Lecture 8 - More Photosynthesis

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Leaf Anatomy

• It is has a large surface area to maximize light harvesting
• They are thin so that light will penetrate through to the bottom cells
• Composed of the lamina (blade) and the petiole (stalk)
• Leaf shape is highly variable - there are literally 1000's of different shapes of leaves
• Simple leaf - developmentally it has one lamina per leaf
• Look where the petiole meets the stem - you should see a bud
• Compound leaf - developmentally it has many distinct lamina per leaf
• Look where each leaflet meets the rachis, there is no bud.
• Look where the rachis meets the stem, there is a bud
• Therefore, the entire structure is a leaf

• The outer epidermis is covered with a waxy cuticle to prevent water loss
• This also prevents gas exchange :(
• Stomates are pores in the surface of leaves which allow for gas exchange
• Pore surrounded by two guard cells
• Guard cells open and close to allow gas exchange
• The density of stomates is dependent upon ecological conditions like humidity and CO₂ concentration
• The underside of leaves is usually covered with hairs (trichomes).
• Many functions:  catch water, reduce airflow, produce wax, etc.

• A "typical" leaf cross section

• Upper and Lower Epidermis - protective function
• Lower epidermis generally contains more stomates than upper epidermis
• Palisade Mesophyll - tightly packed cells on the upper surface
• Contain three to five times as many chloroplasts as those of the spongy parenchyma.
• Chloroplasts remain usually near the cell wall, since this adjustment guarantees optimal use of light
• Spongy Mesophyll - loosely arranged cells
• Creates air spaces to facilitate gas exchange

C₃ Photosynthesis

• The first stable molecule formed after CO₂ fixation is a 3-Carbon molecule
• Most (>90%) of all angiosperms are C₃ plants
• Possess "typical" mesophyll arrangement
• Rubisco is exposed to O₂ and the plant loses energy due to photorespiration

C4 Photosynthesis - a Mechanism to Reduce the Effects of Photorespiration

• The vascular bundles are surrounded by a special type of mesophyll cell which are collectively called the bundle sheath
• The mesophyll cells do not have Rubisco
• The bundle sheath cells have Rubisco and fix CO₂ just like in C₃ plants
• But where do they get the CO₂ ?
• The mesophyll cells have another CO₂-fixing enzyme, PEP carboxylase
• CO2 + PEP (phosphoenol pyruvate) >>> OAA (Oxaloacetate), a 4-Carbon compound
• PEP Carboxylase has NO affinity for O₂
• OAA >>> Malate
• Malate is shuttled into the bundle sheath cell
• The CO₂ is removed, forming Pyruvate, a 3-Carbon compound
• Pyruvate is shuttled back to the mesophyll cell where it is converted to PEP (requires ATP)
• CO₂ enters the Calvin-Benson cycle (exactly the same as in a C₃ plant)

C₄ Photosynthesis is found in many plants, mostly in drier climates

• C₄ photosynthesis hase evolved independently many times
• All of the enzymes involved in C₄ photosynthesis were already present in the plant, so nothing new needed to evolve, just the sequence of operation
• Corn and sugar cane, two of the 10 most important crops worldwide, C₄ plants

• PEP carboxylase has a much greater affinity for CO₂ at high temps than does Rubisco, so it can assimilate carbon much more efficiently
• However, the CO₂ shuttling costs energy, so this efficiency is lost in cooler temperatures
• Also, the CO₂ shuttling becomes saturated at a much lower CO₂ concentration than does Rubisco, so when CO₂ levels are high (such as when the stomates are wide open in moist tropical plants) C₃ is more efficient

As if C4 wasn't enough, there is yet another addition/modification to the typical C3 photosynthetic pathway, called Crassulacean Acid Metabolism (CAM)

• Found almost exclusively in plants in xeric (dry) environments
• mainly in succlents, cacti, etc.
• Plants open stomates during the night - they are kept closed during the day to conserve water
• The light-dependent reactions occur during the day, creating ATP and NADPH as expected
• During the night, the stomates of a CAM plant open, CO₂ is taken up into the plant and incorporated into a variety of organic acids
• During the day, the light-dependent reactions proceed, making more ATP and NADPH - this promotes the release of CO₂ from the organic acids and Rubisco operates as normal (but in a greatly CO₂-enriched environment)
• This is more of an adaptation to conserve water than to reduce the effects of photorespiration

• What type of plant, C₃ or C₄ , would you expect to show the greatest improvement if it were grown in an artificial environment with no O₂?
• Global CO₂ levels are on the rise (don't believe anything to the contrary, this is just big business trying to protect their interests.  CO2 levels are steadily on the rise and have been since the industrial revolution!)  Sorry, off my soapbox.  Global CO₂ levels are on the rise - will this help or hinder plant growth on the earth?  What about agricultural plants?