Plant Phys exam 3: 6 photosynthesis- C4/CAM

What did the evolution of two metabolic pathways lead to and what are they called?

limit photorespiration

• C4 Pathway (C4 carbon cycle)

• CAM (Crassulacean Acid Metabolism)

When does C4 and CAM occur and what is the consequence?

PRIOR to the Calvin Cycle

• physiology is different

• anatomy is modified

• Calvin cycle STILL occurs!

What are the separation types of the C4 and CAM pathway?

C4- spatial separation with the calvin cycle (different system)

CAM- temporal separation with calvin cycle (day= CAM vs night= calvin)

Describe C4 plants

4% of plant species have a C4 metabolic pathway

• maize, sugarcane

75% of agriculture’s worst weeds are C4 plants

• this pathway recently evolved= ~25 mil years

  • 30 different times

  • C3 and C4 plants- some do both

C4 anatomy

• ring of bundle sheath cells- tightly associated with vascular bundles (phloem & xylem tissues)

• bundle sheath cells: thick walled (impermeable to gases), large/abundant chloroplasts, tightly associated with mesophyll cells

• unique anatomy

What is unique about the C4 biochemistry in mesophyll cells?

PEP carboxylase (no RuBisCO is present)

• Carboxylates 3C PEP & produces 4C acid

• Enzyme has no affinity for O2

• Very high affinity for CO2

• Can bind CO2 at very low conc

• 4C acid is transported to BSC and is source of CO2 for Calvin cycle

What does PEP stand for?

PEP = phosphoenolpyruvate

How is PEP made?

starch

• starch is broken down at night and reformed during the daytime

Describe the Bundle Sheath Cell biochemistry

NADP Malic Enzyme: decarboxylating enzyme

• releases CO2 in BSC for Calvin Cycle

• CO2 is trapped in cell due to thick cell wall: C4 keeps conc. high

• RuBisCO is present in BSC

• Pyruvate (3C) is returned to mesophyll cell to reform PEP

• requires 2 ATP

• Enzyme is pyruvate phosphate dikinase

What is the cost of the C4 pathway?

requires extra energy

• All pathways require a minimum of 2 Extra ATP to fix 1 CO2 (overall: 5 ATP/2 NADPH per CO2 fixed)

• Some pathways require extra NADPH to fix 1 CO2 (overall: 5 ATP/3 NADPH per CO2 fixed

How do C4 plants do photorespiration?

No/little photorespiration in C4 plants

• C3 = photorespiration increases as temps increase

What is the CO2 compensation point in C4 plants?

lower in C4 plants

• CO2 compensation point: ambient CO2 concentration when the rate of CO2 uptake for photosynthesis is balanced by rate of respiratory CO2 evoluti

Reason: PEP carboxylase has a much higher binding affinity for CO2

Describe the temperature optima in C4 and C3 plants?

• Temperature optima is higher for C4 plants

• PEP carboxylase activity is maximal at higher temperatures

• At lower temperatures - C3 plants assimilate more CO2 into plant

What is quantum yield?

ratio of light dependent product produced (i.e. sugar or fresh weight) to the # of absorbed photons

C4 = steady as temps increase

C3 = decrease with increasing temps

Why do C3 plants have decrease in quantum yield with increasing temp?

need more O2… photorespiration

Why do C4 plants not change with increasing temperature

water loss- C4 is better at keeping H20 

• can close guard cells and continue to photorespire

What is the transition ratio in C4 plants?

lower in C4 plants- H20 loss through guard cells

• due to PEP Carboxylase; high affinity for CO2 - none for O2

• guard cells can change size of stoma, limiting water loss but isn’t affected by changing CO2/O2 ration

transpiration ratio = moles of water lost/ moles of CO2 fixed

C4 = 200-350

C3 = 500-1000

Describe CAM plants

• Occurs in at least 23 different families of flowering plants

• Most families are not exclusively CAM - exception is the cactus family (Crassulaceae)

  • can also have species within family that are C3, C4, or CAM

• some are facultative: turn CAM on ONLY under stress

• example- cacti, pineapple, agave, orchids

** evolved as adaption to hot/dry environments

When do CAM plants photorespire?

later in the afternoon as malate is used up

• Grow very, very slowly

• But survive in harsh environments where water is limiting (ALOT)

Describe the guard cells in C4 and CAM plants

CAM: Inverted guard cell cycle (open at night-closed during day)

• C4 made at night and stored in vacuole (guard cells open)

• C4 decarboxylated during day (when guard cells close) to release CO2

  • Light reactions and Calvin Cycle can occur during day

CAM plants anatomy

• thick cuticle (waxy covering on epidermis)

• low surface to volume rations

• large vacuoles (for storing malate)

• stomata with small apertures

• tightly packed mesophyll (to help prevent CO2 loss/diffusion during day)

** specialized to limit water loss

Which type of plant has the lowest transpiration ratio?

CAM plants

CAM = 50-100

C4 = 200-350

C3 = 500-1000

** higher ration BUT fixing lower amount of CO2

How does a low transpiration ratio benefit these plants?

• Only assimilate about ½ the carbon of C3/C4 plants

• Ability to utilize respired CO2 keeps them from losing carbon (they maintain their dry weight)

• Energy Requirements/CO2: ≈ 6.5 ATP: 2 NADPH