Photorespiration, C4, and CAM Photosynthesis: Key Concepts and Pathways

Under what specific environmental conditions does photorespiration most frequently occur in plants?

it occurs on hot, dry, and bright days when plant stomata are closed.

What is the primary enzymatic reaction that defines photorespiration?

It is the oxygenation of Ribulose-1,5-bisphosphate (RuBP) catalyzed by the enzyme RuBisCO.

The enzyme _____ is described as bi-functional because it can catalyze both carboxylation and oxygenation of RuBP.

RuBisCO

During photorespiration, what molecule does RuBisCO fix instead of $CO_2$?

RuBisCO fixes molecular oxygen ($O_2$) instead of carbon dioxide.

What are the two products formed when RuBisCO adds $O_2$ to RuBP?

The products are one molecule of 3-Phosphoglycerate (3-PGA) and one molecule of Phosphoglycolate.

Which product of RuBP oxygenation can be directly used by the Calvin cycle?

3-Phosphoglycerate (3-PGA)

Which product of RuBP oxygenation cannot be used by the Calvin cycle and must be salvaged?

The 2-Phosphoglycolate (a 2-carbon molecule) cannot be used and must be salvaged.

Why is photorespiration considered a wasteful, counterproductive process?

It consumes ATP and releases previously fixed carbon as $CO_2$ without producing any sugar.

Photorespiration can drain away as much as ___% of the carbon fixed by the Calvin cycle in many crop plants.

50%

What is the name of the metabolic pathway that salvages the 2-P-Glycolate produced during photorespiration?

the photorespiratory pathway or glycolate metabolism.

Name the three cellular organelles involved in the photorespiratory salvage pathway.

The chloroplast, peroxisome, and mitochondrion are involved.

How does an increase in temperature affect RuBisCO's affinity for its two substrates, $CO_2$ and $O_2$?

As temperature increases, RuBisCO's affinity for $CO_2$ decreases while its affinity for $O_2$ increases.

What is the primary evolutionary driver for the development of C4 and CAM photosynthesis?

These pathways evolved as adaptations to minimize the effects of photorespiration in hot, arid climates.

In C4 plants, photosynthesis is divided _____ between two different cell types.

spatially

In C4 plants, the initial fixation of CO2 occurs in the _____ cells.

mesophyll

In C4 plants, the Calvin cycle takes place within the _____ cells.

bundle-sheath

What is the key enzyme responsible for the initial carbon fixation in the mesophyll cells of C4 plants?

The enzyme is phosphoenolpyruvate (PEP) carboxylase.

What is a major advantage of PEP carboxylase over RuBisCO in low $CO_2$ conditions?

PEP carboxylase has a much higher affinity for CO2 and has no affinity for O2.

In the C4 pathway, PEP carboxylase adds CO2 to phosphoenolpyruvate (PEP) to form what 4-carbon compound?

oxaloacetate.

How is the 4-carbon product (e.g., malate) transported from the mesophyll cells to the bundle-sheath cells in C4 plants?

through plasmodesmata.

What is the function of the C4 pathway within the bundle-sheath cells?

The 4-carbon compounds release $CO_2$, concentrating it for assimilation by RuBisCO and the Calvin cycle.

What is the additional energy cost associated with the C4 pathway?

The C4 pathway requires additional ATP to regenerate PEP from pyruvate in the mesophyll cells.

The extra ATP needed to power the C4 cycle is generated in bundle-sheath cells through _____ _____ _____, which involves only Photosystem I.

cyclic electron flow

What is the primary location of the enzyme RuBisCO in a typical C3 plant leaf?

RuBisCO is located in the mesophyll cells of C3 plants.

What is the primary location of the enzyme RuBisCO in a C4 plant leaf?

RuBisCO is located in the bundle-sheath cells of C4 plants.

Name two agriculturally important C4 plants.

sugarcane, corn (maize), sorghum, and millet.

Water Use Efficiency (WUE)

A measure of the unit of dry matter produced per unit of water used by a plant.

Which group of plants, C3 or C4, generally exhibits higher Water Use Efficiency (WUE)?

C4 plants have higher Water Use Efficiency.

CAM plants, typically found in arid environments, divide their photosynthetic processes _____.

temporally

During what part of the day do CAM plants typically open their stomata to take in $CO_2$?

CAM plants open their stomata during the night.

At night, CAM plants fix CO2 into organic acids, such as malate, which are then stored in the cell's _____.

vacuoles

What do CAM plants do with the organic acids stored in their vacuoles during the daytime?

During the day, the acids are broken down to release CO2 for use in the Calvin cycle, while the stomata remain closed.

What is the ultimate purpose of the Calvin cycle in all C3, C4, and CAM plants?

The Calvin cycle is essential in all three pathways to make sugar from CO2.

Which photosynthetic pathway is generally most efficient under cooler temperatures (15-25°C)?

The C3 pathway is most efficient in cooler temperatures.

Which photosynthetic pathway has a higher optimal temperature, typically between 30-40°C?

The C4 pathway has a higher optimal temperature.

Give an example of a common CAM plant.

pineapple, agave, aloe, and cacti.

What is the term for the normal, productive reaction of RuBisCO where it fixes CO2 to RuBP?

Carboxylation.

In C4 plants, after malate releases $CO_2$ in the bundle-sheath cell, the resulting _____ is transported back to the mesophyll cell.

pyruvate

The C4 and CAM pathways are often referred to as CO2-_____ mechanisms because they increase the concentration of CO2 available to RuBisCO.

concentrating

How does the leaf anatomy of a C4 plant differ from that of a C3 plant?

C4 plants have a distinct arrangement of mesophyll cells surrounding tightly packed bundle-sheath cells, often called Kranz anatomy.

What are the inputs for the Calvin cycle reactions?

The Calvin cycle uses ATP, NADPH, and CO2.

What are the primary outputs of the Calvin cycle that are used to synthesize sugar?

The primary output is glyceraldehyde 3-phosphate (G3P).

In what specific part of the chloroplast does the Calvin cycle take place?

the stroma.

Where in the chloroplast do the light-dependent reactions occur?

The light-dependent reactions occur in the thylakoid membranes.