4.3 Adaptation in C3, C4 and CAM plants

what is plant gas exchange?

CO2 and O2 and water vapour are related and absorbed into plants through pores called stomata, which can open and close

what is rubisco?

• an enzyme that catalyses the entry of CO2 and fixes CO2 to RuBP

• this enzyme can also bind to oxygen and allows for photorespiration to occur - opening extra ATP to ‘recycle’ this molecule

what are C3 plants?

• Comprise around 85% of the plants worldwide.

• grows best in cool-temperate (15- 25 °C), moist conditions

• Always use the Calvin cycle for carbon fixation. Taking it from

its gaseous form and fixing it into an organic molecule

• RuBisCO is the enzyme that catalyses the reaction between

RuBP and CO2

• The entire pathway takes place in the mesophyll cells.

• At high temperatures, the stomata close to reduce water loss, which

causes O2 to build up inside the leaf (unable to diffuse out). A low

CO2: O2 ratio increases the chance of rubisco binding to oxygen

(photorespiration), which results in no glucose.

what are C4 plants?

• Comprise around 3% of terrestrial plants worldwide

• grows best in warm-temperate conditions (30- 40 °C) and

tropical regions.

• Glucose production occurs in two stages

→ Carbon fixation in the mesophyll cells

→ Calvin cycle in the bundle sheath cells

• The enzyme that ‘fixes’ carbon is PEP carboxylase

• This ultimately leads to the production of Malic acid

• The anatomy of the leaves is different from that of C3

plants. Mesophyll cells are arranged around the bundle

sheath cells.

• Photorespiration can not occur due to adaptations of this

plant to ensure photosynthesis can occur in different

conditions.

what are CAM plants?

• comprise about 8% of terrestrial plants worldwide

• thrive in hot, arid environments (> 40 °C) and regions exposed to drought-deserts

• Photosynthetic pathways are similar to C4 plants

• Malic acid is produced when PEP carboxylase fixes carbon; Calvin cycle then forms glucose

separate carbon fixation and the Calvin cycle over time.

fixation at night; Calvin cycle by day (not separated by location - occurs in mesophyll cells)

open stomata only at night to collect CO2 - this also reduces water loss.

• Photorespiration can not occur due to adaptations of this plant to ensure photosynthesis can

occur in different conditions.

what is malic acid or malate?

a temporary storage unit for carbon dioxide during the night which is stored in the vacuole until it is transported out of the

vacuole and broken down to release CO2, entering the Calvin cycle during the day

what are the similarities between these pathways?

They all use the Calvin cycle to produce sugars from CO2

what are the differences between these pathways ?

C3: No separation between CO 2 fixation and the calvin cycle.

C4: between mesophyll and bundle sheath cells (space)

CAM: between day and night (time)

when does rubisco work most efficiently?

• carbon dioxide levels in leaves are high

• oxygen levels are low (as happens when water is freely available)

• when temperatures are moderate

what is PEP carboxylase?

An enzyme found on mesophyll cells that combines carbon dioxide with another sugar called PEP to form malate

• only CO2 as a substrate, not O2

→ to ensue rubisco doesn’t go to the oxygen to allow for photorespiration

what is photorespiration?

a process that occurs at high temperatures and a low CO2:O2 ratio, where rubisco uses oxygen as a substrate, rather than carbon dioxide - competitive inhibition, disrupting the Calvin cycle