C3, C4& CAM, Photosynthesis (Lecture 27)
C3, C4& CAM, Photosynthesis:
- Photosynthesis: CO2 + Water –> Sugar + O2
–Photosynthesis is the production of sugar (stored energy) and oxygen using energy from the sun to combine carbon dioxide and water.
–CO2 is brought into plants and O2 is released from plants through pores (stomata) in their leaves and other tissues.
–RUBISCO is the enzyme in plants use to undergo photosynthesis.
Concepts
- Respiration: Sugar + O2 –> CO2 + Water + E
–Respiration is the burning of sugar in the presence of oxygen to release energy stored in the sugar and produces carbon dioxide and water as by-products.
- Photorespiration: Occurs under high light/heat when RUBISCO tends to react with O2 (undergoing respiration) rather than CO2 (undergoing photosynthesis). This slows rates of photosynthesis under high light/heat (this is not what the plant wants to happen).
Type 1: C3 Photosynthesis:
- Adaptive Value
–More efficient under normal light, temperature, and moisture.
- How
–Uses RUBISCO to collect CO2 during the day and undergo photosynthesis
- Who: most plants (C3. beans, rice, wheat, potatoes. most temperate crops. all woody trees).
Review: C3 Photosynthesis:
- During “regular” photosynthesis, CO2 is trapped into a 3-carbon compound by Rubisco à C3 Photosynthesis
- This 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)
Rubisco:
- ALL plants have it and use it for the Calvin Cycle.
- Rubisco usually reacts with CO2, but it can also react with O2 – 2 competing reactions.
Competing Reactions
- Rubisco can react with CO2 (Carboxylase Reaction) – good for glucose output
- Rubisco can also react with O2 (Oxygenase Reaction)
–not good for glucose output
–Even though CO2 is eventually regenerated, it wastes time and energy (occupies Rubisco)
Photorespiration:
- When Rubisco reacts with O2 instead of CO2
- Occurs under the following conditions:
–High O2 concentrations
–High heat
- Photorespiration is estimated to reduce photosynthetic efficiency by 25%
Why high heat?
- When it is hot, plants close their stomata to conserve water
- They continue to do photosynthesis —- use up CO2 and produce O2 —– creates high O2 concentrations inside the plant —– photorespiration occurs
Type 2: C4 Photosynthesis
- Certain plants have developed ways to limit the amount of photorespiration
–C4 Pathway*
–CAM Pathway*
* Both convert CO2 into a 4 carbon intermediate —- C4 Photosynthesis
Leaf Anatomy:
C4 Pathway:
- CO2 is fixed into a 4-carbon intermediate first
- Has an extra enzyme– PEP Carboxylase
that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate
- The 4 carbon intermediate “smuggles” CO2 into the bundle sheath cell
- The bundle sheath cell is not very permeable to CO2
- CO2 is released from the 4C molecule à goes through the Calvin Cycle
How does the C4 Pathway limit photorespiration?
- Bundle sheath cells are far from the surface– less O2 access
- PEP Carboxylase doesn’t have an affinity for O2 à allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)
- Significance of C4 Plants
- Increase photosynthetic yield 3 to 4 times more
- High rate of photosynthesis even when stomata is closed
- Adaptability
- Reduced rate of photorespiration
C4 Photosynthesis
- Adaptive Value 1
–Photosynthesizes faster under high light/heat conditions.
- How
–Eliminates Photorespiration by using PEP carboxylase (another enzyme) to collect CO2 during the day and hand delivering CO2 to RUBISCO so that it can’t react with O2.
- Adaptive Value 2
–Better water use efficiency
- How
–PEP carboxylase is faster at pulling in CO2 so stomata don’t have to be open as much so don’t lose as much water.
- Who
–many desert summer annuals and grasses
(corn, sugarcane, amaranth. hot, dry. mostly grasses but some shrubs (cold-tolerant)
Crassulacean acid metabolism (CAM cycle):
- First observed in family Crassulaceae
- Fix atmospheric CO2 in dark
- Accumulate large malic acid
- Accumulate organic acid in leaves at night, decrease in day
- Vacoules for accumulation of malic acid
- Stomata closed during day, open during night
- Possess xerophytic characteristics
Type 3: CAM Photosynthesis:
- Adaptive Values
1.Better water use efficiency
2.Cam-idling (defense response against severe desiccation)
3.Drought resistant
- How
1.Uses PEP carboxylase to collect CO2 during the night (when evaporation rates are less), stores the CO2 as acid, closes stomata during day when conducts photosynthesis.
2.Can keep stomata closed all the time, using CO2 from respiration to photosynthesize and O2 from photosynthesis for respiration.
Who: cactuses, agaves, bromeliads, euphorbia
CAM Pathway:
- Fix CO2 at night and store as a 4 carbon molecule
- Keep stomates closed during day to prevent water loss
- Same general process as C4 Pathway
- Has the same leaf anatomy as C3 plants
How does the CAM Pathway limit photorespiration?
- Collects CO2 at night so that it can be more concentrated during the day
- Plant can still do the calvin cycle during the day without losing water
Summary of C4 Photosynthesis
- C4 Pathway
–Separates by space (different locations)
- CAM Pathway
–Separates reactions by time (night versus day)
Comparison
- Light intensity is directly related to temperature
- C4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures
Review of Photosynthesis Types
–C3 photosynthesis
–More efficient under normal light, temperature, and moisture.
–C4 photosynthesis
–Photosynthesizes faster under high light/heat conditions.
–Better water use efficiency
–CAM photosynthesis
–Better water use efficiency
–Cam-idling
–C3 photosynthesis
–Uses RUBISCO to collect CO2 during day.
–C4 photosynthesis
–Uses PEP carboxylase to collect CO2 during day.
–Delivers CO2 directly to RUBISCO to eliminate photorespiration and is faster at pulling in CO2.
–CAM photosynthesis
–Uses PEP carboxylase to collect CO2 during night.
–Stores CO2 in form of acid. Allows idling.
–Delivers CO2 directly to RUBISCO to eliminate photorespiration and is faster at pulling in CO2.
–C3 photosynthesis
–Most plants
–C4 photosynthesis
–A variety of plants, especially our summer annuals and many of our grasses
–CAM photosynthesis
–Cacti, agaves, bromeliads, and others