5.4

Question 1

Gases in Atmosphere

Answer 1

• CO2 makes up only 0.04% of
  the atmosphere
• Concentration 500X less than that
  of the O2
• This ratio has consequences for
  photosynthesis

Question 2

Preventing Waterloss

Answer 2

• Water is component into cytosol and is reqiured for photosynth.
• Plants have evolved adaptations to limit water loss and
  respond to changes in water availability
• Regulated by guard cells
• Waxy Cuticle
  ◦ thin waterproof layer covering leaves that helps prevent
  water loss due to transpiration
  ◦ Stomata
  ◦ Small pores of the leaf’s surface (control the rate of gas
  exchange) that can be opened and closed by guard cells
  ◦ Open during the day, allowing CO2 to enter while losing some H2O
  ◦ Closed during the night = photosynthesis STOPS!

Question 3

Photorespiration:
The Problem
with Rubisco

Answer 3

• Rubisco is a slow enzyme (catalyzes 3 CO2 per second)
• However there is a lot within cells
• Its active site can also bind with O2

◦ O2 + RuBP is not useful

• Consumes ATP and generates CO2

Instead of fixing CO2, the oxygenase of rubisco does the opposite

◦ Photorespiration
◦ b/c O2 is a reactant in the recovery pathway and CO2 is produced

Question 4

PT - 2

Answer 4

O2 and CO2 concentrations are
equal, Rubisco will bind to CO2

◦ Occurs 80X faster than binding with O2

◦ However, our atmosphere is 21% O2
and 0.04% CO2

◦ rubisco will bind
with CO2 ~75% of the time
◦ 25% of the time it binds with O2, and releases CO2
rather than fixing CO2

(this is a drain on the cell)
◦ If CO2 in the cell is significantly reduced, photorespiration becomes a
serious concern

Question 5

Plants in Hot Climate

Answer 5

• Plants in hot climates experience waterloss and photoresp.
• They need to open stomata to let
  in CO2

, but keep them closed to

conserve water
◦ When closed, less CO2

can enter and as
the CO2 present is used in The Calvin
Cycle, its concentration drops and
photorespiration increases
◦ In high temperatures, as much as
50% of the plant’s energy could
be wasted by photorespiration

Question 6

C4 Plants

Answer 6

◦ Plants found in hot dry
climates

◦Bundle-sheath cells
surround leaf veins to
separate them from
mesophyll air spaces

◦ Reduces exposure to O2

◦ Reduces
photorespiration

Question 7

C4 Plants and Steps

Answer 7

• Mesophyll cells also reduce access to CO2
• C4 plants operate a
  second carbon fixation pathway
  called the C4 cycle
• Co2 to PEP, PEP to Oxoacetate, Oxo to malate, malate to NADPH

Physical arrangement of cells + C4 pathway establishes a high
concentration of CO2 around
rubisco while reducing its exposure
to O2

• ◦ For each turn of the C4 cycle, double hydrolysis of
  ATP to AMP to regenerate PEP from pyruvate

◦ 6 ATP per G3P produced by The Calvin Cycle

Also b/c of lots of sunlight in hot climates, the
additional ATP requirement is easily met by
Cyclic Light Rxns

◦ C4 plants can open their stomata less than C3
plants, allowing them to survive better in arid env.

◦ C4 plants also require 3-6X less rubisco, therefore
have a lower Nitrogen demand

◦ This allows them to survive in nutrient-poor soil

Question 8

CAM Plants

Answer 8

Succulant Plants

• The Calvin Cycle and the
  C4 Cycle are separated IN TIME
  for better efficiency of CO2
  fixation
• Use the C4 cycle at night
• Only opens stomata at night
• To releases O2 accumulated during the day from the light rxns
• CO2 can enter, fixes malate to malic acid in cells vacoules

Question 9

CAM Plants (Day)

Answer 9

• Daylight initiates the second phase of the
  CAM processes
• Sun rises and temps. increase, stomata
  close to reducing water loss
• Malic acid diffuses from vacuoles into the
  cytosol,
• Malate—> pyruvate
• Lots of CO2 is released
• High CO2

favours carboxylase activity of
rubisco, allowing The Calvin Cycle to
proceed efficiently
◦ As pyruvate accumulates during the day, it
can then be changed back to malate at
night
◦ This process requires ATP