rubisco and photorespiration

Question 1

rubisco; definition and purpose

Answer 1

key enzyme of the light-independent stage of photosynthesis.

either binds to CO₂ and facilitates photosynthesis
OR
binds to O₂ and creates the reaction of photorespiration

Purpose: bind CO₂ and ‘fix’ the carbon into organic molecules in order to ultimately produce glucose

Question 2

rubisco steps

Answer 2

1. Carbon fixation – which refers to the conversion of CO₂ and RuBP into 3-PGA.
   Here, we say that the carbon from the inorganic CO₂ is ‘fixed’ into an organic compound. Rubisco is responsible for taking carbon from an inorganic, gaseous form (CO2 ) and incorporating it into an organic compound (3-PGA)
2. Reduction – NADPH (with ATP, from the light-dependant stage), donates electrons to (aka ‘reduces’) an intermediate three-carbon molecule in the cycle to produce G3P.
3. Regeneration – the RuBP molecules needed to start the cycle again are reproduced.

Question 3

the problem with rubisco?

Answer 3

Sometimes, rather than using CO₂ as a substrate, it uses O₂ instead.

Rubisco is responsible for catalysing the fixation of CO₂ as the first step of the Calvin cycle, so without this Rubisco-CO2 pairing, PHOTOSYNTHESIS CANNOT PROCEED.

RUBISCO & CO₂ ⇒ Photosynthesis
RUBISCO & O₂ ⇒ Photorespiration

Question 4

how does photorespiration occur?

Answer 4

when rubisco binds to O2 instead of CO2 (which facilitates ps)

Question 5

the process of photosynthesis…?

Answer 5

…is wasteful and unwanted in plants.

In the calvin cycle there is now
no glucose ( as its O2, and not CO2, no CARBON and glucose is C6H12O6)

The binding of O₂
wastes energy, and
reduces the rate of photosynthesis as there’s less glucose decreasing the efficiency of the plant.

plant survival efficiency decrease

Question 6

why is photorespiration harmful to plants?

Answer 6

Undertaking photorespiration stops carbon dioxide from binding with Rubisco and fulfilling the light-independent stage of photosynthesis to produce glucose

Question 7

factors that influence whether rubisco binds to CO2 or O2?

Answer 7

• substrate concentration
• temperature

Question 8

substrate concentration in relation to whether rubisco binds to CO₂ or O₂

Answer 8

The more substrate is present, the greater chance it can bind to an enzyme and undergo a reaction.

Maximise PS: expose Rubisco to a high CO₂ concentration and a low O₂ concentration

1. To facilitate this, the stomata of the plant leaves open to allow CO₂ to enter the plant, while O₂ and water vapour simultaneously diffuse out of the plant.
2. When a plant needs to conserve water it will close its stomata, causing the O₂ produced during the light-dependent stage of photosynthesis to build up inside its cells.
3. As O₂ concentration in the leaf increases, relative to CO₂, rubisco binds O₂ more readily, increasing photorespiration rate.

Question 9

temperature in relation to whether rubisco binds to CO₂ or O

Answer 9

At regular or low temperatures, Rubisco’s affinity for CO₂ is far greater than that for O₂.

At higher temperatures, the affinity for O₂ is higher, leading to Rubisco binding oxygen more often.

As temperature of the plant increases, rubisco binds O₂ more readily, increasing photorespiration rate.

Question 10

C3 plants

Answer 10

‘Normal plants’ – take up 85% of plants on Earth

Undertake ‘normal’ photosynthesis

Have no adaptations to reduce the rate of photorespiration

Question 11

C3 plants light-independent stage

Answer 11

occurs within a single mesophyll cell

In C3 Plants, Rubisco is responsible for fixing carbon dioxide into three-carbon compounds (3-PGA) which then cycles through the pathway

Question 12

C4 plants

Answer 12

Light dependent stage is the same as C3

Separate Light-independent Stage into 2 stages occurring in 2 different cell types → reduce rate of photorespiration

Adapted to hot, humid conditions

Question 13

C4 plants light-independent stage

Answer 13

In C4 Plants, the initial carbon fixation and the remainder of the Calvin cycle are separated into two different cells rather than a single cell (mesophyll) like in C3 plants.

Carbon fixation occurs in mesophyll, Calvin cycle occurs in bundle-sheath cells.

This separation allows for a high concentration of CO2 around Rubisco, encouraging it to bind CO₂ rather than O₂, which decreases photorespiration and increases photosynthesis.

Question 14

process of C4 plants to reduce photorespiration (and also photosynthesis)

Answer 14

light-independent stage

1. Carbon fixation occurs in the mesophyll cells, via an enzyme called PEP carboxylase (which does not bind O2). The enzyme adds the carbon from CO₂ to a three-carbon molecule (PEP) to create a four-carbon molecule (oxaloacetate).
2. PEP carboxylase, has no affinity to bind to O₂ (unlike Rubisco)
3. Inside the bundle-sheath cell, malate breaks down and releases CO₂ , which then enters the Calvin cycle in exactly the same way as C3 photosynthesis, leading to glucose production
4. Pyruvate formed from the breakdown of malate is transported back to the mesophyll cell and converted to another molecule, PEP, with the help of ATP
5. PEP is then ready to contribute to the fixation of CO₂ and production of oxaloacetate and the cycle continues all over again.

Question 15

CAM plants

Answer 15

Light-dependant stage is the same

Light-independent separated into two stages (both in the mesophyll cells) – occurring at different times of day → which reduces rate of photosynthesis

Adapted to hot and dry conditions

Question 16

CAM plants light-independent stage

Answer 16

different times of day

The controlled release of molecules out of vacuoles ensures a high concentration of CO₂ is maintained near Rubisco, maximising photosynthesis and minimising photorespiration.

Like C4 photosynthesis, the CAM pathway requires more ATP than C3 photosynthesis to cycle PEP.

Water is also conserved in CAM plants as their stomata only open at night when it is typically cooler and more humid.
= CAM plants are very prominent in very hot dry areas like deserts.

cam plants incl. cacti, pineapples

Question 17

process of CAM plants to reduce photorespiration

Answer 17

1. At night, CAM plants open up their stomata to bring in CO₂ . The CO₂ is fixed into a four-carbon molecule (oxaloacetate) by the enzyme PEP carboxylase, similarly to C4 plants. This is converted into a different four-carbon molecule.
2. The malate (or other) molecule is stored inside vacuoles within the mesophyll cell until the daytime
3. During the daytime, CAM plants do not open their stomata to prevent water loss. This makes them very resistant to water loss.
4. The malate (or other) molecule is transported out of the vacuole and broken down to release CO₂ (meaning they can still photosynthesis)
5. The CO₂ is then free to enter the Calvin cycle in the same fashion as in C3 and C4 plants, leading to glucose production.