BCM. 32 Photorespiration, C4 and CAM

• As are your eyes: your retina is wired in ‘backwards’, resulting in a blind spot. Cephalopod eyes are wired in in the logical way.

• But there’s no way to get from our misdesigned eyes to the cephalopod-like system, because all intermediate steps (eyes partly wired-in right) would be worse than what we have already, and would therefore be less fit (leave fewer offspring on average).

• We are stuck on our local peak unless the landscape shifts significantly, e.g. if – as a species, over a long timeframe – we no longer needed eyes (like a cave fish) and/or relied on some other sense (e.g. sonar), then the trough of despair would be less deep, and might then become traversable should we later come to need eyes again.

This is like security at an airport: better security (higher specificity) means longer queues (slower reactions) means more security guards (more RuBisCO).

Algae and bacteria can concentrate C02 and RuBisCO

• some algae have HCO3- ion pumps in cell membrane, and carbonic anhydrase packed into ‘pyrenoids’ in the stroma

• some bacteria pack RuBisCO into icosahedral carboxysomes along with carbonic anhydrase

The ammonia is recycled into an amino group in the plastid, using the glutamine synthetase pathway we discussed in BCM. This consumes reducing equivalents. A reducing equivalent is just a cofactor/currency that donates electrons and which could be interchanged with NAD(P)H: e.g. ferredoxin, thiol groups, etc.

C₄ metabolism is a neater adaptation against photorespiration found in many sub/tropical plants

• C₄ plants include maize (Zea mays) and sugarcane (Saccharum officinarum)

• but most of our crops are C₃, includeing wheat (Triticum aestivum), rice (Orzya sativa), and potato (Solanum tuberosum)

• C₄ was discovered by Hatch and Slack feeding ¹⁴CO₂ to sugarcane → first, products were C4 acids, not C3 PGA (Malic acid, Oxaloacetic acid, Aspartic acid)

Suppression of photorespiration by C₄ allows a plant to withstand stress

• Stomata narrow/close during water stress

• [CO2] in mesophyll falls to compensation point

• this is lower in C4 plants

• only half as much water lost

C4 plants show kranz anatomy - photosynthetic tissue is concentrated around veins

In C4 plants, what is the initial CO2 assimilation done by

• assimilation is by PEP carboxylase in mesophyll, not RuBisCO 

• RuBisCO is still needed, but is separated into bundle sheath 

• PEP+CO2+H20 → Oxaloacetate + P_i

General anatomy of the C₄ biochemical pump

• CO2 fixation by PEP carboxylase occurs in the mesophyll resulting in C4 acids translocated into bundle-sheath and decarboxylated

• C02 enters Calvin cycle via Rubisco as usual

• C3 fragment is returned to the mesophyll and metabolised back to PEP

• RuBisCO discriminates a little between ¹³C and ¹²C, preferentially incorporating the latter into PGA.

• PEP carboxylase does not discriminate against ¹³C nearly so much, so the ratio of ¹²C to ¹³C can be used to determine whether fossil plant material (or indeed, a bag of sugar from beet vs. sugar cane) used C₃ or C₄ photosynthesis.

Crassulacean acid metabolism (CAM) plants is like C4++

• mostly succulent xerophytes or epiphytes e.g. cactuses, crassula, pineapple 

• NADP-ME but no kranz anatomy 

• has evolved on more than 30 occasions 

• slow growing, few crops

• CAM separates PEP carboxylase and rubisco temporarily, whereas C4 metabolism spatially separates them

CAM storage mechanism

Night:

• Stomata open

• CO₂ fixed by PEP carboxylase

Day:

• Stomata closed

• Stored malate in vacuole

• Consumed by Rubisco