Rate of Photosynthesis

1
Rate of Photosynthesis

• Rate of photosynthesis Amount of
  photosynthesis per unit of time. The time unit
  provides information on how fast is the process.
  Measured as either mass/volume of product
  produced (O2, sugar, etc.), or of material
  consumed (CO2, water, etc.)
• Amount alone does not equal rate
• Factors affecting photosynthetic rate (can
  become Limiting Factors)
• Light intensity
• Temperature (T)
• Concentration of carbon dioxide (CO2)
• Water availability
• Nutrient levels
• Principle of Limiting factors Each of the above
  factors can limit the rate of photosynthesis in
  an unique manner. In addition, the factors
  interact with one another. Regardless of the
  level of other factors, the factor which is in
  shortest supply has the most effect on
  photosynthesis.

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Too much Light ? Photoinhibition

• Rate of photosynthesis increases with light
  intensity, but is highest at intensities below
  full sunlight, and then declines.
• At the light saturation point, (LSP) the light
  reactions are at maximum.
• Beyond LSP, Photo-inhibition occurs
• - Chlorophyll gets more e- than what the
  e-transport system can handle.
• - Excess Energy passes to oxygen molecules, and
  makes them react with H2O forming hydroxyl ions
  (OH-) or hydrogen peroxide (H2O2)
• - OH- and H2O2 can damage pigments and proteins
  in chloroplasts. This lowers photosynthetic
  rate.

3
Too hot / too cold ? Too much CO2 ?

• ? As T increases, rate of photosynthesis
  increases to a maximum, then decreases.
• _________________________
• As CO2 increases, rate of photosynthesis
  increases to a saturation point (CO2-SP). Beyond
  CO2-SP, additional increases in CO2 have no
  effect on photosynthesis.

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Interaction of Factors on Photosynthesis

• Principle of Limiting Factors The factor in
  shortest supply has the most effect on
  photosynthetic rate.
• _______________________________
• At Light Saturation Point, an increase in light
  cant increase the rate of photosynthesis. With a
  T increase, photosynthetic rate increases beyond
  the maximum possible at lower T. ? T can be a
  limiting factor.
• At ocean surface, nutrient is often the
  limiting factor. Below the surface, light is the
  most serious limiting factor, even in conditions
  of high nutrient.
• A very rich soil in a greenhouse would not give
  maximum plant yield if the T is too cold.

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Too much Oxygen ? - Photorespiration

• Normal air O2 may inhibit photosynthesis by up
  to 50.
• Photosynthesis (favored by high CO2) In Calvin
  Cycle, the enzyme Rubisco fixes CO2 into
  sugars, and 2 molecules of PGA are formed.
• Photorespiration (favored by high O2) Due to
  excess O2, Rubisco attaches to O2 instead of CO2,
  forming only 1 molecule of PGA, and 1 molecule of
  2-carbon acid glycolate. The organism looses some
  fixed carbon, slowing the photoynthetic rate.
• - Photorespiration may help reduce
  photoinhibition, being a way of releasing excess
  light energy.
• - When water is limiting, plants may close the
  stomata during the day in hot, dry weather. This
  conserves H2O but reduces CO2 levels in leaves,
  favoring photorespiration over photosynthesis.

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Special Adaptations C4 , C3 CAM plants

• C4 plants Examples Sugarcane, corn, crabgrass
• - Photosynthesis is favored over
  photorespiration Carbon is first fixed into a
  4-Carbon acid, which only then is passed on to
  the Calvin Cycle (this avoids the joining of C
  with O).
• - Efficient at high T, keeping stomata
  partially closed to prevent water loss.
• - Generally, C4 plants grow faster, specially in
  warm climates.
• C3 plants Examples Most important food crops (
  soybeans, wheat, rice).
• - These plants use only the Calvin cycle to fix
  CO2, using enzyme Rubisco to catalyze fixation
  into the 3-C molecule PGA. Photoinhibition can
  easily happen since O2 (instead of CO2) may join
  the enzyme.
• - Photosynthesis can be inhibited by high T, by
  as much as 40-50.
• - This represents a major limitation for food
  production.
• Crassulacean Acid Metabolism (CAM) Examples
  jade plant and some succulent desert species.
• - At night, CAM plants take-in CO2 just like C4
  plants would during the day. During the day,
  stomata close to conserve water, and enzymes
  break organic acids releasing CO2.
• - Not a very efficient method. Allows survival
  in intense heat, but the plants grow very slowly.

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Leaf Anatomy of C3 and C4 plants
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C4 Photosynthesis

• 2 systems of CO2 fixation in two different parts
  of the leaf
• - Mesophyll cells Dont have rubisco, but a
  different enzyme that can avoid joining to O2.
  CO2 fixation occurs by attaching it to a 3-C acid
  in the outside of mesophyll cells, ending in a
  4-C molecule.
• - Bundle-sheat cells Have rubisco. Cells
  receive 4-C molecule, release CO2 which is
  re-fixed by rubisco, and PGA is formed as in
  Calvin Cycle.
• By concentrating CO2 in the bundle-sheat cells,
  photosynthesis is favored over photorespiration.
  Photorespiration is minimized by achieving a high
  CO2 around rubisco, as a result of the 2-part
  CO2.
• Photosynthesis by C4 plants is less negatively
  affected by high O2 concentrations than in C3
  plants.
• C4 plants grow faster in warm climates.
• Some C4 plants are twice as efficient as C3
  plants in converting light energy to sugars.

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Photosynthesis and the atmosphere

• The largest single biochemical process on Earth.
• - Produces
• 1) Oxygen
• 2) Organic matter (Carbon compounds)
• - Uses
• 1) Carbon dioxide
• 2) water
• Most organisms use O2 and release CO2.
  Photoautotrophs use CO2 again making a cycle.
• Human-caused changes (increased CO2 release,
  deforestation) are altering the cycle.
• Earths O2 has increased over geologic time.
  Recently CO2 has increased dramatically.
• C3 plants are becoming abundant in areas
  previously occupied by C4 plants. Perhaps they
  are favored in the CO2-rich atmosphere.