for warmer climates where C3 plants cannot protect themselves from water loss. They include corn, sugarcane, and sorghum.

C4 - Malate

C4 Plants do not directly form PGA when they fix the Carbon dioxide. Instead they initially form a 4-Carbon compound (malate), hence the name C4.

Bundle Sheath

C4 plants uses 2 locations for sugar production- mesophyll cells for capturing the suns energy, and then bundle sheath cells for the calvin cycle.

PEP carboxylase

Phosphoenolpyruvate carboxylase -

This enzyme is very specific with its ability to fix only CO2 into a 3 carbon compound (PEP). Together they combine to form malate (4C). This happens in the mesophyll cells where oxygen gas is filtered behind.

Malate delivers

Malate is then transported into the bundle sheath cells where oxygen can hardly go into. Here, malate is broken by an enzyme to form again CO2 and pyruvate. Pyruvate goes back to mesophyll cells. Here, the CO2 will be received by RuBP and RuBisCO in a familiar process of Calvin Cycle.

C4 Pathway

The point of the C4 pathway is really just to make sure that when plants close their stomata to avoid water loss, they are able to specifically utilize CO2 and not the Oxygen with the help of PEP.

Multiple Choice

CO2 in C4 plants bind with

• RuBP

• PEP

• Malate

Multiple Choice

In C4 pathway, CO2 in the mesophyll cells is catalyzed by the enzyme

• RuBisCO

• PEP carboxylase

Multiple Choice

PEP carboxylate can fix carbon dioxide AND/OR oxygen into a reduced form of pyruvate.

• True

• False

Multiple Choice

Which of the following is the reason why C4 plants create malate as an intermediate molecule.

• to avoid Photorespiration by oxygen

• to keep oxygen deposits high

• to make water vapor exit the through the stomata

CAM

Crassulacean Acid Metabolism

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Some plants just cannot afford to open their stomata even for a bit during the day time if they live in extremely hot and dry climates. These plants include the cacti and pineapples. They belong to the group of CAM plants.

CAM Pathway

CAM plants live in dry and arid habitats, and the way they manipulate the opening and closing of their stomata is synchronized with day and night. They capture CO2 at night when their stomata is open, and then they close their stomata during the day to avoid transpiration.

CAM Pathway

This pathway is almost literally the same as C4. It also uses PEP to selectively fix CO2 and form malate which will then donate the CO2 to RuBisCO for calvin cycle. The only difference is the timing of getting the CO2 from the outside of the plant.

Multiple Choice

When do plants get their CO2?

• daytime

• night time

Multiple Choice

What is the primary enzyme that fixes the CO2 from the environment in CAM Pathway?

• PEP carboxylase

• RuBisCO